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Model Hydrofoil project

The object of the project

The object of this project is to build a working model of the OU-32 as built by Kotaro Horiuchi

UTube link to promotional video of OU-32

UTube link to video of Journalist driving the OU-32, showing how hard it was to drive

UPDATE - This is Kotaro Horiuchi latest version of the Hydrofoil.

The background

I have built many model boats and cars over the years and wanted to build a boat that would run in the sea, I had come up with various ideas including boats that could pass through waves without getting waterlogged. But one day I decided to look up Hydrofoils on the web, and I came across the OU-32. Fascinated with how a boat like that could work, I bought Kotaro Horiuchi's book, 'Locus of a boat designer' which details many projects he worked on including the OU-32, which sadly never went into production.

This led me to buy Ray Vellinga's book 'Hydrofoils, Design, Build, Fly' which explains in great detail all aspects of Hydrofoil design and explains it all in laymens terms, a very well written book.

The problem

Building a version of the OU-32 that would work under Radio control, insead of having a Human sitting in it to keep the balence would be like building and running a Mono cycle with Radio control, even the real boat was a handful to fly, so making a model to do it could be impossible, but.... I have a plan if that proves to be true.

In 'theory' it will be possible to make a model that flies and manoevers under radio control but if all else fails, I am going to add an Auduino with a self balencing system. I do not want to go that route, but if needs must.

The design


I made a few drawings and calculations and decided it was easier to 'wing it' (if you forgive the pun!), so I basically made it up as I went along.

The first problem was how to power the boat, convential propshafts under the hull would leave the water when it started to fly, so extended shafts were needed and in order to get the props level the shafts had to be bent!

Luckily, you can buy 'Flexible' shafts which use a srtong spring as the shaft, running in a brass tube lined with a PTFE liner, the tube can be bent to the shape required, although the more bend you use, the harder it is for the motors, so a gentle bend would be fine. In fact even with the bend I used, blowing the props made them spin easily.

I decided to use 'Surface piercing props', the idea being they also work under the water so they would get the boat up to the flying height, then be at the correct height to run in surface piercing mode, which is very efficient and fast, so together with the extra efficiency of being airbourne on foils, the boat should really motor along when flying.

Using surface piercing props meant I needed a second wing to smooth the water to the props when flying, this wing I realised could also aid getting the boat out of the water and so it was inclined at a +3 degree angle.

Front strut

The front strut needed to do the following:-

Rear strut

The rear strut needs to do the following:-

The model

So, I needed a starting point and I had this in the garage :-

It was a cheap but fast Deep V speedboat that the previous owner had modified with bigger motors. It looked like a good starting point.

The build

First I stripped out the hull, removing the drive shafts, rudder, stickers etc, than I filled in all of the holes.

I ordered:-

I already had:-

While I waited for the bits to arrive I built the rear strut and wings using NCA 63-412 wing profile, cutting the ribs and supports using my CNC router to get accurate profiles. Not having the prop shafts meant I had to leave the top of the upper wing open for now.

I then built the front wing with a hinge bracket on the top and started to attempt to make some kind of height adjuster (see tech stuff), I tried many different ways but ended up drilling a tough piece of wire into the center of the foil, extending out forwards and fitting a small float on the end.

When the parts arrived I put the bits together and did some basic tests, props spin, steering turns, boat floats without leeking, great.

Mk1 Images

First test

First a bit of advice, unless you live in Somerset or any other area hit by flooding, don't build a boat at this time of the year. As I found out, all of the boating pools have beend drained for winter, and the only one I found with water in was drained the following week!

Anyway I did find a boating pool at Llandudno for the first test, I had to share it with five inquisitive swans but as it happens that was no bother.

I put the boat into the water and slowly powered it away from the side, then banged the power on expecting the boat to shoot forward and hopfully lift out of the water, intead it carried on going slow. Rather than a fast speed boat, it performed like scale battleship!

I then discovered it did not respond to the steering! So having eventually got it back I headed home dissapointed.

