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Sailing: Newsletter Challenge (06/01/10)

Posted May 30, 2010 5:01 PM
User-tagged by 1 user

This month's Challenge Question:

You are in a sailing race on a very wide river. On land there is no wind; it is a dead calm. The race is 10 Km downstream, and the river is running 5 Km/ hr. Your opponents decide that to make the best time, they will streamline their boats to the wind and float downstream as fast as they can. You decide on a tacking strategy, sailing back and forth across the river's width. Who wins the race? What is the winning time?

And the Answer is...

ou win the race, because you are sailing upwind. The winning time is: less than the time your opponents took.

When you all cast off you all feel a wind (a perceived wind) in your face as you float downstream. The wind is really the still air, relative to the shore, that you are moving past.

Relative to the river, in which you float, the wind is in your face, and all modern sailboats can sail into the wind--that is, tacking back and forth, up wind. Your path will be much longer than your opponents', but your downstream time will be faster.

Your opponents' maximum speed into the wind, and as measured by the land, will never be faster than the 5 KM/H current. Your speed will be faster by the speed you are able to sail upwind.

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#1

Re: Sailing: Newsletter Challenge (06/01/10)

05/30/2010 5:17 PM

Help me. I don't think I can use a gravity assist here. ;-)

I think I can see why the idea of tacking. I am thinking that the current across the stern versus that of across the beam is what makes the difference because across beam has a larger cross sectional area and more drag against the current.

The river is very wide (and flat), so there may be some wind. If you tack back and forth and avoid getting too close to shore (where the current is slow), you still move with the current. Any wind is a bonus that allows you to add some velocity down river, so it doesn't matter how much actual distance the ship travels, just how fast downstream you can go. But how much?

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#66
In reply to #1

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 10:57 PM

Hi Guru, it all depends on the class of yacht in the race. Each class has design specifics such as waterline length and weight. being straight up and down with no wind means the least amount of hull is in the water. to answer this question I can see a "Class" of yacht must be applied to the question. the it is simply a matter of findinding out the 'below waterline area' of the design, including any keel and not forgetting the area of the rudder when side to the current too before we can work out drag from the curve of the hull. Another factor would be weight when considering momentum and drag. Lets take it that we are floating sideways in the deepest part of the river away from the shallows which drag the hull down (and where the current is slower). Now the last factor of great importance is the material used in the building of the hull. Magnetic attraction to water can slow things up considerable. Is the hull made of a material which rides on the emissions of current put out by the H2O itself. great speed is gained here.

Now we see why it is easier to put a yacht design sideways in a Current Tunnel and measure time in relation to drag than to work out the maths manually.

it is posible to get a correct answer to this question and when you do please give it to the ownwers of Americas Cup Class Yacht Oracle. Be nice to see a good challenge from them.

Anonymous KIWI (NZ) surrounded by ocean

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#173
In reply to #66

Re: Sailing: Newsletter Challenge (06/01/10)

06/16/2010 12:11 AM

I think it is a simple matter of using the boat as a sail and the sail as the keel . The tacking boat should be able to gain speed relative to the stationary air much as if it had a perfect tailwind

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Anonymous Poster
#288
In reply to #1

Re: Sailing: Newsletter Challenge (06/01/10)

06/29/2010 11:25 AM

As noted below, it depend on the assumptions you make about boat performance. The starting point is to note that air at rest relative to land should be travelling at 5km/h relative to the water. An achievable performance for a modern yacht would be 3km/h boat sped (rel water) of 3km/h at 45deg to true wind. This gives an apparent wind of 7.1 km/h, so leeway should be low, supporting the assumption.

Hence for tacking boat speed relative to land is 5 + 3*sqrt(2) = 7.1 km/h down stream. Time taken is 10/7.1 hars = 1.4 hrs. For the drifting boat, the speed ref land is 5km/h - speed lost due to wind resistance or maybe 4.5 km/h. Time taken is about 2.2 hours.

QED,

Smug B*st*rd.

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#2

Re: Sailing: Newsletter Challenge (06/01/10)

05/30/2010 8:34 PM

I go with the tacking strategy.

Pretend the river is standing still while the land and the air are going by. Since a sailboat can make headway in the water while tacking upwind....

(Although 5 km/h is a very light wind.)

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#3
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Re: Sailing: Newsletter Challenge (06/01/10)

05/30/2010 11:29 PM

5 km/h is the current, not the wind.

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#4
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Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 2:26 AM

The air is stationary with respect to the land; the river moves at 5 km/h; thus the sailboat "sees" a 5 km/h "headwind." Make use of it by ordinary tacking (or beating) upwind.

This is sort of like relativity. Consider the river to be stationary, with the air and land moving by. Sail upwind. That's better than saying the wind is blowing the wrong way, so just furl the sail(s).

Nice puzzle on reference frames.

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#7
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Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 8:45 AM

Makes perfect sense now.

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#10
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Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 10:29 PM

Right. A 5 km/hr breeze isn't much, but with the right headsail you'll get 2-3 knots from it at about 45 deg to the wind. That gives you a 1.5-2 knot advantage over the competition.

Only problem is that they'll see what you're doing and copy you.

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#17
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 12:19 AM

I can understand the relative wind applied to the sails. If each craft has the same amount of sail, the relative wind will create the same amount of relative drag. Unless they break out an orr, ... it's still a raft.

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#20
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 2:44 AM

Check the earlier threads (see my post #19), and perhaps it will start to make sense.

The boats that are trying to drift will take the sails down so that they are pushed back less by the apparent wind generated by the boat's motion. In a 5km.h breeze, a typical sailboat with bare poles will move very slowly away from the wind (a couple tenths of a km/h). (With the nose into the apparent wind, the boat will be moving slowly backwards relative to the water.) With sails up and flogging, it will move faster away from the wind (half a km/h). With the sails trimmed, and with the boat sailing 45 degrees off the wind, the boat will move forward.

The aerodynamic force causing the sailboat to sail upwind is lift*, not drag. Lift is always perpendicular to the apparent wind direction, so in this case it is 45 degrees forward of abeam, tending to both push the boat sideways (resisted by the keel) and forward. The aero drag vector (from sail drag) is aligned with the apparent wind, so tends to pull the boat sideways and rearward. The lift/drag ratio for a typical sail is about 6 or 7 to 1.

* In this case the "wing" is standing on end, so "lift" is now horizontal, instead of vertical as in an airplane.

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#25
In reply to #20

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 7:00 AM

Good to see someone else remembered this problem (just found the thread).

Oh, you're really new here, so your must have used the excellent search facilities this site has.

Hmmm...this is good EinP material....

