How hard to run a Perkins Diesel?

So I have a couple of questions on running my Perkin's diesel engines.
According to TF member Greg, I have these below decks:

2-6.354's made in Britain in 1978.
They have about 1200 hours on them, according to the hour meters.

What is the usual RPM to run these engines? I live in the CA Delta, in semi fresh water, and with the tides, I like to run mine at about 1200-1300 RPM for my 34" Californian. This keeps me in the range 7-11 MPH, depending on which way the tide is going; I could go higher, to 1800-2000 RPM, but the noise level increases along with the fuel consumption, and I only seem to get a mile or two an hour gain.
Doesn't seem worth it to me, the engines are working harder, fuel use goes up, etc.
The reason I am asking is because a friend who has a similar boat and engines, says that he typically runs his at 2000-2200 RPM all day, 'because these engines need to run hard to make the last, and running them at lower RPM's is not good for them'.
As I've admitted previously on this forum, I am not an engine head, and only go on my gut instincts, and I think that running an engine harder than I need to, for a mile or two an hour gain, is not what I want to do.
These engines are, in any event, 40 years old, albeit, with relatively few hours on them, but how hard should I actually push them to keep them running for another forty years?
And what about the transmissions?

Unless you're a super user putting a thousand hours a year on your boat, your engines will outlive you as long as you maintain them. They might be happier at a higher RPM, but in the grand scheme, means they will expire at the human equivalent to 90-years old instead of 100. Maintenance is, by far, a better indicator of lifespan. Do not ignore any leak no matter how small, keep a close eye on the raw water cooling system (including replacing exhaust elbow), and your engine is good for many, many thousands of more hours. Even at 1200-1300 rpm. Classic advice is to run-up the rpms to 80%-85% WOT periodically which is still good advice.

But your post hints at something else - a boats least efficient speed is just before it climbs on-step to a plane. Adding more throttle pushes a lot of water but doesn't add much speed. It takes a lot of work to push water which is where your fuel is going. It's not converted to speed. What your buddy is doing is getting up on plane at WOT, then reducing back to around 85% RPM. It's not exactly fuel efficient compared to displacement speeds, but it is more efficient than bulldozer speed.

Enjoy your boat. Enjoy the Delta. We moved from SF Bay to Florida 15 years ago. We affectionately recall our times cruising in the Delta. ICW is somewhat similar.

Peter

'because these engines need to run hard to make the last, and running them at lower RPM's is not good for them'.

Not true. An old wives tale that has bounced around the docks for years and years. The Perkins will last a long, long time running at 1,200 rpm.

David

1200-1800 will be fine even with wrong props and dirty bottom. You did not say if they are turbo charged or not but in this range you will be good up to at least 2000 if clean. Even more depending on the model. Just watch the coolant temperature and exhaust plume.

I used to run mine at about 2000 rpm. That was in a 1978 Mainship model 1.
It was actually a "anti-clockwise" rotation engine, which is opposite a normal engine.

Guess what? Your engines do not know they are 40 years old. That number is irrelevant.
Hours and maintenance are the key.

Run them so they come up to temp and sound "happy". They will last many thousands of hours.

pete

Nothing wrong with running around at 1200-1300 all day. At the end of the day, good to kick it up to say 1800-2000 for maybe 10-20min to burn off soot and gunk from engine internals and to cook off any moisture that may build up in lube oil. Even though boat is not at an efficient spot with engines at 1800, for 20min the extra fuel burn will be trivial.

When you kick it up to 1800, you may see some blu-ish exhaust smoke for a few minutes, that is the gunk getting cooked off.

OP wrote;
“The reason I am asking is because a friend who has a similar boat and engines, says that he typically runs his at 2000-2200 RPM all day,”

The friend may be right but it dosn’t depend so much on rpm. It’s the load that makes most all engine wear and load is dependent on how your engines are propped. That is .. the dynamic relationship between the engine, trans gear ratio and prop.

