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post #69 of Old 10-17-2012
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Re: Optimal propulsion system


Those numbers are what I remember from an engineer on another forum. I believe he calculated in conversion efficiency, but I could be wrong. But don't forget you also have to calculate in the usable energy portion. Lead acid batteries can only reliably use about 50% of their nominal power, without damaging the cells lifespan.


I actually think diesel/electric is a reasonable way to go. At least under some conditions.

1) there is or needs to be a generator anyway
2) reduced power range is acceptable
3) the size of the house loads correlate reasonably to the propulsion demands (we don't want a 20kw generator running to power one AC)
4) little or no range expected from the batteries
5) propulsion requirements are minimal

In short I think it is possible, but doesn't meet many boats requirements since 3 and 5 are conflicting. The conversion is roughly 1hp=.75kw. So if your boat has a 20hp engine, you would need a 15kw generator to have the same power available at the throttle (ignoring conversion inefficiency). If you have a 40hp engine, which is pretty common on 40is footers you would be in the 30kw range.

Frankly I don't know of many boats that would install a fraction of this size generator for house loads. And it would be massively inefficient to operate it just for house loads. Even if you were willing to sacrifice some available power you would have to make a pretty massive reduction in available power.

The only way electric power is if you can accept a pretty minimal range. Like just in and out of a harbor. For many day sailors this would work fine. But you would also loose the capability to make longer trips under power. I for instance would be fine with this system 99% of the time, but for a week or so a year we take the boat about 100 miles away for a distance race. I don't know that I would be willing to give up the capability to do this even if the rest of the time electric power would be fine.

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