High RPM, Light Duty Diesel Engines

I understand that diesels have changed a lot over the years. As an old farm boy, I understood heavy and tough in diesel tractors, and expected the things to run forever, but somehow I haven't got a clue to what I may be buying into when it comes to choosing a boat with one of the many marine diesels on the market. I am not interested in a newer model, Tier II and above no matter how fancy they are, but I'm having a great deal of trouble figuring out whether I should be steering toward or away from the engines of the late '80s and '90s (such as a Lehman, a Volvo, a Yanmar, a John Deere, a Caterpillar, a Cummins, etc.) for continuous cruising, reliability over tens of thousands of hours, and overall toughness and dependability. Also, recently I've run across the term "high rpm, light duty diesel engine." Are they bad, good, or just different? I'd be grateful to anyone who could anyone provide a few guideposts that might help me navigate the maze of information that's out there?

Welcome Mountain Sailor, from an older Montana native ( born and raised there back in the 50's and 60's). There are a lot of diesel guys (and a couple of girls) here on the forum who are well versed on the subject. It all depends on your type of boat. If you're getting or have a full displacement or semi-disp. trawler, I'd stick with a mechanically injected' non-turbo engine. If you wanna go fast, you'll need a common rail, turbo type, high RPM engine. Your question is akin to asking what's the best type of anchor. Ben

This question of engine longevity is a bit foggy.

If you were to honestly determine how many hours a year maximum you’d run an engine the number would almost certainly be much lower than the expected life of the engine in hours. My opinion.

Most of the skippers here like the sound of lower rpm engines .. about 2500rpm. They think a 3500rpm engine is screaming noise wise. So it’s not really an objective thing. Just a preference.

The usual advantages of “high rpm engines” (3000 to possibly 4000rpm) is considerably lighter weight and lower fuel consumption. Reducing the weight of the main engine or engines means more things like a bigger dinghy, FB enclosure, additional generator ect ect could be employed.

Vibration would depend on the engine, how it is mounted ect. It’s hard for small engines to shake a boat much due to their small size so vibrations caused by bulkheads or decks shaking may be less than w a smaller engine. Parts and maintenance will usually be cheaper. Less oil smaller gaskets ect.

On twins there is more to gain from smaller engines. The weight factor is a bigger issue and the additional room around the engines for maintenance will be welcome.

So I’d give the nod to small high speed engines but most here don’t like their higher pitched sound. But functionally I think the small engine is better.

Now that I’ve said that you should recieve many posts on why bigger engines are better. Either will do fine.

MountainSailor:

Yours is an interesting question, and there are lots of gear heads on this forum that will answer and start a vigorous discussion. I bet this one hits the 100 reply mark. So let me start by giving some history and analysis:

Until somewhere in the 70s all diesel engines were normally aspirated and relatively slow turning. The Ford Lehman 120/135 and Perkins 6.354 and to a lesser degree the Cat 3208 became the go to engine for trawler installation. They were rugged (for their power output), fairly slow turning (under 3,000 rpm) and were perfect for trawler use. Run at moderate loads they will last 10,000+ hours if maintained, propped and operated right.

In the 80s turbo chargers came along and these engines got turbos and others like the Cummins 6BT(A) were developed. These engines produced 40-60 hp per liter with their turbo, roughly double what the NA engines put out. They stayed at or under 3,000 rpm and generally had good life at the higher power, particularly the Cummins as long as they were operated at reasonable, say 40 hp per liter load levels. Others like Yanmar and Volvo also had good engines in this segment. None of these engines were auto related although some were used in light trucks.

The next step in the evolution which ocurred in about 2000 was the development of common rail engines to meet EPA regulations. The Cummins QSB series, loosely based on the 6BTA was one of the trailblazers of this series. It still operated at 3,000 rpm or less and even the 480 hp version could give decent life, but probably was operated at 50-60 hp/liter or more which limited that life. Yanmar and Volvo had products in this series, but Cat dropped out of the under 6 liter market.

