One possible source of the reaction "But what I really notice is its power in reverse." is simply the material and thickness of the propeller. I know that a plastic prop of the correct material and thickness will actually change effective pitch in reverse. The water pressure bends the tip of the prop such that 2 and even 3 extra pitches may be added to the prop. We have been designing props to do this for years. see http://mysite.
Most prop people are not experienced in the plastic material capabilities and flex of a plastic prop is not mentioned because it would be thought of as a flaw when is reality it can be an advantage if properly designed used.
This change in pitch would explain the increase in speed. This is not usually noticed in the sailing world with geared transmission simply because the forward gear ratio is specified and the reverse ratio which is different is ignored. The a common transmission ratio in a geared transmission 2.2 forward and 3.2 reverse is. this is also one source of reverse handeling problems in sailboats.
larry
--- In electricboats@
>
> I now have one of the so called 'speed' props from Torqeedo, and if
> you are considering a Cruise 4, this prop is worthwhile.
>
> But not for the reason you might expect...
>
> First some background. The Australian Torqeedo distributor, Claude
> Desjardins, has provided me a 'speed prop' for testing on Current
> Sunshine. I understand it was designed for use on light planing hulls
> with a view to getting more top speed for them than the standard prop.
>
> Oddly enough it seems to make little difference to speed - and if
> anything is perhaps a little slower for top speed. Since I've had it
> I have not had the opportunity to test it under still conditions and
> so my tests so far have all been effected by wind. And even just a
> slight breeze can effect the results by .1 or .2 of a knot either
> way. Anyway, the top speed is still around about 6.5 knots.
>
> But what I really notice is its power in reverse. It way better than
> the standard prop. Many times better. And this big improvement in
> reverse is also confirmed by a friend who has two Cruise 4 on his Oram
> 45ft cat. He also notices very little change in speed, but a big
> improvement in reverse.
>
> The standard prop is orange and has very fine blades - the speed prop
> is white and its blades have more area especially towards the tips.
> So this is perhaps why it shows such an improvement in reverse. But I
> expect there are alos other more subtle shape changes as well that are
> helping here.
>
> Reverse on the standard prop can be really unsettling on any boat with
> substantial mass - and this would apply particularly to the sailboats
> of this group. For the original design target for these outboards,
> the small dinghy, reverse is not so important.
>
> But for sailboats manoeuvring in close quarters it can be critical.
>
> If you are thinking of getting a Torqeedo be sure to ask about the
> speed prop.
>
> Cheers
>
> Chris
>
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