Dollbot #4: Water proofing

Dollbot needs to operate in a relatively calm environment (swimming pool) but needs to be ready to withstand some abuse, e.g. people pulling it underwater, capsizing or being thrown in the water from a certain height.

Engine shafts

The engines are housed inside the container but the shaft has to cross the hull and spin the propeller in the water. After some lengthy research we have learnt two important things:

• RC boat models all have the same problem, which is typically solved by using a stuffing box, a small tube filled with marine grease that ensures water is kept at bay

• DC motors can operate submerged in water without any seal

We tried an arrangement to simulate a stuffing box but it proved too complex and error-prone, so we went for exposing a part of the motor to the water and sealing it with a glue gun:

Not sure if this approach will hold the pressure, but it is simple and seems to be effective.

We have designed the propulsion system to be fastened using four bolts, which also need to cross the hull, We have used 3mm x 30mm bolts with washers, and covered everything with glue gun to improve sealing.

The first leak testing showed issues with the bolting: while the motor seal held, water started leaking from the bolts which were not perfectly covered with the glue:

After letting the whole setup dry we added a layer of 'american' tape and re-applied glue to the bolts which resolved the previous leaks, as verified after a 48h loaded test in the water: