Hacker News Reader

Comment by sand500 | Hacker News Reader

I wonder if it's possible to do a sailboat that lifts above the water entirely and stays in ground effect.

Fundamentally, sailing is extracting movement from the relative movement of two fluids. This is done by putting an vehicle at the boundary of the fluids with a wing in each fluid in such a way that the vehicle moves.

If you use wing-in-ground-effect then you are in a single fluid, where there is no relative movement, so you cannot extract energy from the relative movement.

Put more simply, you need something to push against, so you need at least part of the vehicle in each fluid.

You can also extract energy by moving between two fluids in relative motion.

If you count that as sailing, this is the fastest sailing vessel.

https://youtube.com/watch?v=4eFD_Wj6dhk

That must be the most amazing RC vehicle flight I have ever seen footage of; thank you for the link!

Yes, but.

You have to remember that speed sailing is effectively an efficiency competition. The theoretically perfect sailing machine would a foil in the air and a foil in the water connected by something with as little weight and drag as possible. Syroco are alarmingly close to making this happen.

An interesting choice to use a supercavitating foil. "Back in the day" there was an assumption that hydrofoils could never run above 50 knots due to cavitation, but the America's Cup people seem to do that (just). There are nice consistent supercavitating surfaces but they're called propellers.

Either which way, my money is on syroco.

My precise thought was "Aw, Syroco's got this, if it doesn't murder everything that gets bolted inside of it.". It's like the engineers made a force vector diagram and just stopped there.

Downwind: probably.

Otherwise, no, because it's the interaction between the keel and water which allow you to sail other points i.e. broad reach.

Downwind wouldn't work, you'd just be a very fancy leaf moving with the breeze with no control. It's really the differential between the wind and the water (not so much the keel) that allows the sail to exert a force on the vehicle. Once you're going downwind at the speed of the wind with no reference to the water, you might as well be in still air.

Wind speed typically increases with height and wind direction varies, so you could have a sail or kite that’s in faster wind, but I doubt the difference is large enough in practice.

(in theory, I think a kite up in the jet stream and a fairly large sail near ground level would work for some definition of ‘work’)

Two sail planes on a tether should be able to do it. How the pilots are going to get good at it without getting killed is probably a bigger problem than constructing it.

It's the keel that makes it possible to sail upwind in any useful sense. There are functional equivalents possible e.g. a catamaran can have very narrow-but-deep hulls, but it's the the fact that a keel provides very little drag straight ahead and a lot at 90 degrees which allows you to sail close-hauled.

I remain unconvinced it's impossible downwind, given gliders and parasailing work.

Gliders and parasails are trading height. Boats only have 2D of movement (sort of), they don't have anything else to trade

A keel doesn't just resist sideways motion, it is a foil that produces lift.

Indeed

Funny that nobody mentioned it so far, but yes, you can do that indeed. And it is almost certainly even now already the fastest type of sailing. It is called ice-sailing :) Even hobbyists reach 80-100 km/h ice wind sailing. (Note though that technically no ground effect is involved)

For a sailboat to work at all, you need a part of it that “stick” in the water.

Wind-powered “planes” can't work for that reason (and no, gliders aren't powered by the wind, and you can't have a ground-effect glider either)

So the best you can do is have just a foil that sticks in the water while having everything else out, which is basically what every modern race boat is doing.

Once it lifts above the water there's no lateral resistance, so it's not going to make way abeam or beating, just downwind. Having said all that, I think WIG is a factor in the CFD sims of Sailrocket 2, just not anything like the primary lifting force.

Only if there's enough wind shear between the aircraft and kite altitudes... kinda like dynamic soaring [0] but without the need for the aircraft to jump back and forth between the two regions. You'd probably need to use a sailplane or something to get high efficiency flight at low speeds. I think it would be super dangerous, b/c in order to access the strongest differential wind speed, the craft would need to be very low to the ground (just a few meters).

[0] https://en.wikipedia.org/wiki/Dynamic_soaring#Manned_aircraf...

So, like, a wing-in-ground-effect craft using sails for thrust, or a hovercraft that uses sails for (horizontal) thrust?

Seems like the sails would be too big of an aerodynamic compromise for the former, but the latter might work (though it would be kind of silly).

There are already hydrofoil sailboats - is that what you mean? https://en.m.wikipedia.org/wiki/Sailing_hydrofoil

No, ground effect is (AFAIK) when the depth of the wing (fore-aft) is greater than the distance above the surface it is flying over. The leading edge of the wing creates a pressure wave that reflects back from the surface and provides lift to the whole wing. You can see it with birds like pelicans which skim the water for a long way without needing to flap at all. I don't know if this would enhance the speed at all though, there are presumably other effects involved with going faster.

I think they mean like flying as in not touching the ground/water. Sounds doable: these speeds are usually ground speeds so the applied wind speed (ground speed+wind speed) could reach 100kmh which is enough for a sail plane.

As other replies have mentioned, you need to push against the water to position the windfoil to produce thrust. Otherwise you're just being pushed by air, and you're limited by windspeed.

Sailboats can go many times the speed of the wind they're in.

Hydrofoils don't lift entirely out of the water.

No, you need lateral resistance from something in contact with the water. Otherwise you just get blown downwind until the point your speed relative to the wind (apparent wind) drops away and you're not getting enough lift any more.

Reply on news.ycombinator.com