Assuming you got this motor for your boat: http://www.graupner.de/en/products/1794/product.aspx I think I know why the motor got hot.
In the video the motor didn't sound like it was revving at 15000 rpm, half that is more likely, meaning you're bogging down the motor severely.
It'll still do it's best to get up to the rpm dictated by the Voltage applied and will draw quite a lot of current, heating things up.
As no brushtab or cancooling was installed, the heat build up has nowhere to go.
Another point (literally) is breaking in a brushed motor; new brushes have just two tiny contactpoints to the commuter.
When you apply full throttle, all current has to flow through these minute points, the brushes will arch badly and possible burn into the copper of the commuter, damaging it for the rest of its working life.
The arching also causes a lot of heat.
A new brushed motor should be broken in, running submerged in a glass of water, on an as low Voltage as possible (1,5 - 3V) to avoid arching.
When the water starts to get dark, check if the brushes have assumed the curve of the commuter, if so, dry out the motor (compressed air work well) in a warm place, put a tiny(!) drop of oil on the bushings and you're good to go.
Hi Steven,
Sorry to hear the motor keeps overheating, I quoted my earlier message about why this may happen, if the motor is the one from the link, it may be toast by now...
You 'upgraded' to a hotter motor, didn't brake it in and kept the same prop as supplied for the stock motor.
I was once told that unless you're racing water cooling is the fallback for a badly matched prop and that's something I try to live by.
As mentioned above, the prop is too large and the motor can't rev out and runs (very) hot.
Watercooling is rarely necessary in scale boats, only the occasional over powered ones (like my MAS) may require some cooling assistance, but as StuartL correctly stated, it's not a bandaid for a poor setup.
Modeling clay will absorb some heat, but if the setup isn't sound, the motor will keep overheating until the magnets get demagnetized and the motor looses all power.
A Speed 400 revs too high for this application; a smaller prop will help a little, but the performance will suffer, as the mismatch remains.
Regards, Jan.