Sailing: Newsletter Challenge (06/01/10)

I'm sure you understand the situation correctly. It is his first post that you may have missed.

His first post (#233), is completely wrong. His VMG through the water is correct, given a 4 km/h boat speed through the water: 2.8 km/h VMG with respect to the water. What he fails to do is add that figure to the water speed to get the speed relative to the ground. So his time of 3.57 hours for sailing is completely wrong. Given his figures, it should be 10/7.8 = 1.28 hrs.

His drifting option is almost correct. However, in practice a dinghy "streamlined" for drifting downstream against an apparent wind would have the pointy end facing downstream (upwind), and the flattish stern facing upstream (downwind). The sails would be lowered. The apparent wind would have little to act against, but the dinghy hull would be stuck in water (water having 800 times the density of air, and substantial viscosity). Under this condition, the skipper may have a hard time maintaining head-to-wind condition, but I'd guess the sternway would be closer to .2 or .3 km/h, relative to water speed. But otherwise he's just about right... the drifting time would be a somewhat more than 2 hours.

His third option, which he thinks is fastest, is actually slowest. Let's assume that he did not turn turtle and get his mast stuck in the mud at the bottom of the river. With the dinghy capsized and perpendicular to the apparent wind, it would get blown upriver with respect to the water. (If the bottom is facing downriver, the wind's force on the dinghy's bottom would cause the dinghy to turtle. This is why dinghy sailors who have failed to jump on the centerboard immediately upon capsize, must then try to float the mast tip (e.g. with a spare PFD) and must get the mast tip pointed into the wind, so that the windage on the hull helps (rather than hinders) righting the boat.*

With the 5 km/h apparent wind up the river, one could also sail up the river, but at a speed much less (through the water) than the speed of the current relative to the ground. (I used to have a spinnaker rated for a maximum apparent windspeed of 5 knots [9.3 km/h] which would fill adequately in this wind, of around 5km/h at the instant of spinnaker hoisting.) A capsized dinghy will be sailing up river, if turtling is prevented. With its hull acting as a spinnaker, it might make 1 km/h upriver relative to the water, for a ground speed of 4 km/h down river. That makes this alternative the slowest option, not the fastest. 10/4 = 2.5 hours. Realistically, however, it is very hard to keep a dinghy from turtling... so this option, combined with the skipper's apparent inability to sail, could result in hours in the water. (In practice, the skipper, boat and any crew would be aided by a motor boat, and the chagrined skipper would drive to the finish line [perhaps the first one to get there... but not by boat, so it wouldn't count] to hear his friends' chortles.)

*Watching new dinghy sailors can be good entertainment. Invariably, they forget that they had cleated the mainsheet before the capsize, making getting the boat upright a real challenge, with the sail full of water and fighting you all the way. Then just as it comes up, after lots of struggle, the boat gets blown right over again, because the sheet is still cleated, and there is nobody in the boat to stabilize it.

BTW,if you're going to capsize, you might as well do it with some drama. These guys are all competing in the Moth Worlds -- so they are not new to sailing. You can imagine how tricky these boats are to sail.

Another good sailing video -- this one with very high video production standards.

depending on the time of day, does the river at it's mass and speed, drag the air above it down stream with it, or because of temp. differential,and expanding air, can the air move upstream? Consider that in a wide river there is allot of "leverage" passing one point at 5 km/hr. "ON LAND there is no wind, it is dead calm"

Craig! For crying out loud, it's a simple thought problem,,, don't complicate it. Do you know what a free body diagram is? If so you'd be the nerd that asked what the friction of the pulley is,,

I give up,,, no more

depending on the time of day, does the river at it's mass and speed, drag the air above it down stream with it

Yes, of course. The effect is so small as to not be worthy of consideration for this problem.

or because of temp. differential,and expanding air, can the air move upstream?

No, not in any reasonable sense. A very large river will have slight sea breezes and land breezes roughly perpendicular to the river's flow.

Neither phenomena has any effect on the outcome of the problem. The sailors win.

Right on!!!

