We have long believed that the prevalence of high revving lightly built engines installed in sailboats, particularly those boats built in recent years, is often more about ease of availability to the builder and initial price than making the best choice to give long term economical service to the owner. Particularly an owner, like many of our readers, who either does, or plans to, put a lot of miles on.
Regular readers of this site will remember that back in the spring we posted several articles about our re-power engine selection process and final decision to install a Perkins M92B.
Now that we have some hours on the new engine we are seeing some real benefits that, at least initially, are making us happy that we took a different path, opting for a slow revving (2400 top end) high torque industrial engine of lower horsepower than would generally be specified for a boat the size of Morgan’s Cloud.
Thanks for this post, John. Your experience seems to confirm what I keep hearing from long-distance voyagers, big ship guys and maintenance engineers- a solid, medium-RPM industrial/marine duty engine is the ideal match to typical displacement hulls.
A lot of what shows up in the marine engine market really seems to be light-duty car and utility motors, modified slightly to meet marine requirements, and turbocharged / ECU-mapped to within an inch of their lives. A rather extreme but interesting comparison at the 600 hp point is the 81 litre Grenaa six-cylinder compared to 12-13 L turbo six-cylinders based on truck blocks. (A friend who’s thinking of building a large motor passagemaker has been posting in detail about this on boatdesign.net lately.) The Grenaa costs much more and is far heavier- but in a boat that’s designed to handle its bulk, the Grenaa will run with significantly better fuel efficiency, needing only routine maintenance, through the truck-engined boat’s first four repowers.
Obviously an 81 litre, 8-tonne engine at 600 rpm is an extreme example, but the principle is clear: a larger, slower, less highly loaded engine tends to be longer lived, more reliable, more fuel efficient, and- for several reasons- easier to coax into compliance with emissions laws.
Hi Matt,
Thanks for the note and another good example. I have a friend that has a Gardner installed in a motor boat. Top end is 800 rpm if I recall correctly. It runs great. But then of course it’s young for a Gardner having been built in the 1950s!
John:
Thanks for this good input. Sensible. We have found that propeller selection may be just as challenging as motor selection. I have mentioned this before, but I have had great experience with the AUTOPROP, a self pitching propeller made by Bruntons UK and imported to USA by AB Marine. It continually finds the correct efficient pitch depending upon load and speed, and we have really benefited from it (especially when motor sailing, you get a puff, the propeller changes pitch and off you go). Feathers when unused as long as shaft is stopped. Have heard very good things about the folding Gori’s as well.
Pete & Kareen Worrell
S/V PATIENCE
Hi Peter and Kareen,
I have been hearing great things about the efficiency of the Autoprop too. A few days ago we were motoring round a point into 25-30 knot winds and a big sea. Generally our MaxProp did well keeping us at a little over 6 knots at 2000 rpm. However, a couple of times a big wave knocked us back to 4.5 knots. At this point the MaxProp started to cavitate due to the high slip. I think an AutoProp would have automatically fined out its pitch and been able to bring the boat back up to speed more quickly.
I did hear of several reliability problems with the AutoProp, including a couple that shed a blade. Do you know if they have this sorted?
Also, because we have a hydraulically activated transmission, we would need to fit a shaft lock for an AutoProp. An additional complication that is not required for a MaxProp.
John:
I agree, I have heard of some problems years ago with the Autoprop, including one or two that threw a blade. I have heard also from some people that they thought that the Autoprop was noisier or vibrated more than other props, but I haven’t had that issue. I do know that it is critical that the prop be fitted specifically to your vessel/power combination and AB Marine did physically install my prop. I agree that a hydraulic transmission adds a complexity with the need for a shaft lock. And as we all know, complexity=bad; simplicity=good!
Pete & Kareen
S/V PATIENCE
Hi Peter,
Yes, I think, on balance, if I was doing a new boat I would try an AutoProp, even if it meant a shaft lock.
But any talk of further engine and drive train work on “Morgan’s Cloud” tends, at the moment, to send me whimpering into the fetal position. Likewise our banker.
John,
If it is any comfort, I re-engined a Bowman 48 with a Perkins M92B, and I have been very pleased with it. There have been only two annoyances: The welds for the water injection cowl on the stainless steel exhaust swan neck failed – poor quality control, and the cooling system is sensitive to being even slightly overfilled.
Overall, I think that it is a good choice – I hope so – as i have plans for going to high latitudes once children are through school!
Thank you for a great website.
Martin Wright
As further evidence of the benefits of a slow revving engine, our 24 year old Perkins TC 6.534 which has max revs of 2500 is still running very well with just under 7000 hours, and our diesel mechanics all say she’s got lots of life left. I hope they are right! These same mechanics also confirm they see many more problems with light weight high revving “screamers” as you call them.
Hi Doug,
Now there’s a comforting comment for we who have just dropped a bundle on installing a new Perkins! Thanks.