Propshaft power generation using an alternator or magnet generator.

[fuss]

Hi all,
My 20 inch prop turns all the time when sailing and I am thinking of putting a pulley on the propshaft to drive a small alternator or permanent magnet generator, just for charging my 24v batterys.
After doing some research, I see that there are not many people doing this with an alternator or a permanent magnet generator. The main problems seem to be noise, enough revs and regulation.
It seems a good idea to me.... no need to have a wind generator or tow something off the stern in order to keep the batterys topped up!!!
Or am i missing the point?
Has anybody any experience with this that could point me in the right direction and help me make a decision.
thanks
John

 

[Mirelle]

I can remember discussions about this.

I seem to recall that one firm made a propshaft alternator commercially, about 20 years ago, and I came back from a Boat Show full of plans, but all the yacht engineers of my acquaintance warned me off it.

I think that the issues include:

1. Difference in speed of rotation with the engine running or not. Probably the major issue.

2. Wear on the gearbox (academic if you trail the prop anyway)

3. Friction losses.

4. Non-ideal shape of propeller

5. (not least!) Access!

 

[BobOwen]

Re: Propshaft power generation using an alternator or magnet generator

I met a German chap in Malta recently who took delight in showing me his system. He had a small ( Off a Jap mini car) alternator driven by a pulley on his prop shaft. The ratios were about 3 to 1 with the smaller pulley on the shaft. He did say he'd measured the average rpm on the shaft and then geared up to give a sensible output from the alternator

The installation was extremely compact and neat and he said it gave him a regular small charge while sailing and an extra boost while motoring. I didn't get to see or ask about all the details I'm afraid, but he was very enthusiastic about it and was adamant it worked well.

Good luck, with what sounds a great idea!

 

[pappaecho]

I too had similar ideas, but ultimately discounted them due to the wear and tear on both the stern gland and the gearbox bearing.
came to the conclusion that a towed array, such as Ampair, was the way forward, giving something like 0.75 to 1 amp per knot - assuming 24 hour passage at 5 knots something like 80 amps to the service battery is not to be sneezed at

 

[William_H]

Re: Propshaft power generation using an alternator or magnet generator

BobOwen I presume you meant that the small pulley was on the alternator large on the shaft. This then raises the question can you get a big enough pulley on the shaft to turn an alternator fast enough. Then as mirelle says you are in trouble with too much speed when motoring.
You would probably need to re wind the alternator to get it to produce more voltage at lower speed however you could try a 24 v alternator on a 12v system. yes if you could get a permanent magnet alternator /generator it could save field current making it at least not drain when drifting or stoped. My recommendation forget it get a folding propeller. regards olewill

 

[ytd]

Re: Propshaft power generation using an alternator or magnet generator

I tried this in the 80's using a bike chainwheel on the prop shaft, and a small bike cog on the alternator and the deurallier (?) used to keep tension on the chain. It had about a 6:1 ratio. Alternator was a 60A one fom a car with the internal regulator bypassed. Never spun fast enough to deliver more then 5A. The friction from the stern gland and gearbox was too great. I thought about putting a lightweight oil in the gearbox to reduce friction but ended up using the alternator as a second one on the motor.

 

[BobOwen]

Re: Propshaft power generation using an alternator or magnet generator

William - oops, quite right mate, smaller pulley on the alternator. I didn't check his system out in detail at all. He just showed it me while I supped his wine.

It was extremely neat and compact - the alternator was very small and I think he said it was off some small Diahatsu or similar jap mini car. All I can say was the guy insisted it worked well. There may have been an element of pride involved, where he was always going to say that, but my gut feeling was that he was telling it as it was. I'm not saying this worked, only that this chap had done it and claimed it worked well.

It certainly isn't complicated and I don't see that it created any undue strain on bearings. The field drain when stopped is a good point - maybe an adapted aerogen with perm magnets is the way to go? No worries then, when ever it spins enough, you get a charge.

 

[Ric]

I have built such a system on my boat, and would agree with some of the remarks so far.

My boat is a 32 ft Jeanneau, with a two blade prop. I started by fitting a Sigma bike computer, to measure propshaft rpm under power and under sail. If you set a Sigma to a 1667mm wheel diameter, it will measure display rpm/10 (ie if the speed says 30kmh, then rpm is 300). The results were that at 6 knots under sail, the prop turns at 700rpm. At six knots motoring (2500 rpm on the motor, 2:1 gearbox reduction) the prop turns at 1200rpm ish (so you can see that the prop is fairly inefficient when being dragged through the water).

I then fitted the largest pulley I could to the propshaft, and worked out a torque v rpm curve by with a string friction brake attached to a spring balance while sailing.

That allowed me to work out what power of generator I could aim for, and the correct gearing ratio for it. I realised that my boat could only drive a small generator. I ended up fitting the generator out of a broken Rutland wind generator.

I am very pleased with the results. Like the German chap, it gives a small but constant charge - usually about 50Ah per day. That is a fair amount on a small boat - more than my 913 RUtland wind generator gives anyway.

I know the towed generators give much better power because they use a propellor that is more efficient - but they are a hassle to deploy and far harder to recover (limiting their use to long offshore passages) and you can't trail a fishing line.

On bigger boats I have seen far more powerful systems. One 90ft boat I recently sailed had a propshaft alternator that produced 70A continuously at cruise sail speed - easily enough to power all the boats DC systems. It also had a propshaft driven hydraulic pump that could power the boat's autopilot and had the advantage of being more silent than the alternator.

 

[fuss]

Hi Ric,
Thanks for this information, it was very helpful. On my boat i installed a cycle computer and I get 300 rpm at around 5 knots. Could you tell me what alternator you ended up using and what ratio you have from the prop pulley to the alternator pulley. I have measured how much room I have and I have calculated a ratio of around 4.8:1 so I am not sure if this is enough.
Thanks again
John