Pondering power!

[Allan]

Pondering power!!!

In a few years I may buy a small long keeled yacht to indulge my liking for medium to long single handed trips. I've see lots of suitable boats around, most of them have either no engine or a knackered engine. Mostly that's not a problem, I'm happy to stick an outboard on the back. One thing I would lose is the charging.
It would be simple to use the original prop to drive an alternator via a belt whilst sailing. Having a belt tensioner which could be slackened to disconnect it when not required would probably help too.
Anyone have any thoughts? (Apart from me having too much time on my hands!)
Allan

 

[pvb]

It would be simple to use the original prop to drive an alternator via a belt whilst sailing. Having a belt tensioner which could be slackened to disconnect it when not required would probably help too.
Anyone have any thoughts? (Apart from me having too much time on my hands!)

Not terribly practical, I'm afraid.

 

[stevie69p]

Won't turn an alternator fast enough. Plus... alternators are pretty hard to turn when under load.

Solar panels and reducing as much power consumption as possible, via LED lighting, etc., would be a more practical solution... IMO

 

[Allan]

Won't turn an alternator fast enough. Plus... alternators are pretty hard to turn when under load.

Solar panels and reducing as much power consumption as possible, via LED lighting, etc., would be a more practical solution... IMO

I agree 100% with the second part, on our present boat we've done exactly that.
The speed factor is just a matter of gearing so that doesn't bother me. That only leaves the difficulty of turning, which is surely dependant on the current draw?
If you look at something like the Watt and Sea unit, they claim 300watts (24amps at 12v) from very small propeller. I'm thinking of maybe 50watt.
Allan

 

[lw395]

The problem with the 'original prop' will be that it's running in a fairly high friction P bracket and stern gland, even the oil seal on the gearbox will be significant in the context of the few hundred watts available. If the gearbox is still there churning oil, that's likely even more power.
This is why people have used towed impellers for power generation.

 

[Allan]

The problem with the 'original prop' will be that it's running in a fairly high friction P bracket and stern gland, even the oil seal on the gearbox will be significant in the context of the few hundred watts available. If the gearbox is still there churning oil, that's likely even more power.
This is why people have used towed impellers for power generation.

I don't think a gearbox would be required. Just a relatively large pulley on the end of a shaft with a couple of bearings. This could be driven via a flexible coupling to the original prop shaft. I could build it for a couple of hundred quid.
Allan

 

[duncan99210]

OK, we've all watched (or heard) the prop freewheeling when sailing. You will be able to harness that energy but at a cost in performance. First off, you already know that you'll need to spin a standard automotive a fairly high speed, so you'll have to have some gearing of some sort in place to step up the revs of the prop shaft to the speed needed for the alternator. Then you need to factor in the mechanical losses caused by the cutlass bearing, shaft seal and any other bearings. Finally, you'll need to bear in mind the loss in water speed that'll be caused by the drag of the prop. All of the above will be things you'll need to factor in to the overall equation.
If you're only looking for a means of generating electricity underway, then I suspect that there are probably easier and cheaper ways of sorting the problem. If you look at things like Watt and Sea unit one of the things to bear in mind is that part of the reason for the high cost of the unit is that the alternator is designed to produce current at much lower rpm range than a standard alternator. The output is also unregulated, enabling it to produce electricity over a much wider speed range, unlike an car alternator. Point I'm trying to make is that comparing purpose designed units like the Watt and Sea with an propeller driven car alternator is like comparing apples and pears.
Before going down the route of trying to convert a conventional propeller to driving a car alternator I'd advise looking carefully at the alternatives such as the Watt and Sea unit or solar or wind. Also look at this site http://www.hybrid-marine.co.uk/ where they're building hybrid regenerative systems to get a feeling for where people are heading.

 

[Allan]

Many thanks for the quite comprehensive reply. A lot better than "Not terribly practical, I'm afraid.".
A couple of things got me thinking of this, one of them was when I realigned the engine on Brilliant I was surprised how freely the shaft spun. Although it is a Sadler it has a Beneteau type bearing and a Volvo seal. The layout I have in mind is similar to the way Lynch mount their electric motors, which also work as generators.
The Watt and Sea are good units but starting at 3500euro+ are not a consideration.
This is only a hypothetical question at present.
Allan

 

[Kelpie]

Everyone seems to be assuming that you'd use a car alternator and then warning about the power need to turn these.
Surely motorbikes also have alternators, which are much smaller?
For gearing, you'd just need a belt drive with appropriate pulleys.

 

[duncan99210]

Everyone seems to be assuming that you'd use a car alternator and then warning about the power need to turn these.
Surely motorbikes also have alternators, which are much smaller?
For gearing, you'd just need a belt drive with appropriate pulleys.

Motorcycle alternators are not separate belt driven items like car ones. They are generally fitted to the end of the crank shaft or one of the gear shafts and directly driven. Remember that they are also designed with significantly lower outputs than car ones as the electricity demands of a motorbike are much lower than a car.

 

[Allan]

Motorcycle alternators are not separate belt driven items like car ones. They are generally fitted to the end of the crank shaft or one of the gear shafts and directly driven. Remember that they are also designed with significantly lower outputs than car ones as the electricity demands of a motorbike are much lower than a car.