As rediculous as some of the comments are, I am beginning to enjoy this blog again, just reading the appropriate sailing reponses to see how many ride-um out floaters are still not covinced. They must surely be hard-headed engineers, as I are one, myself.

"You are in a sailing race......." "YOUR opponents decide to float".... "YOU decide to tack".....

thank you for your answers. I also think the sailors win. (me,myself,&I).

I think you missed my point. This problem is just not that complicated. IF there were wind involved, I'd go with the tacking solution. However - to cut, as it were, straight to the heart of the matter, chop off the masts. Orient one boat 0 degrees to the current (the ones who chose to occupy the middle of the road), and the other at 90 degrees (they who chose to tack). The one with the most hull cross-section is the one who also has the most force generated, and ends up traveling at close to the same speed as the water. The other boat, the zero degree people, will drift quite a bit slower at first, eventually reaching some fraction of the current speed.

But there IS wind ... that is if you go with the common definition of wind as being air in motion. The thing is that if you choose to stick to the static "earth" referential you then have to speak in terms of relative wind as being air in relative motion to an object. There are plenty of posters that explain this better than me, just read a bit of the thread ...

• But there IS wind ... that is if you go with the common definition of wind as being air in motion. The thing is that if you choose to stick to the static "earth" referential you then have to speak in terms of relative wind as being air in relative motion to an object. There are plenty of posters that explain this better than me, just read a bit of the thread ...

...AAAAnnd, it's immaterial. The crew fiddling with ropes is gonna win by virtue of a slight distance gain plus the [greater] hydraulic advantage. This is a word problem, not a math problem. It might help to draw a picture and know what tacking means, but it mostly boils down to relative vector magnitude between the two contestants.

unbelievable????? get with the program !!!!!!!!!!!!!!!

Do please NOT forget that the tacking boat AND the one that just floats BOTH have a basic speed of 5 MPH from the river flow......so even if the sailing brought no speed gain, they would arrive together.......

But a good sailor would be able to increase his speed made good to actually win the race.

A really, really bad one maybe not.....

Where/what are these "Options" you mentioned and did not mark as options?

• Do please NOT forget that the tacking boat AND the one that just floats BOTH have a basic speed of 5 MPH from the river flow......so even if the sailing brought no speed gain, they would arrive together.......

Umm, no. I wanted to minimize characteristics of sailing vessels in particular in this, but there is one thing that needs pointed out. Hulls are designed for and to drag numbers. It will take a measurable amount of time for the floaters to reach their final velocity, which will be close to, but not equal to, the river current. I do not know what that time will be, but I do know that it will be much longer than the other hull oriented 90 degrees to the flow. One of my base assumptions is that the sailors in said 90 degree craft also know enough to keep it from capsizing. I don't know how much steering control they have, but they can even probably zig zag across if they like. And throw raspberries at the other crew.

I can't believe this thread is generating this much static.

what sailor would capsize his boat?

what sailor would capsize his boat?

These ones.

• what sailor would capsize his boat?

Judging by some of these entries, I'd say engineers who are not sailors. I am not a sailor. I do remember floating sideways in a canoe and saying - "watch out for that" right before I got dunked.

Sideways to a fluid flow with an unstable hydraulic foil and no sustaining counterforce (rudder control or wind) will eventually re-orient the craft to the most stable or lowest energy configuration. Some sail will help. A boat with a tapered or curved hull is a hydraulic foil. Of course so is a flat board. The flat is stable, the hull is only stable when it has proper forces applied at control points. It does have a neutral zone though.

This is the type of quiz question we had every day in dynamics, right before we got handed the day's 10 problems for homework.

You are bogging yourself down in unimportant (to the original post anyway) details.....

Take a large step backwards and look at the big picture, I think you then have a good chance of seeing a proper solution......

Most of the guest posts (lets say all for safety!) are completely wrong.

This is a good tip.Study the posts that come from logged on members only. They are not all correct, but the percentage of correct to incorrect is very high!!

