Sailing: Newsletter Challenge (06/01/10)

Absolutely correct. I'm amazed at all these would-be tackers. For a start, since the boat is near enough stationary in the water, there is near enough no force on the rudder to turn it. (Shifting the crew from side to side will roll the boat but not yaw it.) The correct strategy is to minimise the air resistance (sails down and aim to keep bows pointing straight downstream with an oar at the stern) and to maximise the water resistance (drop spinnaker in the water?). Then leave the water to float you downstream. For those who believe they can tack against the apparent head wind, may I point out that the apparent head wind is always exactly equal and opposite to the velocity of progress. If you try and tack against it you will simply change the angle of the head wind.

Thank you for that excellent explanation MB.

I have not been sailing for 60 years (as a young man)

You obviously know far more about this than me,
as I admitted... a sheer guess.

jt.

If there is no true wind and only apparent wind then the fastest a sail boat or sailboard can travel is the current. Check this site and apply the formula. I apply this without a good grip on the arithmetic required so correct me if wrong.

A=√(W² +V² +2WVcosα)

If W is the true wind =0 , then A = √V²

Or A = V which we already know. I don't see how a tack can help in this situation.

Drop the sails and drift is still the best bet to finish first. I like Martin Baxter's analogy of the water skier in an early post. Imagine a sail board with minimum resistance and all sails and spars removed. Just lay on the board and let it drift. If there is a true wind then the sail and tacking will help. You won't get tired and can enjoy the ride...hmmm..can you take a brewski on a sailboard?

That's a good site, but, for it to be relevant you have to change your frame of reference (as several other posters have pointed out).

The question states that the river is "wide": although in practice the slower speed of the river away from the centre would need to be taken into consideration, the questioners intension is that you can imagine a huge flat expanse of water all travelling at the 5km/hour.

Now change your frame of reference to a frame stationary relative to the water. There is no "real" wind, but, there is an apparent wind (not to the yacht, but, to the frame of reference) of 5 km per hour.

Everyone knows that you can tack in order to make progress in a direction opposite to the wind.

Now you can use the equations from the site to do your calculations.

The central point of the question, to my understanding, is a sort of a paradox: As you change the frame of reference the APPARENT wind becomes REAL wind for analysis purposes ... kind of a mental tongue-twister :)

You can tack in order to make progress in a direction opposite to the wind only if it is a real wind. This is an apparent wind caused only by the current. Let us suppose that the boat's motion was caused, not by the movement of the water current pushing it but by a long towline pulling it. This is exactly the same mechanical situation and there will be the same apparent wind. Are you suggesting that tacking will cause the towline to slacken? If so, the whole theory of kite flying will have to be rewritten.

The towline hypothesis doesn't represent the same situation as the one described because of the fact that in that situation there's no relative motion between the water and the air. As several posters have pointed out, a sailboat takes advantage of the difference in velocity of the water Vs. the air, regardless of which moves and which is stationary (in fact if you take the whole problem into a universally fixed reference frame, both the water and the air move, together with the whole earth - it's the relative motion between the two that matters. So the towline approach is not the same mechanical situation, and of course in that different mechanical situation the towline would never slack.

All right, if you don't like the towline try the high-performance sailplane. There is no wind, but the force of gravity is pulling it down. If in a two seater the co-pilot fell out, gravity would take him straight downwards, but fortunately for the pilot left in the plane his wings act as sails and he can tack down the glide path. Both pilots experiences an apparent wind whether dropping like a stone or travelling along the glide path, but please explain to me how the pilot in the plane gets down to the earth by gliding down the glide path (any glide path you like) more quickly than the co-pilot falling by the direct route. Note that a modern sailplane has a minimum sink rate of about 0.5 m/s, so it can take a long time to reach the ground, though the pilot has the option of increasing the sink rate by pushing the stick forward. (The terminal velocity of the co-pilot is about 150 m/s.)

You can tack in order to make progress in a direction opposite to the wind only if it is a real wind.

You're just not managing to change your frame of reference are you?

but by a long towline pulling it.

Ever seen a water skier nearly overtake the motor boat? Imagine lots of boats spaced "diagonally ahead" of the skier with lots of tow lines he can swap to.

Ah, but surely a kite tow-line is in opposition to the wind, or in this case the current...not with it! Is your comment not the equivalent of towing a kite into dead-air?

Bear in mind that the effect of towing the boat does not remove the relative speed of the vessel with respect to the air.

DUUUUHHHHHHH!

Somehow you managed to not read or even try to understand 99% of this blog.......

OMG!!!

Forget him, he got "passed" ages ago!!!

Either the people here understood the (simple) problem, or they did not. He is/was one of the "did nots"!!

Well put.

I think there is a problem with the site when the author calls W the true wind one feels as a stationary observer. I believe that is wrong 9or at least confusing) and W should be identified as the true wind experienced by the boat. My assumption of applying zero as a number for W is in error. So with this in mind I concur that the tacking is the fastest method to get to the finish line. We can apply that formula to get the time. But my arithmetic is lousy so I will leave it to others. Thanks for the second look at that issue.

The only problem that I have with this "problem" is that not enough information is given to REALLY figure it out. For example, How wide is the river? If it is too wide, you are adding a substantial amount of distance to travel. So, even though your speed has increased, the distance could be disproportionally longer. At what angle are you moving across the river? What is the friction between the boat and the water?

All those things matter if you want to "do the math"...