Lessons learnt

Having phoned a few model shops it was clear that the brushed motors I was using were not powerful enough to turn the 50mm props, let alone make it fly. So it was suggested I swap out the motors for brushless ones which are more powerful, that would entail buying two new motors, two new batteries (LiPo) and a new charger at a cost in excess of 150. I was very dishartened as I could not afford to spend that amount of money on top of original costs just for the fun of trying to make this model.

But I discovered I had been using 1500 to 2000 mah batteries and I found a 4500 mah battery, maybe two of those would help, so I bought another 4500 mah battery.

The other problem was NO steering, the answer was to add a rudder fin to the front strut.

Second test

As all of the pool had been drained by now I attempted to run the boat in the sea off the steps on the promenade. The sea was calm but the swell was a bit of a problem as far as recovering the boat mainly.

The boat did go faster and did lift a bit at the bow, but still not fast enough.

Lessons learnt

Obviously I had a power/prop size issue so I decided to get some smaller props to see if that would work, I ordered two twin blade 40cm props and fitted those.

I also discovered that the ESC (electronic speed controler) limits the speed in reverse, and I had the wiring reversed so to go forwards the ESC thought it was in reverse! So I re wired the motors the other way and presto, much more speed!

Third test

This time the weather was not so good and it was high tide, so I had to launch off a wet and slippery slipway (aptly named). I threw the boat in and powered it away from the slipway, looking good, it was traveling faster already on low throttle, I turned it and although it tipped a long way to one side as it turned, it did actually turn.

I let it settle heading into wind and gunned the throttle, it shot out of the water like a dolphin, did a skip on the surface and then settle back into the water? Something had happened, the boat was barely moving and steering at slow speed was difficult.

First flight a very short video!

On finally grabbing it out of the water I saw the problem, the props had picked a large lump of sea weed and nylon string!

Having cleared the weeds and got the boat back into the water, it continued to dolphin out of the water with full power, but then just dropped back into the water.

I then dropped some of my equipement that rolled down the slipway into the sea, ontrying to recover it I fell on my back in the water and was now frozen and very cold so I called it a day.

Lessons learnt

At least we now have the power to get it out of the water, but I did not get enough information to find out why it would not stay there. It was obvious that on leaving the water the foils lost all lift and crashed back in, also the props were loosing power out of the water, so the height control was not working at all, back to the drawing board.

Fourth test

This time I had made a longer height control lever and added a spring to bring the foil back as the boat rised.

The weather was a bit rough but I managed to run from the fishermens pier in the harbour, and the boat tried much harder to get up and stay up as seen below.

But the problem was probably the wind as much as anything, the boat would lift steadily if I increased the power gradually, and as long the steering was dead straight (and the wind did not blow it over) it would lift out of the water for a bit before falling on it's side

But most of the time it fell over and ran on it's side wings in a slow curve

Lessons learnt

Again the height control did not seem to be working very well and I suspected the rear foil was not giving the lift it should be.

Fifth test

This time is was high tide so I went to Conwy morfa and used the ferry plaform as a base, but unfortunatly it was very windy, so I had the same problem of the boat blowing over. But I decided that putting the power on full from stop, just made the boat jump out of the water, it then inevitably crashed back in, so I tried increasing the speed more gradually with better results although it gave the wind loger to catch it.

I tried holding the power on even after it had crashed back in ot fallen over and did at least prove that the boat was struggling to lift, manly from the back.

Lessons learnt

The back foil was set at zero degrees as stipulated by the design books, yes a wing does still five lift at zero degrees, but what exactly is zero degrees on a wing?

I decide to re build the rear wing and give it a hinge so I could adjust the angle, once I have found he correct angle I can build another fixed wing at the right angle.

Mk 2 design

The rebuild of the rear wing will include a hinged larger rear wing and I will take the oppertunity to re make the prop support as that was not good, I will pass the props through the wing and get them level with the waretline as they obviously are pointing down at the moment, hence the boat jumping out of the water on full power.

The story continues here

Keep coming back to see how it goes and thanks for reading all my waffle.