Incredible and very well said

Like it. Also GA from me!!!

the floaters win it takes 2hrs tacking takes longer as the wind resistance of the sail will slow them down slightly. There will be not wind power advantage as there is no wind blowing.

Again, I give up -- for good this time, perhaps.

I know, its very sad, but please stay with us, PLEASE?

Wrong, wrong, wrong and yes WRONG!!

You are no SAILor!!

But compliment, you did use your CR4 avatar....thanks!!

LOL!!

I/We tried but still ................................

I think I may go for a sail down river. The theoreticians won't be able to get me there.

A thought....maybe just maybe they are winding us up......I do hope so if only for the future of techy land.

At what point across the river was the 5Km/hr measurement made.

This rate would not be equal throughout its entire width.

Friction, viscosity considerations at point where river meets land!

You have a good understanding of the incredibly obvious.

If this is a sailing race with all the rules of sailing in place, it would be canceled due to lack of sailors. If i am the only one sailing and 99 others drifting there is no race

Apparently my first answer to this problem got lost in cyberspace as I do not see it anywhere.

After having read most of the arguments for and against "tacking," I stand by my first explanation that the "tacker" will win. No, there is no breeze to help the tacking, but that was never a consideration in the first place. Perhaps a better word to use would have been "steering."

The answer lies in the amount of boat surface that is presented to the current. Going head on offers the least surface for the current to push against, whereas by turning the boat slightly (tacking)(steering), more surface is presented to the current thereby gaining a slight advantage over the head on approach and resulting in a slightly better time than the alternative.

At least this has been my experience in the past when I was still able to get out of this stupid wheelchair and hospital bed to enjoy those things that I used to take for granted. Take my advice gentlemen and avoid old age. Wisdom gained doesn't count for much when you are unable to take full advantage of it!

Of course, old age beats no age!

Ken Leigh

Again, you are relying on black magic rather the natural laws of physics.

From where is the lateral thrust for your rudder coming??? With zero water current relative to the rudder, you have absolutely no rudder control over the heading of the boat!!!

In order to achieve that relative motion, you MUST rely on the relative wind to create that motion relative to the water (Read my Comment #240, as well as many others too numerous to list right now). Now, you have control over your boat, and you win. However, in so doing, you have used the wind. Now, (in my opinion) that is sailing.

RIGHT ON BROTHER!!!

Thank you guys,,,,,,,(Jorrie,MoronicBumble,Blink, Mareng, Switchman, Tippycanoe, Amichelen) among others..... reading back to aug. 06, we only have to win by an inch but we win this brain teaser.

You will win because the current will carry you faster especially with your centerboard down but your keel and hull alone presents more surface area as you tack back and forth across the current than if you stay in line with the current and therefore will allow the current to carry you faster downcurrent. The closer to the center of the stream usually the faster the current will flow as well.The water causes more friction with the larger side presentation

This is an infinitely wide river (not simple stream)! Therefore, there is no edge. Hence, no edge effects. Don't complicate the un-complicated. KISS -- Keep It Simple and Stupid.

I change my mind about sailing/tackin...stow sails, secure tiller centered, tie the bow line around my waist, dive in and breaststroke to victory!

Time 1 hour 20 minutes.

The one who sails, tacks will win. On land there is no wind. When following a current at 5km/hr there is a relative wind. The sailer must sail into the wind. like one would do on a lake.

Consider if they were on a lake, the one who trimmed goes nowhere. The one who tacks makes progress. In this case the lake is moving.