It’s been said 1000 times to get rated (plus a little) rpm at WOT. Once you get a little past rated rpm then you can use rpm in a conversation w meaning. Your friend may be able to (at WOT) exceed rated rpm by 100 to 150rpm. Then his running 2000rpm “all day” has some meaning. But it’s not all said unless one can get a bit over rated rpm. But if your friend can only get to 300rpm less than rated speed his statement has no meaning. WOT first ... then rpm talk.

I’d keep running your engines slow until you get propped right. I don’t know the rated rpm of those engines and you omitting that very important number is very common here on TF. First thing I’d do is ask your friend what his WOT rpm is and if it’s .. say 2800 then his statement is completely correct. Load isn’t high and if oil and coolant temps are correct so he’s “good to go”.

But most importantly run your engines at WOT (if your cooling system and all else is in good order) for 30 seconds so you get a good read on the tachs. And to be sure you should check the tachs also in the 2500rpm range.
I did that when I first got my present boat. The engine quit. I reached 3000rpm and found out right away that my fuel delivery was weak. The engine started back up soon and that was a good thing as I was in a narrow channel. I was going to test for prop load but found out something else. If one does this every few months many to most engine problems will show their ugly face and quite likely save your bacon in the near future.

Peter wrote;
“It's not exactly fuel efficient compared to displacement speeds, but it is more efficient than bulldozer speed.”

Peter should get a blue star for all of that post ... #2.

I really appreciate all of your insights and will now worry less that I am "babying" the engines!
It's just a smoother, more relaxing way to cruise in my opinion.

Mtcabsales said:

I really appreciate all of your insights and will now worry less that I am "babying" the engines!
It's just a smoother, more relaxing way to cruise in my opinion.

Click to expand...

You will be fine running it like that.

Eric talks about an absolute need to size an engine/gear ratio/prop in order to achieve the rated rpm for the engine under WOT.

I've always thought that this concept is fine, until you consider the rather large effect of variable boat weight.

For example, when WESTERLY has 100% water/fuel, provisions, tools, persons, etc. WOT is 2575 on a 2600 rpm rated engine. But it is only in that condition for 3.6% of cruising days, the start of the trip North to Alaska for about 2.5 days, and the start of the trip south for about 2.5 days. The rest of the time (96.4%), it will turn over 2600 rpm due to lower vessel weight.

I've found that cruising at 1600 rpm and producing 45 hp sufficiently loads the engine. And I know that for 96.4% of cruising days, the engine is not overloaded by definition. So, where is the line drawn that makes this acceptable for boats in recreational service?

The biggest historic problem with weight are vessels that are new, and propped to achieve their rating at significantly under 100% wet load, with no provisions/tools/curtains/bedding/persons, etc. Many new boat owners have been caught in this situation, purchasing boats that needed significant pitch reduction after becoming fully loaded, if the engines survived the initial years.

Even in the towing industry, it used to be common to have a new tug built with two sets of fixed pitch propellers, the sea trial props for where the engines will produce maximum bollard pull (static), and another set to be used for actual towing. When the tug finally starts working using the towing props, a heavy tow will keep the engines from reaching their rated output. It is a very common situation, and tug operators are quick to reduce engine power to make sure that the overload condition is not immediately harmful.

But the thing to remember is that over the service life of the engines, they will not be overloaded for the majority of the time (there may be a few exceptions). Can this concept also apply to recreational vessels?

Hey MtCabSales,

If I remember correctly, you've got TC series engines, 20651 builds, so standard naturally aspirated marine 6.354s.

If you can, post a picture of your injector pumps label plate. It will tell us how it is set up. In particular, take a look at the attached photo. That 3rd number is the WOT "no load" max rpm that you are set up for. In other words, if you'd WOT in neutral, that is what you'd get.

