[Public-List] Alberg 30 Sail Plan

[Kris Coward]

On Tue, Mar 16, 2021 at 02:18:33PM -0400, Mike Meinhold via Public-List wrote:
> Diesel electric is a good technology for ships when it serves the
> arrangements well.  If you are forced into compromises such as a very long
> shaft, or increasing section beam aft to accommodate an engine, then it can
> pay off.  It works well for vessels that use as much electricity for house
> or for weapons as for propulsion. In these cases physically separating the
> energy conversion from the propulsion motor can work well.  In an A30 , you
> simply add weight and complexity to go diesel-electric, unless you
> significantly reduce the requirements for speed and range.   The gains in
> propulsive efficiency will be small - we already have a good match with
> small diesel and gas engines - so for matching peformance, your diesel
> generator will have to generate 16 or so horsepower, and you will need a 16
> hp motor.  You could use a smaller engine combined with batteries , and run
> it longer , but that will get heavy quickly.

Except that a 16hp electric motor doesn't match the performance of a
16hp gas or diesel. Hell, even gas and diesel don't match each other's
performance on the same horsepower, but at least they're a lot closer to
each other than either is to electric. The torque curves are completely
different between electric and internal combustion, and on the internal
combustion side, the shape of the curve means that the engine is only
really putting out its rated power when it's spinning the prop somewhere
around 2-3x as fast as is needed to get the boat to hull speed (hence
the existence of reduction gears as an option on a lot of models of boat
engine -- of course it's just an option because, as anyone who's ever
dropped their car into a lower gear for better acceleration while
passing knows, the best engine speed for power is not the best engine
speed for fuel economy or wear).

Now considering that power is torque * revs, and ICEs hit peak torque at
higher revs, as you dial back those revs to something that's actually
useful to the boat, you're also dialing back the torque, so the rated
power of an ICE tends to run about 3-5x the actual useful power you can
get out of it in the applications we're talking about. On an electric
however, the torque curve tends to be a very close approximation of
input voltage * input current * efficiency rating = torque * revs.
On a well-controlled brushless DC, efficiency ratings of 95% or higher
are not uncommon.

So you don't want to match the electric to the ICE horses to horses;
there's no efficiency band to worry about on the electric, and you don't
have a torque curve working against you. You just want to put in a big
enough electric to be able to spin the prop fast enough to push the boat
through the water as fast as you think you'll ever need to go. If you
anticipate being in enough of a hurry or fighting against strong enough
currents that you want that to be hull speed, you're probably looking at
around 7kW which is roughly 10hp. If you're happy topping out around
4.5-5kts, 5kW (roughly 7hp) is plenty.

The difference in torque curves is the big reason you see so much diesel
electric, not just on ships, but on trains, earth-movers and other
mining equipment, etc. -- all applications where low-end torque is
really important, often in the band of engine speeds where an ICE not
only fails to deliver much torque, it straight-up stalls, even when
unloaded. Of course this doesn't apply to our boats; we accomplish
approximately nothing by spinning our props at 10rpm -- even 100rpm --
but the point remains that on a boat, electric horsepower to ICE
horsepower is an apples to oranges comparison.


Of course none of this addresses range: batteries, generators, how much
they weigh, or where you'd put them -- and that's really the tricky
business with repowering an A30 to electric.

Cheers,
Kris

-- 
Kris Coward					http://unripe.melon.org/
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