Engine replacement with another of increased horse power: Is it worth it?

[Bobc]

we often hear of bores glazing. Glaze is smooth. Smooth is low friction. Low friction is low wear.
Getting clogged with carbon deposits is a separate issue. Oil flow should keep those bores clear, if it doesn’t the oil and the filter isn’t doing it’s job.

Not sure you quite understand how cylinders are lubricated.

 

[Bobc]

generatorwhatisglazing

And from Vye's website:
bore-glazing-and-polishing-in-diesel-engines

Summed-up quite well in the first paras of the top article.

This is engine damage caused to generators either through misuse or poor sizing.

A diesel engine is designed to operate at above 60% of it’s maximum load and ideally closer to 75%.
An engine relies on sufficient cylinder pressure to force the piston rings to tightly seal in the bore against the film of oil on the bores surface. When an engine is consistently run at less than 60% load, it leads to a drop in cylinder pressure which in turn causes poor combustion and subsequently low combustion pressures and temperatures.

The poor combustion allows unburnt fuel reside to form an oily treacle like substance that clogs the piston rings creating an even greater loss in cylinder pressure. Now that the cylinder rings are not sealing properly, the hot combustion gases force their way past the rings and flash bake the oil on the cylinder wall into a hard, ceramic like substance. This is known as glazing or glazing the bore.

I don't entirely agree with the last para, as it actually clogs the cylinder honing grooves, but it probably does also gum-up the rings too.

Bottom line is:- Don't fit an engine that's too big, as it is likely to destroy itself sooner than it should.

 

[geem]

You're saying it physically won't rev past 2000 rpm while motoring along in flat water?
At what point does over-propping an engine run the risk of not being able to get to peak torque , if for instance the boat was stuck in some mud?
I don't see this being an issue with a beta, which claims 26Nm@ 800rpm, peak torque about 31 @2600, but the 1GM10's torque looks to be falling fast as the revs go below 1800:
1GM10 - YANMAR Marine International

Yes, it won't rev past 2000rpm. Peak torque at 1200 but my engine cc is huge compared to a 1GM. Check the peak torque of that engine. Ideally cruising at peak torque would be good but you need revs spare to get to acceptable Hp should the need arise. If you can't do this you are over propped and it's a problem

 

[Stemar]

Bottom line is:- Don't fit an engine that's too big, as it is likely to destroy itself sooner than it should.

Having done it - 28HP in a 24ft 2 1/2 ton boat - I'd say it's a waste of money and, perhaps fuel, but engine life doesn't have to be a serious problem. My VP2003 always started and ran well, but over a few months of regular use at around 2000rpm (max 3400, from memory), it would start to smoke a bit. That was the signal for an Italian tune-up. Get somewhere where I won't annoy anyone and open it up as far as it would go. I'd lay a smoke screen worthy of a destroyer on the Murmansk run for a couple of minutes, then it would clear and I was good to go for a while longer. The engine was from 1989, a freebie, replaced on a charter boat, and shoved Jissel along a treat for 16 years and counting.

 

[Wing Mark]

Summed-up quite well in the first paras of the top article.

This is engine damage caused to generators either through misuse or poor sizing.

A diesel engine is designed to operate at above 60% of it’s maximum load and ideally closer to 75%.
An engine relies on sufficient cylinder pressure to force the piston rings to tightly seal in the bore against the film of oil on the bores surface. When an engine is consistently run at less than 60% load, it leads to a drop in cylinder pressure which in turn causes poor combustion and subsequently low combustion pressures and temperatures.

The poor combustion allows unburnt fuel reside to form an oily treacle like substance that clogs the piston rings creating an even greater loss in cylinder pressure. Now that the cylinder rings are not sealing properly, the hot combustion gases force their way past the rings and flash bake the oil on the cylinder wall into a hard, ceramic like substance. This is known as glazing or glazing the bore.

I don't entirely agree with the last para, as it actually clogs the cylinder honing grooves, but it probably does also gum-up the rings too.

Bottom line is:- Don't fit an engine that's too big, as it is likely to destroy itself sooner than it should.

So why do car engines last thousands of hours?

