Matching Propellor to Engine

[pmyatt]

Forever Freedom's engine is a 23 year-old Yanmar 3GM30F, which was overhauled top and bottom in 2004. Maximum rpm is 3600 and maximum continuous rpm is 3000. The original propellor was 2-blade, irreverently called "the egg whisk". It pushed the boat along adequately, but produced some ideo-syncratic handling in harbour. Some years ago we changed it for a 3-blade Brunton Autoprop. The Autoprop made a noticeable improvement to boat handling, it seems to produce extra propulsion at lower revs when motor-sailing (we seem to do a lot of that now!), and has eased close quarter manoeuvring dramatically. Nevertheless, I am now having reservations about it. Recently a boat here in Porto Lagos had to have it's engine lifted out and repaired because of water in the oil. That was all sorted but, examination of the exhaust mixer showed it to be almost completely blocked with carbon, which had to be drilled-out. The engineer who repaired the engine said that the carbon deposits were caused by running the engine at low revs for long periods of time. Since fitting the Autoprop, we can maintain cruising speed under engine at 2/3 rds of maximum continuous revs, i.e., 2000 rpm instead of 3000 rpm. The engineer recommended running the engine at a minimum of 75% of maximum continuous rpm always, and running it at maximum continuous rpm for 2 minutes in every hour. Unfortunately, with the Autoprop, we can no longer reach 3000 rpm, and at 2750 rpm we produce clouds of black smoke and a trail of soot on the water. It would seem we are significantly "over-propped" and that, for the benefit of the engine, we should re-fit the original 2-blade prop so that we can maintain higher rpm - something I never thought would be necessary. I was wondering if any engine gurus have any comments?

 

[garvellachs]

This has been discussed at length not too long ago, concluding broadly, if I remember, that an occasional Italian tune up is good for an engine and burns off excess carbon; that running an old engine at near to maximum revs under a heavy load for extended periods is overly stressful; and that using an old engine as a generator (or maybe using it to motor sail at low revs) is likely to lead to bore-glazing and an excess of kindness that may do it harm such as clogging the exhaust mixer bend.

We have a 30 year old 3HM similar to your 3GM30, with about 1600 hrs. A feathering prop too. I've just changed the original exhaust bend because it was corroded, not blocked. We run it at about 2200-2400 normally, rarely more. At that speed I think it is working but not stressed, the exhaust is clean and the temperature stable. In very rough seas when a wave stops the boat it labours to maintain that speed and may smoke briefly as it does so. If it was a car I would want to change gear! Same when we do an emergency stop or use a burst to turn the boat in a marina.

Maybe your autoprop is slightly over-propped but it doesn't sound to me as if there's much wrong. 75% of max continuous revs is 2250 - can you run smoothly at that? Why not take the exhaust bend off and examine it? For the price of a gasket you will learn quite a lot.

 

[macd]

Why not take the exhaust bend off and examine it? For the price of a gasket you will learn quite a lot.

+1. The fitter to whom the OP referred was remarkably definitive, which I instinctively distrust on such matter (sounds like my mum warning me about chilblains, which I've never experienced and never even seen despite complete disegard for her kindly advice). But why speculate when four bolts and a couple of hose clamps are all that stand between you and finding out for sure?

Also worth checking out Vyv Cox's website re effects of light-load runnings, and noting the difference between bore glazing and bore varnishing.

 

[Ian_Edwards]

Why not send your Autoprop back to Brunton's and get them to re-pitch it?

I had that done when the 3 bladed prop I ordered turned out to be over pitched, it only took them a few days to do it and they serviced the prop at the same time.

 

[penfold]

If you've never been able to reach 3k then the prop is too big; as I understand the Autoprop it is self-pitching, so if it is sized appropriately you should be able to reach 3.6k. If you were able to reach 3k, but now cannot, then it may be the exhaust is becoming choked, or some other problem has developed.

 

[Pye_End]

Perhaps worth getting Bruntons to check the numbers to make sure that you have the right size.

Presumably the prop is clean when you max it out in your trials.