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Sudden deep ventilation ?

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Peter_Frank
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Sudden deep ventilation ?

Postby Peter_Frank » Fri Feb 20, 2015 12:47 am

Hi

I've ridden many different wings, and a few different foils/brands too.

And with all of them experienced something odd:

When you go at really high pace, suddenly the wing loses its lift and you crash forward.

Like the carpet has been tugged away under you, and a deep hole is underneath ha haa !

It is VERY odd, and I would hear if others experiences the same sometimes ?

I am really experienced, and I know for sure it is not because I foil too high when it happens, so not because of surface ventilation.

On the other hand, I mostly ride in quite choppy water - so a couple of guesses :

1. Some kind of underwater bubbles causing abrupt separation - maybe coming from the white caps on the surface going deeper ?
Although, it happens in low winds too with almost or no whitecaps, so not very likely ?

2. Can it be a delayed separation, coming from foiling too high and partially ventilating, thus having air around the wing but still lift - and then much later when foiling deeper for a while, it seperates suddenly ?
Hmm, maybe - but still not convinced this is the reason...

3. Running into seagrass ?
Could be, but it is winter here, so havent seen any seagrass for ages - and it feels different (used to it from summerdays), as it will not always give a total separation like experiencing now, but often slow down only, or partly seperate, or give a tug to one side or the other.

4. Could it simply be when hitting a fish ?
This could explain it happens with every wing and every foiltype, and also when riding deep.
And the reason why it happens most often when at speed, could be the fish dont get away in time, and might also just be imagination because the consequences when at your max speed is much more challenging when wiping out :naughty:


I find it really odd, and I can foil much higher even in chop, at lower speeds or going upwind - can even save a bit of surface ventilation most often even without straps (think straps would help at high speed to keep it level and maybe regain control - but depends on the very cause that is unknown...)

But this abrupt loss of lift at high speed when foiling okay deep, anyone experiencing this too ?
Anyone knows what causes it ?
Anyone who will try to guess what it could be ?

I dont know...
But it means I can not ride "relaxed" at speed for indefinite time without crashing eventually, simply because of this phenomenon that happens now and then.
Which is several times per session typically, or more.

It is not really a "problem" as such (although a bit painful when winter and going 20-25 knots), but I am really curios as to what causes it !

8) Peter

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Re: Sudden deep ventilation ?

Postby BWD » Fri Feb 20, 2015 2:02 am

It's what they call "glitching" right?
Or something else ?
Could it be from foil damage or deflection/fatigue if it's an older foil?
I remember people saying some carbon composite foils seemed to have a certain lifespan and then lost their stiffness. Although usually using carbon is supposed to avoid this in most applications, right?

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Re: Sudden deep ventilation ?

Postby borist » Fri Feb 20, 2015 2:13 am

I wonder about the same thing, although not at high speed, rather medium, as I only get to modest speeds now. Most crashes follow a short vibration (how ever brief), usually as a consequence of surface ventilation (riding higher). The crash (loss of lift) I'm curious about happens at medium depth with no warning and no clues on the water (smooth surface, no bubbles, no weeds ....). I have almost brand new Alpine
Last edited by borist on Fri Feb 20, 2015 2:15 am, edited 1 time in total.

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Re: Sudden deep ventilation ?

Postby Randahl » Fri Feb 20, 2015 2:15 am

Happened to me all the time on my Lift 2013. Usually when headed upwind or crosswind and locked in. I went through and smoothed out all of the bolt holes and all the spots where the flow could get turbulent, as much as possible, it seemed like it helped a little. All the time I would get the wing tip too close to the surface when healed over and it would pick up some air and that air would stay with the wing under water for quite a while, even going deeper. I didn't feel like that really effected the ride drastically. This phenomenon is definitely sudden and catastrophic, and it makes you look stupid.

Hitting things definitely feel different than this. I have hit a fish, I have even blasted through some fairly heavy kelp. This feels like skateboarding and hitting a huge crack.

