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Old 10-14-2015, 08:58 PM   #1
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Diesel RPM

I am in the process of looking for a boat. Have decided that about 45' will have enough room to check all the boxes. One question I have while considering hull styles is this: I know a full displacement hull is going to travel at about 8 knots and I'm OK with that. Most I have viewed have low HP engines and have a low fuel burn rate which is a good thing. Semi displacement hulls carry higher HP engines and will cruise at a higher speed, not important to me. Can I run the higher HP engines at lower RPM's for extended periods, at trawler speeds to reduce the burn rate, without damage to the engines. I have been told that turbo engines need to be run at higher RPM's, at least occasionally, to clear exhaust soot. Any feed back would be appreciated.
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Old 10-14-2015, 09:32 PM   #2
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There are many threads on this subject. The general consensus is that it's ok to run these engines at lower RPMs. Some engines may require running at higher RPMs periodically to keep them happy. This need is relative to the specific engine (some needed, some don't).

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Old 10-15-2015, 12:42 AM   #3
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I never heard of anyone blowing up their motor because of running it slow.
Look at the max rpm rating of the motors in interest to you.
If it is in the 3000's-4000;s range, it is not the motor you want.
Something that tops out at 2800 or less will be a better match for trawler/ cruising style of operation.
The transmission should have a ratio of 2:1 or deeper, and the propeller should have a large diameter, and only moderate pitch.
I know all this stuff is quite general, however if you post some relevant details of boats you are interested in, you can get better answers.
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Old 10-15-2015, 06:35 AM   #4
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Not only larger engines for the speed , but semi disp hulls have a different shape that may also require extra power at crawler speeds , and give a different ride at low speed.

A boat is a complete package for an objective.

If you try to operate out of the design zone , some compromises must be made.
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Old 10-15-2015, 08:55 AM   #5
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My boat's engines WOT rpm is 2150. When I run them at ~1050 I'm doing about 10 kts which is where we spend most of our cruise tine, If I want to run on plane, 80% throttle puts us at ~1750 and we're doing around 22 kts.


It was suggested to me to run the engines at 80% throttle about 20% of the time to keep 'em happy. I'm not sure I do that 20% of the time, but I do it periodically just to keep 'em warmed up. They seem to like that and I want them to be happy!
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Old 10-15-2015, 09:25 AM   #6
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Running at trawler speed is fine as long as the engines get to proper operating temps. An occasional short fast run wont hurt and is generally recommended. It is a good way to test the engines for any developing problems such as overheating.


My two 430 HP 8.3l Cummins were operated that way most of the time for 15 years with no problem.


Fuel use at low speed will be about the same as it would be with smaller engines at the same speed.
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Old 10-15-2015, 11:04 AM   #7
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kapnd wrote;
"Look at the max rpm rating of the motors in interest to you.
If it is in the 3000's-4000;s range, it is not the motor you want."

That is an entirely subjective statement. The boats have "X" amount of resistance as they are are pushed along on the water and horse power developed over time does the work of pushing the boat. 100hp at 4000rpm pushes the boat just as well as 100hp at 1500rpm. The 1500rpm engine may push the boat for more hours over time but it's no better at pushing the boat. Many put great importance on how long an engine will last but almost nobody gets even close to taking advantage of the extended life the slower engine offers. And many or most boat engines go at quite high rpm for very long periods of time. So your statement above is almost totally false.

However having said that the "personality" of an engine has a great deal to do w it's speed ... rpm. People see that a trawler is slow and sluggish so feel it's only fitting that it be powered by a slow and sluggish engine. Most here w trawlers would be pleased if their engine ran at 1000rpm instead of 1800. The sound of the slow turning engine would fit the slow turning engine in that it's personality would be more like the slow boat. But it's completely subjective to think that the 1000rpm engine would in any objective way be better than a higher turning engine ... for pushing the boat. Pushing the boat takes hp and engineers should be the ones that decide what engine speed will be best suited to developing the hp required to move the boat.

There are a sackfull of probable advantages to the higher speed engine but there are only two advantages for the lower speed engine.
The first is that it will most likely make less noise pushing the boat. But that's not a given either. Noise is a result of vibration caused by sudden forces acting on moveable objects from air to steel. Attach an engine to and sourround it w objects that it can move at various frequencies and one will hear the sound of not only individual explosions in the cylinders but from all the moveable things the engine is attached to including the air around it. Some engines in some boats make much more noise than others.