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#63
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 7:49 PM

I would guess that if you were floating down river and you "sculled" the rudder (back and forth) you could generate more speed. So out of all the boats in the water,(and it did not say how many) the crew that was sculling in the middle (and they all would be) of the river wins. Imagine how much energy would be wasted with everyone trying to tack and obey the rules of sailing. Which boat has the right of way?

Side note. In the 60's, my friend picked up a new hoby-cat in Dana point and sailed it up to San Pedro, where he turned out and set his course for Ismus Catalina. A pea soup fog rolled in, so he "sculled" and dead reckoned across. I'll be darned he hit Catalina almost dead on. It's the nature of sailors, and in a race , they use every advantage, (and i do mean every)

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#174
In reply to #3

Re: Sailing: Newsletter Challenge (06/01/10)

06/16/2010 1:49 AM

if the current is 5 km/h it is creating 5 km/h headwind that you can take advantage of by tacking !

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#67
In reply to #2

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 11:18 PM

Tornado,

As with all rivers an airflow is created with the wash of the water going by. If the current is 5 knots....how much wind will it create going by and up to what height off the water will the wind be created....will it help fill the sails if left up?...or will it be low down and only act on the sides of the hull???? This would have to be a factor in momentum as it will be further weight added onto onto side of the vessel versus the drag from mast and rigging and hull when sails are down.

I strongly suggest a handkerchief (smallest stormsail) fored and smallest storm main or maximum reefing in the main. I am judging the windspeed travelling with the current will be approx the same as the current and a little low down sail area will help the vessel along nicely even to the point that travelling 'stern to the current' might show greater hull speed than sideways...all due to the wind created by the current in the same direction of the current. Hence the water has created a 'slipstream' above it!!!!!

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#5

Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 8:04 AM

The clue is in the Frames of Reference, as Tornado said. The streamlined boat stays still in the water's F of R and must wait two hours for the 10k mark to arrive; meanwhile the tacking boat makes a little progress against the wind and goes to meet the 10k mark first.

The tacking boat wins.

I had another reason for picking the tacking boat, too. The question wouldn't have been worth asking if the streamlined boat won.

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#6
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Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 8:44 AM

So, how do you calculate the time for the tacking boat?

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#8
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Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 8:58 AM

I don't know if it is possible to calculate time, the skill of the captain comes into play. And if for optimum tacking, we need to know the area of sail and the boat's "sail" area resisting the sideways drift in the water. Probably some other stuff beyond my ken, too.

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#282
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Re: Sailing: Newsletter Challenge (06/01/10)

06/28/2010 10:31 PM

It is very simple: once you step on a sail boat you are there.

Hours don't count. It never worked out for me sailing the Atlantic. I even made it never twice the same time from the Bahamas to Florida. Only motor boaters need to calculate, to see if they have enough gas or diesel.

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#9

Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 11:40 AM

I'm assuming because winds are more prevalent along the shoreline because of the temperature difference that there is no wind even on the river. Tacking with no wind would be a bitch. With this thought I would suspect my opponent would win in a little over 2 hours. It would depend how long it would take to get up to speed to match the current. If I could I would drop a sail in front of the boat and let the current pull me along with a drag line. Unless this was against the rules.

Just a little outside the box.

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#28
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 7:57 AM

Tacking with no wind is simple enough, you roll tack the boat by having the crew move from one side of the boat to the other. The keep will also create some lift as this is done. A good roll tack will actually accelerate the boat.

Putting the crew on the leward side of the boat will also cause the sails to shape to a better foil in light air. This goes hand-in-glove with a roll tack.

You don't need a drag line unless the current is faster at a depth other than the surface. The whole river is moving so you move with it.

Of course the river motion also moves the air at the boundary layer. Better have a tall rig!

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#11

Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 11:07 PM

I once owned a sailboat and smell a red herring in this puzzle.

The fastest part of any stream is down the middle. But suppose the stream is moving at the same 5kph even at the edges and suppose the edge of the stream is 1 km from the middle, the boat that tacks at 45 degrees to the center line will have traveled the length of the hypotenuse times 2 or 2√2 for every 2 km the middle of the river boat travels. When the middle of the river boat travels 10km the tacking boat will have traveled 10√2 km. Both boats will have traveled at 5kph. So the mid stream boat will cross the line in 2 hours and the tacking boat will take 2√2 hours. Or about 0.8 hours after the middle of the line boat. You will not make the boat go faster by tacking unless the wind is in the face of the boat. As there is no wind you are at the mercy of the stream speed.

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#12
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Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 11:32 PM

I'm not a sailor but if there is no wind the energy to move me is in the water so I'm dropping my sail in the water in the middle of the river and let it drag me to the finish line .

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#18
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 12:19 AM

Remember apparent wind, you get more than 5Kph across the sails, 45sin(ships speed) plus the 5 Kph wind, but you loose leeway and also hull friction (see "wetted surface friction).

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#175
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Re: Sailing: Newsletter Challenge (06/01/10)

06/16/2010 2:11 AM

there is a head wind of 5 kph created by the 5 kph currant, allowing the tacking boat to arrive 10 klm down stream sooner than the floater exact time ? but less than 2 hrs!

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#13

Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 11:44 PM

With no wind, you are on a raft. It's a tie and 2 hours.

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#14

Re: Sailing: Newsletter Challenge (06/01/10)

05/31/2010 11:50 PM

A couple of things here:

To make a better speed towards the finish line, one must extract energy from the system somehow. Tacking side to side may make one feel like he has control, and indeed increase the apparent wind across the sails, but this is at the expense of the increased friction against the keel, the keel water interface not being ideal, leeway comes into plya also.

Imagine two water skiers behind identical boats, skier a maintains a straight line behind his boat doing 25 mph, skier b slaloms like crazy behind his boat, which is also doing 25 mph. Skier b certainly attains some crazy velocities, but the big thing here is "VMG", a sailing term for Velocity Made Good, sin(angle between current course and the direct course to the mark) times speed; skier b's VMG is the same as skier a, they get to the line at the same time. However, in the skier example, b can pick up a lot of speed near the line and turn up stream and cross before a, he gets his energy from the tow boat.

In the sailing example there is no such "extra" energy to be recovered, in fact the tacking boat will give up energy to the system through friction from the hull travelling extra distance through the water and will in fact lose

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#15
In reply to #14

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 12:04 AM

You can't tack anything. There is no wind. Again, you are on an expensive raft.

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#16
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 12:15 AM

Of course there is a wind, the current is carrying you into the "still" air, creating an apparent wind, given a sufficiently wide stream, one could not tell if there was a real wind, or one caused by the boat being borne downstream by the current.

Frames of Reference again, but we do have a fixed start a finish line establishing a sort of "absolute" frame of reference here.

