Hi Alex, This info from a guy who did it and sailed California to Hawaii. http://alberg30.blogspot.ca/2011/02/motor.html
Research will indicate that you do not do a direct HP --> HP when comparing engine to motor.
Daid Gerr's propeller handbook is the best I've sen. Web calculators: http://www.vicprop.com/calculator.htm http://www.rbbi.com/folders/prop/propcalc.htm Cheers and enjoy, Russ Sweet 35 mk-1 At 07:50 AM 01/03/2013, you wrote:
I want to second what Joe has said. You've not said what size boat you have, so perhaps you are thinking about an electric outboard. You may want to look at the last two or three issues of Cruising World for columns written by Webb Childs. He has put a Torqueedo outboard on his latest boat, which is somewhere around 24-26 ft. IIRC he is able to get 3 or 4 knots for about 1 1/2 hours, and 1 1/2 knot for 4 or 5 hours, but has concerns about the ability to "motor in headwinds" and chop. Torqueedo does offer an option that can solar charge the lithium ion battery pack on the motor. They also have a sail drive you could put in place of your existing diesel to avoid the typical problems of an outboard on a sailboat. But I expect that by the time you did hull mods to install a saildrive, you would exceed the modest cost of a new 18 or 24 hp Beta Marine diesel. I have seen one sailboat and one power boat that have had successful electric power systems. The power boat was a smallish sport fisherman with a diesel/electric hybrid system. Something like a 36 ft Black Watch, or the other sport fisherman built on the Jersey shore. Owner had replaced the two diesel engines with high torque electric motors typically used in large fork lift trucks. They were like 18 or 20 inch diameter motors if I recall, and 36 volts. Power came from a 3000+ AH 36v fork lift battery and a 6 KW Onan generator to provide power to/through the battery. Genset ran all the time to provide maneuvering power and the battery provided surge power for acceleration and high speed. Don't know what he used for a motor controller/throttle, but that technology is very available and pretty reasonable since it is what has been used for forklifts up until the advent of AC motor controller technology. I'd guess the owner had $20-25k in the conversion, but the fuel usage had dropped from something like 20-25 GPH to 2 or 3. The sailboat was a larger, mostly custom, offshore boat - maybe a 42 Gossard - and had been built for the electric power. There was an access hatch in the cabin top to let you R&R the 1800 pound 80 volt battery (also used in fork trucks and costing about $5k)that was nestled in a well in the keel under the cabin sole. Electric motors - particularly AC motors - are much more efficient at 72 to 120 volts. The power was by Solomon's Wheel and used two 7 pancake electric motors and AC electric controllers. The motors are only about 7 hp each, but an AC electric motor can produce max torque at I RPM, and it is torque that spins a big boat prop, not horsepower. Also, when sailing, the props continue to spin and drive the motors as alternators that recharge the battery. All in all it was a nice installation that gave almost infinite range and should need very little maintenance except for the battery. But, boy, I expect it was expensive compared to a 50 ho diesel installation. Rick Brass Washington, NC -----Original Message----- From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Della Barba, Joe Sent: Friday, March 01, 2013 9:49 AM To: cnc-list@cnc-list.com Subject: Re: Stus-List PROP SIZE Unless you just need to move a mile or two in and out of a slip, and electric engine WILL NOT WORK FOR YOU. You would literally need THOUSANDS of pounds of batteries to equal the power and range of 10 gallons of diesel. There are plenty of diesel-electric boats around including tugs and cruise ships, but for a sailboat it makes not much sense. Joe Della Barba Coquina -----Original Message----- From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Alex Giannelia Sent: Thursday, February 28, 2013 1:57 PM To: cnc-list@cnc-list.com Subject: Stus-List PROP SIZE I know the list must have dealt with this extensively, so pardon the re-hash. I am replacing my diesel engine and possibly with an electric and want to get a handle on prop size. Is there a formula where you enter the displacement, DWL, RPM and HP and get a prop diameter and pitch? ALEX GIANNELIA Phone (416) 203-9858 Fax (416) 203-9843 Cell (416) 529-0070 _______________________________________________ This List is provided by the C&C Photo Album http://www.cncphotoalbum.com CnC-List@cnc-list.com
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