Albin 25 with Universal M25 engine - too slow

[PugetPete]

"Inherited" 1971 Albin 25 with a Universal M25 (5421) installed by a previous owner. Best speed, full throttle, 1850rpm = 5.5 kts. Clean bottom. Everything OK as far as I know. No previous experience with inboard or trawler type vessel or diesel engines. Don't know what propeller it has, old one from the AD-21 or ?
My GUESS is that it's simply underpowered with that engine, but no idea of ballpark figure to repower with rebuilt whatever. No trailer, so can't think of doing it in my back yard, even if I was able to figure everything out.

Thoughts? I'd like to get the advertised 7 kt cruise.

 

[AusCan]

The boat is not under-powered. They originally came with an AD 21 producing the same horsepower. The difference is the max revs of the original engine was 2300-2400 rpm. The new engine should max out at 3000 rpm, so it seems you are not getting enough revs to develop full horsepower. It is likely you are overpropped with the current set up.

Original gearbox was 2.04 ratio and prop was 16 1/2" diameter x 12"pitch.

Here's the manual if you don't have a copy.
http://www.albin25.eu/_download/albin25manual.pdf

So - the original engine/gearbox/prop setup was all matched. If you still have the original gearbox and prop, and you have a different power curve on the new engine. The prop you now require will have less pitch (or "twist") which will allow the engine to get to a higher rpm to produce it full horsepower.

First - check your gearbox ratio. Then contact a good prop shop to see what they now suggest. As a rough guess maybe 16 x 10 now.