Secondly (and most importantly) the low speed engine will make the boat easier to sell when the time comes. Because there are so many people that think like yourself that slow speed engines are more suitable to the slow speed trawler boat it should be more easily sold.

So it makes no difference what rpm the engine runs if it easily pushes the boat. Untill you sell the boat. But as to making more noise there may be not much difference. Pushing the boat takes a given amount of work and doing that work makes noise. The sound an engine doing "X" amount of work in two strokes will probably make about the same amount of fuss (noise and vibration) as the engine doing the same work w one revolution. But theoreticly the force of the combustion in the engine working w one stroke will be twice as much as the engine doing the work in two strokes. But I can add that if you are passed by a truck turning at low rpm it is much less irritating that a motorcycle turning 8000rpm. But an HD motorcycle passing you doing far far less work than the truck will be far far more irritating than the truck. The noise of an engine is hard to evaluate. BUt the type of noise, the personality of noise has various effects on people.

So the personality of the sound of a slow turning engine is more suitable to many than the sound of a faster engine .... but it is a subjective value and has almost no meaning regarding the task of pushing the boat.

But the suggestion to someone that they "need" a certian speed engine for his boat is totally subjective and not worth mentioning re powering his boat.
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Old 10-15-2015, 11:30 AM   #8
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High RPM engines are generally smaller displacement run at higher RPM to up the power rating. Higher loads per liter generally mean shorter life. Running at 50+ HP per liter is working pretty hard. At 30 HP per liter moats engines will last a very long time. Those numbers are independent of engine size, just loading. engine makers try to up the RPM to sell higher HP but keep it low enough to control warranty costs.

You could put small high HP engines in a big boat that will rarely go fast but what buyer would believe the seller hadn't run them hard??


Look at the data sheets for families of engines from one maker where multiple power ratings are sold from the same engine size. You will see commercial heavy duty use engines at maybe 1/2 the power of recreational rates for the same block. That is the makers way of telling us that run at lower loads they will last a lot longer.
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Old 10-15-2015, 12:18 PM   #9
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And high rpm engines are usually built with the intent to be in light fast boats, where a light engine has a big advantage. This also means using aluminum in lots of parts, like manifolds. I have seen more engines pulled for rotted al manifolds than for being worn out.

No way do I enjoy cruising slow with a fast spinning engine. Not subjective at all. Something that sounds like a weedeater is simply not pleasant to anyone.

A big slow boat deserves a big slow engine.

I get 7.7kts at 950rpm. The sound of that engine is famous for good sleeping. Rowdy kids turn into a pile of sleep on the futon.
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Old 10-15-2015, 01:23 PM   #10
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If one looks at the curves given by many MFG or converters , many will start at 1200RPM , others at 1500RPM.

It might be unwise too long term cruise below the where the ratings begin.
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Old 10-15-2015, 01:59 PM   #11
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Interesting discussion. My MD02030 30HP Volvo has a rated WOT range of 3200-3600 RPM; my WOT is approximately 3400 RPM. I see Volvo reduced the WOT of their newer Volvo D1 30HP engine to 2800 to 3200 RPM. Anyway, my Volvo manual says I should cruise 500 RPM below my WOT, but 2900 RPM is simply too noisy and uncomfortable. I tend to cruise at around 2400-2700 RPM, which seems "just right".
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Old 10-16-2015, 06:56 AM   #12
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The Volvo reduction of 500 RPM is to unload a light engine,

It is a max RPM for cruise , not a minimum.
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Old 10-16-2015, 07:16 AM   #13
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Interesting to me as to how my view of the engine RPMs has changed with a change in geography, or perhaps I and my friends are now just older and their boats are older.

In the US I thought as Eric does that no one will ever (or rarely) wear out an engine. Thus the 3,500 rpm Yanmars were fine. Now in the Eastern Caribbean I find several boats replacing their engines each year. May be an issue of use or just boat age - 20 to 30 year old boats are common. Of course being the Caribbean these are sailboats. The trawlers in the Eastern Caribbean all seem to have 1800 rpm engines and to date no trawler (I am familiar with) has changed out an engine.