Thanks,

I have read many (although by no means all) of the posts on this thread, and I'm afraid I come (at the moment) to the conclusion that it is not possible to use the apparent wind to increase the sailboat's speed relative to the bank. I think it was mentioned earlier, but an increase in speed means an increase in kinetic energy. This means that energy has been transferred to the boat from somewhere else. The only potential sources of energy in this case are the water and the air, however the air is not moving (regardless of which reference frame you use - if you change the frame then it is the frame that is moving, not the air) and therefore has no energy to impart. Therefore the air cannot possibly speed the boat up. Two illustrations that have at least convinced me: 1) The apparent headwind that the boat experiences is equal to its speed (NOT the speed of the water). This means that if the boat speeds up, the speed of the apparent wind also increases. This would presumably mean that the boat could speed up a bit more, further increasing the "windspeed"... to what limit? Surely an infinitely wide river would result in infinite speed? 2) If hypothetically the boat is able to move faster than the river by tacking, this means that the water is no longer powering the boat (it will be causing drag at this point). So where is the power to resist this drag coming from? The apparent wind? And what causes the apparent wind? The movement of the boat! So the movement of the boat is powered by... the movement of the boat. This is perpetual motion is it not? My apologies if this has been covered in any of the middle posts that I skipped!

"Apparent wind is what ALL aircraft fly with......." Aircraft use apparent wind to direct the thrust produced by the engine, they do not use it to increase their speed relative to the ground - only the engine can do that .. take away the jet engine, the plane falls out of the sky. Take away the thrust from the river (by sailing faster the the water is flowing) and the same will happen.

take away the jet engine, the plane falls out of the sky

Reading about sailplanes might help you understand both flying and sailing. Also look into DDWFTTW. A low resistance sailing craft can sail directly downwind at greater than wind speed.

There is a tiny difference between this boat and a palne - the plane is driven by a propellant or jet-turbine (not tour biene) and is (nearly) independent from the environment.

Our Boat is drifting on the water and is undriven or driven by the wind that creates the boat itself by its movement on/in the water.

the blowing drive wind is a force for driving the boat - who much this force is depends on the sails and the trimming of the sails, the course of the boat and maybe many other things (is the sailor drunken?).

What about sailplanes or gliders as they are called.....how can they fly all over, some have remained up to 12 or more hours in the air...

Please explain how this type of aircraft (Caproni Vizzola) manages to have several world records in distance and speed circuit flying??? It will be very interesting to hear you describe how it would not be possible!!!

Look here.

http://en.wikipedia.org/wiki/Caproni_Vizzola_Calif

By the way, it makes NO difference as to whether the wing is vertical (sail) or horizontal, laminar flow laws are the same for both!!! Any engineer will understand that without needing to be told!!!

the reason sailplanes,sailboats,iceboats,etc. do so well, is a mater of competence, and design. The pilot/sailor (s), are so good about keeping their ship in perfect trim, 24/7. The new designs are pretty sophisticated .

re read the comments by sailors to find out who would win. in this case as i have said before there is only 1 sailor in this race.

Thank you! For me, that is the clearest explanation so far. GA.

I had some understanding but mainly hung my hat on the assurance, from others, that a sail boat make progress against the wind by tacking diagonally across it. Now for the pencil and paper and the calculator and some simplified efforts.

I'm not sure that's right - an equilibrium is reached for any given wind speed, but the wind speed will be continuously increasing, which means that the boat will continue to accelerate (although the rate of acceleration will decrease as speed gets higher due to the effect of drag that you mention). Unless you are saying that a point will be reached where the boat will not go any faster even if the wind speed increases?

Unless you are saying that a point will be reached where the boat will not go any faster even if the wind speed increases?

That's essentially what I am saying, if by wind speed, you mean apparent wind speed. The shape of the drag curve varies with the hull type. In a displacement hull, the drag goes up roughly with the cube of speed. In planing hulls, the drag increases similarly at low speeds, but then the curve flattens dramatically at high speeds, so that doubling speed results in less than double the drag.

In planing boats there is, therefore, a resistance "hump" in the drag curve. Sailors of boats that can plane (a small minority of sailboats in general) are very aware of this hump. Windsurfers, in marginal winds will "pump" the sail to get over this hump, and the the boat will suddenly accelerate, generating more lift as it goes. However, the drag on the sailor and anything above the water is increasing just as fast as lift, and the vector direction is becoming increasingly more effective is slowing the boat, while the lift vector direction is becoming increasingly less effective in accelerating the boat.

The top speed of a sailboat for a given true wind speed is that speed at which acceleration falls to 0.

