Which wings works best as hydrofoils?

[JonB]

Hey all!

Been looking at vids, reading threads. But I´m struggeling to get my hydrofoils to work. Is there any difference between different wings or do they all work as good? Or is like elevon´s better?

[RoofCat]

tiny wings. Water has huge drag and also lifting capabilities.

elevons, because you can adjust them on the go. Usually you want front with a tiny bit higher lift than rear and then also the wing placement doesn't have to be as perfect in respect to CoM.
You may (will) need different optimal angles at different speeds and with different weight as you burn down fuel.

wings closer along the main mass. Water has huge drag and thus any directional leverage is even stronger. Wide base might make your boat eager to turn or slow down with bouncing.

some extra air wings can help with steering considerably and lift too, but they will start to work later (air is thin) and using them for lift would be kind of cheating since you are building hydrofoil.

slightly above 100m/s is pretty much stop even for best boat setups. At those speeds wings large enough to carry you on (lift out of) water are almost enough to carry you on air too and due to air having less drag, boat might decide to fly instead. It's easier to fly at those velocities than float. Funny part - boat without any wings can reach 100m/s too.

even with tiniest wings engines on water will use much more fuel for the same distance as planes. Planes can easily travel around the globe with the right engine. Boats would need refuelling and a whole day for that. Traveling with boats for long is a challenge due to "low" speeds. Well, 100m/s = 360km/h which is a lot on water. But it can get boring fast on long distances.
https://steamcommunity.com/sharedfiles/filedetails/?id=1394380070
https://steamcommunity.com/sharedfiles/filedetails/?id=1480081138
the fat one actually has small part of the belly still in water. So it is probably not 100% hydrofoil, no idea. Helps with stability while not ruining the speed either as you can see. In the end it is just about lift and soft gliding with powerful engines. It has also extra engines for quick turning on the spot at low speeds. Turning on water takes a lot of space and time otherwise.

[JonB]

Wow, thank you for a splendid answer. Big thanks.


I´ve set myself a challenge, a quite tough one. I wanna build a vehicle that can do 100 m/s on water, 100 m/s on land and 1000 m/s in air. I want it to be stable on all three arenas as well as carrying drill + converter as well as a lab and sciencegear.

It´s a tough nut to crack I can tell you. At least for me.


To my help I´m using parts from OPT and MK2 Extended as well as just starting with Infernal Robotics.


It´s a fun challenge but hard.


pictures from one of my (failing) testmodells.

https://steamcommunity.com/sharedfiles/filedetails/?id=1523899002

https://steamcommunity.com/sharedfiles/filedetails/?id=1523899051

[RoofCat]

the thing about pontoons is - they are basically useless. Actually make your boat worse.
The reason for that is rather simple - to float you have to submerge part of your ship pushing away particular amount of water. Archimedes law. That volume depends on total mass of your ship (1m­³ water = 1t, salt water actually more). The mass of water displaced equals the mass of your ship. If your ship has larger volume than the same mass of water, it floats. If smaller - sinks.

It doesn't matter which part do you submerge - pontoons, cockpit, main body or engines. The total submerged volume required to float is still the same. That means also the total drag (front plus surface) is rather close to the same value every time (except you go with flat nose which I doubt any reasonable man would). So adding pontoons you just increase total mass and thus drag. Eventually make your ship/plane worse. Also the construction may become more complex, harder to balance. I have done some pontoon tests and they all were slower. Also balancing thrust vector with drag vector and mass vector is tricky on those. You may need fine-rotated engines and what not.

Slopes help you get out of the water the faster you go. Though slopes on both ends have rather neutral impact. So building rhombic or wave like pontons will do absolutely nothing.
Wings and elevons (any inclined gliding surface in fact) are one directional and can trade volume for extra lifting force. Once you start to lift out of water, there is less volume submerged and thus less drag. But it also depends on the angle of the wings. You will notice once you start to fool around with elevons.
For some water planes gliding on the very tips of elevons will be optimal (since elevons have drag too!) and for some elevons as close to zero angle while still lifting a bit will do more due to less angular drag. It depends on the placement, size of those and total moss of the vehicle. Hard to predict. Of course there is some math eventually, but I suspect those would be rather complicated formulas within an optimization equation. I haven't even searched for them.

Some light real life water planes have pontoons to keep them (engine and propeller) above water. Like water wheels for landing. While larger ones have just some tiny balancing/safety pontoons further on wings for parking time and land on specially designed belly. Because pontoons are basically drag and waste anyway.

[JonB]

The idea of pontoons came because I wanted to raise the vessel higher off the water. touching water at high speed often included the engines to fall off.
On the other hand, raising the engines (obviously) makes the vessel dip the front into the water.

I get your point made above. Pontoons are a waste. Guess I tried to make the three pontoons create lesser contact with water (less surfacearea) then having the whole fuselage in water.
Will do some testings with evelons.

The pontoons are now ditched. Allthough it looked cool in my eyes, and looking cool is as we all know half the battle in KSP-designs ;)

// JB

[RoofCat]

I started my own boat building with pontoons too. Two years back. Then I realized my mistake with unavoidable mass vs volume physics.

You can still build with pontoons - for style points. I just warned you because in my experience pontoons aren't the fastest. You mentioned you want fast vessel. And then pontoons have issues with water vs. flight drag. You described your vessel as plane-boat. So you want reliable flight mode too. Elevons are quick to un-deploy or use as assist in flight as well. Unlike volume drag pontoons with rather higher mass offset too.


Chances are high at some point you will realize your boat-plane is actually more of a plane that can land safely on water. And since you have large wings for flying, you don't need much of the hydrofoil in the first place. If I were into new experiments right now, I would probably build some VTOL water landing saucer. Because going fast on water really isn't that cool. You get bored after 10 minutes while the around the world tour would take 3700km / 360km/h ~ 10h in best case scenario. A lot. Flying everywhere is preferable.


This was one of my first hydrofoil planes and it ended up with one tiny wing right below CoM.
https://steamcommunity.com/sharedfiles/filedetails/?id=755839127
One is better than two, as two bounce more. One basically can't bounce forth and back. It either is in water or in air. The problem is with landing on that wing. It's a bit of a luck situation. You can have soft takeoff from water, but as soft touchdown is too much to ask from SAS. The main problem is engine thrust. During takeoff you have full power, while before landing you reduce to minimum to have low speed. As a result during landing you experience very quick slowdown.
Then this plane has takeoff speed somewhere at 60m/s on runway. So going at 70m/s "on" water it basically flies with one submerged part while carefully managing minimal altitude. It has not much to do with hydrofoil. It helps to lift out of water easily though.