What one gets under load varies a lot with props, bottom, etc. It isn't uncommon to see diesels set up for 90% or less, sometimes much less, underload.

A lot of people overprop to run at lower rpms to get less noise figuring they don't want the top end, anyway.

Also, it isn't uncommon to have the controls set up so they can't WOT the pump, but instead have a set back of 100-200rpm, that way an operator doesn't slammed the throttles and damage the pump.

I bet you've you've got an F728 or equivalent pump, just like the one I pictured. I've attached a power curve for what I think is this configuration of engine, or close enough.

You can see from it that the RPM range you are talking about is a nice one for the engines. They've gotten up the hill. Amortized some costs. And are making pretty much max torque per cycle. Beyond that, it actually goes down per cycle,, but makes more horsepower just by more cycles per unit time, i.e. diminished marginal returns, but increasing total returns.

Your engines don't have turbos to soot uo, so you don't have to worry about keeping those spooled up.

I guess what I'd recommend is taking your boat out and, if you are comfortable, WOT it very briefly. Find that RPM, and then cruise anywhere between where you are and 80-85% of that.

By the book, I suspect those engines are set up for max output and max WOT RPMs under load to be 2800rpms and for maximum continuous operation, e.g max cruise rpm, at 85% of that, i.e. 2400rpm.

So, I'd see what you actually do WOT under load, and then take 85% of that as your max cruise. Being conservative, I'd stay at least a couple hundred RPM below that most of the time.

I'd also keep an eye on the smoke. If it starts to darken a lot before you get to that 85%, you might be overpropped. Another indicator is not getting to the top RPM.

But, again, a lot of people overprop. I don't like the practice, myself. But, within reason, and unless you are running with avoidable dark smoke at your preferred cruise, I thinknfkrum searches will show it to be a religious issue.

But, back to your question, there is nothing wrong with a 1200-1300pm cruise, unless something else is wrong, e.g. drastically overpropped, in which case the "something else" would be the real issue.

I think, at the least.

Personally I think DavidM and Ski have nailed it. Running at "lower RPM" does not "hurt" a diesel (usually). That is an "old wives" tale. Tony Athens, a respected marine diesel guru states that he has never seen a marine diesel fail due to being run at a "lower than optimal" RPM. He also states that the engine's lifetime is more related to "fuel burn" than any other factor (assuming good maintenance practises are followed). In other words, your engines will theoretically last longer if run at half throttle than they would if run at 85% (all things being equal). It also makes "common sense". Thousands of hours at 2200 rpm compared to thousands of hours at 1300 rpm. For example 3000 hours at 900 rpm difference means 2.7 million more revolutions. There has to be more wear and tear! This does not even take into account the fact that many boats operate in an "overload" condition which is really hard on the engine.

The talk about over propping is a good idea to look into, and running your engine up to 80% of WOT for 10 minutes on a regular basis is not a bad idea either. Even a few minutes at WOT with some regularity is not a bad idea either, assuming you are not overloading the engine (you are properly propped). This last (occasional 80% to WOT) is even more important with a turbo charged engine like mine.
I am not qualified like Ski, but this is what my rather extensive research has shown from reliable sources like Tony Athens.

@gkesden
Thank you!
I happen to be on the boat as we speak soaking some bait and enjoying this wonderful CA afternoon!
I went below, and was not able to locate the plate you mentioned on either engine, similar to the last time you had me look for the numbers!
And particular place I should look?

Hi MtCabSales,

It is on the injector pump. Follow the lines back from the injectors to a cylinder-ish thing about the size of a small old school coffee can, a little bigger than a soda can. It'll be on the "outer" side. It is directly "behind" (really "inside", closer to the longitudinal centerline of the engine) and above the area where you read the engine serial number.

BTW, I didn't intend to suggest that you shouldn't run slower than what you were talking about. I don't see a problem with slow. I just intended to note that the RPM range you had in mind is a really nice one torque per cycle wise. And, beyond that, figured I'd add in how to find the top end. I didn't intend to suggest that you should go faster.

