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Loss of Lift

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Loss of Lift

Postby LakeMiKiter » Fri Oct 30, 2020 1:00 am

I have a question for the hydrodynamic savvy foilers. My setup is a Moses 633 on an 82 CM mast.

What causes the loss of lift I experience on occasion when cruising along well powered. I will be foiling along stable, don’t breach the wing out of the water and out of no where I crash. It feels as if I loose lift on one side of the wing and pow I crash and go flying.

Any ideas what physics are at play causing this reaction on the foil?

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Re: Loss of Lift

Postby PrfctChaos » Fri Oct 30, 2020 8:29 am

How often do you clean your mast and wings? Also ever touch your wings and mast, especially with sunscreen on hands?

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Re: Loss of Lift

Postby pākihiroa » Fri Oct 30, 2020 10:30 am

Perhaps it is the same phenomena as ventilation in windsurfing. One second you are blasting along in the intended direction and the next you are going sideways, swear the fin has dropped off the board, only to crash/recover, turn the board over, and see that it is still firmly attached.

I am not sure of the exact physics but I believe that somehow air gets sucked down (or forms ?) on the surface of the fin. Not clear how that could happen on a submerged foil though. Perhaps related to stalling the foil ?

In windsurfing, ventilation is also referred to as cavitation but I think that this is a misnomer because true cavitation requires extremely low pressures (to 'boil' the water) and the collapsing micro bubbles apparently can reach the temperatures of the sun, generate light, and form micro jets that can cause pitting in ship propellers. All fascinating stuff.

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Re: Loss of Lift

Postby Peter_Frank » Fri Oct 30, 2020 1:08 pm

Hmmm, I have never experienced this myself, on any wing.

But, if choppy waves, I have, the broken small wave tops sucks air under the water, which causes your wing to ventilate, even if quite deep down.
And on my shortest mast only 83 cm, it happens on occasion, simply because it is so short that it WILL suck air down sooner or later.

These are my best guesses :wink:

Some say they have experienced air gets sucked down along the mast, and causes the wing to ventilate.
Not sure I believe this is true though, it might, but I am sceptical, at least when talking about freeride speeds. Maybe at racing speeds, this can happen.

You ride a very short mast - it might also be the reason as said, so you still get some air from close to the surface sucked down on your wing, without you knowing :naughty:

Does it happen in low winds almost flat water surface too?

How experienced are you?

The more experience and riding time, the shorter mast you can ride without ventilating, simply a fact :D
If less experience, you will think you ride along steadily, while you are actually not :roll:

8) Peter

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Re: Loss of Lift

Postby joriws » Fri Oct 30, 2020 1:53 pm

My crashes on same foil is always surface breach aka ventilation. I am not going that fast that I'd have pure cavitation. I've 360 camera on board nose so I can clearly see it..

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Re: Loss of Lift

Postby nothing2seehere » Fri Oct 30, 2020 3:42 pm

Sea weed or kelp maybe? Causes the problems you described and might be washed off again by the crash
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Re: Loss of Lift

Postby edt » Fri Oct 30, 2020 3:47 pm

I've lost lift before, lifted up the wing and notice a tiny tiny piece of grass about 5 centimeters, 1 or 2 inches hanging on the wing. It takes very little to loose lift or create drag on these wings. There are also bubbles in the water. All sorts of reasons and of course ventilation.

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Re: Loss of Lift

Postby Kamikuza » Fri Oct 30, 2020 4:15 pm

I've noticed that at some angles when chop breaks onto the mast, you can get a loss of lift like air sucked down the mast.

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Re: Loss of Lift

Postby SWO_kite » Fri Oct 30, 2020 6:03 pm

I support the seaweed/grass theory. I've taken a fall or two unexpectedly only to turn the foil over and find a single (or couple) strands of grass on the wing.

Also, cutting across a boat wake has caused me to loose lift. The aeration of the water from the propeller system drastically effects lift - as well as the unpleasant turbulence in the water.

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Re: Loss of Lift

Postby Flyboy » Fri Oct 30, 2020 7:04 pm

I think everyone has experienced "unexpected" crashes. They are more likely in choppy water & they become less frequent as you gain experience. My guess is there is turbulence under the water that is not readily apparent. Sometimes this turbulence is sufficient to cause instability in the flow of water over the wing & sometimes this is sufficient to lead to a crash. I think with experience your body learns to make minute, almost subconscious adjustments to weighting to compensate for the instability in the foil, so you gradually eliminate most of those crashes.

One other thing I have noticed is that riding over a reef (not an actual reef at my local spot, but a ring of boulders that makes the water shallower) the foil will twitch around due (presumably) to turbulence created underwater. Again, with experience you learn to make the small adjustments in weighting to ride through the turbulence.

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