Why complicate an otherwise simple problem?
Keep it Simple and Stupid.
Infinite Width. Therefore:
1. No Tacking back-and-forth is required.
2. The current in the vicinity of all vessels is constant across the width of the river.
3. The relative wind in the vicinity of all vessels is constant across width of river.
Infinite Depth. Therefore:
1. The current in the vicinity of all vessels is constant with depth.
2. There is no bottom effect.
Exact solution not required.

Refering back to my childhood (playing with the water) i.e sheer guess.

The "wind" is a red herring as there is none ("there is no wind..." period.)
So it is all about the flow, force and resistance, of the water.
Any "induced" wind is just resistance to the boat flowing with the river.

Usually, the broadside boat (raft) travels the fastest. (childhood memories)
Because it is acted upon (utilises) the max. possible force of the river.
(The water down stream of the boat is running away from the boat.)

Yes,to minimise (lower) the air resistance of the sails, etc. is needed, but
a broadside boat would win over an inline boat. (other things being equal)

That's my guess anyway.

jt.

(wish I was back there playing now, - think I'll go visit the stream again.)

doen't say what size of boat. dinghy sailors roll tack in no wind to create apparent wind. the mast and sail move in an arc around the centre of rotation so create a driving force forwards. just like a windsurfer pumping sails to move forwards. Time is then down to how fit you are. This is a bit difficult in a big boat. do nothing and drift will take longer than 2 hours due to drag, roll tacking wins.

In this case, however, roll tacking is unnecessary. Ordinary upwind sailing will take you toward the finish line with a speed over ground of about 2 km/h better than drifting (assuming a boat of average performance). If you were out of sight of shore, you would have no way of knowing if you were sailing into a 5 km/h breeze in still water or sailing in moving water through still air.

"dinghy sailors roll tack in no wind to create apparent wind." This is more to give them something to do than to achieve any useful effect. Let us start with the crew to port and the boom to port. The sailors then move to starboard to tip the boat the other way, and the boom moves across. It generates force until it meets the centre line. After that the further movement of the boom out to starboard generates an equal and opposite braking force. The windsurfer has it easier, because he can set the sail at right angles, tilt the mast until the centre of pressure of the sail is in line with the centre line of the boat and pull. On the return stroke he can set the sail along the centre line and tilt the mast forward again. This is, in effect, rowing the sail though the air instead of using an oar in the water. A sail attached to a fixed mast cannot do this.

A sail attached to a fixed mast cannot do this.

In fact it can do this (if by this, you mean propel the boat), which is why kinetics are outlawed in many sailing classes: it is considered little different than rowing your sailboat across the lake. I used to race J 24's (a 24' keel boat) and a J-24 can be propelled easily by roll tacking in 0 wind, although it takes a little crew coordination. A Laser (and almost any single person dinghy) is very easy to propel this way. Sculling (in which the rudder is moved from sided to side to propel the boat) is also forbidden in most class rules for the same reason -- it's more like rowing than sailing. (The roll tack/rudder sculling motion is not as obviously like rowing as the rudimentary pumping used at negligible speeds in windsurfing, but is very similar to the mainly side-to-side pumping used in fast windsurfing.)

The Hobie Mirage system for kayaks uses the underwater equivalent of roll tacking to propel kayaks very effectively. The blades pivot from side to side (in the same sense that a sail on a mast does in a roll tack) and the flex in the blades angles them with the water to provide a vector component forward (and a not so useful vector component sideward), again in the same way that a sail works in roll tacking.

Neverthless, for this question, (and for the similar one in which there is a wind from the west) only normal sailing is required to benefit from the relative difference in velocities of air and water: sailing is impossible only when the relative difference is 0.

I thought about "Usually, the broadside boat (raft) travels the fastest. Because it is acted upon (utilises) the max. possible force of the river." too, but there is the air resistance of the broadside boat to consider, which is why I suggested the sail in the water.

Don't forget, the river is wide. That's a hint.

And it is possible to sail upwind.

Any dead fish can float downstream. A live fish swims upstream, spawns, and dies. Then he floats downstream.

If you read "very wide" to mean "infinitely wide," as is usually the case in an undergraduate physics problem, then we can assume that the boundary layer between water and air will be very thick, and air speed will match river speed when you are "far" from land. Thus no amount of tacking will do any good because it is a dead calm in the middle of the river. Everyone floats down together in 2 hours.

Your alternative presentation is better than the original I think. Makes it even more counter-intuitive.

Making the rather unrealistic assumption that the river is flowing 5 km/hr across its whole width and down to the depth of the keel, I will go with the crew that tacks. We would face an apparent 5 km breeze, and should be able to make some headway against it, depending on the boat. So, we should finish in a bit less than 2 hours.

One thing I've noticed is about this chit chat is that not many of you guys have any experience on a river and almost no one can read. There is no wind! The first thing that I learned many years ago is that if there is no wind when sailing a boat you are at the mercy of the tide or current. I have also observed after countless beers and many hours of fishing from boats and off mud flats that dropping the sail in the water creates as much drag as it does forward progress so forget about it. Ditto for turning your boat sideways to the current. The push of water on the upstream side of the boat will equal the resistance of the water on the downstream side of the boat. I've never seen a boat go "elastic" and somehow stretch in the water to move faster in the current. Everything on that river is moving at the speed of the water that it is in contact with. Everything! No exceptions. I have watched thousands of objects of all makes, sizes and materials roll past on the Mississippi and other rivers and none of them miraculously scoots past any of the others pieces of garbage. The fastest water in the river, regardless of how wide or deep, is at the surface in the center of the river. This is where the least amount of friction occurs. Steering your boat straight with an oar at the back and keeping it in the middle of the stream will put you across the finish line in two hours. You land lubbers doing the tacking will show up a bit later as you are going to spend a lot of time in slack water near the banks.