They win with 2 hours time. One has to stick with the centerline of the river where the speed of the stream is max. As there is no wind taking any action other than following the centerline will slow down the boat.

Total twaddle, but we expect that fron certain Guests......

There are here many well written and simpe to understand explanations as to why a boat sailing or reaching will increase its overal speed against a boat just drifting......

I am not going to waste my time further......

A horse must be lead to water, but it has to drink on its own.

Are you still thirsty?

They state that you have the option to sail, and most sailboats need at least 3mph (about 5K) relative hull speed in order to have stearage way, therefore; the tacking boat will travel at least 5K above the 5K current. Since they are tacking, this is not 100% towards the finish line. Assuming worst case (a gaff rig, not Marconi) we can assume they are tacking at 45 degrees to the straight line path, and not running down wind (since that would be jibing,) so relative progress is half of 5K= 2.5K+ the 5K current= 7.5K relative progress. This will put them across the line at 1 hour 20 minutes.

We will give the passive boat the benefit of the doubt and assume they are travelling backwards (to allow for maneuvering,) with a storm anchor deployed to create more drag (and also some crude maneuvering,) so they will be travelling the current speed minus drag, therefore will finish in more than two hours (assume 2hours 15 minutes)

The tacking boat wins in 1 hour 20 minutes, nearly an hour ahead of the passive, and now possibly quite inebreated competitors, giving them plenty of time to maneuver out of their way.

Great Flying Spaghetti Monsters people! once you start to catch up with the river, you can steer by using a small sail or piece of rigid board as a rudder in the air, before that, you could use an oar at the leading (downstream) edge of the boat, which ever it is at the time.

I officially withdraw the idea of any hydraulic advantage, although I can still make a case for it. It had to be combined with tacking anyway.

My vote is for tacking, the selected multiplier is .37, and the finish time is approximately 1hr, 15min. I cannot calculate the tacking loss or efficiency - both are human factors. If that number approaches .5, then the time would be 1hr, 37min, which is putting it pretty darn close.

If you want to win, you have to work.

I agree 100 percent. See my post #240, which may be a bit wordy.

However, I did assume a fairly efficient sailing rig. Your estimates probably are closer. Either way, we still have time to finish off a few beers before the floaters come in.

Ok folks. We could get into discussions about center of lateral resistance and center of effort and lift til the cows come home but in the real world a boat will not sail or steer if the boat is drifting. This is called having "no way on". PLEASE go out in a boat and try this. I have been in these situations where the captain lost his cool and declared a steering casualty when in actuality, it was only due to loss of motion through the water. Once we got "under way" the boat steered perfectly well.

In this case, you either move through the water or through the air or both. If you can't use one, you can use the other to steer.

Still noodling over this one. Seems to me the tacking boat will be slower to reach the end because the energy creating the relative wind is actually coming from the river. Therefore the longer travel distance and the increase in drag from the sail will ultimately be a minus compared with the boat capturing the river's energy by drifting. We cannot get more energy out of a system than is there in the first place. All that tacking and sail in the wind has to be subtracted from the energy available to push the boat along. If there was real wind, there would be additional energy available and the sailing boat could win, but not under the conditions described.

Wow! We still have non-believing non-convincible land lubbers amongst us!
Unbelievable!

The still water lake with the 5KMH wind is NOT the same logical condition. That wind provides energy from a completely different source. The relative wind created on the river in still air comes from energy provided by the river. All of the energy in the race between the two boats is provided by the river. Please explain how sticking up a sail will produce anything besides additional drag in this situation. If the wind was created by some other source, energy could be extracted by tacking and thus added to the motive force provided by the river. Under the described conditions of the problem, there is no other energy source besides the river, therefore anything that increases drag will only slow the boat down. The streamlined boat wins, assuming identical hulls.

How is the energy source relevant? Whether you hold a pinwheel up in a 5 kmh breeze or walk at 5 kmh in still air, your pinwheel will rotate at the same speed. It matters not a jot that the power source is the wind in one case and you in the other.

Now (as others have exhorted) apply that information to the dozens of excellent posts that explain the mechanics of sailing upwind (in this case, downstream).

