Diesel engine longevity

If your boat is properly powered with 2 engines, it requires that thrust to operate efficiently at 70% load. If you run only one of those engines at 70% load you are severly overloading that engine - actually shortening its life.

Also, you have a free wheeling prop on the other side - what's that doing to your tranny. If you have dripless seals, are they being charged with water? Many other downsides to what you describe, IMO.

Beau

If you run one engine at 70 percent of its maximum continuous rating then that is exactly what it is doing, running at 70 percent.

There is another 30 percent before it even reaches its maximum continuous rating and somewhere beyond that before it can be described as "overloaded."

All the boat will know is that it isn't being powered highly enough to reach the same speed it would if pushed by two engines running at 70 percent each.

Hi,

Please do not confuse load with RPM.

Lets say your high speed marine diesel turns it's lot at 2300 rpm, at this rev level it develops 95-100% of it's full power when loaded with a propeller that matches the power curve of the aforementioned engine.

If you have strain gauge on the prop shaft you will be able to measure the power in kW or HP being delivered to the prop less a bit for some friction further down the sterntube.

If that same prop and shaft are covered in growth you will find that the amount of power ( the load it applies to the engine) it can absorb reaches what your engine can bang out before it reaches the previous max RPM.

If you can get one engine in a two engine application to 70% load you will most likely find the engine RPM is down a lot on what it was previously, this can be managed to a large degree by an electronic fuel system and almost not at all by a fully manual one.

I have a lot of experience in running 3500 CATS with one shutdown I used to always use the EGT as my primary guide when pushing the revs up on MUI engines, I let the PC deal with that stuff on the newer electronic ones.

As for free running shafts and trannys on the non running side - this can easily be achieved if there is an intention to run long distances on one engine by the use of auxiliary cooling pumps.

BTW - You will never 1/2 the F.O. consumption unless you are willing to take a good hit on the speed over the ground when running on one and often find that the boat really drops off to a crawl when you encounter any sort of head sea.

Is there a problem with leaving the non running engine gearbox in reverse while underway? I've always done this, even on catamarans.

Hi,

I have not tried this myself and can only see it working with a fully mechanically operated gearbox where the clutches can be operated without any oil pressure.

I used to work on a very old vessel with all mechanical operations, I was told it was possible but never tried to bump start one engine but running one full power and then dropping the other one into gear kind of like roll starting a car.

If you do it with a hydraulically controlled gearbox you will not achieve anything meaningful other than moving the lever in the wheelhouse because gearbox oil pressure is required to keep the clutch pack closed. Very few gearboxes have a rear mounted pump that can provide this oil when the shaft is freewheeling unless specifically ordered and built that way.

Does doing this lock the shaft?

KIWI

Thanks for the lesson. I'm still scratching my head though. It just seems counter intuitve. I'm sure I'll see the light when I enjoy my next rum. In my mind I was thinking of rpm, and misused the term "load".

Thanks again for the info

Beau

Your statement about overloading is still incorrect.

Using the propeller demand curve of a CAT C19 as an example:

Maximum rated horsepower available to and absorbed by a "well matched" propeller is 575 at 2500 rpm.

If you reduce rpm to 70 percent you will see 1750 rpm on the tach and the propeller will absorb approximately 200 horsepower or around 35 percent of rated horsepower.

If you had a horsepower meter installed on the boat and ran the engine at 70 percent of rated power you would see the power meter reading a hair over 400 horsepower and the tach would indicate about 2250 rpm.

Far from overloaded in either case.

Marmot

I hear you. I just can't get my arms around that concept. My boat weighs 60K pounds it really needs to my two mans to push it thru the water, Never tried it, but with one engine at say 1700 rpm (wot 2300) I think I would barely be on plane. if at all. Isn't that engine working darn hard? I would hate to see my water temp and EGT.

Beau

Oops, as some kind soul pointed out in a PM, there is no C19 (yet anyway).

The engine I used as an example was a C9. Sorry to confuse anyone but that is a good example of an engineer's brain to fingers to eye connection.

Sometimes the fingers move quicker than light and when you are used to never making mistakes the computer just can't keep up. That's my story and I'm stickin' to it.

Sounds to me like it would be loafing along at low power not going very fast. You're not burning much fuel so why should the temps worry you? Why do you think they put an EGT and coolant temp gauge in front of you?

Try it sometime ... don't you want to know the the "envelope" within which you can operate your boat? What would you do if you lost an engine and had to come home on one? It shouldn't scare you to explore the boat's limitations.

I would be more afraid of leaving the dock with a captain who doesn't know his boat's operating parameters than I would be afraid of exceeding the engine's limits, especially with gauges to tell me when I reach them.

Actually running most engines at 70% load, on 1 engine when the engine is propped to achieve lets say 2500 rpm's at 100% throttle/ 100% load using both engines, is very detrimental to most engines longevity. They are expected to see 70% load at let's say 2250rpm's, not 1750 rpms. At 1750 rpm's the engine is throwing fuel for 70% load, except the turbocharger is not providing the amount of boost, airflow, and HP the engine should see at 70% load/ 2250rpms. So the engine is seeing too much load for it's rpm and it bogs it down and creates more wear........

Caterpillar's computer will restrict engine RPM to around 1200 rpm's if they see too much load for the actual rpm range the engine is producing. MANs computer will spit out alarm codes.

