Hydrofoils?

[BHChieftain]

Anybody tried a hydrofoil design? After seeing my son run his Slash for over 20min on a single 3000mah battery, I was wondering if a boat design that really gets most of the hull out of the water (even more so than a hydro or rigger) would get significantly more runtime with decent speed.

-Chief

[Diegoboy]

RC Hydrofoils exist but are hard to get a hold of. I have one I bought from Germany. They are more for scale than for FE.

[Fluid]

Hydrofoils have more drag than hydros do. Cars/trucks have much lower amp draw than boats because the friction of water is so much higher. If you back off the throttle on a car it coasts a ways - the boat will stop quickly. Hitting the throttle accelerates the car with very little drag, just overcoming inertia. The boat has to overcome inertia plus all the water drag as it rises on plane. You just can't compare the two types of vehicles, models or full-scale.




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[BHChieftain]

Hydrofoils have more drag than hydros do. Cars/trucks have much lower amp draw than boats because the friction of water is so much higher. If you back off the throttle on a car it coasts a ways - the boat will stop quickly. Hitting the throttle accelerates the car with very little drag, just overcoming inertia. The boat has to overcome inertia plus all the water drag as it rises on plane. You just can't compare the two types of vehicles, models or full-scale.
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Yes, I understand the drag differerence between boats and trucks... hence the reason for my post...

My question to the forum was has anyone build one and tried it.

Do hydrofoils really have less drag than hydroplanes? Seems the surface area of the foil is a lot less than the surface area of the sponsons.

http://en.wikipedia.org/wiki/Hydrofoil

A hydrofoil is a wing-like structure mounted on struts below the hull of a boat, which lifts the boat partially out of the water during forward motion, in order to reduce drag.

Hydrofoils are similar in appearance and purpose to airfoils.[1] As the craft increases its speed, the hydrofoils develop enough lift to raise the hull up and out of the water. This results in a great reduction in drag, and a corresponding increase in speed

-Chief

[NativePaul]

Thats the thing with getting your info from Wikipedia often it doesn't tell the whole truth and sometimes its flat out wrong, the first being the case here, while it does get the hull out of the water and reduce the drag of a displacement vessel it will always have more drag than a planing craft. To operate in a stable manner a hydrofoil needs at least two foils/wings 2 struts to support them on, often they will have more, but that is the minimum and most efficient amount so I will use that as my example.

Firstly at FE speeds with surfaces the size we are using water has no chance of staying fully in contact with both surfaces of a foil with a proper wing cross section which is why we use wedge rudders not elongated teardrops and with that said using flat or wedge foils you can discount much of the lift from the top of the foil which leaves us needing a similar area of foil to the planing area of a planing hull to support the same weight at the same speed with the same angle of attack, but the foil has an upper surface which doubles the wetted area and a strut with 2 surfaces which doubles it again giving roughly 4 times the wetted area of a planing hull, against that the planing hull needs a rudder in the water but the hydrofoil can twist its rear strut for the same effect so it gets back half its strut drag there making it only 3 times as draggy as a planing hull and it still has disadvantages for models, without sensors and electronics controlling the angle of attack of the foils a hydrofoil wont control its ride height well enough over a speed range to allow use of a surface piercing prop which means you either have to add the drag of a shaft, strut and increase the prop drag massively for a submerged setup or hydrodynamically control the ride hight more accurately, maybe I am wrong here but personally I don't see trim whiskers being accurate enough so the only solution I see is not giving the foils enough area to lift it fully out of the water and having small planing surfaces on the struts to make up the rest of the lift and keep the ride hight stable, making it partially planing would increase the efficiency a little but also defeat the object of the foil which to my mind is to become disconnected from the surface of the water allowing a smoother ride.

[Fluid]

Hydrofoils have more drag than hydros do.

I answered your question in the first sentence of my post. I built a hydrofoil kit years ago, and while it was fun to play with it had issues with speed and turning. It was quite draggy even in a straight line, and turns were...interesting. The same motor/cell setup in a sport hydro of the same size was noticeably faster (no GPS back then). This makes sense for a number of reasons, Paul mentioned a few. Hydrofoils use nothing but hydrodynamic lift, while hydroplanes rely on air pressure for much of their lift. Aerodynamic drag is much less for equal lift than hydrodynamic drag is, so the physics guarantees the hydrofoil will be slower with equal power - because they have more total drag.

Science trumps intuition, and even Wikipedia. The hull type with the lowest drag for the lowest power is a hydroplane. That's why they hold all the records. Hydrofoils have been around far longer than hydroplanes - if they were really faster that's what we'd all be running.



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[BHChieftain]

Thats the thing with getting your info from Wikipedia often it doesn't tell the whole truth and sometimes its flat out wrong, the first being the case here, while it does get the hull out of the water and reduce the drag of a displacement vessel it will always have more drag than a planing craft. To operate in a stable manner a hydrofoil needs at least two foils/wings 2 struts to support them on, often they will have more, but that is the minimum and most efficient amount so I will use that as my example.

Firstly at FE speeds with surfaces the size we are using water has no chance of staying fully in contact with both surfaces of a foil with a proper wing cross section which is why we use wedge rudders not elongated teardrops and with that said using flat or wedge foils you can discount much of the lift from the top of the foil which leaves us needing a similar area of foil to the planing area of a planing hull to support the same weight at the same speed with the same angle of attack, but the foil has an upper surface which doubles the wetted area and a strut with 2 surfaces which doubles it again giving roughly 4 times the wetted area of a planing hull, against that the planing hull needs a rudder in the water but the hydrofoil can twist its rear strut for the same effect so it gets back half its strut drag there making it only 3 times as draggy as a planing hull and it still has disadvantages for models, without sensors and electronics controlling the angle of attack of the foils a hydrofoil wont control its ride height well enough over a speed range to allow use of a surface piercing prop which means you either have to add the drag of a shaft, strut and increase the prop drag massively for a submerged setup or hydrodynamically control the ride hight more accurately, maybe I am wrong here but personally I don't see trim whiskers being accurate enough so the only solution I see is not giving the foils enough area to lift it fully out of the water and having small planing surfaces on the struts to make up the rest of the lift and keep the ride hight stable, making it partially planing would increase the efficiency a little but also defeat the object of the foil which to my mind is to become disconnected from the surface of the water allowing a smoother ride.

Hi NativePaul,
Thanks for providing the rational. That was helpful.

-Chief