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

[Bobc]

Diesel van and car engines seem to have very long lives, spending most of those hours running at a small fraction of their max power.

There might be more truth in what you say for a seawater cooled engine which doesn't get properly warmed up at low power?
Does it apply to a modern freshwater cooled Kubota engine?
Surely those digger and generator engines run all day, most of it at light load?

Yes. Those Kubota engines still need to run at full operating temperature, otherwise they start coking up.

You regularly read reports on here about exhaust elbows blocking, causing seawater to run back into the heat exchanger resulting in holes being corroded in the heat exchanger housing (mainly VP engines). The main cause of this is that the engine hasn't been run hard enough or hot enough. This is usually because the boat has too big an engine.

 

[Bobc]

Whilst not being an engineer, contrary to popular opinion, I think it’s a myth that light loads cause engine problems.

If we think about what causes engine wear…they don’t wear out when not running, what causes wear is moving parts, and we have oil to protect against that. So as long as the oil and filters are changed regularly that should prevent wear.

Diesel engine bores can glaze if not run under load (the minute oil channels in the cylinder bores get clogged with carbon deposits, meaning that the oil is not able to sit in them to lubricate the piston rings, resulting in loss of compression and bore damage).

 

[geem]

Yes. Those Kubota engines still need to run at full operating temperature, otherwise they start coking up.

You regularly read reports on here about exhaust elbows blocking, causing seawater to run back into the heat exchanger resulting in holes being corroded in the heat exchanger housing (mainly VP engines). The main cause of this is that the engine hasn't been run hard enough or hot enough. This is usually because the boat has too big an engine.

Not true. It's poor design.
VP do some very poor marinising solutions. Their exhaust elbows being one such item.
I swapped a crap VP exhaust elbow for a different make and engine performance is markedly improved. It was nothing to do with engine temperature.

 

[Pye_End]

Mine isn't a variable pitch. That would be the Brunton Auoprop. Mine is a Varifoil. It's a three blade folder. The blades on my folder look like Mickey Mouse's ears. Lots of area?

Interesting. Looks as though the blades have some shape to them (presume you mean Varifold?).

Certainly the 2 blade came out well in 'ahead' in the PBO test: Folding and feathering propeller test It is a shame they didn't test the 3 blade - would have been interesting to see how the 'astern' issue in particular improves. In particular the Varifold was a 15 x 11, and many of the other props tested were 16 or even 17" diameter - perhaps due to the hub variations.

When I changed from a fixed, folders were not an option as it would have fouled the rudder, like so many. However if space is not an issue, having shaped blades on a folder must be a big advantage. It is difficult to see why a folder should necessarily be better than a fixed - perhaps more research on shape has gone into these designs, or they are robbing 'astern' performance in favour of 'ahead'?

 

[geem]

Interesting. Looks as though the blades have some shape to them (presume you mean Varifold?).

Certainly the 2 blade came out well in 'ahead' in the PBO test: Folding and feathering propeller test It is a shame they didn't test the 3 blade - would have been interesting to see how the 'astern' issue in particular improves. In particular the Varifold was a 15 x 11, and many of the other props tested were 16 or even 17" diameter - perhaps due to the hub variations.

When I changed from a fixed, folders were not an option as it would have fouled the rudder, like so many. However if space is not an issue, having shaped blades on a folder must be a big advantage. It is difficult to see why a folder should necessarily be better than a fixed - perhaps more research on shape has gone into these designs, or they are robbing 'astern' performance in favour of 'ahead'?

I don't know why it better than fixed. It could be that the old prop was just not a good design. I have no complaints about its performance. We can push hard in to a big sea with surprisingly good performance, allbeit at stupid fuel economy. Far better to motorsail with main up and 20-30 Deg off the wind at economic revs

 

[Bobc]

Not true. It's poor design.
VP do some very poor marinising solutions. Their exhaust elbows being one such item.
I swapped a crap VP exhaust elbow for a different make and engine performance is markedly improved. It was nothing to do with engine temperature.

Agreed, but the coking-up is as a result of not pushing the engine hard enough. This problem is most prevalent on engines that are not pushed hard or allowed to get fully up to temperature.

 

[Tranona]

Agreed, but the coking-up is as a result of not pushing the engine hard enough. This problem is most prevalent on engines that are not pushed hard or allowed to get fully up to temperature.

Agreed. My Volvo 2030 did 3500 hours when it was on charter on the original exhaust elbow. Lot of motoring in the Ionian and instructions to cruise at 2400rpm.

 

[geem]

Agreed, but the coking-up is as a result of not pushing the engine hard enough. This problem is most prevalent on engines that are not pushed hard or allowed to get fully up to temperature.

