Two different engine sizes?

[malbert73]

Why are so many engines dying on relatively new catamarans?

[MicHughV]

Thinking this through a bit....

On my fishing boat, the engines are very close together, and the hull is relatively narrow, in essence the twin engines are really performing as one so to speak.

Yahama has a very interesting video available online, where two identical boats are matched agains each other....one equipped with twin 150's and and the other with a single 300. Performance and fuel consumption is tabulated on graphical form at the end of the video

On a cat, however, the engines are located some 20-25' apart, so I can see where an engine mismatch or rpm mismatch is a different animal. Cats can motor with just a single engine and maybe a touch of rudder correction.
The other engine, smaller or even less rpm, is there to just keep the cat aligned straightforward....

I've never experimented with engine response with a cat, so I could be talking thru' my hat, but wanted to throw this out there to see if makes any sense.

[Tom and Maje]

No, no, and no. After years of maneuvering two engines where, although were the same size, one was underpowered. I would never advise someone to buy a boat with two different engine sizes. Once the boat was repowered I was shocked at how easy it was to dock. Night and day.

Maje

[Jetx]

Originally Posted by StuM

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

Engine manufacturer prop power curves are based on a theoretical prop, which just matches the rated HP for each engine at maximum RPM. So the two Yanmar prop curves assume different theoretical props.

However, if you install the same prop on both engines, then in the real world, each engine is now "facing" the same prop curve. That is, identical props, both turning at 2000 RPM, require the same amount of power (and therefore torque) to turn at that speed. One engine just has more surplus power capability at that RPM, and at all other RPMs of course.

Or another way to put it... if the props are the same, then at any given RPM, both engines are putting the same amount of power into the water, and generating the same amount of thrust.

If the props are different, then I'd expect performance differences to be pretty obvious, and you'd have to run each engine at a different RPM to balance the thrust.

[Marymoon Cat]

Thank you all for your reply's it was very helpful and insightful. We chose to pass on the boat due to big areas of delamination we found thankfully before pulling for survey.

Thanks again

[col50]

Originally Posted by Jetx

Engine manufacturer prop power curves are based on a theoretical prop, which just matches the rated HP for each engine at maximum RPM. So the two Yanmar prop curves assume different theoretical props.

However, if you install the same prop on both engines, then in the real world, each engine is now "facing" the same prop curve. That is, identical props, both turning at 2000 RPM, require the same amount of power (and therefore torque) to turn at that speed. One engine just has more surplus power capability at that RPM, and at all other RPMs of course.

Or another way to put it... if the props are the same, then at any given RPM, both engines are putting the same amount of power into the water, and generating the same amount of thrust.

If the props are different, then I'd expect performance differences to be pretty obvious, and you'd have to run each engine at a different RPM to balance the thrust.

Exactly!!!!

[Trawlerman]

JETX and COL50 Nailed it. Everyone should pay attention to the prop curve.