I am thinking that this is a lot like "Where does the smoke from an electric train go if the wind is blowing at 100 MPH from the east and the train is running 100 MPH to the east."; or, "Where do you bury the survivors of the plane that crashed directly on the border between the US and Canada?".

I am thinking that if I ever get back to Georgia I'll try this experiment out with the necessary amount of beer and fishbait for a relaxing afternoon on the Altamaha. Have fun!

Um. Some of us can't read, that's true. Try reading the problem again. "*On land* there is no wind." If you ever go white-water rafting on a fast river, you will realize that you can feel "wind" even when there is a dead calm on the shore and you are at the mercy of the current.

Of course you can. The "wind" you experience is because you are being propelled downstream by the water. As for the current problem, it is insoluble unless you make the assumption that there is no wind on the water as well as on the land. If there were any wind over the water the answer would depend on the vector of that wind as well as on the water velocity.

Sort of like driving down the road and putting your arm out of the window. Are you feeling the "wind" blowing; or, are you feeling the resistance of the stationary air on your arm?

If you are floating on an expensive raft, would there be any relative movement of water across the hull that would allow the rudder to turn the boat?

You need to create that force yourself by fast moving the rudder. You bring an unbalance in place with the force induced by yourself. This results in repositioning the raft, under a different angle as before. You will need a big enough rudder to to this. If you can take your rudder out of the water it can be applied as a roar.

You can increase your speed through the water by tacking. However, you will increase you distance that you must travel to get to the finish. I could get into the math, but enough math has already been displayed. Outcome is that the increased speed will not compensate for the increased distance that tacking will require. Stay in the highest speed current (in the middle of the river usually, depending on the contour of the bottom of the river at any specific point).

I see there are a few sailors here. Now regardless of wind drag and current speed there are important factors to consider such as...1. Keel Size. 2. Keel Depth. 3. Hull Size. 4. Waterline Length. 5. Are all the Yachts the same class? 6. Depth Of Water (most important as yachts drag down in shallows...as well the current moves slower in shallows). 7. A Keel athwartships to the current creates more drag therefore more speed.

It is illegal in Yacht racing to paddle with your rudder so that is not an option. Hope these comment help.

Anonymous Kiwi (NZ) surrounded by ocean

The tacking strategy would be 'running off' and 'jibing' staying as close to the center of the river current and especially away from the banks and shallows where turbulence and depth slow sails and hulls down. A river can create an apparent land sea breeze as with close to the land on the sea but this would be nearer to the centre where breeze from either side meets. The sail choice is storm jib and storm reefed main to keep sails closer to the jet/slipstrem created by the moving current close to its surface. It may show that more breeze can be gained by sailing just off the middle of the river to catch the upward draft of air when the cold off the land meets with warmer air in the middle of the river and more sail may be required.

So many factors to consider! Regardless a down river breeze will be created by the current! Low down sails will most probably help!

Your speed will be faster than the river flow near the center (if a really wide river then near to the center but not in the center due to the added breeze created through land/sea temperature meeting ['inshore breeze' to a yachtsman])!

Im judging 1.5 Km will be gained from air stream off the current but .5 Km will be lost through drag of water. therefore guestimated speed at 6 Kmph would see us winning in a time of.....100 minutes...if rigged as above because I cant see the slipstream rising high off the river unless the inshore breeze does. The sail choice would be a tactic on the day. When is the race and on what river and what time of day?night? What will the temperature be?? (forgot about ionic temp change at dawn/dusk in my estimates, this creates more or less land/sea beeze) :)

Enough from me. Thanks for the puzzle guys. If I'm not close with my finishing time then hope Ive helped someone understand a bit of yachting. Where there is water current you will find a breeze, where there is temperature change between two surfaces you will find breeze where they meet. When sitting on the banks of a river you can feel the river go by on one cheek. Every morning and night when the air ions speed up throught heat from sunlight coming and going...there is a breeze even if just a Zephyr. When cool air off the land meets the warmer sea (or vica versa)...there is a breeze.

Anonymous KIWI (NZ) surounded by ocean

since there is no wind effect, both bodies will have the same speed with the river flow

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?

I would win the race and the winning time would be slightly less than two hours. The reason for this is due to momentum given as mass times velocity of the body of water acting on the greater surface area of the hull of my boat. Inertia will be greater initially but on a tack more energy is exerted on a larger area, with the benifit of a certain amount of streamlining once my boat's velocity increases. Floating in a streamlined boat will put a greater point force on the stern of the opponent's boat due to a smaller area but due to the streamlined nature of the opponent's hull the kinetic energy will not be transferred nearly as efficiently to the opponent's boat.

I almost always hate the non-specific questions that end up on these types of websites. These questions require you to ignore almost every possible contingency and focus on what the essentials are, even if they don't specify them; good luck with that. With that said, it seems that the crux of the problem is whether or not tacking is faster than floating. One big problem with this question is: "the race is 10km downstream". It does not specify whether that is 10km by land or 10km by water. If it is 10km by land then I would have to go with 2hrs for each boat. If it is 10km by water then I would have to go with 1hr for the tacking boat (assuming no direction changes). These are "ideal conditions" which do not exist in the real world. It will always take longer than 2hrs even for the floater, as it has to go from a dead stop to get up to the 5km water speed. Granted we are talking seconds not minutes, but there are always considerations when doing any calculations, which is why I hate being asked for a specific answer when it should only be an estimation.

