Ideas on a thruster design for your homebuilt R.O.V.
Concept :
I need my ROV to be small, transportable, afordable, but yet eficient to a maximum 50/60 meters depht, wich i consider to be quite enough for an amator project.
There is various options availables to design an home-buildable thruster but i wanted to avoid using a compensator and filling the motor compartment with messy oil: I consider it a necessary choice only for deep-diving ROV´s.
The design i propose is using a brush D.C gearotor inside an air-tight compartment, the propeller shaft being sealed by an industrial  mechanical seal . The  Hyball  observation ROV from  Hydrovision  use this kind of technology.

 

The pressure housing :
We want our motor to be protected from seawater and from the pressure as the vehicle travel down the sea.
A simple option giving the necessary strenght without beeing too heavy is to use a piece of aluminium pipe.
I will be using a 50mm (about 2 inches)  internal diameter neumatic cylinder pipe: It´s anodised and will be less sensitive to corrosion, and it has a very clean internal surface wich is ideal as sealing surface with o´rings. The 3mm thick wall should be more than enough to sustain the pressure down to 60 meters (200 feets).
An even better solution would be using composite materials pipe. If not available off the shelf, graphite or fiberglass pipes can easily be  molded over another pipe.

The gearmotor :
The small D.C motors availables on the hobby market would run too fast and generate too little torque for our application. High torque is desirable because driving a large propeller slowly is better than driving a small propeller quickly in terms of eficiency. A ROV is not a fast vehicle anyway, so we´d rather be gearing down our motor.
The gearbox need to be small enough to fit in the pressure housing. Also, an output shaft aligned with the motor shaft would alow to simplify the design : A planetary gearhead  is the ideal solution.
After considering hobby motors or even some very nice industrial products, i figured out that the cheapest yet reliable gearmotor around is the kind found in some cordless drills. Manufacturers like Bosch, Skill, Makita will offer both motor and gearhead as spare parts at very reasonable prices due to large productions.
I found my PSR 9,6 VES-2 Bosch drill gearmotor ok to begin experimenting.
Useful link : SparesOnTheWeb

The shaft-seal :
In my mind, the biggest problem in a one atmosphere thruster is to seal eficiently the propeller shaft. Water ingress due to bad sealing would rapidly ruin the internal parts of the thruster.
Most of the pumps in the industry use now mechanical seals for the impeller shaft. Some of these seals can sustain over 10 bars (150 psi) and rotate both directions. Burgmann provides with his MG1 a usable seal for a thruster.

Machined parts :
Custom-made endcaps sealed with o´rings  will be necessary to keep everything watertight. They are to be made of aluminium, trying to keep the machining work as simple as posible to be cost-efective. Dimensions will be adapted to fit other parts like the shaft-seal.

Propeller :
Hobby-like propellers with 3 or 4 blades could be used but i would prefer a self-made propeller with carbon fiber blades.

Lighting :
The construction can include frontlight, specialy if you´re making the two horizontal thrusters like on the Hyball
Here´s a description of a diving light that uses an halogen light bulb easily found anywhere.

Drawing :

 

Hardware design

Video Camera

 Main Hull

Thrusters