Two different engine sizes?

[SailingHarmonie]

Originally Posted by StuM

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

Ah, gee there you go spoiling a perfectly good argument with actual DATA! What fun is that?

[Chotu]

since a catamaran motors along just fine on one engine without anything noticeable happening, I’m sure changing the horsepower of a second engine by a percentage hardly affects the boat in any way at all.

many of us motor along on one engine for economy. Running on one engine can save you fuel as well as maintenance costs. Less hours on the engines.

possibly this person had installed two different engines to have one for motoring on fuel economy and one for bringing the power when needed. A perfectly valid approach.

aside from that, they are just used to rotate the boat and move it around when docking. Doesn’t much matter what the power is in that case since you are using like 10% of it.

[roryboy]

Run the boat. If it handles well under twin power and also under single power with each engine, why panic? Might be a blessing. Pontification is cheap. But ya' never know a boat until you run it. It's that simple.

[RaymondR]

Matching speed and thrust whilst two engine maneuvering is probably not very common anyway.

[Cheechako]

I imagine one engine had to be replaced. It is kinda weird. One wonders with the comments above just how many Cats with Yanmars have to have engines replaced. Mine had one replaced also!

[Adelie]

Originally Posted by 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…

With a cat using 2 different motors, I believe you can balance them by observing helm loads.
Is the boat trying to turn one way or the other when rudder is centered?

[Galumay]

Originally Posted by StuM

Interesting, Yanmar manuals say there is considerable difference

As I said, almost no difference in the torque curve.

As I also said, no practical difference that we have noticed.

Maybe a small difference in over all fuel economy, but its so economical anyway that we have not noticed that either.

There is no difference in the helm because we run them both at the same revs with the same props, thats why I say there is no practical effect we notice.

It wouldn't be the setup I would choose, but as I said I had no idea it was the case in our boat until I read this thread, because the boat operated exactly as expected under motors.

[boatpoker]

Originally Posted by 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…

Agreed ! Plus having done over 5,000 valuations, This would get a huge discounted value from me.

[MicHughV]

This is an interesting thread for me.

I run a twin engined fishing boat.....twin 150 Yammie's.

I've often noticed when both engines are not running at the same rpm, that the engine with the higher rpm is just not a happy camper, purely by the sounds of the engine.

Getting the other engine up to the same rpm as the higher rpm engine...often only 100 rpm or so different, makes an almost instantaneous difference in the engine note of both engines and boat runs much, much better.

This requires but a small, almost imperceptible tap on the throttle of the slower running engine to get both engines in synch.

[Galumay]

MicHughV, i think the issue here is you and others are now talking about high horsepower, fast power boats, in this case any mismatch, whether in the way the OP is describing, or in the way you are talking about running at different revs, has a significant impact.

In the case of our boat, and to a lesser extent the OP, it makes next to no difference.

We run the motors around 2000 revs if we are motoring, in the case of the 3JH series the torque curves are so flat there is basically no difference, HP developed at this rpm is about 23hp for the 3JH5 and 20hp for the 3JH3 and fuel consumption about 2.3l/h versus 1.8l/h
(from Yanmar's datasheets)

So the real world differences are imperceptible for us, as I have already mentioned, I would not have known they were mismatched had I not found this thread.

No doubt in your boat, or a big, fast powerboat with high horsepower, turbo engines, any sort of mismatch in the engines would have a large and obvious real world impact - but thats not really a relevant comparison to this situation IMO.

[StuM]

Originally Posted by Galumay

As I said, almost no difference in the torque curve.

I'd call 23% difference in torque at 2000 RPM significant.

[Galumay]

Stu, looks like its quite a bit more for the 4JH series, with our 3JH its nearer 12% - which as i keep saying is unnoticeable in the real world.

Its a pity the graphs have different axis, as it makes them look much more different than they actually are, because the torque curves are so flat both engines are very nearly at their maximum torque at 2000rpm.

Anyway I am done with the subject, my real world empirical experience informs my comfort with a small difference between the two very similar engines. Its up to the OP as to what weight he puts on the issue and whether he thinks it should affect the value.

Its now getting to the point of arguing how many angels fit on a pin head and thats not a discussion that interests me!

[Jamme]

Originally Posted by StuM

I'd call 23% difference in torque at 2000 RPM significant.

Stum, is there any difference in calculation involved between ‘engine’ torque listed on the left pic and ‘crankshaft’ torque listed on the right pic?
Thanks for the clarification.

[boatpoker]

Originally Posted by Galumay

Its up to the OP as to what weight he puts on the issue and whether he thinks it should affect the value.

Its now getting to the point of arguing how many angels fit on a pin head and thats not a discussion that interests me!

You are correct, it's up to the OP what weight he puts on it. The differing opinions on this thread suggest that the free market will have the same discussions. The OP would be wise to consider resale value unless he wants to have the same argument with a future buyer.

[Sos]

When close manoeuvring our twin engine cat I just adjust the throttles until the boat is doing what I want. Two different size engines wouldn't make any difference to me. You would soon adapt and you the advantages of better fuel economy.