[Public-List] Alberg 30 Sail Plan

[Mike Meinhold]

Kris
That’s a good explanation of the torque band efficiency of ICE vs electric.
  Ships are generally designed for optimum efficiency at one speed, which
is partly a consequence of using ICE engines .  A diesel plus a reduction
gear at design speed will be lighter and  cheaper than an electric motor
and a generator. The gains in efficiency at that one speed will be minor so
the generator will have about the same power as the Diesel.

It still comes down to requirements. You mention 4.5 - 5 knots . I replaced
my Atomic 4 with a Beta 16 . For a 5 knot engine it would be an even
smaller diesel .  There is something to be said for the designers intent to
power the boat  at hull speed

As you also say even that is just theory until you figure out how to
arrange a diesel electric system in an A30.

On Wed, Mar 17, 2021 at 00:38 Kris Coward <kris at melon.org> wrote:

> 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/
> GPG Fingerprint: 2BF3 957D 310A FEEC 4733  830E 21A4 05C7 1FEB 12B3
>