using my propeller shaft to charge my batteries

[Tranona]

I don't know why you do not believe they exist and work. Propshaft generators are not common, but plenty of crusing boats have them. 11A at 7 knots in this typical example.


http://www.sailnet.com/forums/miscellaneous/22098-propshaft-alternator.html

Where did I say there were not any? Rather the opposite - I observed that they were nothing new BUT nobody has made them successfully commercially. They are more popular in the US, but you only have to read your own reference to see that what I have said is correct. It is not easy to design, fit and get them to work well.

So my original observation stands - theoretically attractive but not easy in practice. Therefore, as you say, not common.

 

[noelex]

Where did I say there were not any? Rather the opposite .

Here.

In theory yes, in practice no. You cannot get the shaft to spin fast enough to get any significant charge out of an alternator. You can get purpose built water powered generators for towing behind the boat, but they have an effect on speed and output is poor, so only really good for continuous long distance sailing.

You will find many people have tried to harness that so called wasted energy, but none have been successful. It features in some of the hybrid power solutions, but is the bit that does not work. See Niki Perryman's write up of their experiences with it in Classic Boat mag.

 

[Tranona]

If you are ghoing to comment on my posts, please read them all, particularly in this case post 10.

 

[Biscay 36]

Shaft alternator

I fitted a Lucas 15 amp Alternator designed for prop shaft low revs way back. (1994) It cuts in at about 5K and it is very comforting on long passages, it reduces the noise from the spinning prop and pushes about 3 to 5 amps into the batteries, it also boosts the amps input when motoring. A very useful add on and has lasted many thousand miles. No real problems with pulleys and the wiring is simple. Recommended.

 

[ytd]

I fitted a 60A car alternator to the propshaft of a formerly owned boat. I used bicycle components, a big chainwheel welded to the shaft, small rear sprocket on the alternator and a rear derallieur as a chain tensioner. I needed to disconnect the chain when under power because the shaft speed was too high for the chain setup.

The alternator was wired with the field in parallel with the output so no regulator. The setup never gave more than about 5A so I abandoned it after a while. The problem was the low speed of the propshaft once loaded. While it would spin at about 1000rpm at no load it slowed to 300rpm under load and this was too slow to get any meaningful power from the alternator.

I considered using an intermediate shaft to increase the revs but there wasn't enough room above the shaft. I transferred the alternator to the engine making it the 3rd alternator on the engine and got the full 60A from it. Probably quite doable if you had a specially wound alternator that could produce max power at 1500rpm.

 

[Ross D]

This link http://yachtwork.com/report-wind.htm at the bottom of the text gives instructions for rewinding an alternator to make it a permanent magnet motor which will work at much lower revs. So it may be a solution for generating from shaft revs.

 

[CaptainBob]

Reporting back after sitting with strong tides (3/4 kt max I think, log broken) running over our prop.

It didn't budge an inch. No chance of any power generation off it.

Guess it needs more speed, but if 3/4 kt won't budge it I can't see it giving us much at 5 or 6.

Ah well.

 

[fuss]

Reporting back after sitting with strong tides (3/4 kt max I think, log broken) running over our prop.

It didn't budge an inch. No chance of any power generation off it.

Guess it needs more speed, but if 3/4 kt won't budge it I can't see it giving us much at 5 or 6.

Ah well.

Could be true but 5 to 6 knots for a propshaft alternator is much more than 3 to 4.

The problem with propshaft alternators is that many seem to be set up on a hope rather than soundly engineered.

Then other forumites get on the bandwagon and make comments about them not working.

Generally you need the following to ensure satisfaction and production from 4.5kts onwards....
40+ feet of boat
5-6:1 pulley ratio
big prop
hydraulic gb
C bearing in top condition
Clean bottom
the right alternator.... at the very least one that starts generation at no more than 1100 alternator rpm.
If it starts generating at higher revs then forget it.

 

[AntarcticPilot]

Reporting back after sitting with strong tides (3/4 kt max I think, log broken) running over our prop.

It didn't budge an inch. No chance of any power generation off it.

Guess it needs more speed, but if 3/4 kt won't budge it I can't see it giving us much at 5 or 6.

Ah well.

I wouldn't worry too much about this. There is stick/slip friction to overcome; rotating friction is much less than the static friction. It may take a high speed through the water to start the prop rotating, but once it has started, it will keep going at lower speeds. Have you tried starting the engine, then putting it in neutral once the prop is spinning?

By the way, do you mean 3-4 knots or three-quarters of a knot?

 

[fuss]

It didn't budge an inch. No chance of any power generation off it.

Guess it needs more speed, but if 3/4 kt won't budge it I can't see it giving us much at 5 or 6.

Ah well.

The fact that it is not even turning at 3-4 is not a good sign for efficiency... mine always turns from around about 2.5 knots