Hydrofoils Should Use Air Jets
A hydrofoil is a specialized ship which sails by means of wings beneath it. As water passes over and under these wings, the water underneath is slowed, while the water above speeds up. This difference in speed creates a difference in pressure — lift! That’s how the entire ship is held above the water, but it only starts working once the ship is going fast enough for the wings’ hydrofoil effect to strengthen.
Why would you do that? It just sounds too complicated…
Well, when ships are moving through the water at high speeds, they have to push aside the water that is in front of them. That push to the sides creates the ship’s wake, and that energy in the wave is energy that the ship lost. At very high speeds ( 60+ mph…) that “wave-making resistance” becomes insanely strong. Hydrofoils are a way to side-step that problem, by lifting the ship and minimizing wake-formation.
Here is a way to make the hydrofoil lift occur at lower velocities, while improving the overall efficiency and cost of manufacture and maintenance: instead of propellers or water jets, use air jets positioned along the top of the hydrofoil feet. The jets point straight back, pumping air from a pressurized tank in the ship itself that is re-filled by a diesel compressor. Why is it so important that the jets be on top of the hydrofoil feet, instead of sitting behind the feet, as propellers and water jets normally do?
Density.
When those air jets push bubbles into the water that lies above the hydrofoil feet, that water above is now less dense. That gives the same effect we observed due to the difference in velocities above and below the hydrofoil, which generated different water pressures, which gave lift. Here, we get lift because the water above is fluffier, so the water underneath tries to fill the gaps by pushing upwards into that space. Perfect! We’ve generated lift regardless of velocity.
Now, those bubbles won’t be strong enough to completely lift the whole ship! Yet, they would assist lifting, so that the hydrofoil effect begins at lower speeds. This is important for determining the maximum size and range of the ship, because larger ships have more tons of material stacked above each square meter of “footprint”. Meanwhile, hydrofoil wings themselves, underneath the ship, are limited to that same footprint — if a ship grows too large, too many tons atop each inch, then the hydrofoil isn’t strong enough to lift the ship unless the ship travels insanely fast.
So, using air jets would mean that you could make larger ships into hydrofoils, because they’ll generate sufficient lift at normal, manageable velocity. That’s the major insight.
Piston Parallelism
It gets better, though: to generate that air pressure, use an array of small pistons. Each piston fits in a cylindrical slot, for easy swapping during maintenance, and each feeds separately into a central air tank. That way, you can pull a defective piston from its housing while the engine is running. And, not all pistons will fail at once, so you never lose power completely. They’re easier to mass-produce, no heavy equipment for installation or removal.
Critically, these pistons have only a single, simple stage before thrust, when their piston-compressed air enters the pressurized tank. They are a cleaner design — gas into piston, boom, squeezes a mouthful of air into a tank. That tank empties directly into the water, to produce thrust. In contrast, lift your car hood, for a sense of how compact and reliable a direct air jet would be.
These pistons can be diesel (safest bet) or some other liquid fuel. If you really want mad-cap speed, though, I suggest looking into hydrogen-fed combustion chambers. They have enormous power, due to their fast flame front and high heat content. But, a large section of your boat would be devoted to a hydrogen tank, and it leaks unless you seal it with graphene. Good luck re-filling without some solar+water=hydrogen+oxygen system. Solar water-splitters are actually quite efficient, up to twice as efficient as the best photovoltaic solar panels. You go straight from sunlight to fuel. So, there’s that.
A home-brew hydrofoil using this air jet option might prefer a more direct power system, at the cost of usable space onboard the ship; sterling engines and the like can take hot water from solar-trough concentrators and still generate useful power. You can also feed it anything that burns — dry leaves, toilet paper, cow dung. Combustion happens separate from pistons. Sterling engines are huge, but they’re mostly empty space, light.They’re also dirt cheap and stupidly reliable, because there are only a few moving parts, and no internal combustion to clog things up! It’s just not enough power for continuous hydrofoiling — you’d be running the sterling pump for hours just to have enough air pressure for 20 minutes of sea-skiing. Still, that’s exactly what a lot of people want.
When you start the vehicle, begin with only the rear air jets; they’ll generate enough thrust that their own bubbles are behind you. If you let all the jets on at once, while stationary, then those bubbles come up underneath you, and you sink further into the water, potentially capsizing. Don’t do that. Wait until you’re going fast enough for the more forward-positioned jets.
The ideal hydrofoil wing would also need re-vamping, by elongating the curtain of the hydrofoil. Let it drape backwards, like a broad goose wing, instead of being a narrow and sharp dragonfly-wing shape. Put stout walls along either side of each hydroplane’s foot, so water can’t rush in and steal your bubble-power. The broader wing will generate more lift from the air jets, because that froth will hang above wing area for longer periods of time, even at high speed. That’s all I’ve got! If anyone likes it, please, DIY. :)