Since this is a race and I am chosing which is to win or lose. It depend on how the race is started. If it is a traditional start across a line, that is gotten there by other means, I would say the sleak down low air resistance would win. My choice is based on expirence of sailing in currents and rivers. The movement of water will have greater effect on the transport of the vessels down stream. It would also be important to have the winning vessel with as much "grip" on the water as possible. Say a large sail or board in the water. This will pull the vessel through the Air. Any tacking the other boat could do would be of little effect against the real friction of the boat through the water which means the sails only slows down the movement through the air. There is also the problem of lower speed of water toward the edges of the river which would be reached at some point if tacking. Staying in the middle of the river with as little air resistance posible and as much water surface as is practical would win the race.

Imagine the reverse of this concept, The wind blowing and the current static. Imperical knowledge is that the tacking vessel beats the vessel heading dead down wind. Assuming the same type of vessels are used. If you optimize each vessel to its chosen coarse then the problem is how much faster does the tacking vessel need to be to over come the added distance it would need to travel.

I would like to qualify my earlier comment by suggesting that
it comes down to deciding if the "induced" wind will increase
the speed of the boat, by tacking rapidly from side to side.

Put another way, do the sails increase the speed of the boat
through the water by tacking, or do they increase the resistance
to the forward movement, caused by the free flow of the river.

Assuming the boat does not actually "go" any real sideways distance;
i.e no meaningful extra distance, it simply keeps rapidly changing tack;

and, it is accepted that a boat can increase it's speed through water
by using a head wind, (even induced) therefore, with no practical increase
in the distance, I have to conclude.. my guess is... tacking wins easily.

jt.

I can conquer the world... then I wake up and start thinking.

Hello again, after reading many guestimates it appears Moronic Bumble is on a similar tack however the way I read it you are all tacking. tacking also incorporates Running and Reaching. I cant see that heading your yacht into the current wether mono or multi hull will do any good at all. it appears most of you are tackers and not running with the flow at all. It is faster to part the water with your bow in the direction of 'running off' travel than to cut the water tacking across it. Regardless of mono or multi hull it will be important to place all crew to leeward when running off thus to deepen the hull in the water to create maximum waterline length therefore catching more current. A special hull would be required with a big flatspot aft of amidships at the apparent best angle of current collection. Turbulence curves would be rquired to ensure maximum regeneration of energy from the current through voetex to help drive the water on the hull in essence increasing speed of the water...just like regenerating wind wind with the correct curvature at the Head, Luff and leach of sails. Jeepers...i think we could make a race of this...the yacht hulls would cost more than Americas Cup Class!! All the tech would be in water v hull currentage and not air v sail windage. Pehaps this is why Americas Cup races are cancelled with less than sufficient driving air. A river race would never need be cancelled/postponed due to lack of wind. I'll take the main if anyone sets up the race and makes ready a hull. the mast only need be short but with long long boom and gooseswing pole. When designing the hull it must be remembered that water is lighter near the surface so depth of hull will be important. the Yacht Class will need rules!! i.e. waterline length, hull height off water, bilge shape, chine shape, etc.... my original time as offered in Answer 68, It may have been earlier (see "Guest"...Kiwi..surrounded by ocean)

You are getting way to complex, and like so many others failing to grasp the simple concept of changing frame of reference. There are two possible and equally valid frames here: 1). Water is moving at 5k, any wind you feel is created by your motion through the otherwise stationary air, and the finish line is fixed. 2) The water is still, there is a 5K wind, and the finish line is moving towards you from directly upwind. It is vital to understanding this problem that you appreciate the moving finish line.

You cannot, in the absence of outside references tell which of these frames is the more true. Thus you are left with deciding if you can sail a boat and make progress to windward and the advancing finish line faster than the boat who sits there and lets the wind push him away from the advancing finish line. I submit that if you can't see this then you should stay the hell away from engineering.

Beautifully stated, many thanks.

tacking also incorporates Running and Reaching.

A sane skipper does not run or reach to make headway to a point to windward. On a windward/leeward race, a skipper tacks upwind, sailing close hauled to the windward mark, rounds that mark and then puts up a spinnaker (if he has one available) to sail to leeward.

If you leave the starting line in a windward/leeward race on a run, the race committee and observers will think you positively daft.

Moronic Bumble is on a similar tack however the way I read it you are all tacking

The "streamlined" part of the fleet would not have their sails raised, and therefore are not tacking. They are not sailing.

Dear MoronicBumble, they do not understand ANYONE here who has a grasp of the basic concepts of sailing......we both should give up I feel.....I just thought that we could "educate" a few to understand the problem fully, it looks like I am wrong on that point - sadly.

Have a great day in spite of "them"!

A complete and utter load of "Twaddle" as someone else correctly put it.

There is basically NOTHING in what you wrote that has anything correct.....if you had taken the time to read all the answers that have been given GAs, you would not have written what you just wrote.....

Just remember, if you were even a tiniest bit sure of the simple/maths and physics needed, you would post with a CR4 name, but you don't.

When posting an argument I always post under my CR4 name.....

But when I post as a guest, it is just to stop idiots like you attacking back because thats the only strong suite they have!!! No worthwhile knowledge to impart.....

