--- In harryproa@..., "Robert"
<cateran1949@y...> wrote:
> If the electric drive is charging while sailing , do you have a
prop
> only on one end designed to charge while coming and going,
> Robert

Props are a challenge..... of course they have been on the design
table of lots of manufacturers. Coming and going presents the
same problem ever since the screw presented the first
opportunity to move water helically. On a regular outboard with
smaller prop dia and high rpm, cavitation and other efficiency
losses demand optimization for thrust in at least one direction.
The blades are cupped and attack angle/ blade shape optimized
for best performance. Net result is reverse is typically 50% of
forward (if you are lucky to do that well) AND more fun cause
many props are spinning fast plus not aligned level, resulting in
the prop walk phenomenon.

IF we choose the option of a high torque low rpm (1200 max)
electric drive, then prop design makes a different set of
parameters. For high torque/ low rpm we need a large prop with
lots of surface area (3 blade typical). And we are not looking at
high speed say 10 knots max, then we have a scenario where a
tug prop makes sense. Tug props are large, flat blade (not
cupped) and some take advantage of kort nozzles to maximize
thrust behind the prop versus radially away from the prop
(wasted energy). I think that can work with a reasonable degree
of efficiency. The kort nozzel would be double ended, a flare
opening at either end, (forward & reverse thrust) but still have
most of the advantages.

Granted we have NOT optimized for either direction, but my goal
is reasonable efficiency at operating speeds giving the same
thrust no matter which way I choose to go. A more interesting
solution is a voight-schneider cyclonic prop. Those folks are not
ready for commercialization. A few too many small moving
parts, running at higher rpms is not a recipe for success.

Swinging the motor around personally does not satisfy my
sensibility test for practical. Otherwise it would be a common
approach on other vessel types.

Orrrrr you can just accept the status quo of prop design and
recharge/ power at 50% or less efficiency when facing the
suboptimized direction. It can work just not as well. We at least
have a chance of fair performance or better using a larger slow
rotating prop.

Regards,

JT