Robin. Bit of a dilemma here. The 540 will push it along ok, just not particularly fast (perhaps on a two blade 35mm plastic prop and a 7.2v Nimh battery. This would undoubtedly be more than fast enough for a 9 year old. Before anyone says it, there are 540`s and 540`s, they all look the same. some are low power types and on 7.2V will be pedestrian, the "standard" higher revving ones will be quicker. Any makers label on it?

Nowadays I would be fitting a 35mm outrunner brushless, possibly 100-150 watts or so, 1300Kv and  on 7.2V this will be fairly brisk. There are in this size a big range of Kv and power rated motors , to retrofit/swap/whatever. Harry1 claims that the mounting holes are the same for 540 motors, I don't disbelieve him and if that's the case you could swap things about.

This combination would be far too fast for a 9 year old and it would soon me matchwood.

Your choice…..

Ashley

Edited By ashley needham on 29/09/2016 16:45:49

Hi Robin

I would use a 60 Amp ESC(2-4S Lipo battery&nbsp, which would cover these motors 3639-750, 3648-850 and 3639-1100.

As for prop a 2 blade 40mm is a good starting point.

I have not had a problem will Lipo's using a good charger and safety bag.

Do not leave unattended while charging!!!

A 3700mah 40C with give you a good run time without too much weight for the boat.

Depending which motor you get the revs will be between 5500 and 12200 rpm(scale to over speed).

I am using a 1100kv on 3S with a 40mm 2 blade prop in a Sea Commander and is in the over speed section.

Hope this gives you a starting point.

Harry