You are not an engineer.

You will never be an engineer.

You probably need to look it up even to spell "Engineer"!

You may be a school teacher or similar!!! Or a garbage disposal expert.....?

I agree fully on all points.well put.

You are completely right, but its hardly worth replying to a person who has not the knowledge even how to join CR4 fully.....but just post as a Troll.......

You are just wasting your time trying to correct him and bring him to the correct path/method......

Pearls before Swine maybe?

Have a great day still!!

It is hard to say what is best. You will not get a lot of profit - if any - out of tacking with that miserable low wind.

Most sails do not even work with that kind of - sorry to call it breeze - and all the speed you win, you will lose during the maneuvers, to get your boat back to the next course.

With my boat I cannot even participate in that tacking race, because any speed win with a fart like wind is unthinkable.

The floaters will be far ahead of me if I tack.

And your river must be really wide to go for the middle. The speed of the water is always the highest at the deepest outside curves. But this one is a straight one with the given speed? I also read about how to advance and float the fastest?

Test for non believers:

Just try to hold a streamlined hull parallel with the flow direction and feel the tension on the rope.

Afterwards, TRY to do the same with the hull 90 degrees turned and compare.

I think I will try a combination with the sails (I cannot go for 90 degrees) to position my boat to do the race sideways and use the keel, rudder, and hull to become as much as possible one with the flow.

Of course with small, light weight easy maneuverable almost keel less jols the situation might be or might not be different.

These are definitely no race conditions.

bad time or bad place? This is something beautiful to happen in the middle of the Atlantic. It is the time to have the skipper grab the booze, to go faster, better and with more colors.

Don't count me in for this race.

You are not thinking "out of the box". This is not a normal sailing challenge question. It does not follow normal sailing rules and simply ignores many factors that a sailor must take into account in the real world......

Also, you do not appear to have read the dozens of good accurate posts already here either......

A failure of many here (sadly!)....you are not alone.

Have a great day in spite of me anyway.

both should arrive at the same time if it is not the point of interest the boat arrives in the middle of the water or at the left or right bank of the river - the cruising boat may increase the speed as much as it will (but not more than the angle allows), the speed of the boat in the direction of the rivers flow will be the same all times and the drifting boat has the speed of the river so both are using the same time!

It is like ticTactoe - there never will be a winner!

Actually, not quite : While your angle argument is extremely clear in demonstrating that whatever infinite speed the tacking boat could achieve, it could never be faster, down river, than the relative wind generated by the river flow, the tacking boat will necessarly endure more "drift" for her keel resistance to drift cannot be perfect. Hence, the tacking boat will always be second.

Quite a few poster are accounting for the river flow twice : Once for the movement provided by the river and then twice by adding the diagonal movement accross the river by the tacking boat : They are really one and the same.

Is there still no way to build a perpetuum mobile?

If its a conventional single hulled boat, it will probably be best to let the water carry you down and reduce windage by dropping all sail and the mast if possible.

If a reasonably well built light multihull, specially one of the more modern ones then tacking should prove useful and best to tack.

Some good multihulls, under storm conditions, with the tiller lashe 'a Lee and a storm trysail in use, will actually remain where they are or even make up against the wind, wheer almost all single hulled boats will move backwards......even if only slowly.

Simply not enogh infos about the width of the river, the size and type of the boat etc etc to be more than general in answering the question......

The boat that tacks will win, regardless of what the naysayers say, but the available information is sufficient only to say that the winning time will be less than two hours. I'm trying to figure out if there's a fundamental lower limit to the winning time that the "perfect" boat could achieve, but I don't think there is one.

That wasn't too bad, but this (unless it's a hoax -- I don't think so) is doing my head in. If anyone can explain it in terms I can understand, please be my guest!

An ice boat can travel at eight times the speed of the wind, so, it's easy to see how it can achieve a velocity with a component down wind which is greater than the wind speed: if it tacks in perfect conditions it can achieve an overall speed downwind greater than the wind.

In the direct down wind vehicle although the vehicle itself is travelling direct down wind: the propeller blades are effectively tacking.

What you say is true for downwind sailing, but please don't forget that the "wind" is on the nose in this example, its being generated by the water movement only as there is no "other" wind.....

Sorry, I should have made it clear that I was replying to ExPat's last paragraph:-

That wasn't too bad, but this (unless it's a hoax -- I don't think so) is doing my head in. If anyone can explain it in terms I can understand, please be my guest!

Thanks Randall. As to your first statement "it's easy to see how it can achieve a velocity with a component down wind which is greater than the wind speed," I'm not sure about the "easy" bit (for someone as dumb as I am, anyway), because eight times the speed of the wind is achieved at close reach conditions, whereas to proceed downwind implies broad-reach sailing. That said, I think I can convince myself that a high-performance vehicle can generate sufficient apparent wind to achieve the necessary downwind velocity.

Back to the DDWFFTW concept: I get the effective tacking bit, and I don't believe there's any violation of fundamental principles, but I have the damnedest time wrapping my head around the specific mechanics of what's going on; in particular, how does it pass through V_vehicle = V_wind?: Before viewing the videos, I thought the craft might need to by tap into the KE of the sails to get through that transition, but they seem to accelerate even as the tell-tales are dangling limply (especially evident in the second video). I also initially imagined an infinitely variable transmission would be required, and half expected the sail rotation to reverse, but there's no evidence of any obvious gearing changes in the videos.

