The gear ratio that you choose for a Mars Motor should result in an amp draw of 100 amps at 1800 rpm at desired boat speed. You will find that the rpm of the motor increases with the boat speed.
We use a Browning gearbox, they have many different gear ratios spaced apart roughly .2 : 1 1.23:1, 1.45:1, 1.55:1 etc.
There is no real way of telling what the gear ratio should be. Too many variables including the propellor, the location of the propellor (aperture etc.), the underwater profile of the boat, the displacement of the boat, the packing gland, the cutlass bearing and what type of gear reduction you are using (gears, belts, chains etc)
We make the best assessment of the situation based on our experience and then provide a gear exchange until you get the right gear. It usually takes two to three times to get the perfect gear for any boat. We charge a nominal fee for the gear exchange.
You will notice major efficiency gains from the right gear ratio, which translates into increased range.
You will also notice power gains, because the wrong gear ratio will not allow the motor to turn fast enough for cooling, or not provide enough power because the motor is turning too fast and the propellor not fast enough.
I also recommend to start with the best propellor, because you do not want to do it all over again when you get a new prop.
Hope this helps,
James
Propulsion Marine
On Jan 24, 2010, at 9:56 AM, toddbates99 wrote:
I'm trying to use the spreadsheet to estimate the best gear ratio. Its got a lot of info but a steep learning curve. Finally figured out how to match the required HP to the HP for a given diameter/pitch. So now I think that my best propeller RPM should be 1020. So is there a best RPM to operate the 72 volt mars motor at? Best in this case would be RPM rather than maximum power.
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