Hi Lifeboat,
I have a 5 foot Arun lifeboat boat running on 2 Emax BL 4030/10 turn 850 kv brushless motors turning 80mm 4 bladed propshop props with 60 amp Hawk speed controllers and 2 12v gell cells batteries for ballast and power, it runs great at scale speed. I can run it on 24v as well, but a little to fast.
Destroyer42
850kv x 24v = 20400RPM!!! I wouldn't recommend running that unloaded!
Lifeboat, feel free to skip the following man-maths but it should allow you to see how I got to where I am.
Assuming you wish to run at 12v using your existing gel cells, and a target RPM of 4500ish, that gives us a required 375kv (4500/12)
We also want to know how much of a load the boat is going to produce. Take the HP of the real thing, and divide it by the cube of the scale. In this case, that's 570hp/12^3.
That gives us 0.33ish hp, but we want the answer in watts, so multiply that by 746, giving us 246 watts. No scale drive setup is perfect, so we want to give ourselves a 25% safety margin, so 246x1.25=307w, give or take. There are two motors generating that power, so let's say for simplicity, 155w per motor.
All we need to do now, is find a motor which is around 375kv, can handle 12v (a.k.a. 3S in the LiPo world, and what you'll see most motors rated to), and can handle 150w of power. (at 12v, that's 12.5 amps)
A quick search of Hobbyking later, and we have a winner.
https://hobbyking.com/en_us/ack-3515cp-400kv-cw-brushless-motor.html?___store=en_gbThis motor would work at 12v, it is man enough for the job and gives you the 4500rpm you want. They recommend a 45a ESC, but it's just as easy to find a 60a one.
https://hobbyking.com/en_us/turnigy-marine-60a-bec-waterproof-speed-controller-with-water-cooling.htmlYou probably wouldn't even need to watercool it as most of the time it'll be running at 1/4 of its rating.
And the bonus with this setup, if you got all this and found out that it was a bit too fast or slow, you could go up to 16.8v or down to 7.4v and it would be fine!