Engine not reaching Rated RPM during gearbox engaged

Hi,

Recently we had our sea trials in one of our catamaran vessel with 2 Nos of 50 HP Diesel engine. As per our proven hull foam & propeller design, the similar vessel with same displacement was achieved speed of 7.2 knots at 48 Hp. The present vessel is similar to displacements, draft & propeller design.

But what we found is that engine is rev up only upto 1500 rpm instead of 2200 rated rpm for power output of 50 HP. Why we have rated rpm decrease to 1500 rpm instead of the rated RPM 2200?

In short do we expect the rated rpm to drop to 1500 instead of 2200 rpm.

 

Maybe explain what you mean about the gearbox ? Have you different gearboxes ?

 

So many possibile deltas

Gearbox

Prop

Displacement of the vessel not equal weight

Engine limited somehow. Most likely cause is improper throttle adjustment.

Report smoke color.
Report rpms in neutral.

 

What is the max speed of your newer cat?

Many tachometers can be adjusted to accommodate different numbers of cylinders, perhaps you are achieving the proper rpm but the tach is not calibrated properly

 

If the motors are operating correctly, and you have favorable results on a similar craft, I would hazard to guess that the transmission ratio is not the same.

 

Hi Sorry for the longer the delay in reply. The engine rated speed was 2200 rpm. Present gearbox ratio is 1.96:1 & we achieve the required speed of the vessel of 8 knots. But the engine speed is not reaching the rated rpm. If we change the gearbox ratio to 2.96:1 will it help? What will be the reason & we will not change the propeller. The smoke colour coming out was blue smoke.

 

As the other craft is run by electric motors & there is no transmission gears. Based on the sea trial report only we told that this vessel only require half the power of 50 HP. Even this vessel we have 2 Nos of 50 HP instead of one (Catamaran) why it is not reach the rated rpm provided by the OEM.

 

Neutral Rpm - 750 rpm (Idle)
Smoke colour is white & we calibrated the fuel injector- Smoke increased from the exhaust.

 

Here is a very good color of smoke summary with interpretations and more tests.

Diesel Engine Smoke: Blue, Black, or White? - boats.com https://www.boats.com/how-to/diesel-engine-smoke-blue-black-or-white/

Something about the engine seems wrong to me. I mean the engine sounds like it is operating limited. It could be so many things. Restricted air, restricted output from blowby, do the smell and taste tests for white smoke.

You have not reported engine hours either..

 

The engine was a bit old one remained in boat after launching for around 2 years, we did a basic overhauling by changing the hoses - Water cooling, fuel etc but not changing any gaskets of the Cylinder side.

 

Just determine the results of the smoke tests and tell us what you find. The engine may need a major overhaul.

 

This isn't terribly difficult. A boat not making rpm is either overpropped or underpowered. You have a smoking engine, that's where you start.

Moat modern small diesels are incredibly sensitive to injector and fuel pump issues. Puking white at idle and no power.... that's my first check. If the engine is turbo that's my next check, if it's natural aspirated move on down the line to compression related issues.