autoprop performance

[MGambling]

I have recently bought a yacht with an self pitching autoprop . Recently motoring into a head wind and a lumpy sea I experienced "cavitation" ie where the prop lost grip and the engine raced for a second or so. Bruntons say that you have to " play" the throttle a bit in these conditions. Has anyone else experienced this situation and if so has it been resolved :

 

[Robin]

We too inherited a Brunton with our current boat and I really like it. I don't think the prop cavitates in a head wind/sea but what it does do is re-pitch itself to siut the greater load, effectively it drops down a gear (lower pitch) and whilst the revs of course remain the same the speed through the water drops. The counter effect of this is with a helping wind, even a very slight one, it changes gear up (higher pitch) and you then see a higher boatspeed for the same revs. In our case a slight wind assist can raise our 2000rpm cruise speed from 6.5kts to 7kts very easily.

In a head sea my solution with our Brunton is to increase revs by 100rpm at a time until the desired boatspeed on average is achieved. It will still be knocked down at times but the extra revs will bring it back up to speed more quickly. Usually 100 or 200 extra rpm does the trick but in severe cases we have used 400 and even 500rpm over our usual 2000 (our Yanmar 44 revs to 3600).

With a standard prop there would be a similar stopping effect of a headwind/sea and in those circumstances yes I believe cavitation does occur. The advantage of the Brunton is that it adjusts to the conditions and you will always be able to achieve the desired maximum revs under load and hence the maximum HP that the engine can produce.

Welcome to the forum by the way!

 

[dulcibella]

I endorse Robin's remarks - the Autoprop is a very efficient prop. Good sailing (and motoring)!

 

[davidfox]

Me too I love it.

 

[Seagreen]

I second and third that. I found that the amount of pickup to accelerate a 10 ton boat quickly in a strongly flowing tidal river is a fantastic bonus.

 

[Aeolus_IV]

I'll add a fourth recommendation to that.

Aeolus crossed from Beachyhead to the Looe Channel (and back again) at an unstressed 6 Knots through the water with the engine only doing 2000 rpm. The hull had been freshly cleaned and antifouled and the prop itself was polished to an inch of its life only a week earlier.

After a week in the Solent, about 120 miles under engine Aeolus had only used 35 litres of fuel - thats seems pretty good to me.

Is an Autoprop worth the money? Perhaps not in absolute finacial terms, but it is a huge improvement to the flexibility of the boat.

Regards,
Jeff.

 

[Hoolie]

... ... and a fifth. Our 3-blade Autoprop has made a huge improvement in close quarter manoevring compared with the previous fixed 2-blade, particularly going astern.

 

[HermanD]

I heard one disadvantage. You cannot longer go slow speed. A friend had a 14 ton boot, at 1000 rev/min minium 5 knots.
So in a harbour or channel you have to switch every time the gearbox on and off to hold a low speed.

 

[Leighb]

I find this last comment worrying!
I had been considering changing to an Autoprop, but if it won't allow you to go slower than 5 knots then that would be hopeless!!!
I cannot believe this is correct?

Previously in this forum I also read a comment that the Autoprop has no "propwalk" effect. Is this correct, seems unlikely given the standard explanation for the effect, but I suppose the feathering/self pitching might reduce the effect.

Any expert views on these points would be very welcome.

 

[boatmike]

I would from experience, endorse what others have said but disagree regarding slow manouvering. The Brunton is fine at low revs.
What he might be referring to is that if it has been "feathered" and you first start the engine you may need a quick burst of throttle to "engage" it in the drive position. Thereafter it behaves in all respect when manouvering as a normal prop.
Sometimes going from forward to reverse you need to do the same, but a quick burst is all you need then throttle back.

 

[alan]

I have not found this and have had an autoprop on two boats for the last 7 years or so. At tickover the boat goes real slow; i can't say I have ever measured the speed but it is only a knot or so. Also, I would not say there is NO prop walk but there is considerably less. My present boat was fitted with a fixed three blade prop and she was a bitch in astern and pulled to one side really badly. Since fitting the 3 blade autoprop this has been considerably reduced and I can now manoeuvre "normally".
Sorry, Hermann, but I have to disagree with your friend .......... at least in the two boats that I have had fitted with autoprops.

Alan.

 

[Aeolus_IV]

I've only had the propeller for a couple of years and so far I've never experienced a "burst" of ahead when engaging either ahead or astern.

The point about the boat speed at tickover is that the propeller continually adjusts its pitch until the resistance of the boat to be pushed (or pulled) along equals the HP output at the shaft. In the marina I have found Aeolus doing nearly 4 knots at tickover before. But, it takes a long "run up" as the engine is not generating much power, and so the boat (at 5 tons) takes a while to build up speed. Initially when putting the engine into gear virtually nothing happens - the pitch of the prop is so fine.

One common "complaint" about the AP is that when you select astern to slow down, "nothing" happens. The truth of the matter is that the prop is removing/killing forward speed at a rate equivalent to the HP being generated by the engine, and so the boat will slowdown very slowly (as it does when accelerating forward in tickover). This takes some getting used to if you're expecting a similar "performance" to a fixed prop, where when engaging astern in this way the prop "loads up" the engine, the governer kicks in and (behind your back in one sense) increases the fuel injection into the engine (and subsequent HP output) to keep the engine turing at tickover speed. With an AP if you want this level of braking, simply open up the engine above tickover. I don't expect that you'll use any more fuel, you'll just be turing the over at an RPM which it is happier at under some load.

Regards,
Jeff.