Today if you want to meet the current EPA regs and need more than 50 hp you have to go with one of these new common rail designs. But a QSB rated at 240 hp will last as long as any of the old Perkins or Lehmans if run in their 20 hp/liter range. In the 60-150 hp range, Volvo and Yanmar have capable common rail engines.

Common rail also made diesels socially acceptable for automobiles. They didn't smoke and were much quieter. BMW, and VW started making 2-3 liter diesels that produced 150-250 hp and were perfect for small to medium sized cars. These were marinized by Yanmar and Mercruiser. Cars have very short peak hp requirements (like accelerating onto a freeway ramp) but cruise at very low load levels at US highway speeds.

These engines make their maximum hp at 4,000 rpm and are often run at just a few hundred rpm off of top like a gasser I/O in the boats they were installed in. They work fine I suspect for a go fast boat that only runs 50-100 hours each year, but do not give anywhere near the service life of the 3,000 rpm, non automotive based diesels. No self respecting trawler has one of these installed. Some like Ranger Tugs tried, but now they use the less stressed Volvo D3s.

So these later engines represent the high reving, light duty diesels you asked about. If operated like a car, they probably can give many thousands of hours of service. But nobody operates a boat like a car and in a boat, they just won't last very long.

David

For the "turbo" record:

Turbocharged diesels have been around for a very long time. Alfred Buschi a Swiss engineer invented the turbocharger. He especially intended it for a diesel engine to improve efficiency. He took out a patent in 1905. Smart man.

In the 1920s large slow turning diesels began using turbochargers. Diesel locomotives began using turbos in the 1940s. By the late 1960s turbos were common on smaller truck diesels made by Cummins, Cat and Detroit. The rest is history.

Can't imagine a WWII gas engine fighter like the P51 having a service ceiling of 41,500 feet without a turbocharger on those screaming Merlins or knockoff Packards.

On a boat, if new you will have no choice except to get a turbo if your diesel engine is above say 100HP. Lots of higher (3500 - 4000) RPM turbocharged diesels in use, millions in fact if you count autos as relevant.

We have SP225 Lehmans in our semi displacement boat. Love them. They just run and run. They are turboed and intercooled. Just because we have 225 hp per engine doesn’t mean we have to use all of them. We generally run around 1600 to 1800 RPM. It gives us between 9 to 12 knots. It tops out at 17 knots. In a heavy boat I would not want the high speed light weight engines. A planing hull is a different story.

Or you could just take a survey of what is in commercial fishing or offshore service boats of various vintages, especially if you really are going to run 10's of thousands of hours.

I always get a kick out of people wondering if 30 year old engine designs are reliable.. you know the old "I hear these are ticking time bombs" query..

To the OP, there is a forum called boatdiesel.com that can fill your winter interweb time with comprehensive discussion of about any engine you wish.

sunchaser said:

For the "turbo" record:

Turbocharged diesels have been around for a very long time. Alfred Buschi a Swiss engineer invented the turbocharger. He especially intended it for a diesel engine to improve efficiency. He took out a patent in 1905. Smart man.

In the 1920s large slow turning diesels began using turbochargers. Diesel locomotives began using turbos in the 1940s. By the late 1960s turbos were common on smaller truck diesels made by Cummins, Cat and Detroit. The rest is history.

Can't imagine a WWII gas engine fighter like the P51 having a service ceiling of 41,500 feet without a turbocharger on those screaming Merlins or knockoff Packards.

On a boat, if new you will have no choice except to get a turbo if your diesel engine is above say 100HP. Lots of higher (3500 - 4000) RPM turbocharged diesels in use, millions in fact if you count autos as relevant.

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Correct me if I'm wrong but. I thought the Merlin's had a gear driven centrifugal supercharger with a high/low gear box.

Good descriptions and history David and Sun.

Our previous boat had twin Volvo D6. Turbo and Super charged, 3500 RPM. We chose the boat for the boat, not the engines. It will be interesting to see how the longevity of these higher output diesels works out over time.

I prefer the lower range, and that is what we are doing on our new boat where I had a choice. The engine will probably outlive me.

BIG CAT said:

Correct me if I'm wrong but. I thought the Merlin's had a gear driven centrifugal supercharger with a high/low gear box.

