This one is a little tricky to ask without diagrams, I'll do my best...

Scenario:
Let's say you're sailing straight ahead (12 o'clock) position, wind coming from 3 o'clock at say 15knots, happy days. When your board speed reaches 15knots, now combined (apparent?) wind direction is from 1:30, and greater than 15knots.

Questions:
1. Based on the assumption above, faster you go, apparent wind direction gets closer to head on (12:00). So faster you go the more you'll need to sheet in, right?
2. Wind from 1:30 is about the limit of my setup. Past this the wind just becomes too much head on, and hence slower. So, faster you go, do you change course towards 11:00 to keep speed up?
3. Is apparent wind (which is greater strength than actual) one of the reasons that you can travel faster than wind speed? Eg hit 20+knots with 15 knots of wind?

TA.

It's always been a good question. So many ways of looking at it. The big picture view is that "sailing" is extracting energy from the velocity discontinuity between the air and the water. After you've sailed by the wind and the air won't be moving quite so quickly relative to each other. You can use that energy to make your vessel move. If the vessel is really slippery there is no theoretical limit as to how fast it can move, or in what direction it can move, but the drag of all the bits pieces usually catches up and you go a little bit faster than the wind in certain directions.

A while ago we had a post about a machine that goes " downwind faster than the wind". That had us all thinking, but after a bit of lively discussion and a bit of backtracking by some of us we decided it wasn't a hoax. We did pretty well I've just googled "downwind faster than the wind" again and come up with the physics forum. (As well as the youtube video of the device)

http://www.physicsforums.com/showthread.php?s=230a10dc9c4d32982c72695b3f1ec028&t=270041&page=5

Interesting thread, shows even physicists are human, the thread has been locked, then unlocked, names have been called, the very clever constructors of the device have been labeled frauds and they still don't seem to have come up with the correct consensus that it does in fact work.

1. Yes
2. Yes, if you want to go faster.
3. Not really? If you're on a reach for example, the sail + fin net force is across the wind, that will accelerate you limited only by drag and a reduction in forward force as the apparent wind shifts forward. So sailing faster than the wind is more to do with reducing drag (e.g. by planing) and producing forward force in the same apparent wind (e.g. bigger/more efficient sail).

Probably.

Hi GusTee

That is my understanding of apparent wind: simply the apparent direction change and speed from the perspective of the sailor. However, I'm not confortable with the concept of sailing faster than the wind, as that is suggesting that you can break the law of conservation of energy (ie: you can't create energy, you can only convert it). Many sailors I talk to are happy with the concept of getting free energy.

My engineering mind tells me it's a question of measurement and notation and the way you average measurements. An example: The wind speed will be measured every second, and averaged over that second to give you your measurement (ie: 1 second integral), but for say 100 microseconds, the wind may increase 10 knots (for example). Due you the low friction of your board, your sail will react to that 10 knot increase, and before you know it, you going 16 knots... which is 1 knot faster than the measured wind that is measured and averaged at 1 second. To the sailor, he is going faster than the wind and getting free energy, but in reality he is not.

J

I think the "Light WInd Challenge" thread in the GPS seciton has some figures that show clearly that you can easily sail faster than the wind.
While that may seem to break teh whole conservation of energy thing, it does nto have to.
CAlculate how much energy you need to propel the craft at a certain speed, then calculate how much energy you get from X knots of wind hitting a given sail size and they will probably match, with actual wind speed being a figure in the calculation, but not a limiting factor on speed achieved.

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sharkbiscuit said...

Hi GusTee

That is my understanding of apparent wind: simply the apparent direction change and speed from the perspective of the sailor. However, I'm not confortable with the concept of sailing faster than the wind, as that is suggesting that you can break the law of conservation of energy (ie: you can't create energy, you can only convert it). Many sailors I talk to are happy with the concept of getting free energy.

