How to Build a Thruster for a Homemade Submersible or ROV

Some time ago I started building an underwater robot. One of the main difficulties (for me) was building the propulsion system. In this instructable I'll show you an easy to build a quite powerful 12VDC thruster. The original idea came from "Build your own underwater robot" by Harry Bohm and Vickie Jensen, but wasn't really worked out.So here it is.

Step 1: Tools and Materials You'll Need:

Materials:

- 1 Rule 1100 GPH 12VDC Bilge pump.
I used these because I could get them, other sizes or manufacturers might do fine too.

- 1 RC model boat propeller for example Graupner No. 455/10.
This is a 2-wing 70mm diameter prop with 4mm threading. Don't use to big propellers.
4-blade are also ok, but use a smaller one.

- 1 Coupling bush for smaller than 4mm diameter shaft. Graupner No. 3346 (3.2mm) is ideal.

- a 20mm long 4mm bolt with the head removed.

- a 4mm spring washer.

- Heat shrink tube of different sizes with glue or (better)a waterproof cable repair set like Raychem XBL22 (also, this is what I could get overhere).

- round circumference 2 conductor wire capable of carying 4A.

- adhesive rubber pads, like the stuff to put under furniture.
- waterproof cable gland (for example Velleman CPG135)

Tools:

- Hacksaw or dremel
- sharp knife
- electric paint stripper
- threading tool with 4mm bit.
- soldering iron
- pliers, screwdrivers etc...

Step 2: Prepare the Bilge Pump:

The bilge pump is actually a ready-made DC motor in a watertight housing. No idea how deep it'll stay watertight, but I'm hoping 10m.
Remove the lower blue part (klicks off). Remove the little propeller so the metal axis is free.
Take the hacksaw or dremel and cut around the white plastic,about a mm or so from the red area (don't cut in the red plastic).
Remove burrs with a sharp knife. You'll end up with a watertight motor in a nice red housing.
The black stuff in the picture is some adhesive rubber, which comes in handy to distribute the force once the motor is mounted on the robot (see further).

Step 3: Prepare the Propeller:

Take a 4mm 20 mm or so long bolt, saw the head of and make sure the threading still "works" on both ends (use a file if you have to). Using the threading-tool, make a 4mm thread in the coupling bush up to halfway deep. Insert the depacitated bolt. Like in the picture, now fasten a spring washer and the propeller. Put the propeller assembly on the motor axial, and fasten with the supplied little sideways screw. Note the motor axial has a flat side: that's where you want the sideways screw to be pressing against. Make everything really tight... done.

Step 4: Assemble the Cable (optional):

There are two wires coming out of the motor, it would be nice to have one round cable that you can put through a waterproof cable gland. I used the Raychem cable repair kit to make a waterproof connection to a round cable. You can also glue-filled heat-shrink tubing of different diameters. Soldering the connections is the safest way.

Step 5: Test Your Work:

Try it out in a bucket or a bathtub.

Step 6: Mounting on the ROV:

The robot I'm building is work in progress. Here's how the thrusters are mounted.
The motor is placed on a U shaped spacing filed on the end of a piece of PVC pipe.
A screw-clamp keeps it in position.