Jason,
An electric motor acts as a variable speed transmission by changing voltage and inversely changing current.
Most hobbyists are using their electric motors bolted directly to the driveshaft or through a 2:1 reduction gear.
Most of us measure the DC current from the battery, for the Mars motor that would be 100 amps, and call it good.
What we need to measure is the AC current going to the motor, not the DC current. If the motor runs too slow, it will draw more AC current and that is when you will get a stall condition or other failure of the motor.
Horsepower is torque x rpm / 5252. To maximize horsepower, the electric motor needs to spin fast.
The only way to optimize an electric motor is by varying the gear ratio. This allows you to match a propellor to the motor.
By increasing the gear ratio (from 2:1 to 3:1 for example) you increase the rpm of the motor, which is the same as increasing the voltage that the motor is operating at. This decreases the current and allows you to make more power.
Ideally, the DC current from the batteries is 100 amps and that should be approximately the same as the AC current at the motor, also 100 amps, for the Mars motor. I have seen 100 amps DC current and 170 amps of AC current at the motor when I first put a larger propellor on my boat. A situation like this would end up in premature failure of the motor. Measuring AC current is easy, just clamp on an AC current meter onto one of the leads of the motor.
The 20 turn Mars motor is best set up to run at about 1800 rpm. The new 28 turn Mars motor will also run at 1800 rpm, but with far more torque, and operating at a higher voltage for higher efficiency.
You always want to run the largest propellor at the slowest rpm possible for maximum efficiency.
Maximum efficiency translates into maximum power and maximum range.
Every boat is different, there are many different factors including boat speed, hull profile, tip clearance, packing gland, cutlass bearing etc. For this reason, it is a guess at best what your gear ratio should be. When I install an electric motor I can get close with the gear ratio, and sometimes bang on, but most of the time, we change the gear ratio at least once to optimize the system. Optimizing can also be done by switching between the 20 and 28 turn motors, or by switching propellors, or by switching all three components.
On my own boat, a Catalina 30, I switched all three components and efficiency changed dramatically. Before optimization at 3 knots we were drawing 1.6 KW, after optimization we were drawing 588 watts for the same speed. Kapowai now has a theoretical range of 80 miles at 3 knots in calm conditions. We need to use calm conditions for our baseline. I also consider theoretical range as a bias against the elements, an ability to create apparent wind which allows motor sailing in slower wind speeds. An ability to have directional stability into waves and weather. This bias is what makes an electric sailboat far superior to their diesel counterparts as a sailing boat. I think of my boat as a super sailboat, not a motor boat!
Hope this helps,
James
www.propulsionmarine.com
On Dec 11, 2009, at 5:26 AM, jason_d_s_uk wrote:
Thanks for all of the comments.
Just to clear a few things, it's a sailing cat cruiser/racer 30ft and < 2.5 tonnes.
The http://www.psychosnail.com/BoatSpeedCalculator.aspx calculator recons that 5 Knots would use 1.31 KW
I'm guessing that's for only one hull on a cat so should that really be 2.6kw for 5 knots?
Eric, Thanks for your input. I didn't realise the 'normal' voltage of TS cells would be 3.2v and not 3.75 (I was also looking at a 48v system).
Good points about rated wattage on alternative power sources (Solar/wind) and what the realistic values would be on a daily scenario.
Chris, you quoted : 1500 watts = 4.5knots = 6 hrs
I would imagine my cat would be similar and 5 Knots would possibly be < 2 KW?
The reasoning for two motors is for Manoeuvring when docking and backup. If need be, I would only use one for distance travelling (into/outof harbours) and might create a sliding system so that I can Position them on each next to each hull when docking, then slide it into the center of the transom for motoring.
Has anyone taken apart an old outboard? Is the gearing in the top section? Would electric benefit from the gearing or better without?
My main lack of understanding is with prop sizes. With an electric motor, the best torque is in the low revs, so surely a large prop would be best? How would you calculate this!!! I'm looking at using Mars Motors (ME 0308) which have a 3 KW constant use, 9 KW max (1 min). (cloudelectric.com) I found with my current 30hp petrol that at 5-6 knots, cavitation stops me from revving it anymore and it's probably only running at 2500-3000 rpm. It's completely overpowerred and similar cats like mine have 15 hp engines.
How do you work out the max prop size against the motor you're using so that it doesn't stall or burn out?
Thanks for all of the responses, (in adavance)
Regards,
Jason.
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