propshaft driving an alternator , when sailing

[rgleason]

The idea of an adjustible bicycle chain geer is appealing, would it be noisier and be able to take the rpm?
Those big flat low rpm leather belts for farm and manufacturing had a metal connection. I suppose that type would be to noisey. Thump thump.

[rgleason]

How many amps at low epm ?

[john61ct]

Eco-Tech is a top notch alt maker for this use case

designed for high output 24x7 at 1800 rpm

Also Zena?

Of course very much drifted away here from "tiny" standalone DC gennies.

[billknny]

This is getting silly... with a lot of keyboard engineering by people who have never seen such a device and don't really understand how alternators work.

First, there is NO need for an elaborate idler pulley or other mechanical means for disengaging the alternator. Simply shut off the field current and the alternator will happily spin freely without generating or consuming power. You can do this automatically with a relay when the engine is running, or with a manual switch. One brand of boat that came equipped with these as a factory option used the same key (in a different switch) to turn off the shaft alternator as used to turn on the engine so you would have to TRY to run them both at the same time.

The magic number of "5.5 knots" being thrown around as the speed when the generation starts is not at all accurate. The speed at which power generation starts depends totally on the pitch of the prop. How much power you can extract from the shaft depends on the pitch, and the projected area of the prop, and the boat speed.

In a well designed system, you can expect to start to generate usable power at about 1/2 the speed your boat motors at.

All other things being equal, the power available does rise with the cube of the boat speed. So faster is WAY better. (4 knots can generate 8 times as much power as 2 knots, for example.)

A tiny two blade propeller is not the first choice here. A prop with a large projected area and three blades is needed for optimum efficiency.

You can even use feathering props. If you stop a Max-Prop (for example) while motoring in reverse, the blades lock and will not feather. They can then drive an alternator.

These are almost always installed with internally regulated alternators. This is never going to be the only charging source on the boat. In the real world all you are doing is maintaining the batteries at float voltage to balance the consumption of autopilots, lights, fridges, etc. It does not need the expense of a three stage charge controller. So remember, and this is IMPORTANT, when sizing the alternator, you are not using it to charge your batteries from empty, you are using it to generate as much power as you use in real time underway. A 30 or 50 amp unit is more than enough.

Be sure that your transmission can tolerate spinning in neutral.

Yes, it will slow the boat down--trivially. More than if the same prop was free wheeling, and less than if the blades were locked. You will not notice the difference. At very low speeds, when you really care about drag, the power consumed from the water into electrical power is minimal.

If your boat consumes more power underway than at anchor (most do) a shaft alternator can be a useful way of making up the difference. On many boats with rail mounted solar panels they are (or should be!) stowed when sailing. When sailing downwind, wind generators are close to worthless. And if sailing is good enough to generate power from a shaft alternator, who the heck wants to listen to an engine run to charge batteries???

[cal40john]

Originally Posted by 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.

Can you clarify the pictures and ratios for me? It looks to me that that pulley on the prop shaft is close to but a little larger than the 7 cm pulley on the alternator. If I estimate the prop shaft pulley to be 10 cm, then this ratio is 7:10 or 0.7:1. When you put the 5 cm pulley on the alternator, then the ratio could be 1:2 still assuming my guess of 10 cm for the prop shaft pulley.

[a64pilot]

Originally Posted by atoll

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

I think you may be missing a zero there, some may can go as high as 25,000 RPM.
I’m agreeing with your point, your not getting close to the upper limit driving off of a prop shaft.
Most automotive ones though need about 6000 RPM to achieve max output and can go as high as 18,000.
Us little boat guys underdrive our engine alternators, we would get more output and they would run cooler if we turned up the RPM.

If it were me, I’d get a single wire Delco automotive generator, internally regulated to 14VDC, no reg required and no need to disconnect, just connect the single wire to the bank, and your done.
I’ve put too many on farm equipment, all you do is connect the one wire is all, no switch or anything else.
I agree though, make the shaft pulley as big as you can, your not over-speeding an alternator, I’d think about ten to one is about all you could get and twenty to one would be better.
Likely take an idler shaft and two belts to get it fast enough to work well.

[a64pilot]

One question though, alternators have brushes correct?
It’s not usually good to turn a brushed component backwards as the brushes wear faster. It’s obviously done, look at a reversible drill, but I think I’d still look at a way to disengage when motor was running, cause the the alternator is turning backwards

[rgleason]

Billknny
You apparently have not read the full thread. You add some excellent points, but the comment about silly keyboard engineering is unwarranted.

