Reverse prop calculations

I didn't realize leather doesn't burn.

Keep a bucket of water handy.

 

Leather will start to stink, but it will never catch fire.
I will have cooling water available and will always have somebody on deck who can stop the boat while under sail. Stopping the boat will also stop the shaft from rotating.

Thanks for all the advice and concerns guys, I appreciate that!

 

There used to be special shaft alternators made that would produce some amps at 300 or so RPM. E Bay? Google?

A car alt is useless unless spun rather rapidly.

FF

 

That's right, those run at considerably lower speeds, but have problems dealing with higher speeds when the engine runs. i do not want to dis connect the alternator when running at power. Our shaft RPM changes from 250 to 950 RPM.

Another problem is that I have a 24 volts system on board and that the 24 volts version is extremely expensive in low rpm charge version.

 

"I'm thinking of a wooden lever of a meter long for example that I will clamp around the propeller shaft and that has an adjustable slip by means of a bolt clamp. This way I can measure the resistance at different shaft speeds and different boat speeds, plot this in a graph and I see what is happening."

This is a good technique for setting a 2 blade prop brake behind deadwood on a sail boat.

But as a power source,

There is a VAST difference between a stopped prop and in windmilling one in terms of drag.

Ask any prop pilot why engines are feathered when they are shut down in flight.

The spinning prop absorbs power from the entire projected area , and as the blades are efficient while spinning (not stalled) the power and produced drag may be 3-10 times higher.

Mostly depending on the % of free speed the prop reaches.