## hydrofoil at max speed limit

A forum dedicated to Hydrofoil riders

Regis-de-giens
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### Re: hydrofoil at max speed limit

I have a remark regarding the pure theoretical approach, without considering the efficiency of the foil, the vibrations, ect ...
Peter is right in a sense that the hydrofoil principle has no limited speed in itself ; so we could believe that by infinitelly reducing the thrust, your leg power and the cavitation, or whatever you want, we could reach infinite speeds ; same for the kite, it is not theoretically limited in terms of pure speed if its structure allows enormous loading and if the wind is high enough.

BUT ! there is a physical limit actually. It is linked to the Lift/Drag ratio of the kite. If you do the calcs and complete projection of forces, you will see that the rider speed will never overpass the multiplication [L/D ratio * real wind speed]. So here is a definite top speed limit, that will never ever be reached whatever improvement you do on the foil. For illustrating example, a standard race kite (assuming L/D=10) in 30 knots will allows up to 300 knots speed so we have still "some margin" with our current speeds , as of course physics like A/R of the foil, drags in water and air, vibration, legs muscles, ect ... will limit this speed far sooner .... but can still be improved step by step if you progress in all fields.
Last edited by Regis-de-giens on Wed Dec 06, 2017 10:19 am, edited 2 times in total.

joriws
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### Re: hydrofoil at max speed limit

Why do you omit most obvious speed limitation at higher range? Rider's body is a great drag source limiting speed. Rider drag => effectively like turning upwind in regards to kite => pushing kite to window edge => reducing power from kite. This is clearly visible in ice speed records (~120km/h) with very low friction. As with buggy with aerodynamics the record is ~130km/h.

Keep in mind that with 1g acceleration (which is a lot) your "flat body" terminal velocity in lower atmosphere is about 200km/h.

james
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### Re: hydrofoil at max speed limit

plummet wrote:
Tue Dec 05, 2017 10:36 am
What I find interesting about the luderitz trench is that its straight broadreach. Why? the fastest speeds are achieved with a arcing downwind run. The faster you go the further back the wind window shifts and the further you can point down wind. With a high aspect kite you can point even further down wind at speed.
The ludtitz set up incorporates a straight section with a huge bearaway

This acutually does give you the arc to accelerate through.

The only way you can keep bearing away as you get further off the wind is if the wind constantly increases allowing you to keep bearing off deeper.

The practicalities of the real world don’t actually allow this and it’s frequently the case that as you are accelerating and travelling at higher than the actual wind speed you actually have to come back closer to the wind to regain power.

gbrungra
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### Re: hydrofoil at max speed limit

joriws wrote:
Tue Dec 05, 2017 1:53 pm
Why do you omit most obvious speed limitation at higher range? Rider's body is a great drag source limiting speed. Rider drag => effectively like turning upwind in regards to kite => pushing kite to window edge => reducing power from kite. This is clearly visible in ice speed records (~120km/h) with very low friction. As with buggy with aerodynamics the record is ~130km/h.

Keep in mind that with 1g acceleration (which is a lot) your "flat body" terminal velocity in lower atmosphere is about 200km/h.
What limits speed? Drag.

Sources of drag:
1. Foil (hydro drag)
2. Rider+board (aero drag)
3. Lines (aero drag)
4. Kite (aero drag)

If liter on ice skates can go 130kph (70knts) and liter on hydrofoil can only go about 40 knots, then the difference can be attributed to the hydro drag of the foil, correct?

james
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### Re: hydrofoil at max speed limit

The biggest issue is the density of water that you are trying to push the foil through.

Compare that with the friction between ice and the metal blade.

Regis-de-giens
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### Re: hydrofoil at max speed limit

gbrungra wrote:
Wed Dec 06, 2017 6:11 am
What limits speed? Drag.

Sources of drag:
1. Foil (hydro drag)
2. Rider+board (aero drag)
3. Lines (aero drag)
4. Kite (aero drag)

If liter on ice skates can go 130kph (70knts) and liter on hydrofoil can only go about 40 knots, then the difference can be attributed to the hydro drag of the foil, correct?
Yes indeed and Jorist is also right, drag of the rider is important on water and dominant on ice (like for skiers, square of the speed) ; in my above explanation drag of the kite is "hiden" in the L/D ratio for which we have some figures ; I do not neglect the body drag at all , I just say that even if we neglect it, we have a maximum unreachable speed limit of L/D times wind speed, basically due to 90 degree angle that the kite will have toward the rider direction;

james
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### Re: hydrofoil at max speed limit

If at speed the kite is at 90 degrees to the riders direction of travel

B. Your not going very fast
C. It likely that your kite is set up incredibly badly to sit that deep in the window

Regis-de-giens
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### Re: hydrofoil at max speed limit

James, I am not wrong, you just didn't catch my purpose : of course 90 degree is not possible during a permanent ride , as it will generate no effective forward pull, hence no speed ; It is a case that will never happen in real life; it is a theoretical physical limit (assymptote) that proves that you will never ever reach such a high speed (L/D * wind) whatever the extreme improvement you do on all other aspects like Hydrofoil drag or rider drag or hulk legs power ;

If the kite is very badly settled , you will approach this configuration at low speed (low L/D in the formula) ; if you have a wonderfull race kite (high L/D in the formula ) you push this limit further;

james
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### Re: hydrofoil at max speed limit

Maybe, how you wrote it is very open to misinterpretation and basing a theory on something that’s open to misinterpretation is not ideal.

Time to spend less time behind the keyboard theorising and more time speed sailing then

Either way though kite foils in their current configuration won’t actually get near what paul Larsen and sail rocket managed.

The alignment of forces are wrong, the human body is the weak link in transferring the power from wing to foil

And despite posters complaining about cavitation we don’t actually have anyone riding a kite foil with a super cavitating foil section because simply we just don’t go fast enough for it to work

At 68kts you need one!

Regis-de-giens
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### Re: hydrofoil at max speed limit

james wrote:
Thu Dec 07, 2017 11:46 am
Maybe, how you wrote it is very open to misinterpretation and basing a theory on something that’s open to misinterpretation is not ideal.

Time to spend less time behind the keyboard theorising and more time speed sailing then
Unless I have the immediate opportunity to ride, I am ready to spend time behind my keyboard for any clarification request , if constructive
You can dis-agree or mis-derstand what I mean, but no need to judge my english level or judge how I spend my time studying the theory or anything else; please.

ok, back to the topic, you are right on one point : far before approaching this limit, the efforts going through the rider's legs would be enormous (approx 500 kg at half of this assymptot speed with basics assumptions)