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You're correct. My bad.

At the Stuart Boat show, I saw a new caterpillar engine with a supercharger and two turbos! Put out about 1000 HP rated if I recall correctly. It was a major pain trying to figure out all the ducting on that one!

stubones99 said:

At the Stuart Boat show, I saw a new caterpillar engine with a supercharger and two turbos! Put out about 1000 HP rated if I recall correctly. It was a major pain trying to figure out all the ducting on that one!

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Cough cough. It's mfg by Fiat for cat. The supercharger has an electric clutch that's ecm controlled. it engages at low speed then when the turbo boost comes in the ecm will disegage the supercharger.

I don't have a problem with high output, high rpm diesels. Just understand that if you log a lot of hours up near its rated output, it will not last long. If you use a small fraction of its rated output, it can last near as long as an old slow turning naturally aspirated machine.

High RPM, Light Duty Diesels

Wow! I can't believe all of the fantastic responses to my first post. Thanks to all of you.

I think it was Tego -- a good Montana guy -- who pointed out that my question was like asking what anchor is the best. Since I've been there and done that, let me refine my thoughts a little.

As regards boats, my intention at this time is to acquire an older (late '80s-early 2000s), full displacement craft (something like a Kadey Krogen 42) for cruising four-five months a year between Seattle and Sitka, AK (Inside Passage). Since my plan is to explore slowly the fjords, inlets, and bays that are both less traveled and frequently devoid of the trappings of civilization, what I'm looking for is a dependable engine that will push 36-40 feet of fair hull through the water at low speeds (six-eight knots max), low rpms (probably not exceeding 2,000) without me having to worry about if or when it's going to get too tired to go any further. The reality is that marinas, boat shops, etc., will be few and very far in between and because I'm not a gifted mechanic and won't have a boat load of major parts, dependability is a must.

What I've found is that vessels of that vintage have engines that have hours on them that range from 4,000-10,000. Although my research suggests that there may be a couple of diesels out there that can keep on going for up to 80,000 hours with a modicum of care, the longevity that seems to be more agreeable remains in the 5,000 to 20,000 hours range. Understanding that each engine is different and the care and feeding of all of them have diverged significantly, what I'm trying to discover is which engines generally seem to go the farthest, how many hours might be approaching too many on any of them, and especially, are there any I should avoid simply because they don't last or because they no longer have a viable support infrastructure.

As with my first post, I really appreciate your thoughts. Already they have been really helpful.

Ski in NC said:

I don't have a problem with high output, high rpm diesels. Just understand that if you log a lot of hours up near its rated output, it will not last long. If you use a small fraction of its rated output, it can last near as long as an old slow turning naturally aspirated machine.

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I totally agree, Ski. But those 4,000 rpm engines just aren't installed in trawlers any more. I see them in express cruisers and the like. And folks who own express cruisers usually want to go fast. But they probably don't put enough hours on the engine each year to worry about engine life.

But I wouldn't buy a ten year old one that was run at 200 rpm off of top for a thousand hours or so.

David

Well, now that you have defined your parameters, forget the high reving diesels. You won't find them in a Kadey Krogen or similar. And if you are serious about only considering FD hulls then your choices are limited, to maybe a Hatteras LRC or a Willard.

The Krogen and Willards will have the Lehman or Perkins 6s or maybe a Cummins 6BT if repowered. The Hatteras has twin DD 453 engines.

All four of these engines will last well over 10,000 hours if cared for well. And I would expect that all three would be well cared for, particularly as they are in displacement hulls, the power loading is not high.

And as a final comment, I will bet your search broadens to SD hulls, simply because the FD requirement is so limiting. A SD hull can use as much as twice the fuel as a SD one, but if you go slow, fuel cost will be a small part of your cruising budget.