However, I've had the Sword 2 for a week or so and it hasn't happened yet.

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Re: Sudden deep ventilation ?

Postby tkettlepoint » Fri Feb 20, 2015 2:36 am

over the last 4 years of figuring out my foil I have learnt that all wings have a top speed where they loose lift. It comes down to design, profile etc. At a certain speed it will make a bubble and you will fly over it and loose lift. [youtube]https://www.youtube.com/watch?v=U7UgMxMsws0[/youtube] good vid showing you the bubble...

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Re: Sudden deep ventilation ?

Postby plummet » Fri Feb 20, 2015 3:35 am

No foil experiecne here..... But my day job is to work with fluid systems.....

So. I have some ideas.

Cavatation is a possibility. Though I suspect this is not the case. The amount of cavitation required to reduce lift i think would be significant. I doubt you would reach it at foil board speeds.

Moving from laminar to turbulent flow maybe the cause. If the flow up to a certain speed remains laminar then your rear will will have smooth water to carve through and foiling height can be maintained.

As you increase velocity at some point you will reach turbulent flow past the rear wing. At that point it would make sense that erratic foil behaviour could occur.

Resonant vibration may also play a roll. Purhaps lowing the point at when turbulent flow is achieved.

It makes sense that as the board speed increases you will need a smaller and smaller foils to create the same amount of lift. Just like smaller and smaller kites are used with increased wind speeds. It could be that you have found the top end of that foil and now need to step down in foil size to kite at higher speeds.

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Re: Sudden deep ventilation ?

Postby plummet » Fri Feb 20, 2015 3:59 am

plummet wrote:No foil experiecne here..... But my day job is to work with fluid systems.....

So. I have some ideas.

Cavatation is a possibility. Though I suspect this is not the case. The amount of cavitation required to reduce lift i think would be significant. I doubt you would reach it at foil board speeds.

.
Wait.... I had some brain fade for a second

As you increase velocity you will increase vacuum on the upper wing which will decrease the point that cavitation occurs.... Making it a likely possibility.

I wonder if its a combination of cavitation, turblent flow and resonant vibration that sets the upper speed limit of a foil?

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Re: Sudden deep ventilation ?

Postby boardjockey » Fri Feb 20, 2015 5:40 am

Foil farts: My foil started doing that after I hit a reef and scratched up the wings. Sanding the imperfections out of the wings corrected the problem.

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Re: Sudden deep ventilation ?

Postby lander » Fri Feb 20, 2015 8:02 am

I am no expert in design at all, but I think the problem is stifness - or rather lack of stifness in the foil.
Have had 5 different foils now, and it us clear that the stiffer it gets, the less it tend to flex/twist and therefore loose its lift in a split of a secund. I am also on a sword2 now, and it is really stable.
But also the conditions you ride in, makes a difference. Chop, current, gusty wind etc. Is all directed down in the foil and it respond to it.
Therefore you as a rider need to cope with all these differece input in a split secund. So if the current or a chop effect the foil you need to react fast and ajust your angle a bit.
So lots of factors in my opinion...And sometimes you just hit a fish. I even hit a duck on the downwind. Tell you I exploded ;)
Last edited by lander on Sat Feb 21, 2015 8:41 am, edited 1 time in total.

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Re: Sudden deep ventilation ?

Postby flying grandpa » Fri Feb 20, 2015 10:19 am

Hi, Peter.

Have you got any pictures/GoPro movies taken from your lines, when it happens?
That would be the best way to find the reason.
I guess it can be a whirlpool with some air in its core, that sometimes chase foil wingtips for hundreds of meters, and air catches foil when you increase AoA for a short moment.
It is especially valid for heavy riders.

The other reason I crash with deep foils is mast ventilation, induced by small whirls at the top of waves - especially dangerous ar small wite caps - but this is recognizable as the force is sideways and the board runs away from uder my ass.

I envy you can foil, Peter, all the winter. My lake is still frozen.

Greetings

Tadeusz


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