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Speed secrets, something to ponder!

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Created by Roo 9 months ago, 12 Feb 2018
Roo
600 posts
12 Feb 2018 1:34AM
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A question for the boffins amongst us to ponder. If we could double the lift/drag ratio for both our fins and/or sails would this result in higher speeds. Would it then be better to use smaller sails or fins in the same conditions to go faster or use the same sizes in lighter winds and get more efficiency resulting in higher speeds on flatter water when the chop is less. With places like Lake George/Lilacs/Liptons and the weed effect what would the best combination be?

barney831
60 posts
12 Feb 2018 2:22AM
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If you want to ponder I suggest that you read,
HYPERWIND SAILING on SNOW, ICE, LAND and WATER
in Catalyst #52, 2017.
www.ayrs.org

Roo
600 posts
12 Feb 2018 2:38AM
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barney831 said..
If you want to ponder I suggest that you read,
HYPERWIND SAILING on SNOW, ICE, LAND and WATER
in Catalyst #52, 2017.
www.ayrs.org


You need to be a member to read that Barney, care to sum up what it says?

barney831
60 posts
12 Feb 2018 6:22AM
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The AYRS funds its operation with annual membership fees ($15 US) and with a pay-to-read system for recent journal papers (like many scientific journals are now doing). After a couple of years, online access to Catalyst papers are free. While I don't in-any-way profit from their sales, I don't want to undermine the society.

If you don't want to pay for a membership or wait until it is on their pay-to-read system, there is an earlier, much shorter paper that may be of interest. Now free, it is entitled simply 'Hyperwind Sailing' . Go to www.ayrs.org/catalyst and look for it in Catalyst #6 (2001). This paper explains the lack of any further absolute speed records from windsurfers and Sailrocket's success. It also explains why the max speed of a sailboard on water and on ice are essentially the same.

sailquik
VIC, 4121 posts
12 Feb 2018 10:14AM
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www.ayrs.org/catalyst/Catalyst_N06_Oct_2001.pdf

Records have moved on quite a way since this was written, but the general principles probably still apply.

Roo's question is an excellent one.

Does the experience of ice boarders cross over to give us some answers?

BTW. This is the first year at Lake George that I have felt the drag of the weed on the fin (very thick new growth this year) is actually slowing us slightly in places, especially where it is right to the surface where the water is glass smooth.

Also, for the purpose of discussion, I wonder if the weed may actually help reduce drag as well, in two ways:

1. By possibly producing a 'ground effect' type situation where the weed is right to the surface and has some significant mass.

2. By changing to flow off the tip of the fin because of the close proximity of the weed and possible 'end plate' effect.

Any thoughts on this?

sailquik
VIC, 4121 posts
12 Feb 2018 10:35AM
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On smaller sails and fins:

The amount of leverage a windsurfer can exert on his rig is limited by the geometry of holding the rig and his size.
Therefore, it seems logical to me, that once a sailor is fully powered (i.e.. he has reached the limit of the amount of power he/she can hold due to his body weight and leverage constraints), there is no point in carrying a larger sail. The larger sail just has more drag and weight. If you can be 'fully powered' on a smaller sail, it will be more efficient. Of course, this depends a lot on the angle you can run. When running really broad in very strong winds, your apparent wind decreases so you need a larger sail to maintain maxumum power.

I think the same applies to fins. A smaller fin may be more difficult to get enough lift to get going, but at top speed, off the wind, it can have all the lift you can use, even if it is quite small. If the small fin has enough lift at top speed to run at the most ideal angle of attack, then that is faster. The Mathematics to calculate such things is way beyond me, but in practice I have found this seems to be the case for me.

And then there is the question of how the junction between the board and fin affect the fin. Maybe this limits how short a fin can be and still operate effectively?

In any case, I have found that, for me, fin lengths from18cm down to 16cm work very well for speed sailing in hight to medium aspect ratio fins (as opposed to 'Delta' type, very low aspect fins). I have been using 17-18cm symmetrical and 16.5cm asymmetrical.

16.5cm asymmetrical:


decrepit
WA, 8769 posts
12 Feb 2018 9:20AM
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sailquik said.. Also, for the purpose of discussion, I wonder if the weed may actually help reduce drag as well, in two ways:

1. By possibly producing a 'ground effect' type situation where the weed is right to the surface and has some significant mass.

2. By changing to flow off the tip of the fin because of the close proximity of the weed and possible 'end plate' effect.

Any thoughts on this?