David

Thanks, David. By way of further explanation, my last boat was a Nordic Tug 32 (SD) with a Cummins 6BT. The Cummins was quite a nice engine that I got along with very well. But in truth I rarely ran her over 1,800 rpms (about 8 knots). I do believe that I'm one of those rarer souls who knowingly is going from slow to slower (SD to FD) and loving the trip. (Most of my boating experience has been under sail; therein I only owned one vessel with an engine. That noisemaker was a '46 Johnson 6hp outboard that came with the craft when I bought her!) Maybe it's a function of age, or perhaps it's the incredible beauty of the Inside Passage that makes me want to throttle way back and just smell the sea weed, but slow is pretty much where I live when it comes to cruising.

The boats of the age I'm considering have Lehman, Perkins, Yanmar, Volvo, John Deere and, every now and then, a Lugger motor in them. I know the last two and trust them implicitly. I've run sailboats with Yanmars and they probably were OK, but my involvement with them was little more than turning the key and listening for the growl. Perkins I know only by vague reputation; ditto the Volvos. My experiences with anything resembling the Lehmans goes back to the '60s and small agricultural diesels (under) powered by Ford.

Against this backdrop, I'm having a lot of trouble sorting the good marine diesels from the better ones (for my purposes). And, living high in the Rocky Mountains makes walking the docks and talking to lots of people -- beyond a focused buying or looking trip -- difficult. Any help you all can provide would be terrific.

Bob

MountainSailor said:

Against this backdrop, I'm having a lot of trouble sorting the good marine diesels from the better ones (for my purposes). Bob

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When looking at 20+ year old boats, forget brand, to a degree. The good marine engines are those that have been properly maintained. Log books, maintenance records, ER appearance, survey and sea trial will give the best clues.

Why are you reluctant to walk the docks in Seattle? Boat show time is coming up. Easy drive for you. Wish I was that close.

Ski in NC said:

I don't have a problem with high output, high rpm diesels. Just understand that if you log a lot of hours up near its rated output, it will not last long. If you use a small fraction of its rated output, it can last near as long as an old slow turning naturally aspirated machine.

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I disagree with “it will not last long”

Obviously it won’t last as long as a lower output engine but for most pleasureboat owners it still will last much longer than most will ever use them .... If operated and maintained correctly.
Ski you make it sound like there going to go gunnysack next week.

Sunchaser, again, great input. Thank you.

As for being reluctant to walk the docks at the Seattle Boat Show, the very opposite is true. We've set aside the coming week to do just that, if we can get there. Beyond the time and the 1,100 mile round trip, the biggest impediment to such a journey at this time of the year is the weather. With January roads through northern Montana, Idaho, and Washington and two major passes to traverse in the process, getting from here to there on any given day can be an "iffy" proposition. Snow is in the forecast for the day we leave and who knows what even the major highways will look like, but we'll get there sooner or later. We know a couple of brokers that will walk us through a lot of our questions and concerns -- and try to sell us their boats! -- but mainly the docks at the Seattle Boat Show are populated by new or later model vessels that don't put us quite where we want to be. Anyway, we'll see.

MT Sailer

Weather I get, I was born in Butte and get to MT frequently still to enjoy the fun. Snow for the next week between Missoula and Spokane I see. Skiers will be happy.

djmarchand said:

I totally agree, Ski. But those 4,000 rpm engines just aren't installed in trawlers any more. I see them in express cruisers and the like. And folks who own express cruisers usually want to go fast. But they probably don't put enough hours on the engine each year to worry about engine life.David

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True this. High rpm pumped up blocks are mostly in EC's and Sport fishers, and those guys do like to go fast, all the time.

We had twin D6 in a 45ft. EC, and she needed to be at 2800 rpm to be on a good plane which is 80% WOT.

Smart engineers, with R&D, mounds of past data, and test cells design these, and they are not going to send engines out the door that conk out in 5 years, but I still think the jury is still out on the long, long term. IE, will these engines be happy 20 or 30 years from now?

OP, sounds like this is all moot based on your last post regarding what you are after, but it makes for a good discussion.

Fletcher500 said:

Smart engineers, with R&D, mounds of past data, and test cells design these, and they are not going to send engines out the door that conk out in 5 years, but I still think the jury is still out on the long, long term. IE, will these engines be happy 20 or 30 years from now?