The more I think about it, the more I'm sold on the explanation (made in some of the comments on that blog) that the wheels are actually driving the sails, not vice versa (in most rotary sail vehicles, the sails act as a turbine to drive the wheels -- or water screw). Looking at the curvature of the sails, they do indeed seem to be configured as a propeller, not a turbine. That means that the the propeller must be generating a forward thrust sufficient to turn the wheels (and hence the propeller).

Even as I write this, I THINK I'm beginning to get the V_vehicle = V_wind bit, because the mechanical connection to the ground means that the condition is NOT equivalent standing still in zero wind. Still mind-boggling to figure out the forces on the sails and satisfy oneself that there's a net forward component at all operating speeds.

Great answer: and, I withdraw my original explanation, which only really appealed to my gut feeling that there were no physical laws being broken here.

Let's just think about the period when the vehicle is travelling at the speed of the wind. We only need the whole system to conserve momentum and kinetic energy.

1.) The propeller is pushing the air backwards relative to the vehicle.
2.) Because the vehicle is running at the speed of the wind the wind is being slowed down relative to the ground (i.e. the wind is losing momentum and kinetic energy).
3.) Because the propeller is pushing air backwards relative to the vehicle, the vehicle is being pushed forward by the air.
4.) Because vehicle is being pushed forward by the air, the wheels are being forced to rotate faster by their interaction with the ground (not the other way round as with most windmill driven vehicles).
5.) The transmission from the wheels to the propeller is forcing the propeller to rotate faster.

The key thing to notice is that the air (wind) is losing momentum and kinetic energy.

I don't think I've said anything here that you didn't already say. It just kind of helps to try to untangle the brain if we bounce each others ideas of one another.

Very elegant explanation!

Just to labor it a bit more: your points (3), (4) and (5) help understand the mechanism, but are insufficient on their own because they say nothing about the relative magnitudes of the forces involved (similar arguments could be made for any number of perpetual motion devices), but you nail that one with the the observation that the system is extracting momentum and kinetic energy from the air.

Thanks.

Good explanation.

If you have not already read through it, the discussion on Boatdesign.net is interesting. Several folks were convinced that DDWFTTW was not possible. The designers of the craft that has been in the news showed up to explain. Spork (one of the designers) gives a good explanation several pages into the discussion, in which he likens the prop to two sails on a cylindrical surface.

Interesting how obscure these devices have been, especially given the apparent paradoxes that require some thought to reason through.

Thanks: nice to have my instinct validated by someone who clearly knows their stuff.

the captain is the "optimizer " of his sail. like juggling, it is hand to eye stuff. and it is also like music, the left hand is the rock and the right hand is the noodle, but they stay in tune. (holding the sail being the rock and holding the rudder is the noodle)

Here is perhaps another way of looking at it:

When the boat and sails are at about 45 dgs to the wind (apparent in this case), the sails cause the previously still air (still to the outside observer) are to gain kinetic energy, the air molecules are now moving in a direction approximately perpendicular to the current flow. This requires the input of energy, the energy comes from the force of the flowing water acting on the keel of the boat. This also produces and equal and opposite reaction at the sail (remember that Newton fellow?), resolve that force in two vectors, on acting to propel the boat forward, the other to push the boat sideways. The sideways movement is opposed by the keel and results in the boats heeling, and there is some slippage, (leeway). However, the forward portion of the force will be enough to give the boat enough forward velocity that when the leeway is subtracted from that portion of the velocity that is dead down river, the VMG is greater than the drifting boat.

"However, the forward portion of the force will be enough to give the boat enough forward velocity that when the leeway is subtracted from that portion of the velocity that is dead down river, the VMG is greater than the drifting boat"

I beg to differ : We agree that a sailboat can gain against the wind. In this case, the wind as a driving force is only the result of the current : drifting boat drift straight, tacking boat drift sideways at incredibly high speed if you want but still sideways and cannot gain on the wind at speed greater than the windless drifting sailboat. You are in effect counting the current twice !

I really wish you had not voted my previous post as "off topic". Should any one offer a better explanation, I will be more than happy to read it. My previous post is a tentative demonstration. As it stands now, your argument is neither an explanation nor does it demonstrate that my demonstration is wrong. Please feel free to improve both...

First, I did not vote on your reply one way or the other.

Second, it matters not weather the wind is as result of the current and the air is still, or the wind is there and the water is still. If the river is infinitely wide, can you tell whether there is wind or current? I submit you cannot without some external reference. The only question then becomes, does not a boat actively sailing, make progress to windward while one just sitting there is borne by the wind in the direction of the wind?

In this case (the river moving) the boat that is drifting will make some speed less than the river speed, the boat actively sailing will make the river speed plus whatever speed it can achieve to windward.

Cheers

Martin

Second, it matters not weather the wind is as result of the current and the air is still, or the wind is there and the water is still.

(Your spelling of "weather" must be a Freudian [boat?] slip.) You are correct.

Another way of viewing this. Suppose you are on a cruise ship, and the upper deck, with its pool, is free of obstructions to the wind caused by the ship's motion. If the air over the still (still WRT the ground) ocean water is still (WRP to the ocean water) and the boat is cruising at 10 knots, then the wind seen at the pool is 10 knots (WRP to the pool water). An RC sailboat will sail in the pool in exactly the same way as it would if the ocean liner were docked and the wind over land and dock was 10 knots.

(Your spelling of "weather" must be a Freudian [boat?] slip.

) You are correct.

