T-Pain Is On A Boat: Re: [Electric Boats] Re: Electric Catamaran

Friday, December 11, 2009

Re: [Electric Boats] Re: Electric Catamaran

Jason, someone here recommended Dave Gerr's book "Propellar Handbook" as a comprehensive guide to props. I bought the book at Amazon and the subject of cavitation to be very complex. "Not only that, but these complex methods frequently are no more accurate than simpler ones, since they do not allow for such factors as shaft inclination, strut and stern-bearing fairing, and so on, which can make two otherwise similar propellars behave quite differently, even at the same speed."

"Diameter, revolutions per minute and pitch are the three most significant factors affecting propellaer performance and efficiency... Diameter is the single most ciritical factor in determining the amount of power that a propellar absorbs and transmits...For the vast majority of installations, the larger the diameter the greater the efficiency . The only exception is for high-speed vessels---over 35 knots or so---in which the extra wetted surface of large diamter shafts, bearings, and so on causes excessive drag."

I am using the Mars ME 0308 motor (electric yatch) and went from a 10 x 12 two blade prop, to a 12 x 13 three blade prop on a 27' sailboat, and I am content with its performance. I would strongly recommend shopping for a used prop on ebay UK. The british have a large market for used props. I could not find a used one here in the states.

Don Swanson

On Fri, 11 Dec 2009 13:26:08 -0000 "jason_d_s_uk" <jason_d_s_uk@yahoo.co.uk> writes:

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.