I've thought this for a while, I'm sure both the ground effect and end plate thing happens to a certain extent, and could partly explain why the drag feels less the faster you go.
Also the reason low aspect ratio fins work so well in smooth weed.
There's less weed disturbed than a high aspect ratio fin would disturb, and the induced drag is reduced by the end plate effect.

decrepit
WA, 8769 posts
12 Feb 2018 11:10AM
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Roo said..
>>> If we could double the lift/drag ratio for both our fins and/or sails would this result in higher speeds. Would it then be better to use smaller sails or fins in the same conditions to go faster or use the same sizes in lighter winds and get more efficiency resulting in higher speeds on flatter water when the chop is less. With places like Lake George/Lilacs/Liptons and the weed effect what would the best combination be?



So we can double the lift and drag by doubling the size of the foil, but the weight also doubles in the case of a sail, but a fin is probably squared because it has volume.

Doubling the lift and drag without increasing size is a decrease in dead weight so should have a speed advantage.
Also in weed, if the fin is shorter, that means less weed disturbed, so again there should be a speed advantage.

Keep in mind that weed varies greatly. It's felt that some weed in the right conditions, gives off an oil that will keep water smooth, without the weed being too close to the surface. If the weed is low enough a normal depth medium rake fin is no disadvantage. Sailquick's Lockwood above is the ideal shape for these conditions. High rake down low where the weed is but not too low aspect ratio.
However when the weed is right to the surface and laying across it, the max drag is right on the surface, that fin won't be as good as a heavily raked fin all the way down.

Roo
600 posts
12 Feb 2018 11:35AM
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We double the lift to drag ratio, not double both lift and drag. Essentially drag stays the same but lift is increased. You don't increase the size of the foil that stays the same.

decrepit
WA, 8769 posts
12 Feb 2018 11:58AM
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Roo said..
We double the lift to drag ratio, not double both lift and drag. Essentially drag stays the same but lift is increased. You don't increase the size of the foil that stays the same.


Woops, you did say that didn't you, for some reason I read that as double both.

Well in that case, it should be a big speed advantage, both weight and drag are reduced. I have a feeling that there is a minimum depth for fins no matter what the conditions.
I don't go under 16cm, if I want a smaller fin I reduce width.
But saying that I haven't tried anything shorter. I guess with dead flat water and a very narrow board, and a very high lift fin you could go quite short, but there's always the danger of ventilation, from tail walking.

You produce the fin and I'll try it out.

Roo
600 posts
12 Feb 2018 11:59AM
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Where's the most gain to be had, doing it with the sail, the fin or both?

decrepit
WA, 8769 posts
12 Feb 2018 12:51PM
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I reckon if you can do both, --- go for it!

But if you can only do one, in weed, the fin would probably be better.
Otherwise it's just speculation on my part.
Upwind, downwind might be different. the fin is working harder upwind, the sail downwind. Although downwind, drag is not so much of a problem for the sail.

sailquik
VIC, 4121 posts
12 Feb 2018 6:46PM
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I am assuming your initial question about doubling L/D is hypothetical Roo. Since the fin designers who use computer programs like Xfoil to help design their fins have been trying to improve the L/D for many years now, and I understand that improvements to be found now are in the order of small percentages. (Although they still may be large compared with some older, cruder designs). That leaves the sail designers. I am not aware of the extent to which any use aerodynamic analysis programs. Most gains seem to be found by trial and error testing, guided by a smattering of basic theory. In any case, I am not expecting a breakthrough in the order of doubling L/D any time soon, unfortunately.

But I agree that if it were possible to double Lift to Drag ratio, it could result in some significant gain by reducing drag with smaller sails and fins, (and boards?).

However, Barney's 2001 paper suggests that the biggest limitation is simply the drag of the sailors body, and then the limited mechanical leverage we can generate by holding the sail.

Has anyone read his more recent paper?

Stretchy
WA, 385 posts
12 Feb 2018 7:58PM
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I'm not sure about the bodies mechanical leverage currently being a limitation to top speed. As you say Sailquik, if you can change down and stay fully powered up it is usually quicker. I certainly find that with my kit, even when I could still hold onto a larger sail. Current sails aren't particularly high aspect though, they are designed with acceleration in mind coming out of gybes. Perhaps a sail with higher aspect ratio would achieve improvement in a pure speedsailing environment. Yes, acceleration is still important, but if you have a 6.5m speed sail with the same span (boom width) as a typical 5.5m race sail, then in the same conditions you would normally sail fully powered with your 5.5, you might be quicker without the extra drag? It might then be that we find our physical leverage to be a limiting factor?

another thought, are we underestimating the influence of board contact. I stand to be corrected, but it it seems that bigger guys can often go faster with the same size sail and I assume this is because they can get more board out of the water? Perhaps the fillet (Fangy fin) is the logical extension to reducing tail drag?

sailquik
VIC, 4121 posts
12 Feb 2018 10:49PM
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It is pretty well proven that bigger guys go faster, all else being equal, so that is a big indicator for their better leverage, and leaverage being the limiting factor.

