propshaft driving an alternator , when sailing

[GarthGregory]

I am sure someone must have tried this before - but your thoughts - A freewheeling propellor / propshaft to drive an alternator . This is more applicable to long distance sailing .
I noticed that my propshaft at 6 knots sailing was spinning fairly quickly .
So motoring at 6 knots requires about 2200 rpm on my Janmar JH 473 (50 Kw) through a Z F Gearbox . Looking at the ratio , it seems to be 2.69 to 1 , does that mean that my propshaft is spinning at 2200 divided 2.69 = 817.8 rpm , so if I put a 2 to 1 pulley to achieve 817.8 x 2 = 1635 rpm , that would produce a low but steady 12 V charge . I could put in a simple belt tensioner system to engage or disengage the alternator or an electrical clutch on the pulley - many options .
1 The assumption that sailing at 6 knots, the propshaft is turning at 817.8 is unlikely and needs to be tested , but it does spin fast when sailing .
2 It certainly generates a lot of power , by holding the shaft in my hand at 6 knots while sailing , I could slow the shaft a little, but I had no prospect of stopping it turning .
3.I confirmed with Z F that leaving the gearbox in neutral and allowing it to spin is perfectly acceptable .
4. I also noticed this forum , that locking a 3 x bladed prop would cost more than 1 x knot in boat speed .
5.There are specialized alternators that produce power at much lower RPM , but probably require proportionately more input power .
Anyone out there tried it and if so your thoughts , before I set out to re-invent the wheel

[a64pilot]

It has been done, not by me, but I’m led to believe the marginal gain just isn’t worth it.
It will slow the boat some of course, cause that power has to come from somewhere.
There is prop slippage of course and being driven backwards the prop is less efficient so it’s not turning as fast as you think.
However with a cheap optical tachometer it ought to be real easy to find out exactly what it’s RPM actually is so you could gear an alternator

[Wotname]

^^ in addition, keep in mind the rpm (as measured when sailing) will be less once you load the prop with the alternator.

The alternator will also be working when motoring so will have to handle the shaft rpm when motoring.

[john61ct]

Get a proper hydro unit if that's what you want to do.

[HankOnthewater]

Many cruising boats in the eighties had such alternator on the prop shaft, but with solar and windpower becoming more common place, those kind of setups became unfashionable.
If you have the room behind the gearbox and your prop is spinning anyway, why not? Do not expect lots of power, but even 5 amps at 5 knots is beneficial.

Is it worth doing? meaning: the setup, machining, getting all the parts, maintenance etc.... dunno, but the first post in this thread made me think of putting an alternator on my propshaft.....
http://www.cruisersforum.com/forums/...rs-205966.html

[HopCar]

Sounds like a fun not terribly expensive experiment. Give it a try and report back.

[a64pilot]

Originally Posted by Wotname

The alternator will also be working when motoring so will have to handle the shaft rpm when motoring.

A simple idler pulley like you use to engage the blades on a lawnmower could fix that.
Incidentally, it’s how the blades were engaged on the little Hughes 269 and 300 Helicopter, so it works well

[rgleason]

How about a split pulley that is bolted to the prop. Then you'll need to get the belt in place around the shaft be unbolting the flange and sliding it back.

I 've read about these installations...seems reasonable. We have no room or I would have done it.

[Jim Cate]

If you were to use a bicycle chain and sprockets one could transmit a decent amount of power and install the chain without having to uncouple the shaft. I suppose that one could even use a gear cluster from a bike and be able to experiment with different drive ratios. Could be fun for a tinkerer, but of course would require a big fixed prop with its drag issues, even when not generating power.

A trolling generator is perhaps an easier solution.

Jim

[atoll]

i think you need a much bigger ratio than 2:1,more like 6:1.
i built a system for a 57 ft boat with an 80 amp 24v standard truck alternator.
the pulley on the shaft was 25cm,with a 5cm pulley on the alternator.
the alternator would only start producing power after the boat reached 5 .5knots.
you also need to have a disconnect switch connected to the field wire so the alternator does not drain the batteries when not in use.

there should be some pictures on CF on another thread about propshaft generators

[GarthGregory]

A Start point might be to establish , how much power a small alternator requires, at say 1500 rpm and its output at that speed . I certainly have plenty of space , but from past experience with alternators, it needs to be properly mounted . Mine would fit , on a mounting plate , bolted to the top of the gearbox , pulley facing aft . My shaft coupling is right at the back end of the gearbox and is a rigid coupling . It is a very simple 4 x bolt coupling . To make a clean elegant solution ,I could machine a new larger flange , that included a V belt pulley of the right size .The Alternator could be engaged or not , by tensioning an idler pulley or by moving the alternator in an arc .
The whole thing depends on the power required versus the output gained , it just seems a pity to have all that ( almost ) free energy going to waste . I already have 3 x 100 watt ( semi -flexible ) Solar panels fitted , out of sight ,to the top of my Bimini . They run both my Fridge and Freezer very happily .

[cal40john]

Dashew's take on it.

Download Offshore Cruising Encyclopedia

https://www.setsail.com/free-books/

Electrical system section
Towing Generators
Prop shaft power take off
Separate charging shaft

[GarthGregory]

Thanks Atoll , for the input , I posted before seeing your post . A question does a 24 V alternator not need more than twice the input power of a 12 Volt unit .

If the alternator only starts to produce power above say 5.5 knots , its use is then fairly limited .To make it worthwhile the alternator needs to produce power ,even if it is limited from say at least 3 knots .

[atoll]

Originally Posted by GarthGregory

Thanks Atoll , for the input , I posted before seeing your post . A question does a 24 V alternator not need more than twice the input power of a 12 Volt unit .

If the alternator only starts to produce power above say 5.5 knots , its use is then fairly limited .To make it worthwhile the alternator needs to produce power ,even if it is limited from say at least 3 knots .

yes a 24 v 80A would be equivalent to a 12v 160A,BUT most off the shelf automotive alternator regulators are "machine sensed" rather than "battery sensed"so would only produce high current for very short periods,untill the battery reaches 13.8 v,then only produce enough current to supply loads.

a marine "battery sensed" alternator is designed to charge batterys and supply loads so would be regulated to a higher voltage aprox 14.4.

the one factor you need to consider is that any standard alternator needs to spin at a minumum of 1000 rpm before it will energise and produce any power at all,shaft horsepower will not be an issue on a boat your size once those minumum revs are reached.

your propshaft spinning will produce about 1/3 the usable horsepower of the boat under power,at the same speed through the water under sail

so i would suggest fitting the biggest diameter pulley on the propshaft that will fit,this will determine at what speed the alternator will start producing power.,5:1 being the absolute minumum.

alternators can spin quite happily at 2500rpm,so there is no upper end limit when it comes to generated RPM from a propellor shaft.

[atoll]

here is the alternator setup,this is with the original larger 7cm pulley on the alternator which i had to change to a smaller 5cm pulley for it to work above 5.5 knots of boat speed giving about a 5:1 ratio,prop was driven by a 120hp engine.