The lower end of the outboard is likely a 2:1 reduction so you should need no reduction. Reduction only serves to allow the motor to be cooled with the fan that is built in. External muffin fans in the case will reduce the need for motor RPM. Reduction serves to increase torque at the expense of friction.
Kevin Pemberton
On 03/24/2011 03:22 PM, affordableacadia wrote:
Thanks to all for dispelling some advice which may have been motivated by greed. While I'm not without means, I love to put projects together "on the cheap". My motor installation will be an outboard conversion in a well as in the Glen-L Etek example, so I don't think it will be exposed to lots of splashing. Unlike many retrofits, I am designing the boat for this purpose, so I am doing some weird stuff like putting my 6 Trojan T-105s in a hollow keel. Question: If I know my top voltage will be 36V and my top amp draw will be 84 amps, do I need to size my controller for the maximum draw for the motor? (200 amps)
Next, I am using a 10 HP Chrysler outboard for the lower unit. It has a 3 bladed prop (8-1/4" diameter) which according to the specs, spins at a max of 4700 RPM. (or are the specs referring to the engine speed rather than the prop speed?). If I expect to get 3 HP (to reach hull speed) from this unit I assume the IC horsepower would have an electric equivalent of 5 HP, which means 3/5 X 4700 RPM= 2820 RPM. The Mars 0909 spins at around 3200 RPM at 36 V. So if I do a direct connection I will be very close to the motor speed I need. Many here have advised using reduction. Is that for larger props? If I do a 3:1 reduction, according to my limited mental ability I would not get 3 HP out of this setup! Am I missing something?
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