My engineering mind tells me it's a question of measurement and notation and the way you average measurements. An example: The wind speed will be measured every second, and averaged over that second to give you your measurement (ie: 1 second integral), but for say 100 microseconds, the wind may increase 10 knots (for example). Due you the low friction of your board, your sail will react to that 10 knot increase, and before you know it, you going 16 knots... which is 1 knot faster than the measured wind that is measured and averaged at 1 second. To the sailor, he is going faster than the wind and getting free energy, but in reality he is not.

J

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sharkbiscuit said...

However, I'm not confortable with the concept of sailing faster than the wind, as that is suggesting that you can break the law of conservation of energy (ie: you can't create energy, you can only convert it). Many sailors I talk to are happy with the concept of getting free energy.

Conservation of energy doesn't limit you to going slower than the wind. Imagine you're on a reach, the sail is acting as a wing so the lift is what's driving you forward, at say 5 knots. Now make the sail bigger - you are extracting more energy/lift from the wind and going faster, say 10 knots. Repeat until you're going faster than the wind. The energy you're extracting from the wind is being you used to drive you forward, it doesn't matter if the speed you achieve is faster than the wind speed.

So if there's a 45kmh breeze blowing and your in a car driving with the wind at 45kmh and stick your hand out the window shouldn't you feel no wind?

vector addition.

www.physicsclassroom.com/class/vectors/u3l1b.cfm

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choco said...

So if there's a 45kmh breeze blowing and your in a car driving with the wind at 45kmh and stick your hand out the window shouldn't you feel no wind?

Yep! and if your in an automatic, the engine should just be a little bit above idle, just enough to overcome the rolling friction from the rubber tyres. :-)

Or if you landing in an aircraft into a 40knot headwind at 120knots, you touch down at 80, or if your silly enough to try it the otherway around it would be 160knots.

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racerX said...

choco said...

So if there's a 45kmh breeze blowing and your in a car driving with the wind at 45kmh and stick your hand out the window shouldn't you feel no wind?

Yep! and if your in an automatic, the engine should just be a little bit above idle, just enough to overcome the rolling friction from the rubber tyres. :-)

Or if you landing in an aircraft into a 40knot headwind at 120knots, you touch down at 80,

Oh my god!
What happens if there is a 120kn wind??? It would never come down

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GusTee said...


3. Is apparent wind (which is greater strength than actual) one of the reasons that you can travel faster than wind speed? Eg hit 20+knots with 15 knots of wind?

I don't think its the apparent wind that is the reason. Windsurfers with large sails, and other craft with large sail areas relative to the amount of drag, can sail faster than the wind, just not in the same direction!

As you go faster, the apparent wind increases, however at the same time the direction of the force changes.

In your example when you are sailing at 12'oclock the lift force of the sail would be approximately 11'oclock. as you increase speed and sheet in the lift force will rotate with the sail so now the force is acting towards 9'oclock. The fin and the shape of the board stops you going towards 9o'clock leaving some of the force to drive you forward. So with increased apparent wind speed, the sail will generate more power, but some of it becomes 'drag' as it not in the correct direction to drive you forward.

Apparent wind it still important, otherwise you can't trim the sail correctly,

hangon, i thought apparent wind is when someone tells you how good the wind was at the spot they were at, but there was no wind at the spot you were at, or anywhere else for that matter?

depends on vessel and design mabe

Apparently !!

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Haircut said...

hangon, i thought apparent wind is when someone tells you how good the wind was at the spot they were at, but there was no wind at the spot you were at, or anywhere else for that matter?

Apparent wind is better known as a "BOM Forcast"

bom bom

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Ian1 said...

Apparent wind is better known as a "BOM Forcast"

I always thought they used one of these

Ian K - I had a quick look at the link, the youtube vids were a spinout.

174, RacerX and KenHo, thanks, it makes more sense now, but definately this topic is more complicated than it first appears.

Is it correct to assume then; straight downwind produces the most power in the sail due to actual wind. Straight downwind also means the sail creates the most drag, so the net result is not that fast. On the other hand, on broad reach power is reduced a little, but sail drag is reduced substantially, hence faster. See busterwaa's chart.

OR should just go with Haircut's theory? Much simpler....

