Over propped. Is it really a problem?

[koliver]

DW
I am late to this thread, and may be repeating what earlier posters have said, though I am usually the odd man out on this subject.

I bought this boat in 1994 and re-powered in 2000. My boat is about the age of yours, similar style, and I suspect from what I have seen of your posts here that our cruising style is similar.

My Re-power was from Volvo TMD40s, 145hp, to Volvo TAMD41s, 200hp. In the re-power, I first left the original props on, so the only change was the newer generation engines.
After the repower, my cruising speed increased by a knot and my fuel consumption/mile decreased by almost 10%, due to the more efficient engine design. My mechanic, who had suggested the change, thought that I could get still better fuel economy if I re-pitched the props to reduce the cruising rpm.
When I had 4" of pitch added, my cruising rpm dropped 500rpm or so and I gained a further 5% approx in fuel efficiency.
Naturally I NEVER run these engines at rated WOT.
If I had a planing boat, I would pay more attention to "overpropping", oh, wait, I have a 40 Honda on my dinghy. It reaches 25 knots and 5500 rpm at WOT, so I do have that experience. On a big boat like that, I would pay close attention to what rpm is reached at WOT, but when my best speed is around 8 knots, on a boat that would need 2000 hp to plane, driven by 1/5 of that, I am happier to overprop and save fuel in the process. I am also of the belief that the longevity av a diesel engine is measured best by its fuel consumption, so anything that improves fuel efficiency is good.
If there was ever a hint that underloading was occurring, surely that would have become clear within the 23 years since my "overpropping" experiment began.

 

[tomdove]

Several years ago after buying my trawler I asked Brian at American Diesel this question, as my boat has the final version of the Ford Lehman, a normally-aspirated American Diesel 6N140 built about 2001. I typically run it at 1700-1800 rpm and there's no real benefit to going over 2000 rpm on my boat as it is overpowered anyway.

Brian said not to worry about moderate overpropping on those engines and that virtually all boats with them are. The engine is simply stressed so low that you wouldn't hurt them, unlike turbocharged diesels where it did matter.

So that's what I consider the official verdict.

-- Tom

 

[Nomad Willy]

tomdove,
I look to marine mechanical engineers and gu-ru’s at the engine manufacturers.

 

[Jeff F]

You're right. I keep forget about modern high performance diesels with recreational ratings. In that case ther are likely many uncharted territories for the uninformed. But nobody has these engines in non-planing boats.

And that's why 95% of Tony's experience and advice really doesn't apply here. Maybe someone should ask what he thinks of overpropping a 6bt in a trawler and setting a max cruise of 300 rpm under max rpm. I've read enough of his advice to be able to accurately predict his response, but I don't want to spoil it.

It's an incredibly dull question really.