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 Post subject: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 3:54 am 
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Foil boards are in their relative infancy with respect to potential performance.

By example: if a hydrofoil that can carry its rider efficiently at 8 knots, then it could theoretically carry nine times as much weight at 24 knots, which means it's about nine times bigger than necessary when traveling at that higher speed (or 16 times at 32 knots). That wasted potential lift comes with immense parasitic drag, which is a fundamental impediment to hydrofoils reaching extremely high speeds.

I can think of two obvious design concepts to overcome this speed-limiting handicap. I'm starting two separate threads, to solicit and share ideas on each of the two concepts. I expect at least one of them to develop into a much faster, more versatile future generation of hydrofoil. So...

This thread (concept #2): SURFACE PIERCING FOIL

As speed increases, incrementally less foil would remain submerged, and all or part of it might transition to a ventilating condition, submerged on the lower surface only. This might be akin to classical v-foils that have been around for decades, which have been primarily used only at the front of hydrofoil craft to modulate altitude.


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 Post subject: Re: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 4:12 am 
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But more surface piercing = more ventilation may be a dicier situation with a single foil that with a wide cat platform or similar.
Plus fluid boundary issues, tricky ow conditions, other kinds of drag, right..
Nonetheless the ac cats made it work, and carry the great majority of the sailing load
The more Vee-ed one lost though, especially upwind.
But it did post the highest peak speed!
There is possibly some area to exploit for improvements to kite foils here it seems, but not easily.

and a smaller higher loaded wing = higher pressure differentials meaning more risk of cavitation if they really get going fast enough...
There are benefits to keeping wing loading low too I think.
Higher performance foils are smaller than beginner ones,
but how much smaller can they get without penalty?
Indeed it will be interesting to see what develops.


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 Post subject: Re: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 4:20 am 
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BWD wrote:
But more surface piercing = more ventilation

Yes. However, although an ideal "laminar" foil (for a particular speed) is most efficient, if you're stuck riding a foil that's unnecessarily large for your speed, then letting half of the surface dry off is probably the lesser drag-inducing evil.

Also, you mentioned cavitation at very high speeds. Keep in mind that a ventilating foil won't cavitate.


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 Post subject: Re: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 6:39 pm 
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In terms of maximum performance, I can't see how the push foil is going to compete against a pulling foil.
Once the wind strength is high enough, the limitations of the leverage of the rider will limit the power a push foil will produce.

Developing push foils may be like developing raceboards instead of foil boards.


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 Post subject: Re: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 8:11 pm 
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I have the feeling that this foil design would be very hard to ride.
Boats who use this have pretty good speed control and are designed with certain speeds in mind.
They also have 2 foils, front and fear, and do not rely on weight change for A O A changes.
Can we use this for our needs?
Can someone build a surface piercing foil that the A O A can be controlled with a persons weight change?
There was a guy in England who had a grant for a considerable sum who tried this without success from what I understand.
Again like your other thread why not, but lots of R and D, who can do this?
R H


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 Post subject: Re: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 9:13 pm 
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revhed wrote:
Boats...also have 2 foils, front and fear, and do not rely on weight change for A O A changes.

Neither do foil boards. AoA changes are caused by speed changes, not weight distribution. Respectfully, I think you're misunderstanding what angle of attack means.

Once a foilboard lifts off the surface, the lift it produces doesn't increase with additional speed. But drag does, and it approaches a squared function of speed.

The current generation of kiteboard hydrofoils, which don't reduce wetted surface as speed increases, are relatively efficient at low to moderate speeds, but become inefficient at high speeds (painfully low L/D ratios), so they get walloped by speed boards (which, like all non-"foil boards" are actually ventilating foils).


Last edited by JS on Thu Nov 06, 2014 9:41 pm, edited 3 times in total.

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 Post subject: Re: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 9:21 pm 
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JS wrote:
revhed wrote:
Boats...also have 2 foils, front and fear, and do not rely on weight change for A O A changes.

Neither do foil boards. AoA changes are caused by speed changes. Respectfully, I think you're misunderstanding what angle of attack means.

WHAT?
I put more weight on my front foot and change the A O A of the front wing to make the foil descend.
I control the pitch of my wings A O A with weight changes.

So what are you saying?
R H


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 Post subject: Re: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 9:39 pm 
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revhed wrote:
WHAT?
I put more weight on my front foot and change the A O A of the front wing to make the foil descend.
I control the pitch of my wings A O A with weight changes.

So what are you saying?
R H

AoA is relative to the free flow over the foil, not relative to the horizon. When you dive the board, the direction of flow changes. The AoA doesn't.

While you posted, I was adding an edit to my previous post. You might find it helpful.


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 Post subject: Re: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 9:59 pm 
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So JS you define angle of attack relative to the board/mast/fuselage?

The water in waves moves in a circle, so trying to consider the flow of water relative to the wing, you would maybe have a constantly changing flow angle over the wing.


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 Post subject: Re: Hydrofoil evolution challenge #2: surface piercing foil
PostPosted: Thu Nov 06, 2014 10:10 pm 
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In speed records, you are not allowed any external energy input other than the kite and the kiter, so is this discussion about solutions that do not involve the likes of computers or some sort of power unit?


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