Re: [Electric Boats] Torqeedo speed prop

 

Chris,

Did they tell you what the effective pitch of the speed prop is? One would expect it to be greater than the 10 inch on their regular prop. In this case it is not surprising that the speed went down on a heavy boat. You probably are not getting to the motors max rpm or you are getting a lot of slip because of hull speed limiting drag. With no slip and the standard 12 x 10 prop the Cruise 4 would deliver (10in x 1250rpm)/1215 = 10.3 kn. A higher pitch would just make the mismatch worse. I remember 50 years ago going to lower and lower pitch props so we could pull two slalom skiers with a 25 hp Johnson outboard. At about 20 mph max, it all matched and we had great fun.

Ned

On Mar 12, 2010, at 4:03 PM, Chris Baker wrote:

 

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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