I wondered who would catch that.

To further use your ocean liner analogy; put two RC boats in the pool, on a line bisecting the pool athwartships, this is the starting line. The edge of the pool towards the bow is the finishing line.

One boat actively sails upwind, the other drops all sail. The boat that sails makes it to the finish line, the other drifts back until it hits the sternmost wall of the pool. The first boat is making 10+ knots with respect to the ocean, the other is making something less than 10 knots.

Both will cross a point in space where the original finish line was, but the sailed boat will get there first.

Cheers

Martin

Aviatar,

The boat on the open water knows nothing about the world or where the earth is relative to itself. It only knows what is acting on it relative to the water. Therefore, to obtain the VMG, the speed at which the boat advances against what it sees as "wind" is additive to its motion as it drifts along in the river current.

A simpler example is someone walking on a moving train. One cannot ignore the movement of the base object.

BTW, the speed of the drifting boat is independent of the relative orientation of the boat in the stream. Any object in the stream will follow the stream until the stream changes directions. The only significant forces acting are displacement forces, which are gravity forces (displacement, mass) that are subject to changes only in acceleration.

Also note that a drifting boat has no speed relative to the water, so there is no relative current at the rudder against which the rudder must act in order to provide steering thrust. Therefore, the direction of the boat is uncontrollable without motion relative the water. The stationary air provides that force as the boat tries to push through it. This force advances the boat in a direction that is additive to its drifting motion.

I found a great .pdf online which mentions tacking downwind for multihulls and fast monohulls with this comment:-

"even cruising boats usually reach their destination quickest by sailing at 165-170° to the true wind. If you are sailing a fast racing boat, you will be tacking downwind, in which case you don't want to let the apparent wind come aft of the beam until you become overpowered."

I hope this assists anyone who does not understand the principles fully....

I found a great .pdf online which mentions tacking downwind for multihulls and fast monohulls with this comment:-

This article uses "tack" in the sense of "a course sailed at some angle to the apparent wind." When we talk about ice boats (and even soft water boats that are far faster than the ones this article is about) the turns from one tack to the next are "tacks" used in another sense, namely a turn in which the apparent wind passes over the bow. (This is in contrast to gybe or jibe, in which the apparent wind crosses the stern.) For such a tack to occur, the boat must be going in excess of true windspeed even as it is halfway through the turn, requiring that it is going substantially faster than windspeed while on the tack it has just left.

The boats this article discusses (even the relatively faster ones as opposed to pure cruisers) are using traditional spinnakers, which cannot be used on a boat that makes VMG greater that wind speed. Iceboats cannot make beneficial use of a spinnaker, because they are always sailing upwind with respect to the apparent wind. In the boats the article is about, the spinnaker is jibed not tacked... (and the skipper shouts "jibe ho" just as he starts the turn... and hopes than nothing goes wrong in jibing the spinnaker).

The article has very little to do with sailing upwind in a stream and almost nothing to do with sailing directly downwind at greater than true wind speed by using a propeller driven by the wheels of a vehicle (which is the other topic this thread has veered into).

"Fast" is very relative as it applies to sailing (and "fast racing boat" has no specific meaning, because there are active racing classes for some very slow boats). A J-24 is thought to be a "fast" boat, and essentially all of them are now purchased only for racing. (The designer originally dreamed about overnighting in them.) But getting one to plane is very difficult, and in most ordinary sailing conditions, the boat speed is limited by its hull speed: 6.5 knots or so. So in a ten knot true wind, the boat can sail at only .6 times windspeed, no matter what the course. Only when the wind gets quite strong (20 knots and more) will the J-24 plane, and exceed hull speed. But even then, there is no condition under which a J-24 will exceed true windspeed, either up wind or down. J-24s are fastest downwind when sailed about 145-160 degrees off the wind, depending upon wind speed.

Even with very slow boats (some of which are called cruiser racers -- because in sailing you can race anything) there are two advantages to avoiding a directly downwind course. The first is for safety. The likelihood of an accidental jibe (in which the boom slams unexpectedly from one side to the other) is reduced by being a little away from dead downwind. The second it that the blanketing of the spinnaker (or other headsail) by the mainsail is reduced, making the spinnaker more effective.

But as I said, none of this has anything to do with either the challenge question or sailing dead downwind faster than the wind. I include it only for perspective for the non sailors.

in a sailing race, there is a time before the start where the racers jocky for position. each boat trying to get the best advantage to cross the start line just before the gun. the boat that wins the race in this case is the one that crosses the start line first. and since there is a very wide river , many opponents drifting, and me tacking

(does any body know if sculling is leagle before the start?) I win the race in a time of 1 hr and 54 mins.

(does any body know if sculling is leagle before the start?)

Definitely illegal within one minute of the start (and probably even at the five minute gun -- but I'd have to check). However, sculling is now legal to return to the original tack (the course), if you find yourself without steerage way mid-way through a tack (the turn). This is a relatively new change. It would still be illegal to scull through a tack to get established on a new tack. The rule change is mainly, I think, for safety -- you don't want boats drifting without rudder control in the vicinity of the starting line.

thank you for your reply. So at he 5 min. gun, ( is this the actual start of the race? ) if i am on a tack (very wide river) and every body else is drifting, I can make the one minute gun, and the start gun leading the flotilla to the finish line. (although I know that there is a press/temp differential from cold water, to hot land, and there would always be "wind for steerage". it did not say that it was a morning race or afternoon race)

would anybody have rudder cintrol?