Higher aspect sails have proven to be slower in windsurfing. I think this is primarily because of the leverage issue. The world cup slalom sails are very low aspect, and very twisted top, and they typically sail quite a close reach compared with speed sailing. The shape is to put the power (Centre of Effort) low in the sail where the sailor has better leverage over it. This would also apply to speed sailing broad off the wind, and all the custom speed sails I saw at Luderitz were as close as identical AR to the WC slalom sails.

I think a 1 sq/m bigger sail would have higher drag, even if it was higher aspect, and the centre of effort would be higher, trying to lift the sailor. The big question is, would a higher aspect sail have enough of an advantage in Lift to Drag ratio to overcome the other disadvantages? The evolution of slalom and speed sails would suggest not.

Board contact area is kind of self adjusting for the speed and weight of the sailor/rig combo. Some board designers like Chris Lockwood have played around with the nose area and shape of the boards to get a little more lift under it from the wind. This definitly can help 'fly' the board a bit more. If you have a wider tail board, it will have a slightly shorter optimum contact length. A narrower tail board will have a slightly longer contact length, all else being equal. The rake and rigging of the sail can also take a little more weight off the board by making slightly more vertical lift. But this will always be a tradeoff against drive in the forward direction, and you don't want so much vertical lift that you are out of control.

Roo
600 posts
12 Feb 2018 9:46PM
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sailquik said..
I am assuming your initial question about doubling L/D is hypothetical Roo.

But I agree that if it were possible to double Lift to Drag ratio, it could result in some significant gain by reducing drag with smaller sails and fins, (and boards?).


The latest tank testing has shown it is possible, hence my interest. Have already set up some testing for this week to see how it applies in the real world. 3D printer running hot making the necessary parts! Meanwhile AFTR has gone from design to proto build state to begin testing flex/twist of fins in the next few months.

sailquik
VIC, 4121 posts
13 Feb 2018 2:53PM
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Someone has tank tested windsurfing fins?? Who did that

Must be some new direction of design. Look forward to hearing the results. Still at the leading edge Roo.

Pacey
WA, 48 posts
13 Feb 2018 9:00PM
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sailquik said..
Someone has tank tested windsurfing fins?? Who did that

Must be some new direction of design. Look forward to hearing the results. Still at the leading edge Roo.


What, like this you mean?





sailquik
VIC, 4121 posts
13 Feb 2018 11:14PM
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Yes. Who did that and where are the findings?
Oh hang on. This is not the British surfboard study is it?

barney831
60 posts
13 Feb 2018 10:49PM
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Roo said..
Where's the most gain to be had, doing it with the sail, the fin or both?


Windsurfing speedsailors spend thou$and$ on equipment and an order of magnitude more travelling to the windiest places in the world but they won't spend $15 US to find out why they are not world record holders - incredible!

Roo
600 posts
14 Feb 2018 12:30AM
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One of the biggest things slowing us down is the drag created by the board through the water. Initial calculations didn't show this as being that significant but this is the first time I've ever seen it pictured. Look at the the dent the board is making in the water, the energy required to do that is not insignificant like we thought.





Ian K
NSW, 2723 posts
14 Feb 2018 8:31AM
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Roo said..
One of the biggest things slowing us down is the drag created by the board through the water. Initial calculations didn't show this as being that significant






Initial calculations?? If you go back a decade or so on this forum you'll find a beer-coaster calculation where the moments of a hiked-out windsurfer were resolved using an assumption for the height of the sail's centre of lift. The sail generates about 25 - 30 kg of lift. Most of it not in the forward direction and most of it balanced by the fin.

(Alternatively you could look at the area of the optimised fully-submerged foil on a foil board, that lifts 100kg with the optimised size of a fully-submerged windsurfing fin and come to a similar conclusion)

So the planing board is providing 100kg of lift and the fin only 25 kg. All lift comes with a drag penalty and we know the drag penalty of a fully submerged foil is much less than a planing hull. Anybody who're ridden a foil board will confirm they are slipperier than a planing hull.

