Rshipes - "Probably an over estimation of the power needed."
Yes it is. And should be considered a red line if you will.
My 45' sail boat coasts along nicely at 4 to 5 knots with less than 10 hp. I am basically idling my 65 hp Yanmar and burning less than .5 gallons an hour and charging my batteries and making water etc. at the same time.. Lets say 5 hp or so (3.7 KW).
An electric motor turning a larger diameter prop slower (to take advantage of the torque) can do it more efficiently, maybe 70%? especially if it is optimized for 5 knots. So we are looking at ~2.5 KW.
That would be the equivalent to tying two Honda 2000 Generators together their actual sustained output is closer to 1600 watts : ) and I wouldn't be able to charge the batteries and make water at the same time. At full throttle they each burn 1/4 of a gallon an hour so the fuel burn is .5 gallons an hour. Interesting......
Here is the thing though, when I really need my engine it is for short sustained bursts of power like going through an inlet or up a river against the wind and current, or backing down on the anchor or when I am going into square waves that stop me dead.
My hp requirements for those times are big, probably in the 40 hp range, sometimes for hours at a time and I am still only trying to go 5 knots or so.
If my only need for an engine was in calm flat conditions, I would rather not have an engine. I am patient : )
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Posted by: taladorwood@yahoo.com.au
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