The energy source is exactly what's relevant in this example. If you sat on a frictionless surface holding your pinwheel into the 5 kmh wind, would you move forward because the pinwheel is pulling you? The river is the only source of energy in the problem, the drifting boat is using almost all of it to go downstream. The boat with the sails is only creating drag, because there is no energy to be obtained from the relative wind. All the available energy is used creating the wind.

Thank you for your patience. Let's forget the pinwheel and frictionless surface stuff.

The river is indeed the only source of energy or power or motive force in this problem. And yes we must use the shore as a frame of reference because this is a race of 10 km as measured on the shore.

The drifting boat gets all of its motive force or forward motion from the river, at a rate very close to 5 kph, the rivers speed.

"The sailing boat is slowing down some of the water and will thus be able to extract useful power from it." I agree. It will be able to extract useful power from the tacking action in the water, but this will be less than the power used in drag by the sails. If this were not true, where does the additional energy come from?

In the real world, boats sail happily into the wind all over the world, but that is because they are able to extract real energy from the wind, which is actually a great mass of moving air and has a great deal of energy. In this problem, however, the air is stationary and has no available energy. Most replies seem to posit that the sailing boat is able to extract energy from this stationary air simply by moving through it. I think this motion through the air costs energy to the boat and will thus slow it down. The additional distance caused by diagonal travel only makes it worse.

What am I missing? Where does any additional energy come from?

Thank you for the entertaining word picture. It made it easy to get on board with this problem.

I think all you sailors are actually at a disadvantage here, being unable to separate what happens in the real world from the unreal constraints of this problem. All of you seem have assumed that the 5kph relative wind is a constant and always available. I think not. The relative wind speed is a function of the current pushing the boat MINUS the drag, some in the water, most of the drag from that wind. Any increase in drag immediately decreases the relative wind speed. Putting up the sails is rather a large increase in drag, slowing the boat noticeably. Tacking is a way to recover some of that loss, but it will never recover all of it because the increase in speed increases the drag. There is no energy available to add to the equation.

In the real world, there is almost never a situation where there is NO wind, and in any case, all such real winds are another source of energy, not a function of some wimpy current pushing the boat.

Under the constraints of the problem, I think the sails are a drag, and the streamlined boat crosses the finish line first.

The difference between the 5kph wind across the still lake and the 5kph river current is that the current MAKES the wind in that case. That relative wind is a result, derived by the current pushing the boat into still air. The relative wind diminishes as the drag of the sails dimishes the speed of the boat relative to the shore, the shore being the location of the start and finish lines. If paying attention to the constraints and conditions of the problem is a fixation, so be it.

GA from me for a really excellent post.

I still don't think that "HE" will either get it, understand it or even bother to read things that he disagrees with....eg. "He" is a hopeless case.....

As someone else pointed out before, I forget who, "you can lead a horse to water, but he must drink all on his own!"

This Guy is simply not thirsty....

Apologies to the person who first said that that here somewhere on the Blog, I just forgot who it was (senior moment!), but it was well put.

Joke.

Q. "Mrs Brown, where does your son get his thirst for knowledge?"

A. " The knowledge he gets from me, the thirst from his Father!"

Thanks for posting. While this point of view is indeed challenging, I personnaly believe it can be really interesting to question the easy or seemingly obvious answer. More often than not, we all gain a greater understanding and from time to time, we are all allowed to step back and realize that a misconception has been adopted. As much as I appreciate the multiple worthy developments provided by various members, we have also been witness to a few comments that demonstrated greater shooting abilities than actual reading skills.

In any case, I appreciated reading your posts, Thanks.

A pilot, glider pilot and sailor (yes, in heavy tidal region as well as on peacefull lakes :-)

Its the energy from the apparent wind that will be used here by the sailing boat.

Apparent wind is for example the wind you feel when on a windless day, you get in your car and drive at 100MPH and then stick your hand out of the window. The "apparent" wind will buffet your hand, almost painfully, especially if you try holding the flat of your hand forwards. Now that is energy!!!

