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Old 15th June 2011, 10:59 PM   #11
Failed Engineer
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Lightbulb Failed Engineer.

The argument about boards riding on a cushion of air is I`m afraid just another popular myth.

Its easy to see why when you look at the maths. Board load in region of 200 pounds ??? or more ? Riding on around 10 square inches of water/board (can be less can be more) Required air pressure in region of 20 pounds per square inch to affect things??? Two things
a) Those sort of pressures would never be generated under hull.
b) Even if they were water would simply give and move to contact board elsewhere.

Concaves on hulls are to channel the water not air.

It could easily be argued aerated water is worse for planing on; aerated water does not have the same weight/densitry so would contribute less in momentum exchange. Again its why boards plane slightly earlier in salt water.Board slightly more bouyant and water slightly denser so contributes more in momentum exchange. Board planes deeper over white water .(aerated) (feels great though)

Air is introduced into cavitating props/foils/pumps for different reasons. (to induce cavitation; but thats another headache) Pressures in that are way higher than a board would ever experience.
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Old 15th June 2011, 11:03 PM   #12
BelSkorpio
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Let's also make a difference between "drag" and "grip".

I would define "drag" as a negative aspect that is caused by the wetted surface and also the fin surface, which slows us down in the (intended) direction we want to go.

I would define "grip" as a positive aspect that prevents us from going in a unintended sidewards direction.
The key parameters are the wetted surface in combination with the sharp leeward rails and the fin.
Wider boards give us less grip so more fin action and fin surface is required. Did you ever try a formula board with let's say a 40cm fin. I did, it dances around on the water.

It's logical that there's a strong correlation between "drag" and "grip". (same parameters are involved)

Then last but not least, I consider "lift" as a secondary function of the fin.
You always have it, if you want it or not. It's a side effect of the fin. And in most cases it is a positive one because it helps us planing. But like with many advantages, they quickly can become a disadvantage if you have too much of it. Over finned boards will try to fly off the water, causing instability and lack of control. Much of this can be countered by skilled riders of course. Well, I don't want to go into this any further, because there is already another thread of 9 pages about this.
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Last edited by BelSkorpio; 15th June 2011 at 11:08 PM.
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Old 15th June 2011, 11:22 PM   #13
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My take on fins has only two aspects. Lift and drag. The lift isnt against gravity its against leeward movement.(But drag has two aspects.the drag induced by causing lift and parasitic (frictional) drag.???)

We picture a fin going straight through the water but it doesn`t. It goes very slightly sideways.(Its why they need very rounded and not pointed leading edges)

That sideways movement (caused by leeway itself caused by the wind) transforms the "skeg" into a foil ,generating a force against leeway.(Imagine the foil going through the air with the leading edge higher than the trailing; it becomes a wing and generates lift.If it was level (ie leading edge same height as trailing edge it would not produce lift.(dont forget our fins are symetrical unlike a plane wing)
If you tow a board in a straight line the fin produces no lift; it only produces lift when made to "crab" through the water.Towing in a straight line only has drag.(and only parasitic drag)

WS is great..
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Old 16th June 2011, 12:02 AM   #14
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This would also explain why you can't go below a certain fin length on a wide board, because the hull has less "grip". However I think that there is a speed at which the board is still immersed but water release is so fast that only the hull is wet and not the rail (of course not when crossing chops). Unfortunately it is on the other side and we can't see it.
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Old 16th June 2011, 12:06 AM   #15
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Yes that sums it up.

Last edited by Floyd; 16th June 2011 at 01:42 AM. Reason: Previous post changed. I wanted to retract negative comment.
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Old 16th June 2011, 03:07 AM   #16
Ken
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Failed Engineer,

I am not sure who said boards are riding on a cushion of air, not me, but I do believe there is "lift" from air passing under the board.

I bow to your expertise, but if I am moving at 25+ knots of boards speed off the wind on my Formula board, I can keep it on the water and in control without too much effort. However, if I am on a beam reach at slightly slower speeds and then turn into the wind, lift off can be a serious problem unless I add significant downward force to the boom. Any sheeting out or taking weight off the mast foot and disaster strikes. Board speed is slower, but the chances of lift off are higher. The only thing I see is increased air flow under the board causing it to take off.

What am I missing?
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Last edited by Ken; 16th June 2011 at 03:13 AM.
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Old 16th June 2011, 03:30 AM   #17
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Ken, I know what you're talking about. I've had the same experience several times.

My thought is that when you turn into the wind, there comes a lot of pressure on the fin, causing it to bend more and creating more "foil" action resulting in a lot of lift.
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Old 16th June 2011, 04:00 AM   #18
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There must be some aerodynamic lift under board; perhaps not between hull and water but certainly under hull out of water. Perhaps when rig is unloaded this lift is allowed to have an effect. Afterall even in only 15 knots of wind apparent wind hitting hull will be at least equal to board speed (probably more?) I`d have to look at vectors but I`d suspect in 15 knots wind with 20 knots boardspeed (easily achievable on Formula) there must be at least 22 knots apparent wind hiiting hull. That would certainly have a lifting effect.
And turning into wind would initially increase apparent wind !!
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Old 16th June 2011, 09:56 PM   #19
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A board sitting on the beach not moving and not attached to a rig can achieve lift off the ground if there is enough wind. Lift from air moving under the board is REAL I have seen too many boards moving down the beach in big gusts. I am always careful how I set mine down when not attached to the rig.

Yes, the fin does bend in the transition from reach to a higher point of sail, and it does contribute to the lift, but the key is air moving under the board. Remember, the board speed is dropping as the board turns upwind, but the apparent wind is increasing.

Chop hopping is essentially the same, air moving under the board as the board moves into the air with significant apparent wind. You only need a small wave to create a space for the air to move under the board and bingo - LIFT. It's not just the speed of the board over a ramp, although this plays a big role in getting air.

You get bigger jumps when the wind increases, but the board speed and chop size remain the same.
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Old 17th June 2011, 06:19 PM   #20
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Ken

Agree boards blow down beach but not when you are stood on them !!!

Water is 400 times heavier than air;stands to reason any water effects on board are going to have about: well 400 times more effect !

Unreg was talking about planing; air effects on planing are minimal but fair point windage on front of board/under hull especially when unloaded could effect (does effect) the board.

Once board leaves water any wind will only want to blow board down wind (once board has lost momentum).The wind could only produce lift if there is some force opposing it.ie your fin or once in air your momentum.


Wind alone can not produce lift.Its a common misconception. A kite needs tethering; a hanglider needs rising air. (It dives at a slower rate than air (wind) is rising to gain height) Its why a hanglider needs a hill or a tow. (Hill makes wind rise;the tow opposes the wind to produce lift)
Take away the opposing force and fairly quickly you are coming down.
Not sure about wind producing lift on board whilst in air; if it does its only through board/riders own momentum ; which would produce some lift with or without the wind.Would a thrown boomerang go higher on a windy day ??????

Fin only produces lift against gravity whilst not vertical. If it were horizontal with correct AoA all its lift would be against gravity but whilst vertical no force helps planing and the lift from flex of fin would be minimal.(ie flex is only 10 degrees or so and only over last few inches.)
Fins do not help planing at all.Not directly.Towed behind a boat a fin less board would plane at same point as a finned one.Fin is there to stop leeway;this produces motion which produces planing.
But when railing quite a bit of fin lift helps planing but the board as a planing surface is less efficient when angled ???

Windsurfing is a mixture of aerodynamics;hydrodynamics;the itereaction between the two and lots of BS for good measure.!!!!!!!!

Last edited by Floyd; 17th June 2011 at 06:52 PM.
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