Archive for January, 2008

RC Boat project

Wednesday, January 30th, 2008

Ok, some time ago (just over a year now) I decided that the whipper snipper in the shed with a broken flex shaft had more to accomplish in life other than the tasks of a mere garden tool.  It was with this that i decided to build a boat!

It was a Ryobi 31cc trimmer.  These seem to be common for use on 1/4 scale planes with a few mods so i thought it would be worth a shot.  The engine was stripped from the trimmer and cleaned up.  I then removed all the excess material from the plastic back plate, machined down the flywheel and it was mostly ready.

 I then started browsing for a hull to plant this thing in.  Cheap is usually my biggest objective so i ended up scoring some plans for a wooden hull from a man named Doug.  He designed this hull himself and races with it so i figured it was a good start.
I built the base of the hull with the stringers and bulkheads but that was where it stayed up until about January of this year.  I hadn’t even put the side boards on.  Other things had me distraceted – Family, work, RC car, RC heli….

So, over the recent christmas holiday break i decided to get back into the boat.  I stuck the sideboards on and started looking at it more seriously. 

The engine is natively air cooled which i was thinking might be a problem (most RC boats have water cooling).  So i shaved 3 sets of fins off the head in the lathe and proceeded to make a ‘cap’ for the head to run some water through as the boat ran.  The hat was then bolted to the head using some gasket silicone to seal the water in.


I made up some engine mounts to secure the motor into the boat as well.

From here, i was a bit unsure of how to progress.  I knew i needed a shaft from the motor down to the prop but there seems to be very little information on the net about just how this is accomplished.  It was then that i met up with a man named Les.  As it turns out, he has a big zenoah based boat and used to race boats regularly.  Just what i needed – someones brain i could pick from.

 I inspected Les’ boat, asked a heap of questions and i was on to the next stage.

I sourced some Teflon and brass tube for the flex shaft to travel through and also some other gear to make up a ‘skeg’.  I also bought a prop, drive dog and rudder off ebay.  These you can’t really make….


The skeg was fabbed up using stainless and then secured to the boat using aluminium angle.  I made up a brass ferrule to tie the flex shaft and straight 1/4″ shafts together.  It was then all put together…  I am using teflon in the skeg as bearings, we’ll see how well it holds up.

 19/1/08 – I then spent about a day fabbing up a tuned pipe for the engine (with thanks to javapipe).  It was very hard.  I had to manually fold the cones into shape and then mig weld it all up.  Very hard on just ~0.6mm stainless ( Some old heater  flue i had ).  It worked out ok tho, just not as pretty as i’d hoped…


Weekend 26-28 Jan 08 – The engine is now mounted, petrol tank and flex shaft in.  I have also made room for the radio box and got that all sorted.  I also finalised the manifold for the tuned pipe and I tried to start it for the first time in a year or so.  Turns out that the carb was cactus now.  Apparently the fuel pump diapragm goes hard and stops it form working.  Not to worry, i ‘borrowed’ the carb from my current line trimmer and got it started (With the help of my mate Tim)

Stay tuned, hopefully it will hit the water this weekend!!