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#30
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 8:30 AM

Yes. The 'trick' mentally is to ignore the land. Imagine the river is so wide you can't see the shores. Your boat sits on the water and you have no sense of motion. Then you notice a small headwind. You then tack back and forth sailing into this wind.

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#33
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 9:51 AM

Exactly.

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#52
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 1:42 PM

A sail boat does make apparent wind. Check out a magazine for Catamaran & Trimaran sail boats like those in the America's Cup and you will see those boats sail faster then the windspeed by creating their own wind which is apparent wind. I know a lot of factors are being taken for granted or neglected, such as friction, etc. but the current would create an apparent wind then it is just the drift speed versus angular drift and Wind to help with tacking boat winning.

I like all the other varibles being discussed cause they will have an effect but make the overall puzzle to complicated.

Just my two cents worth.

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#38
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 11:39 AM

You extract energy by putting up the sail. As mentioned in earlier posts, relative speed of the air vs. the water is the same whether the air is moving (wind) over a still lake or the water is moving (river current) through still air. The relative speed then determines how much energy you can extract. Tacking is the only way to extract energy in a way that provides force in the opposite direction of the wind, which is what we want in this case, because the alternative (floating) extracts zero energy.

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#44
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 12:18 PM

I must disagree. With moving air, one is extracting energy from the wind (by changing both its' direction and and its speed), this energy is transferred to the vessel, some of it propelling the boat forward against the drag of the water, (and thereby heating the water), you are in fact transferring the kinetic energy of the moving wind to thermal energy in the water,. In the scenario presented you are trying to extract kinetic energy from the moving water and transfer that to the still air, at the same time using a bit to make the boat go faster, I submit you are comparing apples and oranges, the two are completely different and there is no way to make the boat go towards the finish line any faster.

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#54
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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 2:42 PM

The physics are the same whether the air is moving relative to the water, or the water is moving relative to the air.

The keel and sail are effectively identical devices, (both foils) and are differently sized because of the difference in density between the fluids. Both generate lift and both generate drag. Both heat up the surrounding media. (The underwater and above water parts of the hull have the same effects.) In the fastest soft water boats, both the keel and sail must operate at high L/D ratios.

The alternative presentation of this question is interesting, because in the case of the drifting boats and wind both at 10 knots moving eastward, no energy is transferred, and no work is done by the water to the boat, by air to sail, etc. If sails were hoisted they would not flutter or fill. "Sailing" with bare poles or full (sagging) sails would make no difference.

But in the morning (no wind on shore) then work is done and energy can be transferred. The air gets stirred, the water gets stirred (and both heat up) and some of the energy actually moves the boat relative to the water.

Non sailors, when on a first sailboat ride, are suprised when turning from downwind (when there is very little wind across the boat) to upwind. All of a sudden, the day turns from hot and still to "windy".

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#77
In reply to #44

Re: Sailing: Newsletter Challenge (06/01/10)

06/02/2010 8:06 AM

Answer #4 sums it up, but it is an interesting question; where do we get the extra energy to beat the other boat? I think the answer is that our tacking boat is extracting energy from a greater volume of water per second.

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#79
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Re: Sailing: Newsletter Challenge (06/01/10)

06/02/2010 1:13 PM

I think the answer is that our tacking boat is extracting energy from a greater volume of water per second.

A related question: An ice boat can sail at high multiples of wind speed (as much as 8 times wind speed, but 4 times is not uncommon). On a day with a ten knot breeze, a typical boat will go 40 knots. How is this possible?

If you draw the vector diagrams and do the math you find that it is not too surprising that the same ice boat that cannot start by itself in light winds (let's say 5 knots) (and needs to be pushed to get going [partly to generate apparent wind, partly to melt the ice under its blades]) is generating plenty of horsepower when it subsequently sails at twenty knots. Windsurfers, speed sailors, land yachters, and ice boaters all say "the boat creates it's own wind." At first glance, and even after a little thought, that sounds silly... bootstraps, perpetual motion, etc. But in fact it is true. Fast windsurfers in some conditions cannot get on plane without pumping, but once on plane, shoot ahead and are sailing in a 20 knot breeze when their non-pumping friends are sailing in a 10 knot breeze, and doing only perhaps 4 knots through the water. The 20 knot breeze generates four times the force on the sail as does the 10 knot.

You'd think, that because the wind that the faster windsurfer sees is very close to over the bow, the the disadvantageous vector angle would cancel the effect of greater wind speed. (In fact, again, you'd think that it must be so -- otherwise, it seems like perpetual motion again.) But when you do the vector math, you find that the forward component of lift is still much greater (and the sideward component [heeling force] is hugely greater) than the more effectively directed (but lower magnitude) lift created by the (roughly) ten knot wind that the slower windsurfer sees. Looked at in terms of power, the situation is quite interesting: The slow guy has (let's say) 10 pounds force pushing him through the water at 8 feet per second: .145 hp. The fast guy has perhaps only 25 lb pushing him (because the 40 lb total force is poorly aimed) but he is going through the water at 16 fps: .727 hp (These figures are lower than the actual forces and powers involved, but illustrative.)

All of which is a long way of saying, yes, I think you're right.

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#82
In reply to #79

Re: Sailing: Newsletter Challenge (06/01/10)

06/02/2010 3:36 PM

More to ponder:

Interesting polar plots here.

You can see that an 18 foot skiff (sometimes called "Australian Eyedeens") will sail at a VMG directly downwind of about 12 knots while sailing in a 10 knot breeze. Boatspeed in that case is 16 knots on a course of 145 degrees. As you can see, if the sailor headed further down wind by just a few degrees, the boatspeed and VMG fall off dramatically)

Iceboat polor plots are more dramatic. Ice boats tack downwind (whereas the standard water sailboat "jibes" downwind (meaning the apparent wind passes across the back of the boat). When an iceboat tacks downwind the apparent wind crosses the front of the boat. So an ice boat, if sailed well, always has the wind off its nose, even when sailing downwind.

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#41
In reply to #14

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 11:41 AM

I am all with you, and the few others that see the light !!! From a previous discussion :

"One can effectively tack and make way against the wind, at a hull speed even in excess of the wind but one cannot make way against the direction of the wind at a speed exceeding the speed of the wind itself which I believe represents an absolute limit" :

The vector representing the "ground trajectory" of the boat is the result of various forces : one is the apparent wind (river flow). The others are many but are all resulting from the first : Thrust (lift) generated by the sails, drag in the water, drag in the wind from the boat superstructure, opposition to any drifting by the keel... While the trajectory vector can be many times the lenght of the apparent wind speed vector, I maintain that there can be no geometric solution that would provide a resulting trajectory vector that would end "windward" of the apparent wind vector.