If you had a small windmill generator, it would tear round also generating some electricity - all on a "windless" day.......

If you got into a sailboat, on a lake with still water, but with a 5 KMH wind, that is EXACTLY the same conditions for the sailing (and the non sailing boat) as on the river.

No difference whatsoever.

Many weekend sailors sail across such still lakes, back and forth, with the power extracted from a 5KMH wind.

Depending upon the efficiency of the boat (but we can pick and chose any sailing boat we wish) you can sail at a minimum of 45° to the wind and make good in the direction of a point 20KM downstream (if I remember correctly)...

Remember also that the river can be infinitely wide and infinitely deep.....so even on one tack, you can sail from the start to the end....

Remember that some very modern/exotic boats, using the wind can actually sail and make good DIRECTLY INTO WIND!!

Remember, the sailing boat only has in fact to beat the non sailing boat, even 1" is a win! (This implies that every extra inch can be VERY important!!)

Here you can see a model proa, almost sailing directly into wind, note the direction of the waves :-

http://www.youtube.com/watch?v=Oq6ULIcNc9I

Here is another boat with a simple mechanical system, also sailing directly into wind:-

http://www.youtube.com/watch?v=Aw4F3Lk11As

WHAT additional energy??? You are postulating there will be a need for additional energy ... In terms of energy here is a brief and simplified picture of what happens:

a) the boat with the sails down will match the current speed, so without a velocity gradient there is no viscous force acting on it's hull. No force means no work (no energy extracted from the current). This boat just extracts the initial energy from the current to get to 5kph and then just moves at constant speed as predicted by newton's laws of motion.

b) the tacking boat constantly extracts energy from the current, as the drag component of the forces acting on the sail must be balaced by a viscous force exerted by the water on the hull (perpendicular to the boat's motion). The lift component is responsible for moving the boat across the water.

it's very simple, actually, and you are partly right. Specifically you are right to say that on the frame of reference of your choosing the river is the only source of power, but you are wrong both in saying that the tacking boat doesn't extract any energy from the river and in saying that the drifting boat does. It's the other way around. The marvel of the laws of physics is that when correctly applied they hold true regardless of the referencial you use. If you come to two different solutions for the same problem just by chaging reference frames, you can rest assured that one or both of the solutions are wrong!

A GA from me too...

But I don't think that he has the "where-with-all" to understand your, or any of the other good posts we have already had before Mr.Trunner chanced upon this blog......

Nor is he able to understand that he doesn't understand either! Sad!

He could read and learn, but has chosen not to (learn!). His choice, not ours.

"You can lead a horse to water, but he has to be able to drink all on his own!"

Some are not thirsty I guess pardner!!!

You would obviously like to try and prove the errors of thinking that Einstein made?

HA HA HA HA HA HA HA HA HA HA HA HA HA HA HA HA HA HA HA HA HA

You really are amusing us today (again!)

I have not laughed so hard in several weeks (not since you last posted I think!!)

Suppose that you wake up somewhere and you don't remember getting there. You find that it is a dark, cloudy moonless night, you feel that the object you are standing on is rocking slightly, you hear water gurgling, you feel a wind that you guess is 5MPH and you conclude that you are on a boat. You reach down and find that the water is not moving.

Are you:

1. On a boat, anchored in still water, with a wind blowing?
2. On a boat, floating free on moving water, through still or slow moving air?
3. On a boat, floating free on moving water, with a following wind moving faster than the water?

a sail in the wind is like a planes wing moving thru the air or in moved air - both create a force perpendicular to the direction of the flow (or the sails position).

Suppose the river (or lake) is running round part of the equator. Then it and the air are travelling at ~1000 mph relative to the centre of the earth. How can it make a difference whether you chose a frame of reference which is still relative to the banks i.e. doing the ~1000mph OR still relative to the river i.e. doing ~1003 or ~997 mph.

You have just proved to us all here that you are:-

a) not a great person in mathematics

b) not a great person in physics and

c) not an engineer either.

