Stobby

You have asked the question which we are trying to get a comprehensive answer to!

As an aircraft man you can translate motor sizes 46, 61 etc back into power expected - say 1HP (750 watts) from a good 61. (THE MANUFACTURERS EXXAGERATE TERRIBLY) This is a PEAK power, at one point in the rev band - you don't need to copy it to get blinding performance.

So you are looking for a brushless giving say 600Watts

You have a 6-cell LiPo (I look on you with increased respect) that's a nominal 22 Volts* if they are in series and the current for 600W would be 27Amps

*PLEASE don't anyone take exception to this - the voltage quoted for LIPO cells is ENTIRELY arbitrary - corresponding to about the half-discharged condition

Now you havn't said what velocity you are aiming for - is it an out and out speedboat? a scale model of an offshore racer? or a deep Vee tug

To sort out WHICH motor now requires some decisions - are you geared? Direct drive? What Prop? How much room do you have for a prop diameter etc.

Conferssion - I know nothing about boat props so what follows is logical but may be completely wrong

Say you are going direct drive, with a 40mm dia x 62.5mm pitch prop, suitable for the speed you expect

Thats 75mm advanced per rev (but slipping a bit - say 25% (guess) so the actual advance is 62.75 x 75%= 50mm

So every 1000 revs advances 1000 x 50mm , or 50 metres

if the motor did 1000revs/min the boat will cover 50 metres/min or 50/60 metres per sec - nearly 1 metre per second

If you expect to reach say 10 metres/sec boat speed you need the motor to turn the prop at 10,000 revs/min

You are feeding the motor with 22V so you need the version of a motor with KV of 20000/22 rpm/volt or Kv=454

There are a lot of assumptions in here, and some can only be decided by experience .

Nonetheless I hope its a little helpful - if another mayhemmer can suggest a starting point for prop and speed, we can select a motor

andrew