I had the feeling that when previously posted, that question could have had implications on space travel (Jorrie was the original poster :-)

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#284
In reply to #14

Re: Sailing: Newsletter Challenge (06/01/10)

06/29/2010 9:59 AM

Obviously, has never been on a sail boat!

Strictly a powerboat guy, as am I, who much prefers skiing over sailing.

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#286
In reply to #284

Re: Sailing: Newsletter Challenge (06/01/10)

06/29/2010 10:11 AM

Indeed, if you read my later posts on this subject, I realized I was not seeing the problem correctly. One most certainly can sail in this circumstance and doing so will cause you to win the race by a substantial margin.

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#19

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 2:18 AM

Seems there have been a few of these sailing questions over the years.

It's all relative.

Tacking is certainly faster, with a speed of about 3 km/h through the water being feasible on a fast boat in this breeze. If the boat is well sailed, .7 of this will be VMG, so the speed over the ground will be about 2 km/h + 5 km/h or about 7 km/h. Time would be about 1.4 hours.

The drifters would be pushed back a bit by aero drag. So maybe 2.1 hours would be required.

Other threads:

http://cr4.globalspec.com/blogentry/28/River-Sailing-Newsletter-Challenge-09-13-05

http://cr4.globalspec.com/thread/38222/Sailboat-on-River

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#31
In reply to #19

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 8:46 AM

MoronicBumble has saved me the trouble of working it out. Score 5 for good answer.

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#162
In reply to #31

Re: Sailing: Newsletter Challenge (06/01/10)

06/12/2010 5:38 PM

You'll be the priest, then. Seen Nathan anywhere? He's the prophet....

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#129
In reply to #19

Re: Sailing: Newsletter Challenge (06/01/10)

06/08/2010 7:35 PM

This is the only correct answer I have read so far. this situation is not uncommon to regular sailors wether it be current in a river. or more commonly created by the tide on a windless day. You sail on the apparent wind and make headway against stationary boats( boats not sailing)

Westwind

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#21

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 2:51 AM

Assuming that the river is sufficiently wide, I'm going to guess that you could get up to a speed double that of the apparent wind (with the boat sailing at 30° to a line across the apparently still river):-

This seems very unlikely but it would give you a winning time of just 1 hour (½ that of your opponents).

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#37
In reply to #21

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 11:23 AM

Except you have to travel twice as far...

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#39
In reply to #37

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 11:39 AM

No! I've managed to get a distance covered speed of 5 km per hour relative to the apparently still water. Having looked at several of the other sensible answers I have overestimated the speed of the boat in the water, but, the principle still holds good.

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#49
In reply to #39

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 1:23 PM

In "ordinary" sailing boats (those in which boatspeed/windspeed is less than 1) the best VMG upwind is at an angle of 45 degrees off the wind. The angle is surprisingly constant, regardless of boat design (again for conventional sailboats). Even in ultra-fast soft water boats (in which boatspeed can be 2-3 time windspeed (with true wind abeam) and in ice boats (in which boatspeed can be 8 times windspeed) still, the best angle upwind is close to 45 degrees off the wind, somewhat surprisingly. Many of the average performance boats I've raced will make 3 km/h in a 5 km/h wind, yielding 2 km/h VMG directly to windward.

As you say, you overestimated speed for a typical boat, but the principal is correct.

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#22

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 3:47 AM

For those struggling with the concept of 'sailing' consider, for a moment, two sailing boats, one bare poles, the other fully rigged with a capable crew. They are on a lake (no current) with a 5km/hr breeze. Who will make the most progress in any direction? Typically boat speed is a third to one and a half times the true wind speed. Depends on the boat largely. The only snag is the sailing from bank to bank, no competent skipper would do that. The 'sea breeze effect' would be square to the shoreline as the sun heated land warms the air above it. That would be a bonus but only out of the main river flow and time of day and sun strength is not mentioned. Cheers

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#23

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 5:53 AM

The apparent wind of 5km/hr is caused by the friction of the water pushing the boat and this is all the "power" you have.

Any method that causes you to travel a greater distance through the water (tacking) will be directly at the sacrifice of the pushing force of 5kms per hour. This will reduce the downstream speed of the boat.

The boat that travels the shortest distance wins the race, thus the drifting boat that is streamlined against the wind will win in a little over two hours. If the sails are cast into the water to form a type of sea anchor, the time will be best and the bow of the boat can be easily kept into the apparent 5km/hr head wind.

There is no perpetual motion and there is no such thing as a free lunch.

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#305
In reply to #23

Re: Sailing: Newsletter Challenge (06/01/10)

06/30/2010 12:10 PM
  • The apparent wind of 5km/hr is caused by the friction of the water pushing the boat and this is all the "power" you have.
  • Any method that causes you to travel a greater distance through the water (tacking) will be directly at the sacrifice of the pushing force of 5kms per hour. This will reduce the downstream speed of the boat.
  • The boat that travels the shortest distance wins the race, thus the drifting boat that is streamlined against the wind will win in a little over two hours. If the sails are cast into the water to form a type of sea anchor, the time will be best and the bow of the boat can be easily kept into the apparent 5km/hr head wind.

Did you do some calculations on this? Look at the relative vectors, not traveled distance. This discussion is not about secret knowledge of sail boats or even any more than a basic understanding of tacking. Even assuming that tacking avails you no extra edge, the fact that the hull is 90 degrees to the flow is the competition advantage. Possession of a Mensa certificate nor even an engineering degree is needed to solve this. Strip the problem down to ONLY the components offered in the Challenge. Some base assumptions are given, but any more than a rudimentary knowledge of a boat was offered as a part of the puzzle.

Stick to peeling a problem down into pieces and answer the questions one by one.

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#309
In reply to #305

Re: Sailing: Newsletter Challenge (06/01/10)

06/30/2010 1:07 PM

On your main three points:-

1. Yes.

2. Not in this case. You understood the problem badly to think that!!

2. No, distance has nothing to do with it as both boats will move at 5KMH no matter WHERE they are on the infinitely wide river!!!!......to an infinitely wide destination, so the boat that tacks, makes better overall speed......

Your marks were 33/100. That is a fail, sorry!!

Many people find it difficult to understand simple problems, I too occasionally as I look at the problem at "too high a level!" You are dong the same....

There are many really good and correct explanations here, you should try reading through the blog before posting. Look at the links I recently posted too.....

