I came across some some horribly low-res old plans, in a language I don't understand, to build a radio-controlled boat that uses a gas airplane motor and propeller for propulsion. I don't have a gas motor either and have never built a boat, so naturally I couldn't get the project out of my head.

It has taken a lot of tweaking, but I think the boat is finally working well enough to share here.

I have taken the wonky old scanned plans and completely recreated them in 3D CAD. The plans I have attached are ready for laser-cutting or CNC-routing (from 3mm ply - the bounding box is 1200x600mm).

Parts List - Mechanical

1200x600x3mm Plywood (Marine grade would have been nicer, but I went cheap)

1mm Imbuia/Oak Veneer/Ply (or 2mm if you can get it)

3mm Balsa sheets

Glue (I used various, but waterproof and relatively quick-setting are beneficial)

Paint (waterproof)

Parts List - Propulsion

I would seriously do some of your own research, since my choices were based on what I could get cheaply/freely. I feel like this boat could perform a lot better with a better motor/battery/ESC combo.

10x7 APC Prop (Aliexpress/Amazon)

2s 2800mAh LiPo

1200kV 3530 Brushless Motor (Aliexpress - What I ordered/Amazon - Similar Search)

ESC (Mine was an unknown Amperage, make sure your's meets your motor's requirements)

Parts List - Steering

1mm Stainless Steel Cable

Bicycle Brake Cable Sheath

Waterproof RC Servo (Hobby King - What I used/Amazon - similar search)

Parts List - Control

Radio Control Transmitter

Radio Control Receiver (I used a 5 channel, but only 2 or 3 are needed, depending whether you want to tilt the motor or not)

Step 1: Vintage Design, Modern Methods

I found the original plans here. I tried to contact the site owner to see if he had ever seen one built, or knew where the plans came from, but so far have received no response.

I spent far more hours than I would like to admit on digitizing the old plans. I have never attempted a model with so many strange lofted curves, and Solidworks 2007's surface modelling is sketchy, if you'll excuse the pun. Rumour has it that the recent versions have some rather fancy tools for unfolding surfaces, but I don't have access to those. In the end I decided to model the whole boat as a solid, and then use a multitude pf planes and carefully shaped surfaces to cut it into profiles. The "sheet metal" tool was able to unbend one or two curved surfaces, but I anticipate skinning the whole boat with hand-cut 1mm ply, so I am not going to fight with it to do the trickier ones.

Step 2: Cutting the Profiles

While you could cut these profiles by hand on a jigsaw, I specifically designed them to be cut on a CNC machine of sorts, they have been laid out to fit on a 1200x600mm piece of 3mm plywood.

If you need to dimension the drawing, then scale it so that the slots match the thickness of the wood you are using, I designed the slots for 3mm plywood.

I was fortunate in that a friend runs a CNC router at his furniture factory and he was good enough to guide me through using it to cut these profiles.

This design is actually far better suited to laser cutting, what with all the tight inside diameters, but when you have a generous friend who will do it on a CNC router for the price of takeaways, that's the way you're gonna go!

Step 3: Assemble and Glue the Profiles

Dry Fit

Before touching the glue I strongly recommend assembling it dry, make sure that everything fits and that you know where it all goes.

I marked the various pieces with numbers and alignment lines with a pencil, you don't want to have to do any thinking when the glue is involved.

Right Angles and Parallel Lines

It is imperative that you keep the whole assembly straight, the profiles must all make a right angle with the two long profiles that run stern-aft.

In the photos you will see that I used two long straight beams to clamp the profiles to which helped keep everything in line.

Stringers

This is also a good time to install the stringers, these are 12x12mm wooden beams that fit at an angle down the length of the boat, they will help keep things true.

These beams also form the support for the lid/fuselage.

Step 4: Assemble and Glue the Fuselage

The "fuselage" for lack of a better term (really it is kind of a lid), is the same as everything else, simply slot the profiles together and start gluing.

I cut some extra notches in the horseshoe shaped pieces so that I could run some long strips of wood (about 3x6mm in profile) from the front to the back. I also glue some extra scraps of balsa to the vertical profiles, in order to give the skin more surface area to bond to.

Zip ties were used to hold it together while the glue set.

When it came to gluing the skin I used 3mm balsa, because it gave some "meat" to sand into nice curves.

Step 5: Bend and Glue the Sides

Up until this point I had glued the side profiles on as far as I could before significant bending was required.

There are probably better ways to bend the plywood, I have read about methods that use steam, others that use ammonia (particularly window cleaning spray), but I just went low-tech. I used cable ties to pull the wood in a little bit more each day that I worked on the boat. Once the sides had been sufficiently bent I drilled 1.5mm holes and used copper wire to hold them in place while they glued.

Step 6: Skin, Top and Bottom

Material Choice

I used a variety of materials, because I was using what I had available and experimenting as I went along.

3mm balsa (for the bottom of the two hulls)

2mm oak ply (for the large surface of the tunnel between the hulls)

1mm oak/imbuia ply/veneer

Thin Ply

I used a lot of 1mm and 2mm ply to do the decking and the tunnel, it is nice and flexible which makes it conform to the curves relatively well (although the Oak was much more prone to cracking along the grain than the tight-grained Imbuia). The 1mm ply can be easily cut with a craft knife.

