I already have a radio control TX and RX and suddenly fancy building a very fast little boat. Saw a guy on Gautby road lake when I was a kid (what a memory I have!!) who had a hydroplane/airboat powered by a little glow plug engine and airscrew - wow that went FAST!!!

I want to use an electric motor with a an ESC and lipo battery but not sure what sort of power I need to go FAST!!! Also what size/type of prop.

Anyone here know anything about what I'm talking about and can offer any advice??? Thanks Gerry

I’ve seen those sort of things and they are stupidly fast like you say. Trouble is that the smaller they are the stiller the water has to be and you might not get very many “sailing” days in.

The main one I saw was about 3ft across which would be a bit big for some boating lakes especially at those speeds.

You are probably much better off sticking to combustion engines, with racing model cars and aeroplanes the running time of electric is hugely less than fuel.

I’ll try and work out some power figures later if I can.

Wow DD you are the 'master' - thanks for replying so quickly!!

I'm thinking around 30" -36" size with a powerful brushless motor and 3S lipos's - should give around 10 - 15 minutes running but not sure what size prop and motor.....problem with glow engines is they are very noisy even with a 'silencer' fitted....also I have a few 3S lipos from my drone (which never gets used now).

What sort of sped do you want to go to?

In my eyes, fast is probably 30mph, racing 60mph, straight-line stuff over 100mph (real speeds, not scaled).

I haven't gone to detail yet but very very rough work is showing that for 18" wide (36"length, they are normally a 2:1 ratio) to do over 60mph it the motor would probably probably have to be around 800w. I'd guess it is best to keep the airscrew speed down (water hitting a high speed blade is probably going to destroy it) so limiting to say 7000rpm is going to be something like a 6X5 blade.

The other extremely rough estimate was 36" wide (72" long) needing 1.2kW (around 1.5 horsepower) to get above 60mph and maybe a 10X5 blade.

You'll probably need two 3S's in series as you will need at least 20V to be efficient at those sort of powers and will need relatively low Kv motor ratings as you might not want high revs.

Part of the problem is the inefficiency caused by having to drive the front of hydroplane down into/onto the water to ensure it doesn't do a Donald Campbell. If it truly skims the water with very little pressure then the slightest ripple could kick it over at high speed, this will be worse when de-accelerating, the thrust being above the centre of gravity will help keep the nose down when accelerating or keeping speed constant.

Trying to design an aeroplane that will never take off is one heck of a challenge and not dissimilar to formula 1 other than you don't need the downforce on the rear for traction.

Designing these things to be stable in a straight line is very difficult, to make stable fast turns is an even bigger problem.

Hi DD ...my 'new' thinking is a normal speedboat V type hull with inboard motor and propeller and either single or twin rudders....so not really a hydroplane but more practical.

I've got my eye on a secondhand bare hull that's 70cm long and 24cm wide. Interesting about the Kv size of motor as I don't know much about the volts/rpm linking. If I was to use a normal 2 or 4 blade prop, what sort of rpm for the prop are we talking..I'm wondering whether I may need a reduction gearbox of some sort.

I would like to achieve 30mph if possible...any faster and it would frighten me .....and any poor kids near the pond!!!

Must add I am impressed with your knowledge:-)) Thank youGerry

If you want high speed you have to get the drag down, and the way to do that is to get the hull out of the water.

You can do that with active (controlled) hydrofoils on legs with only these and the propeller in the water.

You'd need a microprocessor (An Arduino might be up to the job) to control the hydrofoils of course, but as I think you are a drone enthusiast which must use very similar control algorithms and sensors, it shouldn't be a huge problem.

It will be a lot faster on the same power than trying to shove a hull through the water. Also, as it is out of the water it will tolerate much rougher conditions.

I'm sure from your own argument you can see that its not active hydrofoils you need in order to minimise drag, it is active aerofoils.

Active hydrofoils on a model are almost impossible to control, it is extremely difficult to sense a reference position, 2mm height difference can be the difference between ploughing through water and being airborne.

Propeller in the water is also an increased drag as it has to be in a significant depth, at least the diameter of the propeller and more to reduce cavitation and centrifugal splatter. Worse still a propeller tries to lift the nose when you accelerate unless it is angled downward which is inefficient and lifts the rear end.

There must always be a downforce to prevent it being an aeroplane, you cannot hydroplane out the water, the faster you go the more downforce you need unless the water is absolutely ripple free.

Lots of studies have shown that legged hydrofoils/hydroplanes are not stable enough at really high speeds especially with turns, a design more like concord is often found to be best, the wide rear gives it stability without significant drag, the light nose just needs sufficient downforce to stop it lifting, the wide heavier rear will naturally skim the water and needs little control or downforce.

Fast averaging will give you an adequate reference position.

I don't accept that its almost impossible to control hydrofoils. given that fast averaging system. It's a straightforward control problem which you can probably best solve by a relatively simple analogue circuit using op-amps. Although I guess a slower digital system would be preferred these days.

Average what, from what sensor(s)? I don't know of a g-force sensor that doesn't drift and I struggle even more to think of a possible analogue sensor.

Ripples and waves are always complex, peaks can be tens of seconds apart or one-offs, there is no Bayesian solution.

Near instantaneous height correction would not work, you can't attempt to steer the contour of each ripple without knowing the future contour.

The attitude of the hydroplane isn't constant, you could attempt to force it to be constant but sooner or later you are going to plough into the water.

Much like a hovercraft, it is the physical design that what does most of the work not an electronic control system.

Wow guys ..... I only want a 'relatively simple little boat' not planning to cross the Atlantic !!! :-))