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#311
In reply to #309

Re: Sailing: Newsletter Challenge (06/01/10)

06/30/2010 2:12 PM
  • On your main three points:-
  • 1. Yes.
  • 2. Not in this case. You understood the problem badly to think that!!
  • 2. No, distance has nothing to do with it as both boats will move at 5KMH no matter WHERE they are on the infinitely wide river!!!!......to an infinitely wide destination, so the boat that tacks, makes better overall speed......
  • Your marks were 33/100. That is a fail, sorry!!
  • Many people find it difficult to understand simple problems, I too occasionally as I look at the problem at "too high a level!" You are dong the same....

Actually, I was replying to a very early comment because it pretty much summed up some of the points people were missing. Reading the bottom of my post should have cleared that up. I used bullets to quote since this editor doesn't import the identity of the original post or the originating message number. Sorry for the misunderstanding. Thread through my other posts and see what I've been trying to point out.

The origin of my comment came from a response to #23.

-Dennis-

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#313
In reply to #311

Re: Sailing: Newsletter Challenge (06/01/10)

06/30/2010 5:30 PM

Make it clearer, I am no genius, but I understand sailing!!

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#314
In reply to #313

Re: Sailing: Newsletter Challenge (06/01/10)

06/30/2010 7:16 PM

Make it clearer, I am no genius, but I understand sailing!!

  1. Boat one is adrift, but skillfully piloted straight down the middle. It runs at slightly slower that current speed due to drag. It's travel time will be roughly 2 hours plus the coefficient of drag disadvantage. It presents a small cross-section to the current. It never actually reaches 5kph because of mass and drag. (yeah, the little stuff does, but this is actually different)
  2. Boat two elects tacking, which you understand, and has been explained in depth numerous times. It gives some 1.x speed advantage. A very kind person here has posted a couple of vector diagrams indicating that the tack angle should be 30 degrees. ANY tacking advantage results in a vector addition of speed down-river. Regardless of how much extra distance they travel. Zig-zagging down-river picking up speed with every change in vector cuts a lot of travel time. BTW - the initial push #2 got was sufficient to begin tack.
  3. Both craft start with a vector magnitude of zero. Both are released from restraint at the same time. Craft one will slowly start to pick up speed because it has mass. F=mA. Both F and A are vector numbers. Craft two immediately turns their craft 90 degrees to the current and waves good bye to the other one as they are swept away. Their hull cross-section is much larger, and generates far more Force. If you plot their speed vs distance, you will see that number two immediately has an advantage. The mass also is the same, of course but F is far larger due to the cross-section of the hull, the energy imparted by the same drag the other boat is experiencing, and the apparent wind transferring energy while the sails are in tack configuration.
  4. No, I do NOT subscribe to the theory that the water drag on the other side of craft two will equal the force generated. Try this. Hold a board parallel to any large water flow. Make sure you select a stream that is reasonably fast. Now turn it 90 degrees and see if you can hold on to it. You can even try it with airfoil shapes like boats. They always seek the minimum energy configuration, which is parallel to the current.

I stayed away from all the math involved except for Newton's principle. Since this involves vectors, you can use linear algebra but the equations are too simple to bother. Sailing I leave up to sailors, but I understand the basics of tacking enough to realize that the apparent wind is enough gather energy from.

The time advantage at the beginning of the race was enough to win.

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#315
In reply to #314

Re: Sailing: Newsletter Challenge (06/01/10)

06/30/2010 11:56 PM

You seem to be advocating turning the boat so that it is perpendicular to the current. A sailor would not do this. The upwind tacking angle for a large range of sailboats (both slow and fast) is 45 degrees off the true wind (which is this case is the induced wind). This results in an apparent wind of 40 or 35 or 30 degrees off the bow etc, depending upon the efficiency of the boat (the ratio of true windspeed to boat speed).

You can not be expected to know this, but for five minutes prior to the start, boats are milling about jockeying for position. The boats are not restrained prior to the start. Therefore your boat could not steer quickly to put its beam to the wind and current. Its speed would already be very close to the water speed, giving you very little steerage way. Five minutes prior to the start, the boats would have to be a ways up river, to avoid crossing the line early.

As viewed from the side, sailboats have far more area above the water line than below it. So placing the boat beam to the wind will increase wind resistance and tend to slow its progress downstream, relative to one facing pointy end to the wind. Whether the increase in drag on the keel (from being beam to) is enough to overcome this would depend upon the actual profile of the boat and keel.

But in any case, sailing directly across the river would be the wrong course to sail in a race.

Re: picking up speed with every change in vector

This works the other way around. You lose speed with each tacking maneuver.

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#321
In reply to #315

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 1:17 PM

"You seem to be advocating turning the boat so that it is perpendicular to the current."

I was, as I saw the advantage that fluid dynamics gave at the beginning of the race.

As I said, I am not a sailor, and I did misunderstand the actual maneuvering involved in tacking. Thanks for pointing that out, and I stand corrected. I do know that tacking adds energy to the foreward vector above whatever the water current is under the right conditions.

Under either conditions, the victors are the ones who work for it.

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#322
In reply to #315

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 1:55 PM

"You can not be expected to know this, but for five minutes prior to the start, boats are milling about jockeying for position. The boats are not restrained prior to the start. Therefore your boat could not steer quickly to put its beam to the wind and current. Its speed would already be very close to the water speed, giving you very little steerage way. Five minutes prior to the start, the boats would have to be a ways up river, to avoid crossing the line early."

IF this was a normal boat race. However, I don't think that was part of the initial conditions. Only given two boats and a river, their initial velocity must be zero. Otherwise it would have been given. Yah, I don't know a whole heck of a lot about boats or sailing.

As viewed from the side, sailboats have far more area above the water line than below it. So placing the boat beam to the wind will increase wind resistance and tend to slow its progress downstream, relative to one facing pointy end to the wind. Whether the increase in drag on the keel (from being beam to) is enough to overcome this would depend upon the actual profile of the boat and keel.

Also ,as you say, it depends on the actual profile of the hull. If you think racing, you get only as much as you need. If you think merchant, quite a bit of the hull is under water. And, of course, there are others that are in between. I will admit that my vision of a sailboat gave me a hull distributed equally.

Aside from all this, I'm getting a great education in boats - thanks!

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#317
In reply to #314

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 5:56 AM

Now that is good detail, lets see if I can give you a good answer.

The original problem as given is here:-

You are in a sailing race on a very wide river. On land there is no wind; it is a dead calm. The race is 10 Km downstream, and the river is running 5 Km/ hr. Your opponents decide that to make the best time, they will streamline their boats to the wind and float downstream as fast as they can. You decide on a tacking strategy, sailing back and forth across the river's width. Who wins the race? What is the winning time?

We should not take regard to any effects that have not been given, by adding them to the above for instance.

There is no mention of any difference in river speed in the middle or near the banks.

There is no mention of any differences to be accounted for with regard to water pressure/effect/resistance on either of the boat's hulls.

