Twin Engine R/C Boat

Instead of getting bored during the Corona-crisis I decided to build a boat for my grandchildren with some old and obsolete R/C parts from my workshop . In the past I scratch-built many Speed400 planes , the common used batteries were Sanyo 500AR Nicads . When the LiPo's came they were forgotten . But when using motors and props that aren't power hungry they might still serve . Here's how ...

I found this at Banggood :

The motor/shaft set : motor and shaft set

The propellers : propellers and nuts

NiCd or other cells , NimH or even Lipo (max 2S!)

Transmitter and receiver , 2 channel ( throttle and rudder)

Speed controller , brushed ( mine is an old Simprop RS 07-8) . If using Lipo make sure the controller can handle it .

3 and 6 mm Depron sheet

1mm light grade balsa

scrap balsa

plywood for the rudder

UHU Por Glue

UHU acrylit glue

Foam safe superglue or foam safe glue (like the one used by flower arrangers)

super light filler like balsa filler or Polyfilla light

Tools : X-acto or similar cutter and blades

The hull is made of depron covered with balsa . Cut the top deck (6mm depron) and draw the centerline . This line will simplify your work . Cut out the formers (6mm depron) , align the with the centerline , make sure they have the correct dimensions ( 3mm space at each side of the top deck ) . The nose block is made of foam , it's easily made with the tools shown . Use 80 grade sandpaper to put it to shape , glue into place and allow to dry .

Now add the sides made of 3mm depron . They have the same height as the formers and are glued together with the top deck and the formers. Its a good idea to mark the formers position on the top deck . We will cut a hatch for the "engine room" and this will prevent cutting through the formers ... When the sides are in place and the glue has dried put the 3mm depron bottom in place . When done sand and add light filler to end up with a nice hull :-)

When the filler has dried sand very carefully until you're satisfied with the shape of the hull .Now we'll cover the hull with 1mm balsa using foam safe glue . I used glue used by flower arranging hobbyists . Start with the rear , then the bottom and secure your work with rubber bands .Notice the grain direction of the balsa sheets as seen on the pictures . Now cover the sides with some overlap .

When done cut off excess balsa and sand carefully . Now it's time to cover the bow ... This is a bit tricky and you'll need some patience . I tried covering using 15mm strips made of 1mm balsa but it didn't went very well , pins were needed to keep the steps in place . 5mm strips are much easier and needed no pins , the glue keeps them in place so you can continue working ... till you've had enough . Time for a break :-)

When almost done we add 2 1,5mm balsa triangles , use tape to secure them while the glue dries . When dry sand with 80 grade and 180 grade sandpaper , take care when using the 80 , the sheeting is only 1,5mm thick .

Now it's time for an easy job , covering the upper deck . Sand away excess balsa and use the Polyfilla all over the hull . Sand with 800 grade , this will give you a smooth finish .

Hull : Done !

The motors are tiny and powered by a 1S Lipo in the Flytec V002 R/C crocodile as seen here :R/C Croc .

This may be a problem because our old receiver needs at least 4.8V , and the controller BEC output is 5V .

Speed controller is NiCd 6cell minimum input.

A series resistor for each motor will do the job but this will transform electric energy into heat ...

So let's test a setup with a 6cell NiCd , 2 motors in series and a classic receiver/speed controller configuration .

A test bench setup was made with a piece of foam as seen on the picture . Test was witnessed by a few curious fish in my backyard pond . There are some minor things to notice , sometimes one motor doesn't run at startup .

Pushing the throttle forward firmly will fix this issue . Thrust is sufficient , we don't need a race boat because it's designed to be handled by my grandchildren . Current draw is 800mA at full throttle so a 500mAh pack will provide about 30 minutes of fun ( no servo/rudder were used during test ). Testing was quite a juggling act but it worked out fine :-)

Both motors are wired to run CCW , see picture . Make sure to pick the CCW props , in the set you'll find 2 CCW and 2CW props . If you like you can configure 1CW and 1CCW , just select the proper propeller .

Now it's time to install motors , shafts and props . Take your time for this job and make sure motors and shafts are aligned properly . When motors and shafts are well aligned power loss is minimal . Motors are kept in place by 3M "GoPro_fixing_stuff" ( very strong double-faced tape ) . (Note : because the motors get warm and cause vibrations I added some "construction Glue" to be on the safe side ...)They are mounted on 2,5mm balsa and 6mm depron . The shaft is glued into the hull with UHU acrylit ( aka Pattex Stabilit Express ) . When both shafts are installed add some more acrylit , it can be sanded afterwards . When done add super light filler to the hull and let dry . Sand and you're done !

The servo is mounted in a 6mm Depron box , held together with Uhu Por . Let dry before working on your servo setup . The rudder assembly is held in place with 2 drops of superglue to allow all the necessary adjustments and locate the position of the rudder control horn . Just use your favorite setup , I used "CA hinges" and a plastic control horn . The servo to rudder connection is made of a 0.8mm piano wire .

When you're happy with the finished hull ( or had enough of balsa/filler dust ) it's time for waterproofing the hull .

My boat just had a coat of primer and a coat of common semi-synthetic paint ( I painted my meranti windows last year ) . It worked out very well but the top deck can be damaged quite easily ...

Use your favorite method , here are some more tips : sealing the hull

Master modeller Glynn Guest uses a tissue/dope method as discussed here : tissue/dope discussion

Finish your boat as you like , take your time , and enjoy the result of your hard labour !

Testing was done at park Romeinse Put ( Edegem ) because this was the nearest useable location , all other locations were taken by algae ... The boat was not completely finished but I wanted to make sure it performed well . It did ! I had to move the battery inside the engine compartiment to raise the bow , but there's ample space .

The 6-cell 500mAh will provide you with at least 30 mins of fun if you like to sail relaxed . Rudder efficiency is OK , full left/right stick commands will cause no unstable reactions so the boat is OK for children .

But if you're a speedboat fan you will be disappointed , the engines were not designed to break records but to simulate a swimming crocodile ... Positive side of the story : low power consumption :-)