 

[Wing Mark]

Yes, it won't rev past 2000rpm. Peak torque at 1200 but my engine cc is huge compared to a 1GM. Check the peak torque of that engine. Ideally cruising at peak torque would be good but you need revs spare to get to acceptable Hp should the need arise. If you can't do this you are over propped and it's a problem

Thanks for that. I need to understand this, because I may be boxed into a situation where I will re-prop a boat without having done much mileage on the old prop.
I may be buying a boat with a fixed prop and change it to a folding, which will be a lot of wasted cash if I get it wrong!.
Ideally I will keep the boat afloat and do some local trips before xmas, enough to evaluate the existing prop size fully, but if my mooring doesn't come good, it will have to be ashore for the winter and that will be the time to change the prop.

 

[Bobc]

So why do car engines last thousands of hours?

because they are under load. Every time you accelerate, then engine is under a high load. Speeds and loads vary constantly, whereas boat engines are run at the same revs for hours. Car engines don't spend 80% of their running life at a constant 2,000rm in second gear.

Diesel cars that do lots of short trips (and never get really hot), do also gum-up and don't last anywhere near as long as the motorway mile-munchers do.

 

[geem]

Summed-up quite well in the first paras of the top article.

This is engine damage caused to generators either through misuse or poor sizing.

A diesel engine is designed to operate at above 60% of it’s maximum load and ideally closer to 75%.
An engine relies on sufficient cylinder pressure to force the piston rings to tightly seal in the bore against the film of oil on the bores surface. When an engine is consistently run at less than 60% load, it leads to a drop in cylinder pressure which in turn causes poor combustion and subsequently low combustion pressures and temperatures.

The poor combustion allows unburnt fuel reside to form an oily treacle like substance that clogs the piston rings creating an even greater loss in cylinder pressure. Now that the cylinder rings are not sealing properly, the hot combustion gases force their way past the rings and flash bake the oil on the cylinder wall into a hard, ceramic like substance. This is known as glazing or glazing the bore.

I don't entirely agree with the last para, as it actually clogs the cylinder honing grooves, but it probably does also gum-up the rings too.

Bottom line is:- Don't fit an engine that's too big, as it is likely to destroy itself sooner than it should.

I understand this to be true if it's a new engine. The advise being to not have new diesel engines sat on idle. Glazed bores and all that. Once the run in period is met then do bores get glazed? My own engine has 2600 hrs in 15 years. Uses no oil and doesn't smoke. It spends most of its life producing about 30-40 hp so less than 50% of its rated power, allbeit sat at its maximum torque output most of the time.

 

[scottie]

The original bore glazing was attributed to stand by engines ie pumps or emergency generators which were started on a regular basis and never put under load .

 

[Wing Mark]

The original bore glazing was attributed to stand by engines ie pumps or emergency generators which were started on a regular basis and never put under load .

Bore glazing is also talked about in motorbike circles, too gentle a running in process, so the cylinders run too long at a low temperature.
Some people say that cars which are used for many very short journeys when they are new, end up using a lot more oil.

Many boat engines don't seem to have a temperature gauge, ISTM a good idea to fit one? Be sure the engine is getting properly warm?

 

[geem]

Bore glazing is also talked about in motorbike circles, too gentle a running in process, so the cylinders run too long at a low temperature.
Some people say that cars which are used for many very short journeys when they are new, end up using a lot more oil.

Many boat engines don't seem to have a temperature gauge, ISTM a good idea to fit one? Be sure the engine is getting properly warm?

If the engine has a fresh water cooling system and the thermostat is present, it should get up to a proper working temperature without much concern, assuming it's not sat on tick over.
Raw water cooled engines have a lower operating temperature. They do this I believe due to the problems of exponential corrosion as you heat seawater. My old raw water cooled Yanmar, used to run at 60degC and still corroded the exhaust elbow away

 

[eddystone]

Specifically on the subject of weight, I replaced a VP2002 with a Beta 30 which was physically a bit larger but slightly lighter, in practical terms about the same. I went for the 30 over the 25 because a) after equalising alternator rating price was about the same and the 25 was significantly lighter than the old Volvo and might have affected trim slightly, especially if I wanted to put more chain in the bow. Think older raw water cooled engines were heavier to provide more margin for corrosion.

 

[Wing Mark]

If the engine has a fresh water cooling system and the thermostat is present, it should get up to a proper working temperature without much concern, assuming it's not sat on tick over.
Raw water cooled engines have a lower operating temperature. They do this I believe due to the problems of exponential corrosion as you heat seawater. My old raw water cooled Yanmar, used to run at 60degC and still corroded the exhaust elbow away

I expect to be doing short runs under engine. A mile from mooring to sailing at most if there's a breeze, and some of that would be quite slow to avoid wash.
If I manage to fit a calorifier, then the temperature will make itself known of course!