Your points are all totally invalid, but you cannot see or understand why.

Are you in book keeping or similar? It would fit well if you are!!!!

We gave you the opportunity to think things through.

Either you did not do that or you simply could/would not understand it. Pick one or the other!!!

Mr Trunner, try and improve in the future.....a well wisher!!

The 5kph relative wind is not a constant. It is a function of the current pushing the boat minus the drag, mostly from that wind. Any increase in drag diminishes the relative wind speed. Putting up a sail is a significant increase in drag under these conditions, slowing the boat. Tacking is a way to recover some of this loss, but no set of the sails will overcome the losses incurred, because there is no energy available except the current in the river. The streamlined boat should win, because it has reduced its drag.

Mr Trunner, draw yourself a vector diagram of the forces involved in sailing a boat. Now look closely at the diagram, it consists of forces generated by the interaction of wind and sail and keel and water. You cannot determine from the drawing if the wind forces are generated by the wind moving relative to the sail, or the sail moving relative to wind: I know that sounds stupid at first, but that is the key to this problem. The same applies to the keel/water part of the system, it's all relative and the boat sails to windward, whether that wind is generated by the current acting on the keel or the keel pushing through the water,, you simply cannot tell without an outside frame of reference,,, if you can't grasp this , welll...

In reading all your comments and replies, I see that you are hung up on the energy here and the energy there. In reality there is no energy transfer from the river to anything, ignoring the friction at the interfaces between the river and earth and the air. The simple fact is that the river is simply flowing past the still air and the earth, which are both assumed to be stationary. Now, we all know that there energy here, but it is POTENTIAL energy, waiting to be used. The energy transfer occurs on the object (our sailboat) at the interface between the two fluids that are moving relative to each other. The relative motion of these two fluids imparts forces on the object caught in the middle. It makes no difference which fluid is moving how fast relative to what frame of reference other than relative to each. They both could be moving at 1000 MPH relative to the earth, the sun, or some galaxy somewhere. The only fact of importance to this problem, at this moment, is that one moves 5 KPH relative to the other. This relative movement between the two fluids imparts movement to the trapped object (our boat) relative to both of the two fluids. Now, the direction of movement of the object relative to some other frame of reference is a function of which media (frame) is moving relative to that frame of reference. In our case, it is the water in the river moving relative to the earth. Therefore, our advancement through the water in the river is additive to the movement of the water in the river relative to the earth in order to obtain our total movement relative to the earth.

If this does not clear things up for you, a have a detailed explanation of the basic principles of the actions and reactions on objects caught in the interface between to frames, where a sailboat being driven on water by the wind is merely a special case.

See you guys next week.

Thank you. I get it now.

The guy with the red face.

Trunner, well done. I was at the point of suggesting that someone take you out in a boat and demonstrate the whole thing by placing you in the water, to act as the drifting boat, and then to sail away down stream.

LOL

He saved himself in the nick of time!!!

Thank you for your graciousness. I was so far off on so much, I suspect I still don't have it all. I am (obviously) not a sailor. The last time I was on a sailboat, I spent the whole time puking, and it had a motor anyway. Thanks to all for attempting to educate me.

I had to add this in just for fun.

It is NOT the same problem as we have been discussing, but still a very interesting one, probably even more difficult to understand fully for most....

Here:-

http://www.youtube.com/watch?v=DPvGTjmn9y0

http://www.youtube.com/watch?v=xHsXcHoJu-A

Here is another fun one to watch:-

http://www.youtube.com/watch?v=k-trDF8Yldc&feature=related

Now turn upstream and run before the wind. As your speed cancels that of the water you are standing still w/r to shore, there is no wind. What happens then?

Dear guest, your thinking processes are flawed....nor do you understand any thing about sailing. Also that was not the question.

So switch the brain on Mr Simpson.....

Your speed is only about 1/2 to 2/3 that of the effective wind, so you are still moving down stream, but at only about 1/3 to 1/2 of 5 Km/Hr, or about 2 Km/Hr. For this scenario, you are racing to see who gets to the finish line LAST.