Only with regard to reduced wind resistance of the opponents (drifting) boat.

The race is defined only as 10KM downstream, not to any single point!!!! So it is to a line drawn from bank to bank, as in most races!!!

The river width has not been defined, so it can be taken as being infinite as well, though this does not affect the answer, but could mean that the taking boat takes a close Haul from start to finish.

The points of sail. A. In Irons (into the wind) B. Close Hauled C. Beam Reach D. Broad Reach E. Running ("No go zone" is shaded)

The efficiency of the tacking boat's "pointing ability is left undefined. A reasonable value that most modern boats should achieve is 45° to the wind. There are actually boats that can drive directly into wind and still make good. See here:-

http://www.youtube.com/watch?v=oKqC5JsurOk&feature=related

and here:-

http://www.youtube.com/watch?v=NNbNNSDljGI&feature=related

So point 1) is wrong as you are taking in account something that was left completely undefined. Ignore these effects.

Point 2) The tacking boat can with the right design and a good sailor, still make good against the wind. Sailing boats have been doing this for hundreds of years, even in very light airs. Think of the situation on a lake of still water and a 5 KMH wind over it, a good sailor can sail to ANY part of that lake. What is the difference when the wind is still but the current takes the boat along at 5KMH?

Physically no difference, so if the boat on the lake can make good into wind, that speed in the direction of the wind, the tacking boat can have this extra speed added onto the river's current of 5KMH....he only needs to be 10KM downstream even one second before the other boat to still win!!!

But with a modern boat well sailed, the differences will be far greater as modern boats can easily make 1.8 x the wind speed, at 45° to the wind or sometimes even better!!!

Do remember that each boat retains its basic river speed of 5KMH ALL THE TIME, EVEN IF TACKING!!! They both still move at this basic speed at all times, the effects of tacking will be added to this..

Point 3) why would the tacking boats people turn at 90° to the wind (broad reach)??? When most boats can tack at around 45° either side of the wind or better???? No logic there. So point 3) is also based on incorrect assumptions.

Furthermore, if the tacking boat was an experimental boat as the YouTube videos show, they would be driving directly into wind at quite a good speed, which would be added to the basic river speed of 5KMH!!!

We cannot say for sure just how much extra speed they would actually have as we do not have a boat design specified, but it would have to be a very poorly built/maintained boat and badly handled for them not to gain substantially over the drifter!!

Ergo:- The tacking boat winds with a substantial, (but undefinable) lead.

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#325
In reply to #317

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 5:59 PM

GA. The best.

A reference is like a thousand pictures -- or a million words in the case of this blog stream.

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#323
In reply to #305

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 4:58 PM

at each river bend, would not a boat without steerage be pushed to the outer bank? (and slow down) So if you had some steerage you could travel a slightly shorter course. Also the pilot of the only sailboat (me), would calculate the time (5+ min.s before the start) to get there just before the gun, and if ahead there with some steerage, I'd pop a brew and look back on all the other drifting contestants, all the way to the finish. Hard to calculate how much time it would take, and any slow moving river would meander, so 10 km downstream is ?? If we all started at exactly the same point in the river, I still could sail a shorter course.

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#326
In reply to #323

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 6:01 PM

Where did it state that this river had bends????

Read the proble through again.....and expect NO BENDS!!

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#342
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Re: Sailing: Newsletter Challenge (06/01/10)

07/02/2010 10:16 PM

ON A VERY WIDE RIVER, means the sailing boat only will tack once at the start, then it's a bee line for the mark, (the finish line ) and get there way ahead.

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#343
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Re: Sailing: Newsletter Challenge (06/01/10)

07/02/2010 10:40 PM

Right on!

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#345
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Re: Sailing: Newsletter Challenge (06/01/10)

07/04/2010 4:13 AM

I find no fault with that reasoning at all!

Fully correct acording to the OP's statement!!

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#324
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Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 5:55 PM

As I have indicated in several previous posts, the orientation of the vessel in the flow makes little (or no) difference to its progress down the river. No matter what its orientation is, it is going to ride along with the current. Besides, without oars, you will not be able to control this because your rudder will be totally useless for steering the boat.

Have you ever watched a bunch of logs floating down a river. It has been many many years since I have, but I am sure the ones that lined up with the river current traveled the fastest.

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#327
In reply to #324

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 7:17 PM

As I have indicated in several previous posts, the orientation of the vessel in the flow makes little (or no) difference to its progress down the river.

Have you ever watched a bunch of logs floating down a river. It has been many many years since I have, but I am sure the ones that lined up with the river current traveled the fastest.

I totally agree... under steady state conditions. Initial conditions, however, are very different. If you can steer your log, the one that's sideways gets a bigger push than the one parallel. What I am trying to point out is, the vessel sideways will reach terminal velocity sooner than the other one. After that point, you can orient them any way you like. Both now have equal energy, but number two has a distance advantage.

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#329
In reply to #327

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 9:11 PM

My point was that I don't hold to the concept of the sideways vessel having any advantage over the parallel one. Anything you toss into a river is going to travel right along with the river, no matter its size or shape, assuming the uniformly perfect lamellar flow of this infinitely deep, infinitely wide river with no side or bottom roughness (none of that information having been specified or implied in the OP). Given that, there is no water traveling past the rudder as your vessel travels along with the river current. Therefore, you have no control over direction! (No mention was made in the original post of any available tools or paddling apparatus aboard the boat). Maybe your vessel wants to turn to face a different direction than you desire -- maybe not.

Also, let's be fair at the start of this race!

1. Attach each vessel to a long boom controlled by the starting vessel.
Maybe this is not practical or even possible in real life, but no matter.
2. Let each contestant pre-arrange the heading of his vessel within the river, while being dragged along on the boom.
3. After EVERYONE is ready, release the boom from all the vessels, simultaneously.

Remember that this is not a normally sanctioned race that provides the advantage to the more experienced contestants. It is a hypothetical, scientific ,theoretical problem for which all un-controllable parameters are ignored, ESPECIALLY the less-than-ideal starting situation for normal sailboat races that give the advantage to the more elite, to the detriment of the novice, un-controllable by the novice. This is not a test of skill. It is question of the laws of physics and how best to proceed in the race (after starting).

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#330
In reply to #329

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 9:31 PM

It doesn't matter a fig. Start how you want. If you sail you win, you drift you lose. Countless posts have clearly presented the reasons, if you can't see it, I find it hard to believe that you are an engineer.

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#331
In reply to #330

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 11:09 PM

First off, I am an engineer. Other than that, I concur with you 100 percent!

I was just trying to poke this ridiculously long thread of non-engineer posts along to see if any non-believers might convert. Apparently not. I haven't seen any yet. Oops, except for yours!