Not my experience on a Perkins diesel engine used a generator. Ran at 3000rpm all its life. Fresh water cooled. The cast VP exhaust elbows are terrible. They corrode and carbon up due to poor design. I swapped it for a Northern lights s/s elbow about three years ago. The generator has never ran so well. The VP exhaust elbow needed decoking every 50 hrs. Never needed it yet on the Northern light elbow after three hundred hours. Design it everything. VP manage to turn reliable Perkins engines into maintenance nightmares

 

[geem]

Yes. Those Kubota engines still need to run at full operating temperature, otherwise they start coking up.

You regularly read reports on here about exhaust elbows blocking, causing seawater to run back into the heat exchanger resulting in holes being corroded in the heat exchanger housing (mainly VP engines). The main cause of this is that the engine hasn't been run hard enough or hot enough. This is usually because the boat has too big an engine.

I don't understand this. Fresh water cooled engines will quickly hit there operating temperature due to thermostatic temperature control. If VP supplied good quality exhaust elbows in S/S instead of crap cast iron elbows these problems would not arise.

 

[Tranona]

Diesel van and car engines seem to have very long lives, spending most of those hours running at a small fraction of their max power.

There might be more truth in what you say for a seawater cooled engine which doesn't get properly warmed up at low power?
Does it apply to a modern freshwater cooled Kubota engine?
Surely those digger and generator engines run all day, most of it at light load?

It is not power that is the issue, but load. Cars are geared so that the power and revs match the load, unlike a boat where there is only one gear and that is maximum power at maximum revs. at all other speeds a boat engine is producing less power than the power available at the revs the prop demands. Industrial applications are much the same as vehicles, the load (compressor, generator, digger, tractor etc places higher loads closer to power available.

What makes matters worse for boat engines is the usage pattern. Short periods of running , much at below operating temperature and low load followed by long periods of inactivity. Typical "weekend" sailors do less than 150 hours a year, compared with my charter boat that did over 4000 a year, mostly constant running and plant equipment often as much as 2000 a year (and don't have saltwater injected into their exhausts!)

 

[Wing Mark]

It is not power that is the issue, but load. Cars are geared so that the power and revs match the load, unlike a boat where there is only one gear and that is maximum power at maximum revs. at all other speeds a boat engine is producing less power than the power available at the revs the prop demands. Industrial applications are much the same as vehicles, the load (compressor, generator, digger, tractor etc places higher loads closer to power available.

.....

That just does not make sense.
The power(or torque) any engine develops at any steady speed is equal to the load on it.
If the load was more than the power, it would slow down, if it was less it would speed up.
The governor sets the power to match the load.

When my car is crruising at a steady 70mph, its 150BHP engine is developing about 20HP.
When a generator is not heavily loaded, its engine only develops a few HP.
When a minidigger is not actively moving its buckets or tracks, the engine is lightly loaded.
Yacht engines probably run at a higher % continuous load than most things when cruising at 80% revs.
Cars or motorbikes rarely use a big fraction of their available power for more than a few seconds at a time. The gears only match the max available power to the load at full throttle .

 

[Wing Mark]

...

What makes matters worse for boat engines is the usage pattern. Short periods of running , much at below operating temperature and low load followed by long periods of inactivity. Typical "weekend" sailors do less than 150 hours a year, compared with my charter boat that did over 4000 a year, mostly constant running and plant equipment often as much as 2000 a year (and don't have saltwater injected into their exhausts!)

Maybe boat engines would last even longer if they were not left idle 5 days a week with water in the exhaust, but ISTM that most of them bang on for more than 20 years, I've seen several boats lately where the original engine is still working at over 30 years. So it's not done a huge number of hours in that time compared to a Mondeo. That's not the point of an auxiliary engine in a sailing boat IMHO, it's more about 30 years of being available if wanted or needed. It seems that some designs could do better in the life of service parts like exhaust elbows? I've lately seen a few 1GMs which don't look like the elbow has seen a spanner in at least 5 years.. Lots of consumer goods 'die' of old age rather than actually wearing out.

 

[jwfrary]

In my experiance engines which are installed well, kept dry externally and maintained properly, clearances cleaning and proper inspection last a long time irrespective of how they are used (within the design parameters)

Additionally diesels like to be run hard, is an old adage from a time where they were built heavier and produced less power. Most yacht engines are not rated for continuous duty and therefore should be run at 80 % of achivable rpm.

Convention wisdom of over propping is not healthy, especially in turbo charged engines but also in n/a engines

This elevates exhaust temperatures causing in extreme cases melting valves (which usually deposit themselves on the exhaut turbine blades which then also knackers that!) fatigue in the valves and cracked seats is generally the result.