As you can see from the photos, I use electrical tape and oodles of weights to hold it down while the glue set. Apparently bags of rice or sand also would have worked really well.

Balsa Supports

I added a bunch of balsa bits along the edges, and then sanded them flush, to give some more surface for the cladding to adhere to.

I stubbornly insited on cladding the top of the boat with the 1mm veneer, since it was given to me, so I put in some diagonal balsa cross-braces to support it as well as give more gluing area. It would probably be a lot easier to clad the top of the boat in 3mm balsa instead.

Step 7: Painting/Waterproofing

This part is up to you, so long as the paint is waterproof it should be ok.

I asked one of the friendly maintenance guys at our office park to spray the boat with "anything waterproof left in the gun after a job", but he really went above and beyond, hitting it first with a few coats of waterproof primer, then some military green-brown, followed by a couple of coats of clear.

If I was doing this myself without access to a spraygun, I would either go for a rattle-can, or even just use acrylics and a foam roller, since they cure to a fairly robust waterproof finish.

Step 8: Rudder

As you can see from the design, my original plan was to use a plywood rudder, it's shape is still included if you would like to try it. In the end I decided to use a slightly larger piece, made from aluminium.

The rudder servo sits between two blocks of wood, hot-glued into the boat. Each of the wood block has a M3 hex standoff epoxied into a hole drilled in it, which is used to fasten the servo down.

The rudder cable passes through two tubes (bicycle brake cable sheath, but I replaced the actual cable with some thinner, more flexible, stainless steel cable) out to the back. The pull-pull arrangement means that I can use thin flexible cable, since it doesn't have to apply any pushing force.

The cable passes through a hole in the rudder and is held in place one each side with a doodad who's name I can't remember (in the hobby shop you will find used for holding wheels onto RC plane's landing gear axles).

Step 9: Mount the Motor and Propellor

I tried a number of different mounts and have attached photos of all of them which should be self explanatory.

Version 1: All Form, No Function

Aesthetically the small motor mounted directly on the nose was the nicest by far (and was where I had always designed it to go), but the 7 inch prop I had designed it around simply wasn't big enough.

Version 2: Adjustable Tilt

The second attempt included a hinge assembly and a second servo to adjust the up/down tilt of the motor. This worked and was fun to play with, but not really necessary (however, with a hotter motor, or more battery cells, it may become more important to be able to tilt down when the boat begins to take off).

Version 3: Simple and Effective

The final mount was a simple L-shaped piece of aluminium, bent with about 10 degrees of up-tilt and high enough to mount a 10 inch prop. Simple and effective, but rather ugly. If I built the boat again I would adjust the fuselage in order to fit a larger prop on the nose.

Propeller

The size and pitch will be determined by your motor, ESC and battery voltage. Most motor manufacturers recommend a specific range of prop sizes per motor, so I would start with one of those. My experience was that I needed all the thrust I could get, but I was only running a 2S battery which was limiting. I ended up with a 10x8 propeller on a 1200kV 3035 motor.

If anyone can find a 7 inch 3-bladed propeller with a fairly high pitch I would love to hear about it, but I couldn't find one myself.

Step 10: Electronics

I won't go into the details of the connections, since it is a bog-standard RC receiver, servo, esc setup, like any RC car or boat. If you need some help there are plenty of tutorials, here is one on this very site.

Watertight Container

I found the smallest snap-shut "tupperware" tub that I could find. Unfortunately, it is just too tall to fit in the boat, so I just cut a slot in it (with a hand saw) and then resealed it using hot glue and scrap plastic.

This container will house at least the RC receiver, but depending on your setup, it could house the ESC too.

Heat Dissipation

Depending which of your electronics are waterproof, you may want to change the layout. In my first build, as you can see in the picture, I put the receiver and the ESC in the waterproof box, which worked, but you will need to consider heat dissipation. If you are pushing the ESC anywhere near its current limits, then you are going to need to get rid of significant heat, which can't happen inside a plastic box.

In my second iteration I sandwiched the ESC between two old heatsinks, making sure to waterproof the whole lot liberally with hot glue. You can see this setup in the photos and the video..

Step 11: Fasten the Fuselage

The fuselage already fits rather tightly, but since all of the thrust is applied to the hull via it, it needs to be firmly attached.

It would be easy to use nuts and bolts, but I wanted it to be easy to detach without tools, so I came up with a solution using steel pins (actually screws) that pass through the hull and into brass tubes glued into the fuselage, the tubes have magnets at the end that stop the screws from falling out.

The pictures tell a better story than words, so have a look (and read the annotations).

Step 12: Go Boating

Some tips for your first test run:

Be ready to swim, according to Murphy's law it reduces your chances of needing to.
Make sure that the rudder works (and isn't reversed)
Make sure the motor is turning in the right direction before putting the boat in the water.
Put a thin strip of electrical tape down the joints between the hull and fuselage, in order to keep splashes out.
Take a camera, you never know if the maiden voyage will be titanicesque or not, but it would be sad if it was and you couldn't post it to Youtube.