appropriate gear ratio

[gharai]

Hi every body
I’m taking part in an rc model boat competition. I’ve made a catamaran hull which is 1300 mm long, 500 mm wide, 150 mm high and the overall weight of the boat is 15 kgs. The boat has 2 propellers both of which have 45 mm diameter and 50 mm pitch. I am using 5 LiPo batteries each are 3.7 V and 5 Ah. The specifications of the motor are as follows:
XM3548CA, 6 turns, 720 RPM/V, Max efficiency current is 25 Amps, the Max power is 650 W (15s for the mentioned batteries), Max efficiency is 80%.
According to these informations what gear ratio should I use?

Thanks alot.

[Guest625101138]

Direct drive is almost ideal. Each motor will absorb 31A at full voltage and boat will be doing 20mph. (I am assuming they are submerged props and not surface piercing)

Rick W