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Forever Freedom · 06-10-2013, 03:28

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?

djmarchand · 06-10-2013, 04:41

Your Autoprop is putting too much load on the engine and won't let it rev to maximum rpm. That is the cause of your black smoke. This could be solved with a smaller diameter Autoprop or going back to your fixed prop.

If you haven't had it very long, I would have a conversation with Bruntons. They sold you the wrong prop and may be willing to exchange it.

Certainly the excessive load contributed to the exhaust elbow plugging. But that engine is notorious for needing a new elbow (or cleaning) every 1000 hours or so, no matter what prop is used.

David