Using spinning prop to charge batts under sail

[beej67]

This discussion was already covered at length, somewhere on the forums here. My Search Fu is lacking, though. I seem to recall it going on for pages and pages, arguing about whether freewheeling your shaft was good or bad, whether it produced more or less drag, and whether you could tie an alternator to it.

For what it's worth, a simple conservation of energy relationship can show that braking your prop produces more drag than letting it spin freely, if you're smart in how you set up the math.

Tying an alternator to the shaft will increase that drag somewhat, but not as much as braking it would, and chances are if you're moving through the water with enough force to spin the prop, the drag in any of the 3 discussed configurations (braked, free, turning a generator) isn't significant. I hear the only time feathering props really make a significant difference in speed is in light wind, when your prop shaft probably wouldn't turn anyway.

[chucklesR]

On the other hand, if you are already constrained in speed by hull length and therefore not hurting boat speed - I'd call it free electricity.

[Valiente]

Originally Posted by beej67

I hear the only time feathering props really make a significant difference in speed is in light wind, when your prop shaft probably wouldn't turn anyway.

Slightly off topic, but I noticed a pretty big difference on my IOR-style racer-cruiser when I went from a fixed 12 x 6 two-blade to an 11 x 8 Gori folder. I seemed to accelerate faster and had to alter when I thought the wind speed merited a sail change.

The new passagemaker will shortly go from a 18 x 13 three-blade to a 19 x 15 four-blade feathering VariProp, which I got to accomplish what you mentioned: better light-air performance (important in a relatively undercanvased boat).

I also got it for the ability to tweak its pitch differently in forward and reverse to get the desired performance out of the engine. Basically, I am willing to sacrifice speed for torque, stopping power and "first gear", as I want more thrust at lower speeds out of the same engine as I tend to hit cruise speed at relatively low RPMs. The way I use my engine, it's more for close-quarters maneuvering than for hours of doing seven knots, and the current prop doesn't bite well enough.

I think shaft alternators are a good choice if the tranny supports freewheeling and if the flow is good, but most people would care about the potential speed penalty. A towed generator is designed to work away from the stern in a clean stream of water, and isn't attached to a heavy shaft and coupling. It's inherently more efficient unless the boat's size doesn't permit other solutions.