Why do I get low RPMs in gear only?...

sounds like you are over proped. you should lose some rpm in gear but not that much.

Has it always done this??

Too much pitch or a foul prop?

Check and make sure the prop is clean first. Then go from there.

hi ,
I have the same problem with a perkins m20 18hp on my 1993 catalina 270
it revs up fine in neutral but wont go above 2800rpm under load , around 4knots through calm water , which is well below hull speed , the engine is rated 18hp at 3600rpm
two diesel mechanics have been unable to diagnose it , fuel lines filters injectors , hull and prop all good , has anyone any ideas ???

mixing elbow choke.

The general cause is overloading. Gearbox, too tight bearings, oversized prop can cause this.

A buddy of mine has the same problem with his Ben.

He was told by his mechanic that his prop was too big. Great for docking but the boat will shake if you try to get it up to the proper rpms.

Assuming that the bottom and the prop are clean it is probably that the prop too big or that it has too much pitch.

If the Maxprop is clean and still loading down the engine, you can adjust the pitch. You still don't want to run the Perkins continuously at more than 3000 rpm.

john232 said:

I have a Perkins engine on My Beneteau first 38 and when the engine is out of gear I get 3800 plus RPMs but when I shift into gear I am lucky to get 2200 RPM. I have A max Prop. Could use some advice. Thanks in advance.

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john232, is this problem new and it just happen or you've been having it this way ever since ?

It really is imposible to trouble shoot problems without answers to questions so I will just throw out the most simple thing I can think of.

Cheepest and easiest to fix.

Throttle is not opening all the way.

Running an engine over-propped is likely to result in a short life, so it's a situation that needs to be resolved or it will get really expensive.

John, are you out there?

It's been almost a year now, have you fixed your boat?

Hum, maybe he sunked it for the insurance.

I hate one sided trouble shooting.

john232 said:

I have a Perkins engine on My Beneteau first 38 and when the engine is out of gear I get 3800 plus RPMs but when I shift into gear I am lucky to get 2200 RPM. I have A max Prop. Could use some advice. Thanks in advance.

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