Wooden boat has a write up on an electric boat repower sept/oct 2013 - SailNet Community
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Wooden boat has a write up on an electric boat repower sept/oct 2013

I’ve been very interested in electric powered yachts for some years.
They sound great in just about every way. They are known for being quiet having full torque at low rpm etc.

The big problem of course is range. One guy summed it up well. He said that when he left the dock he had the equivalent of of a gallon of gas with him.

Solar, wind, Honda generators do not generate sufficient amps for long enough to solve the range problem. The real solution of course is a diesel generator. This seems a little silly of course because of you are going to put in a diesel anyway you might just as well have it drive the propeller directly.

Big catamarians are an exception of course. You can replace two diesel engines plus a genset with a larger genset and actually maybe even save some money and maintenance and give the owners lots of power for toys.

The following are the specifics and some of my comments on the repowering of Vagabond, a 1909 wooden launch that originally had a 30 HP engine that was replaced with a 40 HP engine in 1923.

The system they installed included:
  • Elco 35Kw (equivalent to 70-hp) electric motor
  • 6-kW Kohler 1,800 rpm diesel generator
  • 8 Odysssey PC1800 batteries (214 amp-hour)
  • Charger that would work from shore power or genset putting out 25 amps at 110 volts
During sea trials they found that:
At 600 rpm they would go 4.4 knots and the generator could keep up and not drain the batteries. They had a 12 gallon tank and only used 0.3 gallons per hour. So they had a range with diesel of about 48 hours with a 20% fuel reserve at 600 rpm.

They can do near hull speed on batteries alone but the range would then be only 3 hours the generator can not keep up the the battery drain at that speed.
They figure they have a range of maybe 5 hours at 4.4 knots under battery alone.
Obviously a larger generator would change things but they selected the 6kw unit because it fit well in the boat.

My observations are the following.
The weight ended up being very close even with the batteries. (They decided not to go lithium due to cost and risk)
The cost if you are considering a genset anyway is probably still higher but maybe not that much higher.
I’m thinking that placement and sound control of a genset may be better than a diesel primary motor.

The thing that really struck me was the 0.3 gallons cost to run the genset. I’m guessing that pushing that boat at 4.4 knots would take any regular engine about a gallon an hour.
For someone at a dock using only a tank per year this wouldn't save much. But someone going from NYC to Miami and back it could be huge.

So the with .7 gallons per hour running the genset saved plus even more saved when plugged into the dock plus quiet motoring for 3-5 hours at a time. Super control at low speeds this might not be as stupid an idea as it sounds at first.

And as a freebe you get almost unlimited power for either 110v or 12v toys.

Obviously if you only plan on replacing your current shaft anchor with a new diesel and have no ambitions of putting in a genset this idea is going to cost a lot more. But if you are thinking of a genset anyway a slightly bigger genset plus electric drive motor and some batteries might still be more expensive but would come with some significant advantages including the maintenance of only one diesel.

It is certainly a premium solution but very interesting.
I'm figuring 16k for the motor (I have no idea why this motor is so expensive), 7k for the genset, 6k for batteries and maybe 2k for charger, I'm just guessing.
Lots of expensive switches and wire and labor too.
Over 31k in parts

I'm also aware that the engineering is not trivial. This guy had some of the best people in the planet work it out for him. To get the most from an electric drive you need to swing a big prop for example and modifying some boats to use a bigger prop may be expensive or impossible.

The lesson from the Icarus story is not about human failing.
It is a lesson about the limitations of wax as an adhesive.
If you have an engineering problem solve it.

Last edited by davidpm; 08-20-2013 at 11:46 PM.
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