Underpropping wastes power and efficientcy
Use the correct prop!

I often find the most compromised area on a yacht is the prop as the diameter is restricted by the proximity of the hull. This limits the transmission of power more than most other factors no matter how much power is installed.

For the OP don't underestimate the cost of changing mounts and wiring, the exhaust and with larger engines you may need a larger shaft!

If the one you have is in good condition then you might find it economic to rebuilt or repair it. Buhk prices are a bit steep.

 

[gavin400]

I'm currently replacing my MD2B (23hp) with a Beta 30 - originally was going with a Beta 25, but have been persuaded to go a bit larger (not just by salesman)
The new engine is 100kg lighter than the old Volvo which I find quite amazing - probably most of that is in the Volvo flywheel

 

[Tranona]

I often find the most compromised area on a yacht is the prop as the diameter is restricted by the proximity of the hull. This limits the transmission of power more than most other factors no matter how much power is installed.

That is an advantage of saildrives where there is enough clearance to fit large diameter props - the same basic drive like the 130 Volvo can do from 13-65hp and props up to 18" diameter, then the 150 up to 19"

 

[Mark26]

Diesel engine bores can glaze if not run under load (the minute oil channels in the cylinder bores get clogged with carbon deposits, meaning that the oil is not able to sit in them to lubricate the piston rings, resulting in loss of compression and bore damage).

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.

 

[geem]

In my experiance engines which are installed well, kept dry externally and maintained properly, clearances cleaning and proper inspection last a long time irrespective of how they are used (within the design parameters)

Additionally diesels like to be run hard, is an old adage from a time where they were built heavier and produced less power. Most yacht engines are not rated for continuous duty and therefore should be run at 80 % of achivable rpm.

Convention wisdom of over propping is not healthy, especially in turbo charged engines but also in n/a engines

This elevates exhaust temperatures causing in extreme cases melting valves (which usually deposit themselves on the exhaut turbine blades which then also knackers that!) fatigue in the valves and cracked seats is generally the result.

Underpropping wastes power and efficientcy
Use the correct prop!

I often find the most compromised area on a yacht is the prop as the diameter is restricted by the proximity of the hull. This limits the transmission of power more than most other factors no matter how much power is installed.

For the OP don't underestimate the cost of changing mounts and wiring, the exhaust and with larger engines you may need a larger shaft!

If the one you have is in good condition then you might find it economic to rebuilt or repair it. Buhk prices are a bit steep.

I think you are over simplifying the issue of over propping. it's not one size fits all. I have discussed at length with Brunton and Perkins about prop selection.
Taking my engine at an example of over propping.
Engine is a Perkins M92B 4.4 litre. 86hp.
Theoretical max revs is 2400rpm. With my 22" prop is won't rev past 2000rpm. Maximum torque of the engine is 308Nm@1200rpm.
At 1200rpm we do 6kts. This is our normal cruising revs.
The power curve for the engine is very flat. At 2000rpm the engine makes 80hp.
The engine doesn't smoke. The engine has an MD rating. Medium duty. Typical application being tug boats and fishing boats. We rarely ever run past 1500 rpm partly because our fuel use hits 5 l/hr but also as we never need to. We can push into big seas with no problem at these revs.
Both Perkins and Brunton are happy that the prop selection is ideal for my use. I agree.
I think if you have an engine with a steep power curve and relatively small cubic capacity where the maximum torque is well up the rev range then you are in a different situation.

 

[Iliade]

duplicate post

 

[Iliade]

generatorwhatisglazing

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

 

[Wing Mark]

I think you are over simplifying the issue of over propping. it's not one size fits all. I have discussed at length with Brunton and Perkins about prop selection.
Taking my engine at an example of over propping.
Engine is a Perkins M92B 4.4 litre. 86hp.
Theoretical max revs is 2400rpm. With my 22" prop is won't rev past 2000rpm. Maximum torque of the engine is 308Nm@1200rpm.
At 1200rpm we do 6kts. This is our normal cruising revs.
The power curve for the engine is very flat. At 2000rpm the engine makes 80hp.
The engine doesn't smoke. The engine has an MD rating. Medium duty. Typical application being tug boats and fishing boats. We rarely ever run past 1500 rpm partly because our fuel use hits 5 l/hr but also as we never need to. We can push into big seas with no problem at these revs.
Both Perkins and Brunton are happy that the prop selection is ideal for my use. I agree.
I think if you have an engine with a steep power curve and relatively small cubic capacity where the maximum torque is well up the rev range then you are in a different situation.

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