Yanmar 1gm rpm neutral vs under load

[johnm5858]

I heard a mechanic once say what the normal difference should be on
Max Rpms between /in neutral versus under way/ in gear. (Can't remember). My Yanmar 1983 GM1 seems to top out under load( in gear) at 2500 -2600 rpm but in neutral cranks up to 3200 , 3100. Should I be able to get more while in gear? All should be good based on last service except I may be due to change my exhaust elbow, or am I getting typical rpms?

 

[Capt jgw]

It depends on your prop. The engine's max horsepower is a little below max rpm, check the manual for that rpm. You should be just able to hit max power rpm in forward gear. If not, you're over- propped and need to reduce prop pitch. With a max rpm of 3200, I would think the max power would be at around 2800-3000. With only 10 hp available, you need all you can get. Reverse is usually geared lower and the prop is less efficient turning backwards so you'll reach higher rpm in reverse.