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Wow!! You have a lot of faith that the engineers are running the business and making the decisions about what sells. I don't have that faith.

Look at Yanmar's new 250 hp, 2.7 liter 4LV engine. Making 90 hp per liter it would be a perfect installation in a 30' express cruiser. Then run it at 200 rpm off of top at 70 hp per liter and I bet it doesn't last 1,000 hours at that load. But it will last well beyond the warranty period.

David

djmarchand said:

Wow!! You have a lot of faith that the engineers are running the business and making the decisions about what sells. I don't have that faith.

Look at Yanmar's new 250 hp, 2.7 liter 4LV engine. Making 90 hp per liter it would be a perfect installation in a 30' express cruiser. Then run it at 200 rpm off of top at 70 hp per liter and I bet it doesn't last 1,000 hours at that load. But it will last well beyond the warranty period.

David

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Yes, I do have faith in engineers. Always have.

I have been out of the marine field for 30 years, and don’t pretend to know more than I do about diesels. That’s why I come here...to learn.

Back to engineers. In large, advanced engineering organizations, engineers get promoted into management and end up making large scale decisions. Yes, there are business pressures involved but they don’t send out products with the intention of getting 10 yards past the warranty period.

Fletcher500 said:

You partially quoted my last post.

Yes, I do have faith in engineers. Always have.

I have been out of the marine field for 30 years, and don’t pretend to know more than I do about diesels. That’s why I come here...to learn.

Back to engineers. In large, advanced engineering organizations, engineers get promoted into management and end up making large scale decisions. Yes, there are business pressures involved but they don’t send out products with the intention of getting 10 yards past the warranty period.

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I was one of those engineers that you speak of. 'Nuff said about that

I do believe that the 4,000 rpm, 90 hp per liter engines were consciously developed in the marine world to serve a niche- light express cruisers where the owner wanted a diesel, not a gasser. It is kind of like the gazillion horsepower DDs used in sport fishermen. They routinely get a cylinder/piston kit installed at 2,000 hour intervals to keep them going.

Unfortunately you can't do that easily on these high revving engines. When they get hard to start and use lots of oil, they are junk. But since they have only been around a half dozen years, this hasn't happened yet, but I'll bet it will.

David

MountainSailor said:

Thanks, David. By way of further explanation, my last boat was a Nordic Tug 32 (SD) with a Cummins 6BT. The Cummins was quite a nice engine that I got along with very well. But in truth I rarely ran her over 1,800 rpms (about 8 knots).

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Interestingly, this is a boat/engine combo that I am interested in finding out more information about!

Given that I would bet that most folks run their Tugs in the 8 knot range, I wonder what would be the expected life of a well maintained 220 h.p. Cummins 6BT? I see many of these boats (1998-2002 ish) with 2000 - 2400 hours on them. Wonder if they would easily go another 2000 - 2400 hours? I would also wonder about the 270 h.p. version of the Cummins 6BT that came out in 2003?

Jim

If you drove the boat the same speeds .. no problem.
As long as you operated the engines per the owners manual.

Re the qusetion about high rpm engines .. it’s misleading.
Should be about loading, not rpm. Running an engine at 4000 rpm should give a very long life if loads and temps were well into the safe zones.
Heat and pressure is the enemy .. not rpm.

Fortunately, purchased my new boat when non-turbocharged engines were not needed to meet pollution standards.

I love my Gardener 6LX 8 litres per hour at 8 knots 15000 hours TBO

Siesta has a Cummins 6BT 115Hp naturally aspirated engine with around 4000 hours on it. I run it at 1800 rpm to get around 8 knots average (depending on the tide). Any rpm higher is just a waste of fuel for very little gain in speed (the relationship between rpm and speed is definitely not linear).

Where we boat we have found the journey to get anywhere is a much fun as when we get there, so what's the hurry.

EDIT: I should add that the "fuel consumption to rpm ratio" is not linear as well ;-). For Siesta 1800 rpm is around 8 litres per hour, whereas 2400 rpm is up around 25 litres per hour and all this for less that 1.5 knots gain in speed.