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Permalink Reply by Tom Hruby on May 10, 2012 at 1:56am

Hi Lawson,

Good question, and it brings up some interesting differences between electric motors and combustion motors.

1. An electric motor will draw as much power as it needs (if the speed controller can give it that much power) to maintain a specific rpm up to its rated amp capacity. 
2. You are considering two motors:  the first one is rated at 47 rpm/volt.  To achieve 2400 rpm you would need at least 51 volts (2400/47).  Since you are considering a 48 volts system, this motor will never achieve the needed rpms.
3. The second motor is rated at 68 rpm/volt.  This means you only need 35 volts to reach your necessary rpms (2400/68).  This is where your speed controller because a critical component of your system.  You should go with a controller that uses pulse width modulation to provide the necessary current and voltages.
4. So at 35 volts and 140 amps the motor is putting out about 5 Kilowatts of power which is about 7 horsepower.  If this is more than you need to maintain hull speed (2400) then the motor would still be running at 35 volts but the current would be less. 
5. If you need more power then you would increase the current and that would increase the volts to the motor and increase the rpms up to the 48 volts of your system.  The maximum rpms of your electric motor would be 68 rpm/volt x 48volts = 3264 rpms.
6. At 48 volts and 140 amps you would have 6.7 kilowatts of power or almost 9hp (real horsepower) at 3200 rpm. 

 

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. .

 

Tom

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Permalink Reply by Braith Miller on June 3, 2012 at 12:03am

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.

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