By the way changing out engines is a pain, not just the cost, but getting everything to work correctly. I have seen a couple of seasons ruined while the replacement is getting adjusted/finished.
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Old 10-16-2015, 11:52 AM   #14
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Marty my views have changed considerably over the years on TF. I would now buy a GB or NT 32 and run the engine underloaded .. yes I would. I'd much rather have a smaller engine working at least 1/2 load though. And there was a time that I considered FD boats to be hopelessly slow wallowing tubs.

Yes there are boats that outlast their engines. But I think all of my posts here on TF were thinking of one owner. And I do think these engines you speak of were probably mostly to all killed by mistreatment and/or neglect .. or both. But there are some that really pile on the hours like so many in the PNW that go to Alaska every year. But for the most part trawler engines don't wear out from too many hours. Putting an extra long lasting engine in a trawler is much like me at 75 putting a 30 year roof on my house.

Re the deed of repowering I have heard of sad tales where everything went wrong and the cost effectiveness gets worse and worse. Same w remodeling houses. I, not too long ago bought an 87 Suburban w a nearly new "crate" engine. Somebody spent $4000 doing this and the Burb just wasn't worth repowering. I think the trans was rebuilt too.
Probably the best repower on a trawler would be buying for cheap an engine out of a boat that had a fire or whatever that had been reciently rebuilt. Then everything would match up ect. Changing engine mounts or exhause sides can by difficult but more often very easy. And of course the DIY guy is way ahead of the repower game if he knows what he's doing. Even a yard or a skillful owner needs to scope out the potential problems ahead of time. My repower was mostly yard done and went well .. but not perfect. Would I do it again? Yes. But my Mitsu is going to last forever .... right?

My motive for whomever when I urge a poster to consider a repower is mostly to attain a modern engine. "Rebuilt" engines are much more questionable as to their quality of "manufacture" than new engines. And some old engines are better than new engines .. perhaps the DD falls into that catergory? But the weight and noise .....
But the rebuilt route is almost always to save money.
So many variables.
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Old 10-16-2015, 12:00 PM   #15
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Quote:
Originally Posted by Bay Pelican View Post
Interesting to me as to how my view of the engine RPMs has changed with a change in geography, or perhaps I and my friends are now just older and their boats are older.

In the US I thought as Eric does that no one will ever (or rarely) wear out an engine. Thus the 3,500 rpm Yanmars were fine. Now in the Eastern Caribbean I find several boats replacing their engines each year. May be an issue of use or just boat age - 20 to 30 year old boats are common. Of course being the Caribbean these are sailboats. The trawlers in the Eastern Caribbean all seem to have 1800 rpm engines and to date no trawler (I am familiar with) has changed out an engine.

By the way changing out engines is a pain, not just the cost, but getting everything to work correctly. I have seen a couple of seasons ruined while the replacement is getting adjusted/finished.
Sailors are known for not taking good care of their engines. Boy they can tell you how to operate and maintain and replace a Boom Vang....but the engine space in many sailboats is an afterthought to its owner.
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Old 10-17-2015, 07:03 AM   #16
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The tiny high speed engines , when run all day wide open ,Carib style have power ratings far above their 24/7 ability , so when operated at max power do not last.

Think of a car engine pulling a load up a hill.

Fine for 15 nin but when it becomes hours , ..

3 cubic inches (CI) of engine displacement per hp usually will last thousands of hours

A different way to view it is 1 GPH tales 45 CI . of engine...
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Old 10-17-2015, 11:03 AM   #17
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A mix of responses mostly what I expected.

What rpm an engine turns to push a boat is not worth considering.

Worth considering is;
How heavy an engine is.
How much fuel it burns.
Will it still be in good condition when I sell the boat if I take good care of it.
How much noise does it actually make.
Will I have sufficient space in the engine compartment to service the engine.
How much readily availible service will there be in the waters I cruise.
Mow much vibration will it generate.
Is it's exhaust smelly or/and visable?
Does it provide good responsive throttle response?

If you look at the list above and objectively think about it smaller higher reving engines are better in most respects. In all the points above the high speed engine is better w one or two exception and on that issue it may be the same.
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