Autoprop ?

[hellosailor]

Once you are near hull speed in a displacement hull, i.e .all monohull keelboats, drag means nothing compared to the total stopper of trying to get the boat to climb out of the displacement hole.

When you are ghosting along, a hard sneeze can stop the boat. When you're blessed with 18 knots, life is much easier.

[Yachts66]

I also have an over the transom generator, with equal results, but we're not talking the small prop and drag of those units, we're talking the boats main propulsion system and it's (probably) very larger propeller, so I would think the drag would be far more significant. Sure if it's blowing a gale drag is less significant, but we don't normally sail in gales, do we? I fear this is one of those things that perhaps looks good on paper, but in reality is not so good at all.

Regards,

Thomas

[GALAWA]

There is a lot of buzz about shaft alternors on French web sites.
The actual loss of speed due to shaft altenators charging our batteries ranges from 0.3 knots at low speed and slight wind , to zero knots when the boat is sailing near hull speed.

There is also a lot of misunderstanding about the drag caused by fixed versus freewheeling shaft and prop.
The relevant parameters are the blade /disk area ratio and the residual torque resistance.
Disk area= area of the full circle around the blade tips
Blade area = area projected by the blade on a plane perpendicular to the shaft:
Blade /disk area ratio of a hélicopter rotor is about 5%
Blade /disk area ratio of a 2 thin blades fixed prop is about 20%
Blade /disk area ratio of a 3 wide blades fixed prop is about 90 %

Residual torque resistance is caused by friction of the shaft bearing, packing gland , gearbox, and friction of water on the spinning blades

When the engine of a helicopter fails in flight and no longer delivers torque, the pilot adjust the rotor to negative collective pitch , the rotor autorotates against the residual torque resistance as the helicopter descends and keeps generating enough lift to glide the helicopter into a mild forced landing.
Same with 20% Blade /disk area ratio 2 thin blades fixed prop : Allow it to spin and the Residual torque resistance causes a low incidence laminar flow of water on the blades and a lift, read drag, comparable to that which saved the helicopter and comparable to that which would be caused by a full disc.
So: With a low blade/disk area ratio prop, you have every reason to lock the shaft when sailing : Les drag, vibration and wear !


On the contrary, with an 90% blade/disk area ratio 3 wide blades props , the drag when the shaft is locked is 90 % of the drag which would be caused by a full disk.
If you leave the shaft spinning and if the residual torque resistance is small enough, the prop will spin fast enough, with low incidence and low lift (read drag)of the laminar flow of water over the blades, and the drag wil be much reduced. At low speed, say 4 knots on my 40' 9 tons sailing boat, I notice a .3 knots gain of speed when I let the shaft spin.
At hull speed expect no measurable gain .

[chala]

Originally Posted by GALAWA

Release the brake or disengage the gear box and the autoprop should start spinning the shaft immediatly.

Maybe your’s Autoprop behave this way. Mine require a speed in excess of 5 knots to start to rotate?

Originally Posted by GALAWA

Apply a resisting torque such as caused by a shaft alternator and the autoprop will drive it very efficiently.

The resisting torque of a break will also stop it efficiently.

Originally Posted by GALAWA

I hope we shall get contributions to this forum thread from sailors with first hand experience of running shaft alternators with autoprops

So do I.

[chala]

Originally Posted by Yachts66

Aren't shaft generators a tremendous source of drag? You loose a knot or so in speed, which is a lot on a long voyage. I remember reading that somewhere, sometime.

Thomas

Yes which also impart far more stress on the rigging, often not accounted for.

