Can large diesel engines be run at minimum RPM continually?

I am considering*types of boat to purchase, I have never owned a boat with diesel engines. I*plan to purchase*a trawler with one diesel engine (to reduce operating costs for extended cruising the ICW) and I do not have the need to go fast. Some motoryacht style boats interest me but usually they have two massive diesel engines (designed to run the boat on plane), can a boat with two large diesel engines be run*at minimum RPM*without negitively effecting the engines? Will running them at low RPM be cost effictive? Thanks for any input.

No. Unless* the engines coolant and oil temps get into normal operating ranges, minimum RPM is a recipe for disaster.* Boats are (hopefully) designed for how the intended victims expect to run them. Some older engines will suffer early demise if not run at a 60 to 80% load (as measured by fuel burn) *rating, Newer engines can operate at lower load ratings with occasional bursts to 60% or so.

When* you get a specific engine and vessel in mind, come back with the same question.

I think the rule of thumb is that diesels are happiest when run at 80% power 80% of the time.* The problem with that formula is that the power to get up on plane or move a displacement hull through the water at hull speed against some wave and wind action is geometrically more than the hp required to move the boat at cruising speeds.* Consequently, underloading a trawler diesel is inevitable, unless the power plant is smaller than needed for hull speed operations.* Example, my boat needs 200 hp to move at hull speed of 9.8 knots, and about 50 hp to move at 7.8 knots.* Unless I want to burn a whole lot of fuel and kick up a heck of a wake I will run well under the 80/80 rule most of the time.* The answer, for me at least, is to run up to hull speed an hour out of 24, or for a half hour on the way back to the dock to blow out as much soot as I can and get the EGT as high as possible.

So no, diesels aren't happiest when underloaded, but yes, under cruising conditions they frequently are.

sunchaser wrote:


When* you get a specific engine and vessel in mind, come back with the same question.

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That's a great point.* Some engines will suffer a lot more than others under the same operating conditions.

*

I've been strongly against under loading on this forum for years and I repowered my boat w an engine that I thought was the absolute minimum amount of power necessary to drive my boat. I got it close to perfect but could still do w a little less power. Full displacement boats are quite different regarding power loading than semi-disp boats.
The range of deviation of good power loading is very small on FD boats and more open to variation and opinion on SD boats like most all boats here on TF. If your boat was not powered with a good responsible amount of power (power loading) you may need to run your boat much too fast for it's hull design to load the engine properly so if you attempt to run the boat at a good responsible speed you will be under loading the engine big time. Perhaps most of our boats here on TF are in this category. Fortunately the vast majority are powered by the once popular (and sometimes still popular) Ford Lehman engine. The Lehman can and will burn 6gph at WOT so to achieve a 50% engine loading one must burn 3gph. Ideally (as Tom pointed out) we should be burning 4.5gph to be properly loaded but I'm quite sure most skippers on this forum are burning 2 or less. I don't hear about engines going belly up because of under loading but I'm sure there are a very high number of engines that are but their owners think a haze of blackish smoke is normal and hard starting is relative to how a new but broken in engine starts but new Lehman's are not common. A Lehman, Perkins 6-354 or similar engine is severely under loaded at 2gph but there's thousands of them out there that have run for years like that. I recently came close to buying one of many boats that I would have under loaded just like POs for a long time likely and then repowered. The only "trick" I have come up w to running gracefully under loaded is to disconnect the oil cooler and monitor the oil temp carefully and commit to NEVER operate the engine at or above the rpm where the oil starts to get too hot. This can, I'm sure be done but it's a bit like FFs idea of over propping (he calls it "cruise propping") whereas one cannot EVER operate the engine in it's normal working fairly hard mode. Under loading is bad but the bad effects are a long time coming*** ...there will be no sudden death** ..and years of boating fun can be had doing it so it's a bit like eating fat foods like deep fried foods but one will have to pay at some time most likely.

The real issue with running underloaded is with an engine with a raw water aftercooler. This will keep cyl temps very low when it's loafing along.

Otherwise you will be fine in most cases like David described.

sunchaser wrote:

No. Unless the engines coolant and oil temps get into normal operating ranges, minimum RPM is a recipe for disaster. Boats are (hopefully) designed for how the intended victims expect to run them. Some older engines will suffer early demise if not run at a 60 to 80% load (as measured by fuel burn) rating, Newer engines can operate at lower load ratings with occasional bursts to 60% or so.

When you get a specific engine and vessel in mind, come back with the same question.

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Part of the issue is how big are the engines. If you are looking at a 50'+ boat with twin 500 HP motors, you may find that the in gear idle speed may be faster than you want to go.

