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Mast/strut ventilations: what to do

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bigcane
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Mast/strut ventilations: what to do

Postby bigcane » Sun Nov 05, 2017 9:57 pm

Hi, I wanted to start a specific thread on ventilations that occur on the strut when pushing hard upwind. They are pretty annoying and wanted to see how common it is with other riders. I feel like I can’t push as hard as I want, does this happen to you as well? It seems to be happening on every foil I have owned. My 2 current foils are the Moses Comet 2017 and Spotz Shark. I sanded the Comet to 1200 which helped and I also notice that soap and water applied with a sponge just before a session really helps ( greasy fingerprints on the strut seem to suck Air). But still ventilates from time to time upwind and going into tacks. The Spotz also ventilates at the top of the mast when really cranked over and pushing. So I just sanded it for the first time according to the Rift boards method, the foil seems faster after sanding and slightly less ventilations but there is a strange gurgling/disturbance on the mast when going downwind. I wonder if the 1cm strip of 400 grit sanding near the leading edge is causing that? Is that the vortex generator? What other methods are people using to cure the strut ventilations? I also figured out that riding the kite low upwind helps too. Any other tips on riding technique?

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Re: Mast/strut ventilations: what to do

Postby plummet » Mon Nov 06, 2017 7:16 am

So.
Ventilation is when you breach a wing and introduce air to the lifting surface which means you loose lift and usually crash.
Cavitation is when the pressure drop is so great between top and bottom wing that and the velocity is high enough to cause vaporization of the low pressure zone on the wing.

The solutions you propose will help reduce cavitation potentially and may reduce the ongoing effects of ventilation. Ventilation can propagate cavitation in the right conditions.
BUT. it is not the solution!

The solution to perminantly stop ventilation is to never to breach a wing... ever. Longer mast, ride with the foil deeper in the water, be aware of the chop/swell combinations and adjust to the condition

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Peter_Frank
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Re: Mast/strut ventilations: what to do

Postby Peter_Frank » Mon Nov 06, 2017 8:54 am

I know many confuses cavitation and ventilation at first yes, but I think bigcane is right on this one, as I have heard many of the early hydrofoil designers having trouble with this very issue, and as far as I remember Gunnar also have had these experiences, years ago, with different profiles.

Namely that some strut profiles will for some reason suck more air down, which can even translate into a more jittery ride as the rear stab can be affected from this air.

Have never experienced it myself, but I dont crank hard and push ha haa, too slow.

So yes, I believe this can happen, when going hard upwind, some foils have the strut lifting a bit (as otherwise the strut is normally a pretty neutral non-lifting device) which can make what you describe happen :naughty:

And in tacks where you turn (yaw) it sharply the same can of course occur - air gets sucked down a bit on the strut creating some turbulence :wink:

Can not really help on this one, just say that I understand what you experience, and it is not cavitation, as this only happens on really bad profiles and usually at extreme high speeds only.

8) PF

bigcane
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Re: Mast/strut ventilations: what to do

Postby bigcane » Mon Nov 06, 2017 1:21 pm

Hi PF, yes you are right, I am talking about strut ventilations when riding lit up on race gear (race kite and 110 struts). This strut ventilation is happening when pushing hard upwind, it is not happening when free riding or riding downwind. Also wingtip ventilations are a separate issue that are mostly already solved by using the 110 struts. I think what is happening is that when cranked over the strut is at a non-neutral angle of attack, which creates low pressure on one side which can in turn suck air down from the surface. So the question i am trying to answer is what strut surface preparation best solves the problem. Or maybe there is a riding technique that helps

bigcane
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Re: Mast/strut ventilations: what to do

Postby bigcane » Mon Nov 06, 2017 1:30 pm

Also i notice that the strut ventilation always seems to be on the downwind side of the strut, which is consistent with the idea that there is low pressure on that side and that the strut is providing some vertical lift

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Re: Mast/strut ventilations: what to do

Postby revhed » Mon Nov 06, 2017 1:58 pm

Yes strut suck sucks!
I just used one that would ventilate on almost every run, really was no fun!
I feel there might be many things involved and even combinations?
Strut twist
Strut flex
profile
L E roundness, sharpness
surface texture
Quality of profile shape
Water chop VS flat, waves, Temp?
NOT to be confused with ALGAE on LE of either strut and, or wings, although can sometimes feel exactly the same and maybe even cause?
So,
I think twist adds a lot to this problem as the one that did a lot did have considerable twist BUT
Flew one with even more twist flex and no ventilation?
Have seen wet sanded 400 g L E made (STRANGE as EITHER) sharper OR rounder helped to stop altogether?
Do know of one case where JUST wet sand 600 g circular was all that was needed on ONLY sides of strut not even touching either L E or T E profiles...
And then zero vents!
Know of guys racers who clean intensely their struts and wings JUST before race!!!!
Just some thoughts and if anyone knows FOR SURE the cause sure would like to know as well to help stop mine though current strut seems ok.
R H
It feels like twist more than anything else as can almost make happen on purpose by aggressively turning up wind...???

bigcane
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Re: Mast/strut ventilations: what to do

Postby bigcane » Mon Nov 06, 2017 2:25 pm

Yes revhed, i think you are right, turning and twist seems important. Strut Ventilation definately gets worse when going into a tack or turning upwind from a downwind track. Maybe something caused by having the kite high and pulling up during these moves


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