Two different engine sizes?

[Marymoon Cat]

Hi all you seasoned people. We're looking at a L47 cat that has two different engines. Same year 4jh5e on port and a 4jh3f on starboard. Is this something to be concerned of?

[StuM]

The main issue will be in manouvering using both engines and balancing the power. The RPM required for a given HP is very different between them (both have similar max HP, but one has max RPM of 3000 and the is 3800).


Apart from that, many parts will not be interchangeable, so you will be carrying two lots of spares.

[denverd0n]

Definitely not an ideal, nor very normal, situation. I would expect the boat to sell for considerably less than a comparable boat with matched engines. And I do mean considerably. Like, maybe, 1/3rd less.

[RaymondR]

Yanmar engines of the same series tend to have a lot of common parts, just more or less cylinders.

[Galumay]

Not sure of the specific differences in that case, but we have a newer 3JH5E on one side and an older 3JH3C on the other, makes absolutely no difference, the only thing I have noticed thats slightly different is the 5E has an electric fuel pump. When I serviced them all the serviceable spares were interchangeable. If what StuM says is correct and yours have different RPM then I imagine that could be an issue that would take some getting used to!

EDIT - are you sure its a 4JH3F? I cant find that model, maybe its a 3E?

2nd EDIT - I have just realised that if i am correct about our model numbers, we have the same issue - one is 3800 max and one 3000! Have never noticed any issue at all with it motoring at same revs on both engines, so I will have to double check.

[malbert73]

Originally Posted by denverd0n

Definitely not an ideal, nor very normal, situation. I would expect the boat to sell for considerably less than a comparable boat with matched engines. And I do mean considerably. Like, maybe, 1/3rd less.

Really? A 6 figure discount?
You could repower the whole boat for 50K most likely

[jt11791]

My friend's cat has two different engines. This was done intentionally to allow him to motor sail very efficiently using only the smaller motor, while still having lots of power when needed.

[Sailorichiban]

I really like the idea of 2 differently rated power plants . As long as components are mainly interchangeable.
Perhaps I should be in the market for a craft needing one replaced.
Thanks for the idea!

[Montanan]

Those two engines being very different and having hugely different output ratings. The smaller displacement being more powerful due to being turbocharged and intercooled versus the larger displacement utilizing natural aspiration.

Can't explain why one would equip a vessel with such. Totally

4JH3-DTE

Specifications
Rated Output
91.9 kW / 125 mhp
Rated Speed
3800 rpm
Displacement
1.995 L / 122 cu. in
No. of cylinders
4 cylinders
Cylinder Bore x Stroke
84 mm x 90 mm / 3.31 in x 3.54 in
Combustion system
Direct Injection, Bosch Distributor Type VE Pump
Aspiration
Turbocharged & Intercooled
Alternator
12V - 125A
Dry weight without gear
229 kg / 504.8 lbs
Dimensions
1001.3 mm x 581 mm x 661 mm


4JH5E

Specifications
Rated Output
39.6 kW / 53.8 mhp
Rated Speed
3000 rpm
Displacement
2.19 L / 134 cu. in
No. of cylinders
4 cylinders
Cylinder Bore x Stroke
88 mm x 90 mm / 3.46 in x 3.54 in
Combustion system
Direct Injection, Mono Plunger Pump
Aspiration
Natural
Alternator
12V - 125A
Dry weight without gear
201 kg / 443 lbs
Dimensions
871 mm x 560 mm x 623 mm
Controls
Mechanical

The fuel consumption is crazy different.

[StuM]

Originally Posted by Montanan

Those two engines being very different and having hugely different output ratings. The smaller displacement being more powerful due to being turbocharged and intercooled versus the larger displacement utilizing natural aspiration.

Can't explain why one would equip a vessel with such. Totally

4JH3-DTE

He didn't day it was a DTE! If it were, that would be a very different situation.

[catsketcher]

My cat has two different sized engines. I did this because it went well with one and then I added another. Both are mostly used for manouvring so I don't need as much power. They are outboards however so that may make a diff - it was hard to get twop nice outboards for a 38ft cat - now Yammie make a much lighter 25 but still 2 x 25 is way over thew top for my cat - at 3/4 throttle with two I will be spending lots of fuel money making waves.

For any distance I motor on the single 25. I can motor at 5 knots quite cheaply on the 9.9.

No worries from me about the assymetry.

cheers

Phil

[RaymondR]

"His" and "Hers" perhaps??

[Galumay]

Yep, after some more thinking about it, its irrelevant, and I suspect fairly common. The reason it happened on our cat was that one of the 3JH3C engines died, rather than rebuild it they replaced with a 3JH5E, which makes the 39hp at 3000rpm rather than the 3800rpm of the old engine. The thing is the JH3 series was the only one with the higher RPM, all the other JH series were rated at 3000RPM so if you wanted to replace just one engine you had no choice really.

Nearly all parts are interchangeable, no real world difference detectable, I cant see it as something to be concerned about if you are otherwise happy with the boat.

There is no noticeable difference, and had I not read the OPs post I would not have even realised we had the same issue, the boat performs perfectly under engines with no hint of the difference. I suspect that is because the torque curve is so flat with these naturally aspirated engines. Basically no difference at the 2000rpm that we normally motor at.

[StuM]

Originally Posted by Galumay

There is no noticeable difference, and had I not read the OPs post I would not have even realised we had the same issue, the boat performs perfectly under engines with no hint of the difference. I suspect that is because the torque curve is so flat with these naturally aspirated engines. Basically no difference at the 2000rpm that we normally motor at.

Interesting, Yanmar manuals say there is considerable difference


At 2000 RPM, the prop power curve and fuel consumption:
4JH3 about 6 kW (8 HP) - and 2.2 lph
4JH5 about 12 kW (16 HP) - and 3.3 lph

[SailingHarmonie]

When operating with two identical engines under power, the optimum way to run the engines is at exactly the same speed. If the speeds don’t match, one engine is doing all the work and the other is just wasting fuel. On multiple engine powerboats matching engine loadings is a significant efficiency issue.

With two different engines how do you manage two engines so both are doing work when motoring? Seems a major issue to me…

You could assume identical slip in the two different props, and run them at rpm’s offset by the pitch ratio of the props—I guess…