[chala]

Originally Posted by hellosailor

the mfr/rep responses

What mfr/rep responses? You mean the reported mfr/rep responses. When I purchased my prop I was told by the Autoprop agent that, even if the propeller was kept in its packing box and never used, it would require to be greased every year and also that like any piece of machinery the greasing will depend on its use etc. Like any other piece of machinery consideration must be given to the age, provenance and how well it was maintain in the past. Water coming out of the propeller when greasing is a tell tale that the prop was not properly greased. Under “Greasing procedure item 4” the instruction are clear. Working the new grease around the bearing is not an easy task because the grease as a tendency to escape more easily by the exit hole.
Page 6 see

Originally Posted by chala

more info

[Blinky]

I have read with interest what seems to be a very long thread!
I have an Auto prop two bladed propeller which I fitted to my Bavaria 32 in November 2010. The Boat had a Volvo MD2020 and MS25SR-A sail drive fitted with it’s original 2 blade aluminium fixed blade prop. I purchased the prop at the Southampton boat show for about £1200. At the same time I replaced the sail-drive gator seal, the lower leg oil seals, O rings and drain plug O ring/ATF oil.
The Brunton has performed exactly as its literature suggested. We have increased our sailing speeds an average of 1 knot throughout our normal sailing conditions. We have also increased our top speed by 1.5 knots when motoring, up to 7.5 knots. We can achieve this at about 2500 rpm. However, any additional application of throttle only serves to cause the stern to dig in and the bow rise as the boat tries to climb over its bow wave. Stopping speeds are greatly improved, and we no longer have any discernable prop wash (not that we had a lot previously) Using slightly more throttle when manoeuvring at close quarters has become necessary but it is something you get used to. On Brunton’s advice I coated around the prop anode’s stainless steel fixing screws with a mastic sealant preventing, or slowing the anode dissolving in this area. We keep a small B&W underwater CCTV camera on the boat so we can keep an eye on things should we need to. (and at £70 its been a valuable tool that save me having to put my dry suit on or lug my dive gear in and out of cars, tenders and onto the boat!).
Now here’s the rub…………………… In January 2012 I noticed that the sail-drive anode and the prop anode required changing, and our anti-foul was in need of a pressure wash and recoat. Using our local scrubbing grid (piles) rather than an expensive lift out I carried out the work replacing/renewing the prop bearing grease with the specified Lithium based grease and re-fixing the prop securing bolts with Loctight tread lock (blue). I also recoated the prop with two coats of out-drive/sail-drive anti-foul, as the existing coats were wearing very thin, almost non-existent near their tips. I also changed the sail-drive ATF oil. There was no noticeable wear in the sail-drive, IE end float or back lash, prop bearings or any other components. The tide duly came back in, and on motoring back to our mooring all seemed smooth and there were no issues. The next morning we headed off to Poole and with the wind almost on the nose across Christchurch bay we motor-sailed. After about an hour at 1800 rpm I could hear a distinct cavitation sound and a rumbling from below the water line. This appeared to be coming from the sail drive area at the lower end. I suspected that the “Stripper” rope cutter may have a problem, i.e. we had hit a submerged pot buoy or that the sail drive anode had come loose. (this is not uncommon if the countersunk machine screws do not seat properly or they are not also torque sealed in position). We limped home, and two weeks later on the right tide we dried out again against the piles for another inspection. I stripped off the prop, rope cutter, and the sail drive lower casing. I even re-greased the prop again. The anti-foul had already worn from the outer edges of the blades so this was re-coated. Everything was reassembled and again we waited for the tide………….. On our way back to the mooring all seemed fine again. Next day after a few hours motoring the noise returned, worse than before accompanied a slight vibration. Both in forward and reverse. Having checked with the camera and dived the boat there is nothing loose, no bearing end float or damage. However, the anti-foul is stripped from the blades unevenly 2-3 inches from the tips.
I will test Brunton’s after sales in the next few days to see if they have any suggestions.
In the mean time based on my current experience of the Brunton prop to date I have been extremely pleased…………………..This view may change, unless it’s a Volvo problem now don’t get me started on their after sales or engineering ability!!!
Blinky

[chala]

Originally Posted by Blinky

The Boat had a Volvo MD2020 and MS25SR-A sail drive fitted with it’s original 2 blade aluminium fixed blade prop.
……
Next day after a few hours motoring the noise returned, worse than before accompanied a slight vibration. Both in forward and reverse.

Have you tried to reinstall the original 2-blade aluminium fixed blade prop and see if the problems persist?

[Valkyrie654]

With almost 1,000 engine hours with our Autoprop, if I could offer some observations.

It is very efficient, but we also run a lot of gear reduction, so at max engine rpm, shaft speed is less 1,100 RPM.

Very smooth and vibration free.

