2400 rpms with the existing set up a Yanmar 1GM10 and a prop 13 x 9 RH 3-blade to reach hull speed in calm conditions.
MOTORs being considered. (other motors?)
Agni 143… 47rpm/volt, 85 max volts, 150 continuous amps at 48V 6.4 kWcontinuous at 48V, 4000 max rpm
So how do I calculate gear ratios? I’m assuming I need to gear down from 4000 rpms? How do I make sure I have enough reserve power to push against a tide or wind?
Good question, and it brings up some interesting differences between electric motors and combustion motors.
So my answer to your question is that you do not need a gears unless you go with the first motor. To get the necessary rpms using your 48 volt system you would need to increase the speed of the prop shaft relative to the motor rather than the other way around. The reserve power is based on the capacity of the motor to take a high current for a long time. the second motor has a capacity of about 9 real horsepower. If you had a diesel rated at 10 hp then this would be great. The actual power put out by gas and diesel engines is quite a bit less than their "rated" power. For example a a Torqueedo is compared to a 5-6 hp gas engine but it only puts out between 1-2 "electric" horsepower. .
Your going to have to do your homework here, always start with torque data / amp draw for the motor at hand. Do you see the 60% of rated for continuous? May I suggest the prop needs to be changed to the largest possible on boat with a higher pitch. Generally most electrics are most power efficient at lower than their max rpm and amp draw. Controllers are critical to power consumption. Electric motor 'hull speed' demands, prop. and gearing are interconnected and not analogous to diesel gas engineering (stall rpm being main issue lowering pitch and prop size in diesels/gas). You are not going to reach hull speed on any reasonable power usage or with those motors I'd bet, though you haven't listed boat specs. Does the law of massively increasing power demand approaching hull speed come to mind? A tide can always be faster than even hull speed, wind is relevant to the aerodynamics of your boat as equipt. I think by 'power' you mean torque advantage against low water speed to the propeller under high demand. Most electrics don't have this low speed stall problem. You will probably be engineering/gearing for top speed (or top speed attainable by the continuous rating or efficiency peak) to the motors most efficient rpm to amp draw. Lower rps/lugging conditions will likely take care of themselves. There is nothing short of experimentation to find what is best. calculations only get you closer. Many books on this matter, read one or more.