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

Saturday, June 6, 2009

Re: [Electric Boats] sailboat

Jim,

Here's the back of an envelope estimate:

#1 For 4 hours at 4 mph with 20% reserve, i.e. draining 80% of the energy out of the battery bank (this is the absolute max you should plan on) you need at least a 1.7 kw (2.2 hp) motor and 10 kwh of battery capacity.

#2 For 3 hours at 3 mph with 20% reserve you need an 0.7 kw motor (1 hp) and a 3.6 kwh bank.

watts = volts x amps; kw = watts/1000; kwh = kw x hours. A 100 amp hour 12v battery holds 1.2 kwh of energy.

For example #1 you would want (8) 6v 220AH batteries weighing about 500#
For example #2 you would want (4) 12v 100 AH batteries weighing about 250#

If your boat originally had an inboard, an inboard electric motor can use your existing shaft, stuffing box, strut, etc. You can probably use the existing prop if its not too far on the low-drag-for-sailing end of the spectrum. Bigger props are much more efficient but create lots of drag under sail.

You can experiment with various pulley combos to get the right speed reduction to match the motor to the prop.

A 48v system using a MARS motor (either brushed or brushless) and a 100 amp controller would give you a nice extra margin of speed and, setting up the reduction ratio to draw about 75 amps at wide open throttle would give a good margin of safety for not over heating anything. This would produce about 5 mph wide open and drain the batteries three times as fast.

Don't know why you specified only two batteries - you could do it but the battereries would be too big to lift by one person. I wouldn't want batteries more than 60ish # because more than that and I couldn't install them by myself.

The 3 mph and maybe even 4 mph could be achieved with a regular trolling motor - much cheaper and much much much easier to rig up than an inboard motor. Much less efficient too as they use a small high speed prop (just what you don't want for efficiency). That means you would need to carry more batteries to achieve your range targets.

Another easy to install and efficient system would be a Torqeedo electric outboard - very efficient with its 12 x 10 prop - much better than a trolling motor but a lot more expensive too. Still cheaper than an inboard installation and you can tip it op or even stow it below deck for best sailing performance.

Denny Wolfe
www.wolfEboats.com

PS: Does your 17' boat really weigh 4000#? That's really a lot for such a short boat.

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