Now, I have a problem with you saying "I find it hard to believe that you are an engineer" when you seem, obviously, not to be an engineer, by your own definition. Check out your posts #14 and #44, in which you denounce the sailing notion. Then, all of a sudden, you swapped camps in your #99 post. Then, with your post #253, you presented what I thought was the best answer of the thread, as I acknowledged in my post #260.

In one of my later posts, I mentioned "hardheaded" which caught your eye, judging by your immediate reply. If any of the current hold-outs is an engineer, he definitely is hardheaded! There is no other explanation except incompetency.

Congratulations on your conversion and strong convictions, Mr. Engineer Martin.

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#332
In reply to #331

Re: Sailing: Newsletter Challenge (06/01/10)

07/01/2010 11:21 PM

"Congratulations on your conversion and strong convictions, Mr. Engineer Martin", well thank you. I clearly misunderstood you, just as I did this problem when I first encountered it. My only excuse is that it was very late when I first read the problem, and for some reason my brain was not working correctly. Double shame on me for I have been a sailor for many years, (I bought my first sail boat when I was 14,, I'm 53 now, you can do the arithmetic.) :-)

I am humble enough to admit my errors.

Cheers

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#337
In reply to #332

Re: Sailing: Newsletter Challenge (06/01/10)

07/02/2010 2:32 AM

Well done!!

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#336
In reply to #331

Re: Sailing: Newsletter Challenge (06/01/10)

07/02/2010 2:31 AM

LOL!

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#335
In reply to #330

Re: Sailing: Newsletter Challenge (06/01/10)

07/02/2010 2:30 AM

I think that he also feels the sailing boat will win, or have I missed the point somewhere along the lline?

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#334
In reply to #329

Re: Sailing: Newsletter Challenge (06/01/10)

07/02/2010 2:29 AM

That was "On-Topic", why did you put it "Off-Topic"?

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#340
In reply to #334

Re: Sailing: Newsletter Challenge (06/01/10)

07/02/2010 1:18 PM

I agree. It was more on topic than about 40 percent of the other comments.

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#333
In reply to #327

Re: Sailing: Newsletter Challenge (06/01/10)

07/02/2010 2:27 AM

If the acceleration was very sudden and to a high speed, it might make a difference.

But with a river at 5KMH, we are talking maybe of a centimeter or two of water till the log (boat) is up to river speed!!! In this world you can ignore that completely...

In a real world system (not here!), the occasional times when it hits a rock in midstream or touches the bank for a few seconds will lose far more time......

I would even go as far as to say that if you took 49 logs, numbered them, sent them all downriver at the same time, the first 6 crossing the line after say 10 KM would be as random as the lottery numbers.....

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#338
In reply to #333

Re: Sailing: Newsletter Challenge (06/01/10)

07/02/2010 7:34 AM

There is no resistance when a log is moving downriver, except as noted, if it hits bottom or a snag. The river is moving but the log is stationary relative to the general body of water and since most logs float just at the surface there is no wind resistance.

A sailboat on the other hand, will 'see' the relative wind and a broadside boat will see a great deal more relative wind than one pointed downriver and therefore move slower than one that is kept in line with the flow of the river and the relative wind.

Tie your sea anchor to the front bow eye and toss it into the current for steerage as even for the boat aligned with the flow, there will be some wind resistance to the relative wind.

Tacking boat will still have a .37 mph advantage tho but one bad tack or two and the drifting boat wins.

I still think I could beat either boat by swimming and pulling my boat though. I say this from experience when as a 16 year old water skier I had to pull my Whaler down stream for 3 miles in a similar situation when my girlfriend and I ran out of gas. I swam, she steered and the Whaler went directly downstream without meandering and we made the marina in record time. Mostly it was fear of her Father's reaction to getting her home after dark that drove into the gator and snake infested waters, lol.

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#346
In reply to #338

Re: Sailing: Newsletter Challenge (06/01/10)

07/04/2010 3:35 PM

IMHO there is no way the tacking boat can lose. No matter haw many badly performed tacks, they won't make the boat go "backwards", so it always wins!

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#347
In reply to #346

Re: Sailing: Newsletter Challenge (06/01/10)

07/04/2010 6:06 PM

200% correct!! (if possible!!)

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#348
In reply to #346

Re: Sailing: Newsletter Challenge (06/01/10)

07/06/2010 7:40 AM

Not true. The "wind" can knock it backwards, relative to the water, if the sail and/or boat is/are improperly positioned.

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#349
In reply to #348

Re: Sailing: Newsletter Challenge (06/01/10)

07/06/2010 8:02 AM

They would not be "Tacking" if what you imply is true.

The word "Tacking" implies a specific range of angles to the wind and even the world's worst sailing boat can "tack" into wind, albeit slowly if not a very "pointy" boat!!

If they were on a "Broad or beam - reach or running" they would get to the destination at the same time as the drifter or later.

But surely no sailor is that stupid?

Ergo. The sailing boat wins.

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#351
In reply to #349

Re: Sailing: Newsletter Challenge (06/01/10)

07/06/2010 8:40 AM

Replying to #350, as well. We must agree to disagree.

It depends on the interpretation of "no matter how poorly executed, a tack is still a tack". I see a whole spectrum down to failure and beyond, into the negative. The boat will still be making way through the water and will only detect the difference by watching the floater move away.

In short, It does not seem logical to say that only successful tacks may be called "tacks". I have never sailed a boat but I think I understand tacking, I have a knowledge of statics that will help, but whether I could actually do it is a point of some conjecture.

If the sailor were broadside to the stream, with sails showing, the wind resistance would slow them down compared with the "floater".

I stated in earlier posts that the sailor wins, I did so because the question would only make sense if the sailor was a competent sailor. Somebody else introduced, "no matter how poorly".

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#352
In reply to #351

Re: Sailing: Newsletter Challenge (06/01/10)

07/06/2010 8:49 AM

It's a problem of semantics, then ... the issue seems to be wether you can call being "broadside to the stream, with sails showing" a tacking manoeuvre ... I was assuming you can't. If you could, then I'd totally agree with you.

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#358
In reply to #351

Re: Sailing: Newsletter Challenge (06/01/10)

07/07/2010 1:05 AM

This is a problem resulting from the use of jargon. A poorly executed tack means, in sailing, a tack that does not proceed smoothly. (There is a large portion of the tacking maneuver during which the boat has only inertia to carry it forward, and if you lose speed, you might not make it through the tack [the turn] onto the next tack [the course].) If the tack is done very poorly, the boat can end up "in irons" headed into the wind and unable to resume sailing until the nose falls off away from the wind to significantly beyond 45 degrees.