I like this point in particulat
"Be sure that your transmission can tolerate spinning in neutral."
This is a real problem for most Yanmar transmissions I believe.

Also I like the fact that you are advising the shaft alt can be 30a because it is intended to provide a good reliable power source to make up for power usage.

The earlier thread "tiny alternator" has changed but the idea of tiny is still there. Rather than running your main engine for hours, why not have a reasonably light & quiet diesel or water/prop power source of with alt that can charge at 14.6-14.7v max and 4a-20a for the many hours required to bring FLA from soc85% to soc98%, saving fuel and main engine time.

Using shaft power seems to be is a perfectly reasonable approach and any information available about that gives us a better idea of the issues and power to be gained, seems to me to be appropriate.

[foggysail]

There are no free lunches. If such a scheme produced any power at all, it came from slowing the boat’s speed during sail or from added fuel consumption if under power.

[a64pilot]

Originally Posted by foggysail

There are no free lunches. If such a scheme produced any power at all, it came from slowing the boat’s speed during sail or from added fuel consumption if under power.

Of course it does.
However 1 Hp is close to 750 W. If you extract 10 amps at 14 V, that is only about 140 W or about .2 HP.
Of course nothing is 100% efficient, but if you more than double the power extracted a little your at .5 HP.
It’s more significant than you may think, likely as much difference as there is between a folding and non folding prop.
You have to determine is its worth slowing the boat that much or not.

Most spend thousands of $$$ for a feathering or folding prop and think it’s not, and a $1,000 Honda can supply over 100 amps at battery voltages.
You decide if it’s worth it or not, most want the speed and buy a Honda, some don’t.

[atoll]

Originally Posted by cal40john

Can you clarify the pictures and ratios for me? It looks to me that that pulley on the prop shaft is close to but a little larger than the 7 cm pulley on the alternator. If I estimate the prop shaft pulley to be 10 cm, then this ratio is 7:10 or 0.7:1. When you put the 5 cm pulley on the alternator, then the ratio could be 1:2 still assuming my guess of 10 cm for the prop shaft pulley.

the exact size if i remember correctly was about a 23cm inside diameter on the prop shaft pulley
and after i changed the pulley on the alternator i used a pulley with an inside diameter of 5cm.
giving a ratio of 1:4.6

this resulted in the alternator kicking in at a boat speed of 5.5 knots.
a switch between the grey field wire and positive battery cable allowed the unit to be switched on and off.

this was a real world ,actual installation i built for a boat i delivered from mallorca to the canary islands.
unfortunatly we never had wind strong enough untill the last 24 hours whilst i was onboard to fully test the unit.

but the owner reported that it supplied all their power needs when he crossed the atlantic later that year

but i would suggest at least a 1:6 ratio would have worked better at lower speeds as the alternator will not produce any power untill it revs above 1000 rpm

[bailsout]

What about a generator instead of an alternator. Less spin resistance and rpm need? Or more?

[billknny]

Originally Posted by foggysail

There are no free lunches. If such a scheme produced any power at all, it came from slowing the boat’s speed during sail or from added fuel consumption if under power.

Of course. The real question is if you would notice the difference.

When you are going upwind, ALL the power you get from your wind generator comes from added drag, slowing the boat down in exactly the same way, as you say no free lunch. Why do people never complain about THAT??? Becasue it is too small to notice at speeds where you are generating real power.

[billknny]

Originally Posted by a64pilot

One question though, alternators have brushes correct?
It’s not usually good to turn a brushed component backwards as the brushes wear faster. It’s obviously done, look at a reversible drill, but I think I’d still look at a way to disengage when motor was running, cause the the alternator is turning backwards

What controls the rotational direction of alternators is the fan. All alternators I know of run either direction from a mechanical or electrical standpoint, IF they have a symmetrical cooling fan. Some (usually high output) models have a "handed" cooling fan, and only work one way.

While alternators DO have brushes, the brushes only carry the field current of a couple amps, so wear is negligible because there is little or no arcing. Brush were under no load conditions would not be significant.

[tml]

Max Prop used to mention in their literature that it would work with a shaft driven alternator. If you stop the engine in reverse the Max Prop will spin in reverse driving the alternator with the engine off.
Stop the engine in forward the Max Prop feathers as normal and the shaft and prop don't turn.
Never saw one hooked up but always seemed an elegant solution.
Tom