Very good point, thank you. I'll look into them. I had thought of an old fashioned dynamo as an option to reduce the gearing. A motorcycle alternator may take the load down to an acceptable level.
Allan

 

[duncan99210]

Very good point, thank you. I'll look into them. I had thought of an old fashioned dynamo as an option to reduce the gearing. A motorcycle alternator may take the load down to an acceptable level.
Allan

Hmmm, a motorcycle alternator will still need gearing, as it's designed to spin at the same speeds as a motorcycle engine to produce it's output: in other words somewhere between 5000 - 7000 rpm. Remember that a boat diesel runs at say 2000 rpm and generally has a reduction gearbox that drops the propellor rpm down to less than half that figure, so 2000 engine rpm equals 1000 prop rpm. So to use a conventional alternator will mean you need to use gearing to replicate the 2000 rpm you were getting from the engine if you're going to get a conventional alternator to work. Not impossible but you'll need to think it through carefully.
A dynamo is inherently less effective as a means of generating electricity: that's why they've been superseded by alternators. Spinning a dynamo will take the same amount of energy as an alternator, still needs to be spun at a fairly high speed and won't produce as much electricity as an alternator for the same energy input.

 

[Allan]

Hmmm, a motorcycle alternator will still need gearing, as it's designed to spin at the same speeds as a motorcycle engine to produce it's output: in other words somewhere between 5000 - 7000 rpm. Remember that a boat diesel runs at say 2000 rpm and generally has a reduction gearbox that drops the propellor rpm down to less than half that figure, so 2000 engine rpm equals 1000 prop rpm. So to use a conventional alternator will mean you need to use gearing to replicate the 2000 rpm you were getting from the engine if you're going to get a conventional alternator to work. Not impossible but you'll need to think it through carefully.
A dynamo is inherently less effective as a means of generating electricity: that's why they've been superseded by alternators. Spinning a dynamo will take the same amount of energy as an alternator, still needs to be spun at a fairly high speed and won't produce as much electricity as an alternator for the same energy input.

From dim and distant memory, the pulley on a dynamo was much larger than the alternator that replaced it. As a teenager I fitted an alternator to my Cooper S, I took some persuading that the pulley was the right size! It was only 40 odd years ago!
Allan

 

[jiris]

From dim and distant memory, the pulley on a dynamo was much larger than the alternator that replaced it. As a teenager I fitted an alternator to my Cooper S, I took some persuading that the pulley was the right size! It was only 40 odd years ago!
Allan

Don't get too focused on the revs. That's a a problem than can be solved. What can't be solved is the small amount of energy the prop is able to deliver. Your prop works efficiently at the revs delivered by the engine - quite high. While turned by the water when sailing, it operates well below its optimal revs and delivers very little power [W], so, as a turbine it appears to be pathetically inefficient. I don't want to play with guesstimates, but we are not talking 100s of W in case of a small yacht. Now deduct losses in bearings, gearing (if necessary) etc - and you are left with a cumbersome gizmo without any practical value. The towed generators have impellers tuned to the low speed water flow and operate with much higher efficiency. Still, you have to live with the fact they do slow the boat down and they are generally accident waiting to happen.

 

[Iliade]

Bicycle hub dynamo - Yes the power would be risible, but hey?

Chains are generally more efficient than belts, especially when you wish to limit the side load on the shaft. Of course you could just feed the shaft into a little gearbox specified to suit the dynamo of your choosing and do away with chains & belts altogether.

Add another in series (might be complicated as would require a non-conducting structure) or an inverter if the voltage isn't high enough for your taste. Actually a simple transformer would work as you could rectify the output after stepping it up to 12V.

With LED lighting, gas refrigerator and only occasional use of the plotter you just might get away with it...

https://www.bike-discount.de/en/buy..._PJTtGsxz77mhotaBrDKi2t2SwfgX6sEaAlCXEALw_wcB

 

[solent clown]

if you want a generator with a built in gearbox cheaply to experiment, get yourself a cheap 12v battery drill, connect it's chuck to the shaft, and you will, if the prop will turn it, generate some power... That could be your stage one test

 

[macd]

Allan: forget automotive-type alternators: too many revs required. You idea may have some mileage with a small permanent-magnet alternator, which can generate useful power at much lower rpm. The most basic ones are very cheap...handy if you're going to be experimenting. Google will find lots of them.

 

[doug748]

An old pensioned off windgen might fill the bill. It might be a lot of effort for limited return as jiris has suggested.

A long keeler commonly has a prop in a small aperture which is not great for propulsion and even worse for your project. You might find something with an offset prop (or one of the Scandinavian designs with prop above the rudder) which should operate in better water and might let you fit a bigger turbine.

 

[Norman_E]

The whole scheme looks like very a bad idea to me. Firstly I would not fancy sailing long passages with only a petrol outboard as an auxiliary, even if I could get one with a long enough shaft for fitting to a long keeled yacht without it being drowned in any swell so that it would not start when needed. Secondly under sail the drag of the original propeller turning an alternator will slow the boat in exchange for little or no more electricity than could be provided by a decent solar panel setup. There are good reasons why working inboard diesel engines are fitted to most cruising yachts.

 

[Allan]

Allan: forget automotive-type alternators: too many revs required. You idea may have some mileage with a small permanent-magnet alternator, which can generate useful power at much lower rpm. The most basic ones are very cheap...handy if you're going to be experimenting. Google will find lots of them.

That's nice and positive. I'll have a look, it's certainly an area that is expanding.
Allan