Energy recovery from freewheeling prop

[markxengineerin]

I have always tolerated the noise of my propeller shaft spinning while sailing. Finally today I decided to stop it, used a piece of wood jammed against the engine coupling OD as a brake, then once it was stopped, inserted a pry bar lock it. Noise gone.

But what was interesting was the difficulty in stopping the shaft. This is a 37' boat doing 6 knots, a 3 blade fixed prop. The energy was enough to start machining away/possibly burning the wood "brake". As another datapoint, the whole transmission heats up after a long sail, warm to the touch, when I let the prop freewheel.

Has anyone run the numbers on how much energy might be recovered using an alternator? The shaft RPM does not look like much, so maybe a belt or chain drive would be needed. Something that would disconnect the engine/transmission so that as much energy as possible comes out as electricity.

What about designs that would be the opposite of a feathering/folding prop? Something that was "streamlined" during normal mode, then fans out to an "ideal regeneration" shape on demand? It seems this would cost very little in speed, nothing in some cases (if hull speed is reached already), and maybe be a good extra power source, like a towed hydrogenerator but less hassle, more elegant.

Solar is not really enough on a monohull when cruising in winter/cloudy areas, and more power is always nice.

[donradcliffe]

There are successful examples of boats which put an alternator on the driveshaft to generate electricity. From memory, they were all larger (50-60 ft) boats, and used a belt or chain drive to double or triple the shaft speed.

I used a Ferris 200 watt towed generator on passages. It had a real output of 120 watts, but was high maintenance. Typical problems were sargasso weed and the line jumping out of the water and kinking. However, when it was running well, it powered the whole boat. Boat speed loss was a strong function of windspeed. At hull speed of 7.5 knots the drag was less than half a knot, but beating in light air it was ore like 1.5 knots.

I also experimented with a pelton wheel driven alternator which ran in an island waterfall. It was based on a cheap japanese alternator. It was belt driven, which was high friction. It lacked torque, so I had to cut back the field current to keep it from stalling.

[senormechanico]

Alternately, you can just leave the transmission in reverse.

[markxengineerin]

For whatever reason, Yanmar says not to keep it in gear. I recall seeing a tech bulletin about this. Transmission is KANZAKI, I forget the number off-hand but possibly KM4A.
Does anyone know why this is the official advice/ have had success ignoring it? Certainly easier than the pry bar method.

@donradcliff, 120 watts continuous 24 hours a day on passage would be great.

[capt jgw]

Originally Posted by markxengineerin

For whatever reason, Yanmar says not to keep it in gear. I recall seeing a tech bulletin about this. Transmission is KANZAKI, I forget the number off-hand but possibly KM4A.
Does anyone know why this is the official advice/ have had success ignoring it? Certainly easier than the pry bar method.

@donradcliff, 120 watts continuous 24 hours a day on passage would be great.

Do whatever the engine/trans manual recommends. Some say to put it in reverse, others say leave it in neutral and let it spin. I don't know of any that recommend forward.
Connecting an alternator could work, but the more load on the alt the harder it is to turn. It might be a more effective shaft brake than a charge source. Then again it would also charge with the engine running in gear, supplementing the normal alt.

[wholybee]

Without running any numbers, the prop is significantly larger than what is on a hydrogenator like a watt&sea, so the power available will be much greater than that.

But you need to get the gear ratio correct. On the front of the engine, the crankshaft pulley is larger than the alternator pully. On the back of the engine, the propshaft will spin much slower than the engine. So, you would probably need a series of pulleys to get the speed back up.

[Wotname]

Originally Posted by markxengineerin

For whatever reason, Yanmar says not to keep it in gear. I recall seeing a tech bulletin about this. Transmission is KANZAKI, I forget the number off-hand but possibly KM4A.
Does anyone know why this is the official advice/ have had success ignoring it? Certainly easier than the pry bar method.

@donradcliff, 120 watts continuous 24 hours a day on passage would be great.

AFAIK, it causes problems for the cone clutch. The cone clutch is designed to work with the torque supplied from the engine side. When sailing in gear (engine off), the torque is being generated from the prop side of the clutch which in effect is trying to de-clutch the cones.

Disclaimer - this information was not generated by Yanmar so it could be inaccurate.