Putting all theory aside, do you just trim the sail in and out till you're exerting maximum effort on the boom, hence the most power?

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GusTee said...

Ian K - I had a quick look at the link, the youtube vids were a spinout.

174, RacerX and KenHo, thanks, it makes more sense now, but definately this topic is more complicated than it first appears.

Is it correct to assume then; straight downwind produces the most power in the sail due to actual wind. Straight downwind also means the sail creates the most drag, so the net result is not that fast. On the other hand, on broad reach power is reduced a little, but sail drag is reduced substantially, hence faster. See busterwaa's chart.

OR should just go with Haircut's theory? Much simpler....

Putting all theory aside, do you just trim the sail in and out till you're exerting maximum effort on the boom, hence the most power?

I appreciate that it is interesting to think about these things.
However after thinking about it for years I have come to the conclusion that there is no other wind than apparent wind. You only ever sail in apparent wind so TWD TWA ect are quite useless.

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GusTee said...


Is it correct to assume then; straight downwind produces the most power in the sail due to actual wind. Straight downwind also means the sail creates the most drag, so the net result is not that fast. On the other hand, on broad reach power is reduced a little, but sail drag is reduced substantially, hence faster. See busterwaa's chart.

Straight downwind only produces power in the sail when your standing still, the sails does not work like an airfoil in that case, much more like a spinaker or just a big sheet catching the wind.

A sail can act as an airfoil or just a great big sheet, catching the wind.

As soon as you accelerate downwind, you catch up with the wind so the sail no longer catches the wind. You have to move across the wind, with the sail somewhere between 0 and 15 degrees to the apparent wind for it work like an airfoil.

That is why a broad reach, which keeps the apparent wind reasonably high, and the apparent wind direction relative to your direction of travel NOT TOO much on your bow is fastest for a planning hull like a windsurfer.

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Haircut said...

hangon, i thought apparent wind is when someone tells you how good the wind was at the spot they were at, but there was no wind at the spot you were at, or anywhere else for that matter?

Especially if its a kiter talking..

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Haircut said...

hangon, i thought apparent wind is when someone tells you how good the wind was at the spot they were at, but there was no wind at the spot you were at, or anywhere else for that matter?

Especially if its a kiter talking..Sorry about the double posts..it took so long to come up I hit it another time..it was working after all..

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174 said...

Imagine you're on a reach, the sail is acting as a wing so the lift is what's driving you forward, at say 5 knots. Now make the sail bigger - you are extracting more energy/lift from the wind and going faster, say 10 knots. Repeat until you're going faster than the wind. The energy you're extracting from the wind is being you used to drive you forward, it doesn't matter if the speed you achieve is faster than the wind speed.

So the sail (in conjunction with the sailor forces, the fin forces etc) acts as some sort of force/acceleration multiplier ? (ie: similar to a lever)

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Straight downwind only produces power in the sail when your standing still, the sails does not work like an airfoil in that case, much more like a spinaker or just a big sheet catching the wind.

A sail can act as an airfoil or just a great big sheet, catching the wind.

As soon as you accelerate downwind, you catch up with the wind so the sail no longer catches the wind. You have to move across the wind, with the sail somewhere between 0 and 15 degrees to the apparent wind for it work like an airfoil.

That is why a broad reach, which keeps the apparent wind reasonably high, and the apparent wind direction relative to your direction of travel NOT TOO much on your bow is fastest for a planning hull like a windsurfer.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Racer, thanks for the explanation above, it brought it all together, makes a lot more sense.

So to put the theory into practice, to achieve the highest power/speed, do you simply sheet the sail in/out till you find you're exerting maximum force holding the boom in place (assuming you dont want to change direction of travel)? Then, combine this with turning upwind/downwind till you find maximum force on the boom again for max possible speed for the conditions?

Thanks.

Busterwa's graph is interesting, the 18 footer does 15 knots at 145 degrees off the wind in 10 knots of wind. That's a vmg in the downwind direction of 12 knots! One good gybe and it would beat a balloon released at the top of the course to the bottom. Which makes you wonder why the "downwind faster than the wind machine" (which used broad reaching propellor blades ) was so controversial.

