raitl said:
The motor in question is an electrical grass trimmer motor which I'm trying to convert into a small boat motor. (Talking about a light ply boat, for 2-3 ppl)
First: check if your motor has brushes. If it has it belongs to the 'universal' type of motors which 'could' run on DC. But be careful there, the voltages and power figures might be quite different.
Second: what you are trying to do does not promise a lot of success.
The motor RPM must match the Propellor specs to get any worthwhile efficiency. A gearbox would really complicate the job and suck up more power in the bargain.
Its NOT just a question of spinning a propellor underwater!
Then, driving a submerged propellor demands power, very likely considerably more than your grass trimmer motor could provide.
Some people have tried to use a petrol (benzin) powered weed trimmer motor to propel a boat. They have a bit more power but at high RPM so there is still the problem of finding a suitable propellor. A plastic model aircraft type prop might stir up some useful water- experiment.
For small electric boat motors look up the 'Minnkota' series of motors on the net and see how they do it. You might get some ideas. They use 12V high current motors, something you will NOT be able to rewind your electric motor for. Its simply physically not practical to wind heavy gauge wire where previously you had many turns of very thin wire. Nor will the communtator or the brushes be able to handle the large current.
And, if you are thinking to use high voltage DC, remember that water conducts and you could end up with a lethal trap.
Good luck with more research. BTW, 2-3PPL do increase the weight of a light boat substantially. I would think a 2Hp motor might be required if you want to move faster than rowing speed.
Klaus