An interesting alternative would be to see if you could pull them out, sell them as "running takeouts" and use the money to cover much of the cost of much smaller rebuilt engines that would be of a more ideal size. A customer at the yard that I use recently purchased a ten year old sportfisher for $10K with twin Cummins with less than 1K hours on them. The engines and transmissions are worth over $20K.

If you did this, what would you do with all the extra space in the engine room?

Ted

*

I'm assuming that all suggestions are related to naturally aspirated engines. The turbo adds a wrinkle....from experience loafing doesn't generate high enough exhaust gas temp to allow the turbo to do what it's designed to do. Higher rpm can actually generate a more efficient fuel consumption/performance graph if the turbo is working properly. MHO

The usual soulution is called a cruising prop.

This lowers the engines cruise RPM by adding pitch and diameter , so the engine IS well loaded at the cruise RPM.

A fine solution is a CPP , (controlible pitch prop) but it would be really expensive as a retrofit, so you would never get the cost back.

Most cruisers carry a spare prop so a cruising prop could be a better choice than a somple duplicate of the stock unit.

With a cruising prop you will need an exhaust temp gauge , another $100 or so to NOT overload the engine , should you wish to travel faster (catch a bridge opening , whatever.

While the underloading problem is real, the question on the usual yacht use boat , is who cares?

A fine 10,000 hour engine might only get half or a third of the proper loaded service life , but 5000 hours at 200 hours a year is over a lifetime for most boaters.

Killick

Engines*with a properly designed turbo wastegate begin to operate at pretty low RPMs, mine kick in about 1250 to 1300 RPM. Boost at a modest 1700 - 1800 normal cruise RPM occurs with attendant benefits (and of course negatives as some will say).

Jleonard makes a very good point - one I had not considered. Fortunately, the Cat 3406B is JWAC which probably helps keep the temp up at the low revs we run at.

The price of diesel has meant we run at 9.2 kn (1250 rpm) nearly all the time. This is using*only 95hp from the available 540hp. However, it's enough to keep the temp at 175 degF and 1-2 lb of boost.

On our*2.5 week cruise at Christmas we averaged 3.75 USgals/hr. Of course, much of this was even slower running - we tend to potter about at 8kn (1100rpm) when going fishing etc), *but*also did*1.5 hrs at 12 kn in heavy weather (1600rpm).*

The history of this engine (1980 vintage) is*a lot of idling while running hydraulic haulers as a lobster boat and slow running between pots.*It was rebuilt at 18,000 hrs, when*we purchased the boat.*

I suspect the JWAC configuration is*a key*reason for the demonstrated long life even at low revs.

Jeff, I spoke to one of the CAT engineers who worked on the 3306 (my engine) and 3406 design team.* He said they were about as good an engine as he could imagine for a low rev trawler application because it was designed to idle in North Slope applications where they couldn't ever turn the motor off if they wanted to be able to re-start it.* These engines then proved more trouble free than alternatives in other applications, like the one you mentioned, where low load and/or long periods of idling were common.

Thanks for the info Delfin, *I guess it will outlast me...I'd hate to do another rebuild!

I think David on the previous page nailed it. The Guru he is referring to is Tony Athens and I have read the same thing. However, from personal experience; I purchased my boat from an owner that had not used the boat in nearly 2 1/2 years but had started the engines monthly. Regrettably he did not bring the engines up to temp, about 180 deg coolant temp. The result was a lot of soot in the cylinders that had to be cleaned out and is still being cleaned out.

If you're really concerned about the cost and use of fuel, buy a single engine trawler with a normally aspirated engine. Much less maintenance. Why buy a go fast yacht if you don't intend to go fast. If you buy a go fast boat you'll waste tons of fuel anyway because what you will do is get tired of lugging along at 8 kts and go fast anyway.

What I have found and what Marin has said all along, fuel will probably not be your greatest expense regardless of the boat you buy.

David; I just bought an IR gun, I'll hit the oil filter with it and see what I get. Never thought of that.


-- Edited by timjet on Tuesday 22nd of February 2011 08:36:06 AM

"*Regrettably he did not bring the engines up to temp, about 180 deg coolant temp"

I have had 2 diesel boats (3 engines makes, one was a repower)**and the highest temp I could get at the dock, even with the boat in gear for a while (meaning 30 minutes roughly), was something like 120 F.* I could get my Cummins 6BTA to 130 F but that was only *because I had a block heater running.

jleonard wrote:

I have had 2 diesel boats (3 engines makes, one was a repower)**and the highest temp I could get at the dock, even with the boat in gear for a while (meaning 30 minutes roughly), was something like 120 F.* I could get my Cummins 6BTA to 130 F but that was only *because I had a block heater running.