We do get a good rumble in the aft cabin, but I think if we had a bit more tip clearance between the hull and prop it would be better.

The prop is in clear water, not in a apature

As much power in reverse as in forward. We can stop very quickly in reverse.

Very little prop walk, just a hint to port.

They require a special prop zinc that you can only purchase from them. Expensive and they don't last very long because the little tabs the bolts go through corrode off first then the 99% of the zinc that is left flies off, leaving you with zero protection. The simple solution is to have a flange machined that matches the bolt pattern with a short stub, say, 1.5" long that is the same diameter as the prop shaft, then use a standard shaft zinc that is available anywhere at 25% of the cost. I am now getting close to a year out of a zinc. Use bronze for the stub.

You do need to check the play in the bearings at each haul out. You want just a little bit of play in the blades. I know of one boat that lost a blade motor sailing. They had never checked their bearings. it did give me the opertunity to test ours, towing a boat that was almost as heavy as we are. I was very impressed, we only lost about 20% of the speed we would have had.

The prop will cause the shaft to free wheel. By putting our mechanical transmission into gear will stop it.

Our 63 hp diesel runs a 21" three blade Autoprop on a 1.25" K-500 Monel shaft.

We are very happy with this prop and highly recommend them.

[chala]

Originally Posted by Valkyrie654

We can stop very quickly in reverse. Very little prop walk, just a hint to port.

I do have a lot of prop walk in reverse, so the prop is not immune from prop walk.

Originally Posted by Valkyrie654

They require a special prop zinc that you can only purchase from them. Expensive and they don't last very long because the little tabs the bolts go through corrode off first then the 99% of the zinc that is left flies off

Are you using the nylon screws that they provide with the anode? The anode in the pic was more than 1 ½ years old when replaced and it looks like the top would have came off long before the tabs.

Originally Posted by Valkyrie654

The simple solution is to have a flange machined that matches the bolt pattern with a short stub, say, 1.5" long that is the same diameter as the prop shaft, then use a standard shaft zinc that is available anywhere at 25% of the cost.

I am not sure that the autoprop zinc is of the same quality (ie suitable for alloy hull) as a standard zinc. What I have noticed is that the zinc migrates to the prop.
Also it is good to remember not to use heat when removing the prop. The heat will melt the bearings cage, an expensive finding by some Autoprop owners.

[fstbttms]

Originally Posted by Valkyrie654

The simple solution is to have a flange machined that matches the bolt pattern with a short stub, say, 1.5" long that is the same diameter as the prop shaft, then use a standard shaft zinc that is available anywhere at 25% of the cost.

No, that's not the simple solution. This is the simple solution:



Autoprop Adaptor page

[Valkyrie654]

The photo is what we did. Perhaps it is now available ready made. We had ours machined in Mexico for about $25 US. We use a standard shaft zinc so we don't have to carry two different types of zincs.

To answer the other post, yes we used the nylon screws to attach the zinc to the prop. Ours is a steel boat rather than alloy if that makes any difference. We switched out our hull zincs after 8 years, even though more than half of the zinc was remaining. Prop zincs seem to last 12 to 18 months on the average.

In regards to prop walk our shaft is parellel to the waterline. That may have more to do with the lack of prop walk more than the prop itself. Don't know for sure on that point though.

[chala]

Originally Posted by fstbttms

No, that's not the simple solution. This is the simple solution:

Looks great. Let hope that the zinc will wear evenly and not at the holding bolt.

[osirissail]

Originally Posted by chala

Looks great. Let hope that the zinc will wear evenly and not at the holding bolt.

The key to that is to make sure that you have a good hard metal connection between the two parts.

If you examine the better prop shaft collar zincs you will see a hard metal "point" inside that "digs into" the prop shaft as the two parts of the zinc are assembled around the shaft.

Something similar would be a good idea to ensure a continuous "electrical contact" between the two parts. Relying only on the mounting bolts may not be too good.

[fstbttms]

Originally Posted by chala

Looks great. Let hope that the zinc will wear evenly and not at the holding bolt.

These standard prop zincs are fairly robust pieces of metal and do not usually deplete at the bolt hole excessively quickly. But a lock washer is definitely recommended.