Yanmar engines not getting max rpm

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Gardiner

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I am in the process of buying a Power Catamaran - It has two Yanmar 4BY3-150 Engines.
The engines are rated 4000 RPM and the surveyor only got 3700 RPM on the port engine and 3610 RPM on the starboard.
The engine hours are 1040 and 1070
Is this a concern for me? Should i walk away from the boat?
Any information will be appreciated.
 
Is that WOT in open, reasonably calm water? Notwithstanding, lots of installations are slightly over-propped so your figures don't condemn the engines IMHO.
 
Is that on the analogue instrument tachos? Sounds very precise if so. What sort of load is the boat carrying fuel/water/kit etc?
 
Gardiner said:
I am in the process of buying a Power Catamaran - It has two Yanmar 4BY3-150 Engines.
The engines are rated 4000 RPM and the surveyor only got 3700 RPM on the port engine and 3610 RPM on the starboard.
The engine hours are 1040 and 1070
Is this a concern for me? Should i walk away from the boat?
Any information will be appreciated.
Click to expand...
The revs rating you are quoting is probably without load. Diesels with mechanical governors suppress revs under load - the drop is actually needed to operate the "throttle" - that's a normal function of a regulator with proportional characteristics. The "normal" drop should be somewhere in the manual, but as a rule of the thumb it could be somewhere around 5-8% of max revs without load - if the load is exactly as per specifications that is. The port engine seems to be perfectly normal, the lower revs on the other one can have its root in fouling of the prop, wrong size or pitch etc. If it is a wrong prop, a difference as small as this should be no a major concern. Check for excessive smoke or temperature under load - if no alarms bells there, investigate further but you will probably won't find anything major. If the engine doesn't get to the max specified max revs in neutral, the problem will be most likely only the governor.
 
Gardiner said:
I am in the process of buying a Power Catamaran - It has two Yanmar 4BY3-150 Engines.
The engines are rated 4000 RPM and the surveyor only got 3700 RPM on the port engine and 3610 RPM on the starboard.
The engine hours are 1040 and 1070
Is this a concern for me? Should i walk away from the boat?
Any information will be appreciated.
Click to expand...
What is the hull speed and can the engines drive the boat at that speed?
 
Sandy said:
What is the hull speed and can the engines drive the boat at that speed?
Click to expand...
Gardiner said:
I am in the process of buying a Power Catamaran - It has two Yanmar 4BY3-150 Engines.
The engines are rated 4000 RPM and the surveyor only got 3700 RPM on the port engine and 3610 RPM on the starboard.
The engine hours are 1040 and 1070
Is this a concern for me? Should i walk away from the boat
Any information will be appreciated.
Click to expand...

You should aim to achieve to prop the boat so the engine are running at there maximum rpm or slightly over to help reduce load. to help bring the engines up to their rated power, i would try 2inch drop on the props 1inch=150~200rpm depending on prop type.

If both engines gearboxes running 1 clockwise and 1 counter clockwise, you can have the scenario, when one engine is driving harder ie..port running in reverse can change due to slight differences in gear ratio and prop design.

Checking boost pressure will confirm this.
 
Last edited:
jiris said:
The revs rating you are quoting is probably without load. Diesels with mechanical governors suppress revs under load - the drop is actually needed to operate the "throttle" - that's a normal function of a regulator with proportional characteristics. The "normal" drop should be somewhere in the manual, but as a rule of the thumb it could be somewhere around 5-8% of max revs without load - if the load is exactly as per specifications that is. The port engine seems to be perfectly normal, the lower revs on the other one can have its root in fouling of the prop, wrong size or pitch etc. If it is a wrong prop, a difference as small as this should be no a major concern. Check for excessive smoke or temperature under load - if no alarms bells there, investigate further but you will probably won't find anything major. If the engine doesn't get to the max specified max revs in neutral, the problem will be most likely only the governor.
Click to expand...
Thank you very much for your reply.
 
If we take out of the equation the throttle and connection and tacho, and assume you are in the engine room moving the engine throttle and gearing... how is best to test? Assume we have a handheld tacho that’s accurate, should the gear be in neutral and go WoT? if the engine should be at XX rpm, what’s wrong if we can’t get to XX?

now if we put her in gear, and go to WoT... if we can’t get to XX rpm, what’s likely to be wrong?


My understanding is that for the in-gear, you’d either be overpropped (can’t get to XX rpm) or underpropped (go over XX?!).
 
Gardiner said:
I am in the process of buying a Power Catamaran - It has two Yanmar 4BY3-150 Engines.
The engines are rated 4000 RPM and the surveyor only got 3700 RPM on the port engine and 3610 RPM on the starboard.
The engine hours are 1040 and 1070
Is this a concern for me? Should i walk away from the boat?
Any information will be appreciated.
Click to expand...
Interested in the boat too (we have a power cat :))!
 
Bluemu said:
now if we put her in gear, and go to WoT... if we can’t get to XX rpm, what’s likely to be wrong?
Click to expand...
Assuming you’re running both engines at wot (one engine alone won’t reach full revs) overpropped, damaged prop, dirty prop/hull, over loaded (both engines under revs), in the case of outdrives damaged hub on the OTHER prop (’good’ engine spinning prop and not pulling its weight), partially blocked air/fuel filters etc.

Assuming all else is equal, prop sizes, gearbox ratios etc.
 
Is the speed lever high speed stop on the throttle quadrant on the governor stopping the governor speed lever moving any further. if pedantic you can readjust if you think you trust the RPM gauges but I feel as others have said. No big deal. Also the adjustment might have a wire seal on it.

You don't appear to have clarified if these readings are in gear or neutral which would help.
 
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