Any autoprop experts.. How Autoprop uses RPM?

[wipe_out]

Hi,

We switched to an autoprop last year but didn't manage to get much use of the boat so it fouled up a bit and so I couldn't do any real testing of the prop.. We have done a good amount of motoring last weekend and I wanted to get some feedback..

We have a Yanmar 2GM20F engine, which is probably a little under powered for the boat, and our hull speed is about 7kn.. With the boat going back into the water a couple of weeks ago everything is clean.. With the autoprop we are now hitting 6.9kn through the water at 2200RPM.. The max revs we can get is about 2300-2400 before we start getting lots of black smoke.. I know that were this a fixed prop it would mean the boat is over propped but it's a variable pitch prop so I am not sure how to read this..

Does this sound right that it's simply reaching hull speed easily and just won't go any more than that anyway regardless of the props pitch?

I am planning to verify the accuracy of the rev counter next time we go out because that may explain it if the counter is under reading but I thought I would ask other autoprop users about their experience.. Perhaps it's a case of choosing what speed your want to cruise at and then simply tune the revs until you hit that speed and let the prop do it's work?

Thanks..

 

[vyv_cox]

Sounds like you may be starting the 'Italian tune-up' process. Run for about 20 minutes at max revs and the smoke may well clear and revs increase.
With my Autoprop I can pull over 3000 rpm, 3GM30F. I have heard that the blade area can be too big and Bruntons will exchange foc for new supply.

 

[penfold]

If the throttle at the engine is definitely wide open then the prop is too aggressively pitched(in so far as an Autoprop can be), the blades will need attention from Bruntons. Given the large rev deficit it may even require the next size down.

 

[rgarside]

When I acquired my Parker 275 it had an Autoprop driven by its 1GM10. When I finally got a reasonable rev-counter, I found that with the governor wide open I could only get 2900 rpm - this explained the quantity of black smoke being produced above a certain rpm level. This indicated that the propeller was effectively over-pitched.

I spoke to Bruntons, and they said that it is difficult to get their smaller props matched to a boat and engine without a trial. If I could provide them with rpm - boat speed information from tick-over to wide-open they would be happy to adjust the prop to suit without charge. As I live not too far away from Clacton it was not difficult to arrange. I did the rpm - speed trial on a suitably calm day at slack water [noting both log speed and GPS speed]. At the end of the season I removed the prop and took it to Bruntons. After a few weeks it had been adjusted. On re-commissioning, the engine would get to 3600 rpm, and the black smoke was no longer a problem. I haven't redone the speed - rpm trials as it's not too easy to do, but things are much improved.

Apparently the adjustment is done by a small amount of grinding of the trailing edge of the blades to adjust the position of the centre of pressure of the blade, which controls the pitch. I understand that Bruntons run each propeller in their water tunnel dynamometer, so they have data on water speed, rpm and torque. They could then see the change in the propeller performance before returning the prop to me.

Once I had taken the step of contacting Bruntons they could not have been more helpful.

 

[Bilgediver]

Yes as others are indicating your prop seems to be overpitched and this is easily rectified by Bruntons as said by a previous poster. Just a matter of removing the correct amount of material from the right place. Have had to do this with much bigger props and unfortunately didn't get to keep the removed metal

Ideally you should approach hull speed at around normal full operating speed. Otherwise you are imparting excessive load on the engine when reaching this load at lower revs than the engine is designed for, as indicated by the black smoke.

This does require attention as otherwise you may be overloading the engine when in adverse weather conditions or when the hull becomes fouled..

 

[wipe_out]

Thanks for the replies.. I have dropped Brunton an email so will see what happens..