Personally, I think a little time at higher output is good for the injectors. But, I can't support it. I've just seen a bunch of boats running rough, get used hard, and come back and run well after that to totakly dismiss it. Having said that, more often than not,, I think, "It just needs run hard..." is just a used boat salesperson's line!

(Sorry for the delayed reply, I was out on the water).

Jay N,
Very good copy.
And you should be aware that I’ve had great respect for your posts for a long time. I see also that you’ve been w us since 07.

Whenever I post about engine load I try (believe it or not) to not ramble on and on.

But most boats are old and have had the weight you speak of for a long time. In the old days we needed to be aware of how much weight has been soaked up in the wood in the hull. Thankfully that’s in the past.
But if most prop up the way their boat is usually good enough boat weight wise. But your bringing it up and putting the right importance on dealing w it is excellent.

But now that you mention it when I replaced my ballast w lead I filled the Laz up to the level it was w concrete/steel punchings so I’m now heavy in the stern ... assumably. I thought I’d gradually pull out weight until trim was achieved. Never got around to it. That extra weight could pull my rpm down about the amount that it was. (50-75rpm)

So you’re right. Boat weight is not a fixed thing. Since I haven’t done a free WOT run yet since I took an inch of pitch out I still don’t know my own status. I did, however run a bollard pull at the float. And I got 2500rpm. (3000rpm engine). Well up from the rpm w the previous prop. But I haven’t been out w the boat since before the covid stuff started. Soon.

Re the attachment on gkesden’s post the power curves are said to be “vehicle” curves. Wouldn’t that be a rating for a truck engine?

Thanks! And good for you, fishing?
I’m still out so I will go back down and look.

Re load I’ve thought for a long time that a 50% load is about as low as one should go.

But re posts here quite a few seem to be at only 40-45% load and not seemed to have suffered from it. But the “suffering” may just be not observed and unknown. One could find out by tearing the engine down and inspecting the rings and pistons. I’ve been running my engine for all of it’s 1000hrs and I do notice a little blackish smoke on startup. And when the engine was closer to new there was almost no smoke at all. Always run at 2300rpm ... almost always. Perhaps I’m one of the ones “suffering”? If so 50% load is not enough.

One thing’s for sure if an engine’s blowing black smoke there is a load problem.

So I found it and it appears to be the same as your pic.
3130. I will try to post pic when back at my desk; can’t figure it out on my iPhone, asking for an HTTP address?
Not sure about that.

"For example 3000 hours at 900 rpm difference means 2.7 million more revolutions. There has to be more wear and tear!"
Sorry, made a "small" (not so much) math error. If one engine runs at 2200 rpm and another runs at 1300 rpm (900 rpm difference), the actual difference in revolutions after 3000 hours is 162 million revolutions not 2.7 million. The premise that common sense indicates there should be more wear still stands (even more so). Obviously you do need to load a diesel and ensure it reaches proper operating temperatures so this is not a case where less and less is better and better.

Mtcabsales said:

So I found it and it appears to be the same as your pic.
3130. I will try to post pic when back at my desk; can’t figure it out on my iPhone, asking for an HTTP address?
Not sure about that.

Click to expand...

The upload and url-link are two different buttons. Uploading is the paperclip. You might need to select "advanced".

But, no worries. You got the 411.

"By the book" your /engine/ is set up for a max WOT under load of 2800rpm and a max continuous rpm of 2400 rpm.

...the upshot is the numbers and chart in my earlier message do look like they apply to your engine (with usual disclaimer about the rest of the boat).

So, the bottom line is to feel free to cruise as slowly as you'd like, but not continuously above 2400rpm or 85% of your own WOT loaded rpm. And, I'd recommend less than that. Why push limits?

