The Spinning Prop


When we’re sailing, the prop is left spinning — freewheeling. It appears that either some transmissions say “never let this happen” or sailors have determined through the rumor mill that this is bad. The idea is that the transmission has no pressure to circulate fluid and this causes wear.

The Velvet Drive manual says this:

There’s no damage from letting the prop spin.

(In spite of this, Red Ranger has a Sarns PropLock brake. Similar to the existing ShaftLok brake. We have the brake shoe backed off far enough that it does nothing.)

Can the freewheeling be put to use?

Start here: They talk about a Valeo alternator that works are low RPM’s.  

Some folks call this “propellor regeneration,” a “shaft generator,” or a “prop shaft alternator.”  From what I can gather, it’s not terribly efficient because of some hydrodynamic concerns regarding pushing vs. being pulled. 

But. It can turn propellor motion into electrical power rather than uselessly heating up the transmission.

Here are some more pictures of an installation.

Some Design Considerations

It appears that the issues involve (1) mounting the alternator with a proper belt tensioning arm, (2) fitting an appropriately-sized wheel onto the shaft, (3) supplying a switchable voltage to activate the field coils. A self-exciting alternator is a permanent drag; a switchable alternator can be disconnected when trying to drift in light airs. It also needs to be disconnected when running under power: there’s no reason to have the extra alternator running when under engine power, it’s just more drag. 

Since alternator fans are unidirectional, the fan will only properly cool the alternator in one direction. We only want this alternator to work when the shaft is spinning in reverse. When the shaft is moving forward we have to be sure the exciter circuit is off or we’ll bake the alternator. We might want to rig some kind of safety interlock based on the transmission’s neutral safety switch. We should only be able to power the alternator’s exciter when the shifter is in neutral.

How big a wheel for the alternator itself?

Generally, the alternator RPM’s need to be over 1,000 to generate a reasonable level of output. A 12” pitch propellor will (ideally) turn 1 revolution when dragged through 12” of water. Banging along at 6 knots (437,480 inches per hr) should turn the shaft at something close to 600 RPM.

Sanity Check. We use a 2:1 transmission, so that means 1200 engine RPM’s should yield 6 kt. We often have to dial the RPM's it up a bit higher to overcome friction and other losses. So this 600 RPM at the shaft seems right.

If we use 5:1 ratio of wheels, we can get good voltage all the way down to 2 knots. For 1 knot, we’d need 10:1 wheels. Below 1 kt, we might want  to turn off the exciter circuit: we’re barely moving as it is. If we put a 12.5″ wheel on the alternator, we can just put the belt on the 1.25″ shaft itself — no wheel needed.

When we size an alternator for an engine, we have to consider the number of horsepower involved. There’s a direct relationship between power generated and horsepower consumed. Volts × Amps = Watts. 746 Watts = 1 HP. If we want to produce 100A at nearly 15V, we’re putting an additional 2 HP load on our engine.

When dragging the prop through the water, we’re losing 2 HP of forward power. A Whitby (23,000 lb) sailing at 2 kt (202 ft/min) is creating about 140 HP of power. The 2 HP alternator load is a negligible 1.4%.


It’s only an 80 HP engine. Where does this “extra” power come from? AFAIK, the answer is inertia. Our 80 HP engine can accelerate us to a comfy 6 kt (420 HP overall) because water is essentially frictionless. Water is, however, relatively adhesive (“wet”) and this tends to hold us back. More importantly, water is heavy: we must displace about 23,000 pounds of water up (creating wake) so that we can go forward into the space the water used to occupy. Once we’re at a cruising speed, we are adding 80 HP to push water aside so that we maintain 420 HP overall. We can watch our 340 HP of built-up inertia bleed away when we kill the engine.

  © Steven Lott 2016