Hi Paul

The way you discribe the motor setup with parallel batteries and a 8.5v speed controller all suggest that the motor is 6v.

In my opinion I would fit a new 12v motor and electronic speed controller to give this model some get up and go (the 4ft is a very heavy boat)

Hi

A car windscreen washer unit is great for driving fire hoses, I had one in a 6ft boat that worked a treat.  Try to find a high capacity model that will shift more water this will make for a very impressive display.

All the best.

Hi Paul.

If I may be contentious, Voltage will not destroy your motor, it is the current ,Amps, that cook motors.

OK to try for more for less:-

What I would do is take the speed control out of circuit, and useing 6V battery and an ammeter check what current the motor is taking (in the water with the existing prop). Make a note of this

Now useing 12V battery and an ammeter try different props (they will need to be smaller and/or less pitch than the original) untill you are drawing about the same current as before.

This gives you twice the voltage with the same current hence twice the power.

If this gives you bags of thrust, select a suitable ESC that will handel at least 12v and (i would say at least twice the current you are useing (if you use the resistance you measured

Quote"I have tried measuring resistance which is of course very low "

and use Ohms law.  current=voltage divided by resistance

this will give you the maximum current your motor will take if things go wrong, ie a jammed prop. If funds allow I would use a ESC cpable of handeling this current , and fit a fuse in the circuit at about 75% to 80% of this current

I have used a 6V hectoperm with gearbox remove on 14v with a small prop and watercooling coil on the motor for fast electric it enabled me to have a go at club events

Cheers Tom