Turning the boat in entirely the wrong direction, such as onto a beam reach or run is not a poorly executed tack.

I cannot tell if you are aware of this or not, but tacks can be either the course sailed (and this usage extends into idiomatic English -- "Let's take a new tack." ) or the turning process connecting one course with the next. There are several things that can go wrong during a tack (the turn). Ordinarily, little goes wrong once established on a tack (the course).

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#350
In reply to #348

Re: Sailing: Newsletter Challenge (06/01/10)

07/06/2010 8:08 AM

Andy just took the words right out of my mouth ... no matter how poorly executed, a tack is still a tack.

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#359
In reply to #350

Re: Sailing: Newsletter Challenge (06/01/10)

07/07/2010 1:06 AM

exactly - and a boat is still a boat!

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#353
In reply to #348

Re: Sailing: Newsletter Challenge (06/01/10)

07/06/2010 11:31 AM

"Not true. The "wind" can knock it backwards, relative to the water, if the sail and/or boat is/are improperly positioned."

Except, as someone will surely point out, this is a defined condition problem, and the apparent wind is always steady.

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#354
In reply to #353

Re: Sailing: Newsletter Challenge (06/01/10)

07/06/2010 1:12 PM

"this is a defined condition problem,"

Agree.

"and the apparent wind is always steady."

Disagree. The apparent wind, in this problem is a function only of the boat speed.

Consider two boats, identical except that one has the sails up. both broadside to the river. They will both start together, holding the broadside orientation, picking up speed, but the boat with the sail will feel much more wind resistance than the one without the sail and will trail.

The one with the sail will know that he is not tacking because he will have no velocity through the water, but I wondered if a novice like me might actually tack in the wrong direction. If I had my boat only a few degrees off broadside, and my sail a little more to give me a resultant, I would progress through the water; but if my judgment was off and my few degrees were the wrong way, my boat would still go forward through the water, but we would be going upstream instead of down.

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#355
In reply to #354

Re: Sailing: Newsletter Challenge (06/01/10)

07/06/2010 2:34 PM

Disagree. The apparent wind, in this problem is a function only of the boat speed.

I considered this statement when I made it, and debated about saying anything at all. Yes, you are correct. The tackers, however, will build on the f(x) river speed to attain their velocity.

Consider two boats, identical except that one has the sails up. both broadside to the river. They will both start together, holding the broadside orientation, picking up speed, but the boat with the sail will feel much more wind resistance than the one without the sail and will trail.

Technically correct, and totally irrelevant.

The one with the sail will know that he is not tacking because he will have no velocity through the water, but I wondered if a novice like me might actually tack in the wrong direction. If I had my boat only a few degrees off broadside, and my sail a little more to give me a resultant, I would progress through the water; but if my judgment was off and my few degrees were the wrong way, my boat would still go forward through the water, but we would be going upstream instead of down.

Umm, I don't even understand this. However, to settle the argument, look at the original question. CR4 gave the answer. Tackers win.

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#356
In reply to #355

Re: Sailing: Newsletter Challenge (06/01/10)

07/06/2010 3:27 PM

I said, way back, that the tackers win, I was just following others up a side alley.

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#363
In reply to #355

Re: Sailing: Newsletter Challenge (06/01/10)

07/27/2010 11:48 AM

You would be able to tell which way you are going by observing the bank. If you were headed the wrong way, your boat would be progressing stern first down the river, because you cannot go fast enough to overcome the river current. Change tack to point the bow downstream.

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Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 6:33 AM

The tacking strategy will be the winner because it uses the relative windspeed to move forward. The passive strategy in fact will have drag that slows it down to below the speed of the downsteam current. But to put down exact numbers on the task seems very difficult. As a cualified estimation I'd say that the tacking strategy would do it in 1,5 hours and the drifting strategy in just over 2 hours, depending on skill, sails, hydrodynamics of the hull, and many more parameters.

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#26

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 7:15 AM

The tacking boat will win. As the boat is not turned down stream the keel will catch more of the river current and propell the boat down stream faster.

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#27

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 7:22 AM

I know nothing about sailing, so I would try to ride the current. Try to enter the stream perpendicular to it's flow to get some nice "push" on the fat side of the boat, then point the bow down stream to lose the drag. Enjoy the rays, dangle toes in the water, let the guy with sails tack back and forth across the river. I think my time will be better, but who cares as long as you're in a boat on a river.

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#344
In reply to #27

Re: Sailing: Newsletter Challenge (06/01/10)

07/03/2010 10:07 AM

I think that is why there have been so many comments, boating is a great way to relieve stress.

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#29

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 8:28 AM

Dead heat. The boat is carried downstream at 5km/h in both cases. That's my guess anyway.

Cheers.......Codey

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#32

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 8:56 AM

Assuming zero wind and assuming both boats are of equal construction and hydrodynamics, both will benefit the same water speed of 5km/h so no advantage here. If my assumptions are correct, than tacking will take you through a much longer track, And finally will bring you to second place. that is as if you are two competitiors...

Wangito..

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#35
In reply to #32

Re: Sailing: Newsletter Challenge (06/01/10)

06/01/2010 11:03 AM

I am a Moron, and you are not. Because you are not a Moron, your answer is excusable. Here in Moronia (a village on Phuket island, Thailand) we have many sailors (and Phuket is a famous sailing destination) so we all know the physics of sailing.

Consider a very similar question, in which the correct answer might be even less obvious, because one could assume that a wind blowing in the intended direction of travel would have to help the boats go faster:

  • A river runs straight from West to East at 10 knots. A 10 mile race is held: the boats sail downstream, from West to East. The first heat is held in the morning, when there is no wind. The second heat is held in the afternoon, when there is a 10 knot wind from the West. In which heat are the faster times recorded?

The faster times are recorded in the morning. The sailors in the afternoon race would complain about being "becalmed" because from the boats' perspective, there is no relative wind to fill the sails: the boats are drifting a 10 knots eastward, and the wind is moving at 10 knots eastward (both relative to the earth's surface... which itself has an apparent surface speed of about 1000 mph at the equator, when viewed from the moon).

Explanation here.

Here is an unlikely seeming link having to do with holy spirit, apparently. But if you scan down to about the fifth paragraph you will find this:

  • The energy that drives a sailboat is harnessed by manipulating the relative movement of wind and water speed: if there is no difference in movement, such as on a calm day or when the wind and water current are moving in the same direction at the same speed, there is no energy to be extracted and the sailboat will not be able to do anything but drift.

Think of trying to fly a kit without a string -- the kite falls to earth.

(There is a famous sailing aerohydrodynamics book called something like "The Holy Interface" which I was looking for when I came upon the link above... but alas I could not find a link to the sailing book.)

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