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Ian1 said...

Apparent wind is better known as a "BOM Forcast"

Gee i thought if your parents fart it's "a parent wind"

Cos physicists tend to think theoretically more than observationally and often let their large brains obscure what their eyes are seeing.
I recall someone asking Dr Carl once about a bumpy horizon on a calm day and he was totally stumped. Of course, everyone here knows it was the strong offshore wind causing a dead flat beach with rough as guts conditions 5 miles out.

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Ian K said...

Busterwa's graph is interesting, the 18 footer does 15 knots at 145 degrees off the wind in 10 knots of wind. That's a vmg in the downwind direction of 12 knots! One good gybe and it would beat a balloon released at the top of the course to the bottom. Which makes you wonder why the "downwind faster than the wind machine" (which used broad reaching propellor blades ) was so controversial.

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Ian K said...

Busterwa's graph is interesting, the 18 footer does 15 knots at 145 degrees off the wind in 10 knots of wind. That's a vmg in the downwind direction of 12 knots! One good gybe and it would beat a balloon released at the top of the course to the bottom. Which makes you wonder why the "downwind faster than the wind machine" (which used broad reaching propellor blades ) was so controversial.

While we're talking about pure efficiency how about BOR90 in the AC this year. In race 1 winds were 5-10kts and her VMG into the wind was about 1.8 times the wind speed or 13kts, downwind it was about 19kts or 2.5 times the wind speed (averaged); and we're talking VMG here not acutal boat speed! Sure, BOR90 has a 60+m tall mast so there would have been a wind velocity gradient up the sail but there is no doubt that the boat sails a VMG which is significantly faster than the true wind speed both into and off the wind....incredible. I think on one of the reaching legs in race 2 its boat speed was way over 3 times the true wind speed.

i think on the upwind/downwind race. the commentators said that if you released a balloon at the upwind mark, bor90 would have completed the course before the balloon made it to the downwind mark.

that boat is EPIC

I think the downwind faster then the wind thing was interesting cause it was specifically about going directly downwind... boats can generate crazy VMG numbers sailing angles down wind but I think they would struggle to match those numbers if you tried to sail it directly downwind.




from sailmagazine.com/cupdate/we_predict_the_winner/

those are predicted VMG numbers, actual boat speed would be higher. And I think based on the actual race times and results, the boat exceeded those numbers.

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sharkbiscuit said...

Hi GusTee

That is my understanding of apparent wind: simply the apparent direction change and speed from the perspective of the sailor. However, I'm not confortable with the concept of sailing faster than the wind, as that is suggesting that you can break the law of conservation of energy (ie: you can't create energy, you can only convert it). Many sailors I talk to are happy with the concept of getting free energy.

My engineering mind tells me it's a question of measurement and notation and the way you average measurements. An example: The wind speed will be measured every second, and averaged over that second to give you your measurement (ie: 1 second integral), but for say 100 microseconds, the wind may increase 10 knots (for example). Due you the low friction of your board, your sail will react to that 10 knot increase, and before you know it, you going 16 knots... which is 1 knot faster than the measured wind that is measured and averaged at 1 second. To the sailor, he is going faster than the wind and getting free energy, but in reality he is not.

J

I use a large windmill in a 5mph wind to wind up a huge catapult attached to a golf club. Release the catapult, the golf club swings, the golf ball that it hits goes 150mph...

<highdea>
So, like, is there apparent gravity too? Dude, what's that called? Conservation of momentum, or something? Like if I was falling and then I was to like turn I'd experience more Gs and like maybe even accelerate? Like in a gnarly skate bowl.

So while falling, given a particular path, perhaps not straight, could I accelerate to greater than terminal velocity, the same as I can sail faster than the wind?
</highdea>

update: D'oh! Slingshot via orbit. Answer = yes. That's pretty cool if you think about it.

update 2: an orbit is a straight line. Mind = blown.