Some engines can be detuned to make less HP either through the computer, or through injector and turbo changes which would allow you to run 70% load at a lesser fuel burn and speed. The propellors would need to be re-pitched in order to accomodate the lesser HP.

You could also run most engines at a lower load factor such as 40% and then run them up to cruise for 1/2 an hour every 4-6 hours without shortening their life much.......sometimes if any.......

Actually in the real world, with a C9 propped to achieve it's 2500rpm top end. At 1750rpm's you will usually see around 67% load. At 2250 rpm's you will see 82% load. Give or take a few percent. In most planing hulls. The C9 usually see's 67% load at 1750rpms, but the load factor from there to 2250 goes up very little with RPM's and very non linear. For example........2250 rpm's 82% load, 2200 rpm's 80% load, 2150 rpm's 78%, 2100 rpm's 76% etc etc.........Most all of the modern CATS are this way once they reach around 1650rpm in ones that turn 2350rpm's max.....and 1750rpm's in ones that turn 2500rpms....... Once they start developing a decent amount of boost

MARMOT
I have run my boat on a a single engine several times due to a failure on the other side. I never ran it up over 1000 rpm because of what my instruments were showing me. Am I an anomally?
When both engines run well, which is most of the time, I "loaf" around 30 knots, top out at 36. I plane on 1300 rpm with both engines wifi a squat.

If you read my post closely, you will see I have never attempted what is described in previous posts, but I would hate to see what my various temp gauges would show at that rpm on one engine. I monitor those gauges religiously

Hi,

This is a load of ill informed and mis leading drivel to put it mildly.

I would very much like to know what the gauges would show at all combinations of power and speed.

It is good for a laugh if it was not that some one may believe some of it.

I know what you are trying to say Capt. J. The condition you are describing is what I believe is the problem described in the other thread where the Volvos are overheating when reaching a certain rpm.

The difference is that 70 percent load is just that, 70 percent load. It cannot be anything else.

The load at 70 percent rpm is capable of varying widely in relation to boat speed, hull condition, sea conditions and other variables. But ... and this is the operative condition, if the throttle (speed setting) is not firewalled, and the governor fuel stop (or electronic fuel limiter) at maximum delivery with the hope that the boat will eventually accelerate to normal twin engine speed, the engine will not be overloaded.

If the boat is propped so as to allow matching the prop load to the engine rated horsepower and rpm at a certain speed through the water it is possible to overload the engine at high throttle settings below that speed due to propeller operating conditions. But ... in the case of a twin screw boat operating on one engine, as long as the throttle was not set at max rpm (you don't set power with the throttle lever) the boat would run along at a speed less than it would on two engines at the same rpm. The power consumed is limited by the propeller curve, not (in most cases) by the engine power curve.

Going back to the C9, the engine is rated to provide around 575 hp from 2100 rpm up, it will safely produce over 500 hp from around 1700 rpm. A "well matched" propeller will only absorb around 150 hp at 1700 rpm. There is a big difference between what an engine is rated to produce and how much a fixed pitch propeller can absorb over the rated output range of the engine.

This is very easy to see in real time on larger twin screw boats that are equipped with torque and horsepower instrumentation. When a sharp turn is made at high speed and maximum power (on the fuel limiter) settings the inside engine can easily overload due to the slowing of the water inflow to the inside prop. The load on the outside engine changes very little. But ... and this is the operative one, if the governor of the inside engine is moved off the speed stop so that the fuel limit is not reached, it is impossible to overload the engine.

I disagree with some points. If an engine is designed to achieve 2500 rpms at full throttle/load in a twin engine installation. If you run the boat on 1 engine, regardless of the fact that you are seeing 70% load on the gauges. The engine is now in a seriously overpropped and overloaded situation since it might only achieve 1600 rpms at 100% throttle/ 100% load. It is not producing 70% load in the proper RPM range the manufacturer engineered it to, but a much lower RPM and much lower HP and outside of its design parameters, creating a lot more wear because it's the same as trying to climb a hill in a tractor trailer in high gear.


Also, if you run a yacht on 1 engine that sees 70% load at 1750 rpms with both engines, you'll be at around 70% load with the one engine at 1000 rpms, below turbo boost, way outside of the torque/hp curve of the engine, and yes the coolant temp and EGT starts to really rise beyond this as Beau has seen. This is no different in regards to wear than running a yacht where both engines are severely overpropped and overload and achieve top rpm's 20% lower than they should achieve. The engine also does not have the airflow to properly burn the amount of fuel the rack will give it at 70% load, and the same with the computer controled injectors......

If you can get any large engine manufacturer of high speed marine diesels that you commonly see in yachts(CAT,MTU,CUMMINS,MAN, Etc.) to e-mail you stating otherwise, and that it would not void warranty to continually operate an engine this way, then I will believe it. But I'm willing to bet not 1 manufacturer will.

I've run a lot of boats on 1 engine because I had to......You start going much above 1000rpms with most engines and they start spewing black smoke out the exhaust and coolant temps start climbing above normal operating temps.

The only instance I have found this not to be an issue is in a boat/yacht that is severely over-powered and shutting the 1 engine or multiple engines off doesn't increase the load factor much on the engine you're running.