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One of the main problems with water landings is that you need a bigger engine to counter the various drags caused by the pontoons. The specific case of "run[ing] across our own wake to be able to get up on the step on glassy water" brings some interesting thoughts.

[B]asically a floatplane has to break the suction of the water - kind of like pulling your boot out of mud - and if the water isn't moving at all then it has to do that all by itself. If there are some waves, or a wake, or any kind of discontinuity in the water, then that gives the aircraft the momentary break it needs to overcome that initial suction. "The step" in this context refers to the step-up shape you see partway down the float (or hull); when you're "on the step" then you're using the part of the float forward of that step to hydroplane.

It is well known that water's significantly higher density makes high speeds difficult. A solution to that problem is the hydrofoil - a small underwater wing that raises some or all of a boat's hull out of water to reduce friction and increase speed.

So here is my dumb question: would placing a hydrofoil under a pontoon save fuel on takeoff? And how likely would the increased drag during flight cancel that advantage on a typical flight?

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A hydrofoil is a wing that would only really work while on water. All sea planes already have wings that work while either on water or flying. I find it very hard to imagine a situation where adding the lift to the wings you already need anyway wouldn't be more efficient than adding another set of wings that only work part of the time. Especially since sea planes spend much more time flying than they do taking off, which is the only part where the hydrofoils would help. (And only at a specific short part of the take off too.) – Ville Niemi Jan 22 at 6:53
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@VilleNiemi it's all about drag. There's a ton of power required to get up on the step and into planing mode, and anything that could be done to reduce that requirement would aid in overall efficiency. Engines are heavy; some of that could get turned back into payload. – egid Jan 22 at 7:24
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@egid actually it is not all about drag. You have to also consider mass, specifically the hydrofoil needs to be fairly robust and hence heavy or you risk fairly serious accidents when you run into waves. – Ville Niemi Jan 22 at 8:18
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@egid Yes, but there may only be a very brief window where it helps you break onto the step (ie: start hydroplaning). My instincts tell me that it would just become a massive sea anchor for speeds above that, and dragging a wing through the water all the way to takeoff speed seems like a hundred problems in one. Consider just the effective AoI and lift differential between the air wing and the water wing, especially at speed, and I'd be surprised if such a contraption didn't cause the plane to jerk and pitch uncontrollably everywhere in the envelope. Seems a highly unstable arrangement. – J... Jan 22 at 12:00
    
It seems like a fundamental problem would be that the hydrofoil would create extra drag - a LOT of extra drag. So it would make it more difficult to get up to flight speed. Seems like it would do more harm than good. Seems like @Terry's answer indicated that the suction problem is really only a problem in a dead calm anyway – TomMcW Jan 22 at 20:38
up vote 18 down vote accepted

Yes. I mean, sort of. Nobody's ever built a production hydrofoilplane.

It's not a bad idea, but seaplanes are a relatively niche product these days and there doesn't seem to be a lot of innovation in the field. Back when there was, there was innovation everywhere so the idea of hydrofoils didn't stick.

The Convair F2Y Sea Dart, a 1950s prototype jet-powered fighter, used two 'hydro-skis' for takeoff and landing. It was more or less awful compared to the crop of land-based aircraft coming along and after a prototype disintegrated they (probably wisely) chose to shut the program down.

https://en.wikipedia.org/wiki/Convair_F2Y_Sea_Dart#/media/File:F2Y_Sea_Dart_2.jpg

There are some other prototype aircraft that used actual hydrofoils that I found after some quick googling. A company called Lisa Airplanes seems to have an LSA - the Akoya - that's under development using tech similar to the Sea Dart.

The 1929 Piaggio P.7 used hydrofoils, and was intended to compete for the Schneider Trophy, but never made it airborne. Interestingly enough it had a screw (marine propeller) at the tail and a proper aviation propeller up front, and the pilot would've had to do some juggling between the two.

enter image description here

It's likely there were others, but I'm running low on sleep.

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Found several interesting things googling around. The one that most intrigued me was this: youtube.com/watch?v=yolgS1bn7P8 – slebetman Jan 22 at 12:35
    
It looks like the Piaggio P.7 was designed for the main hull to actually touch the water, rather than using separate floats. This seems worth highlighting. – Kevin Reid Jan 22 at 16:20
    
Same is true of all the other examples. I don't think I've seen a floatplane with foils. – egid Jan 22 at 16:47
    
More on the Pc-7 (soon funded by Fiat and renamed Piaggio), but in French, and the original article by Giovanni Pegna, in Italian. – mins Jan 22 at 19:18
    
@mins You could translate it for us. ;-) – TomMcW Jan 22 at 20:32

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