[skipperpete]

Wottie’s 100% correct about the gearbox, not only Yanmar , it’s surprising how many Hurths damage the ahead clutch pack and thrust washer from sailing with the gearbox in ahead and the engine shut down.
Regarding the power available from a spinning propshaft, it’s not much unless you’re sailing fast, I tried this on my first yacht with very little success, maybe 10 amps at best on what was basically a slow heavy motor sailer despite a very large variable pitch propeller (Hundested) that had enough power to start the engine if the decompressors were all engaged. I could have geared the alternator up more but just left it as a cruise alternator until it eventually stopped working.
Digressing briefly …..I recently read an article about those pesky Technodrive gearboxes where it was stated that running them in neutral when charging batteries caused internal damage….. I thought this through and I can’t see why this would be true, it’s a constant mesh gearbox where all the gears spin whether in neutral or engaged so there’s plenty of splash, the ONLY time there’s no splash lubrication is when sailing in neutral and no gears are turning , just the rear flange and its bearings are partially in oil anyway. Does anyone have any knowledge of this?

[s/v Jedi]

Don’t do it, it’s a path of hurt. Install more solar panels.

[lockie]

Originally Posted by Wotname

AFAIK, it causes problems for the cone clutch. The cone clutch is designed to work with the torque supplied from the engine side. When sailing in gear (engine off), the torque is being generated from the prop side of the clutch which in effect is trying to de-clutch the cones.

Disclaimer - this information was not generated by Yanmar so it could be inaccurate.

An almost, but not quite, perfect answer with respect to the Kanzaki KM-XX cone clutch boxes.


In "forward" the forces are such that the torque from the prop is trying, via the shaft helix, to force the cones apart. This introduces the risk of slippage and consequent glazing and loss of forward function if the linkages and shifter mechanism aren't set just right.
But in reverse, the reverse is true (!) and the torque from the prop is trying to force the cones together, preventing slippage.


In neutral it just spins which I doubt would cause any damage given the splash lubrication system and low speeds and forces, so long as the cones aren't touching (which could happen if the lever was inadvertently knocked). It's just annoying, so I always put it in reverse. There is no advice on this in either the workshop or the operating manuals that apply to my 2GM/KM2A combo, hence my "derived from first principles" approach.

Cheers, Graeme

[Wotname]

Originally Posted by lockie

.........
But in reverse, the reverse is true (!) and the torque from the prop is trying to force the cones together, preventing slippage.


In neutral it just spins which I doubt would cause any damage given the splash lubrication system and low speeds and forces, so long as the cones aren't touching (which could happen if the lever was inadvertently knocked). It's just annoying, so I always put it in reverse. There is no advice on this in either the workshop or the operating manuals that apply to my 2GM/KM2A combo, hence my "derived from first principles" approach.

Cheers, Graeme

Are you sure this is correct? Going by the 'first principles' approach, if the prop torque was forcing the reverse cones together, then the engine torque would be forcing them apart - or have I missed something basic.

There two sets of cone clutches in the gearbox, one for forward and one for astern. In both cases, the engine torque must be greater than any induced prop torque for the clutches to operate correctly. When the prop torque is greater, the cones get forced apart, irrespective of forward / astern gear selection.

[lockie]

Yes I'm quite sure, having contemplated the gizzards when I've had mine open on the bench a few times.

[markxengineerin]

The transmission is actually a KBW20. Has anyone held one in reverse for long term sailing? I would feel better about it knowing I'm not the first.

Further research indicated that the concern with reverse while sailing is the cones could get stuck.

Yanmar saildives (which are the work of the devil, imho) can not be locked into reverse when sailing to stop the whine. The infamous cone clutches will lock and you'll have to start the engine in gear to free them.

Another person said this: "Putting the transmission in reverse puts undo stress on the gears and shifter lever horse colllar that are not made for to this."

I can't see the gears being overloaded like this, it's not a lot of torque relative to what they see while driving the boat. But it does feel more difficult to shift in/out of gear when the engine is off.

For those that keep it in reverse, is are you shifting it in/out while the boat is stationary, or ok to do it underway? Engine on or off? I can imagine some shock load if doing it underway, but have never tried.

Don’t do it, it’s a path of hurt. Install more solar panels.

I'll always have as much solar as my project budget allows, but there are niche cases (like north atlantic climates in winter) where even doubling it wouldn't be enough. The weight and windage becomes excessive. Those who cruise in sunny climates won't understand, but it's true!

Without running any numbers, the prop is significantly larger than what is on a hydrogenator like a watt&sea, so the power available will be much greater than that.

It sounds like the numbers need to be run. I don't know much about prop calculations and have too many other projects at the moment, but will add this to the list.

[MicHughV]

Some boat engines have a hydraulic gearbox and can't be locked in reverse as once the engine stops so does too the pressure required to operate it.

This happened to me and frustrated me no end. I tried various ways to stop the shaft from turning, using vice grips, etc, but it was a PITA no matter what I tried.

I had to resort to fitting a feathering maxprop to stop the shaft from turning.
Besides no turning noise, also a bit of extra speed.
Max props aren't cheap, but worth every penny to me.

[markxengineerin]

The fancy props do seem like the best choice if ignoring the desire to generate electricity.