Also, by the book, you can run at the lesser of 2800 and WOT for 1 hour bursts with a minimum of 15 minutes of 2400rpm or less in between such bursts. But, again, why push it?

If you want the docs I have collected for this type of engine, they are here:
-- https://drive.google.com/folderview?id=1oBq3YiiqKh9sVLVCWbt90JQRNcKQ8RGj

No fishing today. I had a small few non-boater friends over for a socially distanced gathering on the flybridge (which is large on my Europa-style boat) for a lazy cruise along the ICW.

When I was shopping for this boat, I really wanted a 46' Hatteras Convertible, but it was a project boat and I couldn't resist either the large fkybrisfe or it calling my name to finish what the last owner had started.

Turns out, it is, imho, the perfect COVID-19 boat because, even though large gatherings aren't my style -- small ones can be done totally safely, even w.r.t. optIMHO,

Enjoy your ride! Perkins 6.354s are great, reliable, simple engines.

Firehoser75, or Tom,
One would think there would be wear. And there is IMO.

But I believe the bulk of engine wear is between the piston and the bore and rpm mostly is not the reason for wear. The wear comes from pressure. The piston pushing sideways against the outboard cylinder walls. It is common knowledge pistons and cylinder walls wear to an oval shape .... not round.

This happens because the crankpin is off-center .. especially during the power stroke. Some engines have cylinder bores that are off-center to compensate or to accommodate other aspects of the engine design but most are not.

But the bottom line is IMO that most engine wear is caused by the pistons offset crankpin causing high side forces to the piston side and cylinder bore.

So reducing prop pitch can produce the same power and thrust at a higher rpm with LESS engine wear and frequently less fuel burn.

Here's the injection pump; have yet to learn how to rotate the picture, Looks right until I try to upload.

I wouldn't sweat the rotation. It is a pain point for all.

And, yep, we're talking about the same pump and configuration!

Thanks to all for the input and advice!

A very simple ROT (rule of thumb) for any non turbo engine is the engine cubic inch displacement , divided by 3 should be fine for HP at normal cruise.

Happy Perkins

My 1982 T6-354 Perkins pushes along 8.5 knots at 1,150 RPM. with a 22" prop. At the time of replacing the shaft and prop I spent considerable time with several prop shops and a shipwright going over the best speed/prop/horsepower calculations. All came up very close to each other and the realization was that ultimately at 90% of my cruising I did not require more than 35-40 HP to push my 32' Apollo. I simply did neither myself or the engine any benefit by running 1800+ RPMs, even at light loads and the engine easily produces useful power, under tolerable load, without just pushing itself up out of the water.

1800 all day long. Sweet spot ?

2300 for 10 minutes every 5 hours . The best oil and filters every 100 hours. 2800 hours on her now

How hard to run a Perkins Diesel.

All the boats that I have owned (some 20 over 60 years) have been second hand. Thus the engine and props have already been sized and paired off.
My usual practice over the years is that I get up to WOT and check the revs. Then I compare the revs with the engine specs. I take her down to 90% of WOT revs and that becomes my maximum. At WOT I am probably flogging the engine so at 90% I will have taken the load off.
Now if I have a displacement hull I need to know the economical hull speed which will be √WL knots. The maximum displacement speed will be 1.3√WL knots.
Depending on how the prop was sized in the first place I would hope that the engine revs will be much lower at my economical cruising speed.
For an older engine with many hours, I would only run it a what I would term a reasonable speed below the 90% speed which I would hope will not be more than 1750rpm. This would be for the Lehman 120hp and the Perkins.
The worst-case will usually be if the engine is trying to pull the boat at the maximum revs at which point the heat being generated will be overloading the heat exchangers and cylinders. Not a good place to be.

Count your blessings

You have two of the finest, long lived, low maintenance diesel engines every produced. They will be chugging along just fine long after you are underground if they are maintained. Maintenance is pretty much just keeping the oil changed and being sure they are fed with clean fuel.