All adds up to at least ~6 times the drag from the board as from the fin. What did your latest calculations give?

(I think your picture is of a gybe., A board on edge is much draggier. And at the lower speeds half way thru a gybe the wake is correspondingly deeper. The trench will be much shallower at speed)

Roo
600 posts
14 Feb 2018 7:19AM
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Side force generated by fin calculated at 68 pounds. Lift created by foil calculated at 200 pounds across the wing, or 100 each side. Those are the figures we will be using in the AFTR to test flex/twist.

The picture I posted is actually around 40 plus knots in a straight line. I have a 4 second video clip but not sure how to post it.

Ian K
NSW, 2723 posts
14 Feb 2018 9:32AM
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decrepit said..


Keep in mind that weed varies greatly. It's felt that some weed in the right conditions, gives off an oil that will keep water smooth, without the weed being too close to the surface. If the weed is low enough a normal depth medium rake fin is no disadvantage. Sailquick's Lockwood above is the ideal shape for these conditions. High rake down low where the weed is but not too low aspect ratio.
However when the weed is right to the surface and laying across it, the max drag is right on the surface, that fin won't be as good as a heavily raked fin all the way down.


It would be interesting if someone could mount a Gopro looking at the wake as a board passes from deep water to shallow water, from clear water to weedy water.

Any board or foil generating lift in a fluid has to throw water down to generate lift.

www.grc.nasa.gov/www/k-12/airplane/thrsteq.html

It's rocket science equation 101. F = the rate of generating momentum in the opposite direction. If the fluid can be coupled to solid earth or you can involve more mass of water accelerated downwards to a lesser v you can get do better than the deep water case.

Maybe the weed constrains the wake below from spreading laterally, therefore involving a greater depth of water in the disturbance? It's an advantage to generate that momentum(mv) in a downwards direction using more water and less velocity.

(Energy loss is 1/2 mv^2, keep the squared term small if you can). Another way to look at this is to reduce tip vortices, the same thing. less tip votices, the more water below that has to be involved in generating the downwards momentum. ie. you can generate the downwards mv with a smaller v, and therefore win on the v^2 losses.

Anyway the generation of downward momentum shows up as the wake. A video of it as you go across different weed banks at constant speed would be interesting.

Ian K
NSW, 2723 posts
14 Feb 2018 9:37AM
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Roo said..
Side force generated by fin calculated at 68 pounds. Lift created by foil calculated at 200 pounds across the wing, or 100 each side. Those are the figures we will be using in the AFTR to test flex/twist.

The picture I posted is actually around 40 plus knots in a straight line. I have a 4 second video clip but not sure how to post it.


OK you're going a straight line. Now we agree on everything so far.

Drag on hull vs. fin?

choco
SA, 3357 posts
14 Feb 2018 12:11PM
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Roo said..
One of the biggest things slowing us down is the drag created by the board through the water. Initial calculations didn't show this as being that significant but this is the first time I've ever seen it pictured. Look at the the dent the board is making in the water, the energy required to do that is not insignificant like we thought.






Noticed the same thing last year when using the gopro although this is while jibing.

Stretchy
WA, 385 posts
14 Feb 2018 7:18PM
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Roo said..
One of the biggest things slowing us down is the drag created by the board through the water. Initial calculations didn't show this as being that significant but this is the first time I've ever seen it pictured. Look at the the dent the board is making in the water, the energy required to do that is not insignificant like we thought.






Which brings me back to my earlier comment?

sailquik
VIC, 4121 posts
14 Feb 2018 9:32PM
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Well, if board drag is such a large factor, how do we explain that ice boards, which have very little drag on the ice, don't go much faster?

sailquik
VIC, 4121 posts
14 Feb 2018 9:35PM
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How could we test board drag?
Tow a board and sailor behind a speedboat at 40 knots with a rope that includes a strain scale?
How do we take into account air drag of the sailor? Stand a person in a wind tunnel?
Just to be clear. I am volunteering - to drive the boat!

NordRoi
250 posts
14 Feb 2018 8:57PM
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Is the body creates that much drag!? I can wear a baseball cap while going at mach1 with my slalom gear?

Stretchy
WA, 385 posts
14 Feb 2018 9:08PM
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sailquik said..
Well, if board drag is such a large factor, how do we explain that ice boards, which have very little drag on the ice, don't go much faster?


Good point. Do they carry the same size sail for the wind strength?



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"Speed secrets, something to ponder!" started by Roo