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I will generally get to 140 deg after idling for about 15 minutes but no more. Once I get to 1400 rpm, temps go quickly to 180. I take engine readings twice a day and these temps seem pretty consistent. I generally use only 3 rpm settings, idle, 1400 (low cruise) and 2000 (high cruise), except no wake zones then 1000 rpm.*

Perhaps a change in the thermostat will up the temps at idle.*

*

timjet wrote:Perhaps a change in the thermostat will up the temps at idle.*

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I tried that on my 330B but it didn't make any difference.

*

I run my twin TAMD 60 Volvos at the factory recommended minimum operating speed of 1450 RPM.*This gives me a fuel burn that equates to 80 HP per side and pushes my 36 IG through the water at 8.2 Knots with a fuel burn of 2.1 L / Mile. once per trip*I pop the engines up to 2000 RPM for 5 min to keep carbon build up down and to load up components to prove they will not fail when I need them. A good indication of carbon build up is when you run up above you normal cruising RPM see how much black*smoke you are making. You can expect some build up*from the exhaust but if it cleans up quickly you are doing fine.

Every now and then I have to out run weather*at a 80% load gives me 18 Knots however I am washing away small coastal villages with my wash!






-- Edited by Screwdriversteve on Wednesday 23rd of February 2011 12:16:26 AM

-- Edited by Screwdriversteve on Wednesday 23rd of February 2011 12:17:21 AM

normal cruising RPM see how much black smoke you are making

ANY black smoke should only be during the time you are actually accelerating, not afterward.

IT is a goo idea to go full throttle 100% for 5 to 10 min every so often.Your engine can take it.

This will let you know of problems , as well as allowing you to know what high cruise RPM can be carried with no overload.

Down 10% or 300RPM is usual. There should be NO black smoke at 100% (or your overloaded) and at at lower RPM.

"You can expect some build up*from the exhaust but if it cleans up quickly you are doing fine."


I Agree with FF however if you can achieve the WOT that is specified for your engine you can safely assume all operating speeds will not be an overload condition, except for that awkward plowing*period between planeing and displacement.

What HP and do you have turbos?

If no members chime in, your Perkins operating manual*may*have a fuel burn curve for various RPMs and prop loading. If not there, try either the Perkins website (that is where I got mine for my PS 225s) If not there, maybe Jay Leonard would have one from his old Perkins days. If not there, post on boatdiesel.com.

Can you achieve top rated RPM with your current props? If not, I would* not advise running at 80% +*load without knowing your exact (use IR gun as well as gauge) engine temps.

"maybe Jay Leonard would have one from his old Perkins days"

I only have the curve for a T6.354 at 160 hp (turbocharged version). *That was the engine I had.

Run the engine at full throttle OUT of gear for 10 seconds. Note the RPM in the log

Take her out and see if you get that , or less RPM under way.

A 2400 RPM mfg rated will usually pull 100 to 150 RPM over the rating with no load .

IF you get 2400 the prop is correct for the mfg, full load.

I agree with the folks that slightly over prop , and get longer engine life ,less noise and better fuel use.

However, BEFORE doing any WOT rpm test, you must first make sure your tachometer(s) is calibrated. You can do this with a hand held photo tach.

I'll second the phototach.** The dash tach can be off a lot.
Get a phototach that has the laser pointer for about $50-100 online.

After I got mine and did my engine I found quite a few more uses for it.

Screwdriversteve wrote:

*
I run my twin TAMD 60 Volvos at the factory recommended minimum operating speed .................*

Where did you find this information?* My TAMD41P manual lists a maximum RPM of 3900 but not a minimum.

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*

I found it in the operation manual for the engine, not the service manual. Often there is very good information in the small leaflets the come with equipment that gets disregarded because we "think" we know how to use it.

jleonard wrote:

"maybe Jay Leonard would have one from his old Perkins days"

I only have the curve for a T6.354 at 160 hp (turbocharged version). *That was the engine I had.

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Hi Jay, I just PMed you with my email to ask if you'd send this to me.* We have the*wo T6.3544s with 210 HP so not the same but we are curious to see your graph anyway if you don't mind.

*

jleonard wrote:

However, BEFORE doing any WOT rpm test, you must first make sure your tachometer(s) is calibrated. You can do this with a hand held photo tach.

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And you must also ensure that your engine can turn up the required and specified NO LOAD RPMS in neutral. If it cant then you will never reach max specified WOT .

*

R

Jay!

I have those exact engines, could you post the info you have?

Thanks!
Brent