In the conditions of the original question, the people sailing would have good rudder control, once established on a tack. The 3km/h possible sailing speed (through the water) of an ordinary cruiser-racer is enough to give adequate control. With an ordinary boat, this speed is also enough to carry you through a well-executed tack, but not enough to carry you through a poorly-executed one (in other words, you run the risk of finding yourself "in irons" halfway through the turn, with other boats bearing down on you... very uncomfortable situation to be in.) *

The people not sailing would have virtually no rudder control initially, and would be drifting slowly backwards relative to the water flow. But before too long, their rearward speed would be high enough to allow them to steer slowly while going backwards through the water. By this time, they would realize that they should be sailing, and will put up their sails as they are making their rearward turn. Once they are about perpendicular to the apparent wind, they'd trim the sails to get underway, gain some boat speed, and then start to head upwind. They'd have no chance of catching the people who started out sailing, but at least they might arrive at the finish line before all the beer is gone.

* If a sailor is prone to panic, the start is the time to do so. There are many things going on, and all the sailors tend to want to be in the same place at the same time (namely, going full speed across the line at the "favored" end of the line, right when the gun goes off). At any given time prior to the start, about half the boats have right of way over the others, and "staying clear" can mean, in practice "to avoid the boat with right of way by 6 inches". Sometimes you are forced to tack to avoid infringing a rule, and if you loose speed in the tack, and find yourself without enough speed for good steering, it can mean that the right of way boat has to alter course to avoiding hitting you, meaning that the boat next to him must alter course, etc, etc.

The recent change in the sculling rule (allowing you to scull to gain enough speed for enough rudder control to fall back onto your original tack, but not to allow you to get onto your intended new tack) is, I think, mainly there so that you are not yet another obstruction in an already overcrowded situation. For safety, a prudent sailor would use every means to regain control under such a situation if not racing, and the racing rules now allow this for that reason. If would be unfair to allow sculling to enable a boat to make it onto the intended new tack, because it could enable such a boat to complete a tack more quickly than a boat not sculling.

it seems as though I am the only boat in this race with some sail up and able to use my rudder. My opponents (how many?), have elected to streamline their boats to the wind and float as fast as they can. I will make sure that I start in the right part of of the line.

just say that everybody is at speed and equal at the start gun. Nobody has rudder control except one boat on one tack, starting at the right or left, gaining some rudder control.

If everybody is at speed at the start, then everyone has rudder control.

The recent rule change allows for the case in which, during the sailing and jockeying for position in the five minutes before the starting gun, a boat attempts to tack and looses steerage way (because of a poorly executed tack) in making the turn from (for example) port tack to starboard tack. Now that boat is allowed to scull to return to port tack but is not allowed to scull to help get established on starboard tack.

The race, and the winner is the vessel tacking with a time of 1hr and 58.89 minutes.
As with any good competition the ingenuity of the opponents can make the difference of who wins.
I would take the sailing craft with a keel and sails. Utilize all my crew to rock the boat side to side. Say 3 crew of 175 lb each, from side to side , 525 lbs of added force to overcome friction of the water and other factors. I would estimate a speed of .2 knots. Taking into account leeway, angle of attack, apparent wind, slip and other factors I would estimate speed of advance of 5.22 km/hr for the coarse, which would beat the other boat by 2 minutes. I would not try any tacking as turning would only slow down movement thru the water.
In the theoretical world of ideal liquids and air foils and simplified vector analysis the tacking vessel would win.

Tied in two hours.

Both boats finish at the same time, 2 hours.

Now this is ridiculous, after more than one hundred responses, to offer something so mundane, with no reasoning, or logical argument, is quite frankly inane.

SAILING IS FUN insane is how Texas oil has bamboozled us since 1929

what have they done????? we have had sooo much natural gas for so long

back to sailing, i liked YOUR comments on this question. what if you had no keel in this case and your rudder could be used as sort of a movable keel, sculled to maintain a certian long tack. you would only have to gain a foot or so lead then coast. And as they would go the long way, a boat making any kind of headway would take slight short cuts.

The opponents win in a time of 2 hours (assuming their "streamlining" is ideal, meaning that it is effctive enough to avoid any slowing due to apparent wind. Apparent wind is that generated not by the atmosphere, but by the movement of the craft. The tacking strategy would magnify the impact of the apparent wind reducing my boats speed downriver due to resistance of the boat and rigging as it moves downstream against the still air. In contrast, the streamlined boat would minimize this resistance, staying as close as possible to the speed of the current.

Even assuming that my boat could use the 5 Km/hr apparent wind to tack effectively and that we could maintain a 45 degree angle of attack (which would make it even) we will lose. As sailors know, each time you tack you lose momentum and must regain it on the new tack. With each tacking manoeuvre we will lose ground on our opponent. This is why, all things being equal, the boat that makes the windward mark in two tacks will beat the boat that uses four. In the real world, other things such as covering and wind shifts take a toll, but frequently, and with luck, a boat that departs from the fleet will arrive at the windward mark 1st, because they made fewer tacks.

I win the race in less than 2 hours. Both boats will float down the river at the same pace. My boat will have the additional aid of wind generated by the river current and any additional winds that occur during the race.

I win in two hours!. I win because sailing across the river, water flow makes my boat equal it's speed (in this possition my boat's hull offers maximun resistance to water flow) and I do it in two hours because water speed in the river is 5 Km/hr and the race long is 10 of them.

Mario Gasca