Paul,
Well, its a boat, so there are going to be trade offs.
First off, yes, a 2 blade prop will have less drag than a 3 blade, but it will also be less efficient in using your limited electric power for motoring.
Using regen by spinning the prop will slow the boat down, but if you're over powered and already pushing past hull speed its probably wasted energy anyway, so using regen can be a good use for that over-power.
A spinning prop, whether its got additional drag from spinning the motor for regen will create more drag than a fixed one in any case (check our Gerr's excellent 'Propeller Handbook.')
Regen is only going to make a difference on a long passage anyway, it just isn't going to make that much power no matter how fast you're going and you've got to be really going before it makes any at all. Regen is a compelling idea, but from what I've read (no personal experience yet), the practical reality is there isn't much energy to be had there.
The only way to figure out what works best for you; which prop, the drag/power/regen/speed relationship; will be to try different configurations on your boat.
If I were day sailing I'd forget about regen, you're not going to get enough back in an afternoon to make it worth bothering, and if speed under sail is more important than motoring efficiency then I'd go with the two-blade too, it really will cut the drag over a three blade.
If you've got multi-day passages planned and motoring is an important consideration, then the three blade is going to give you more miles and more thrust and you might even power your auto pilot and instruments with regen.
With any luck some other group members with some practical experience will weigh in.
David
Hi everyone,
I have what may be a simple question about regeneration under sail for little boats like my Tartan 27 (LWL=21.42).
In conditions where the sails have the power to push you at hull speed (6 kts for this boat by my calc), would the excess power turn the prop and keep the boat at hull speed, or slow it down?
Any comments on the real practicality of sail-powered regen in a boat of this size are welcome...seems to me that if one only ever goes 6 knots, every tenth of a knot is precious, and it may be wiser to retain the current drag-minimizing prop configuration, which is a 2-blade which can be locked vertical in the aperture behind the keel.
Hope you can help, Paul
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