When we got our Amel (SM #160) it came with what was then the standard 22” diameter MaxProp. All of our documentation pointed to using a setting of 20 degrees (15” Pitch)  With the boat set up like this, it checked all of the standard performance boxes.  Maximum engine RPM 3000: Check. Boat speed with a clean bottom: 8.0 knots, Check.  Should be perfect...

Except... under some conditions, the boat was just not happy.  Motoring into a strong headwind, or steep chop, and the engine RPM could not QUITE climb up high enough that the turbo boost come up enough to continue up the power curve.  It would “get stuck” at 1800 RPM and the boat would really struggle. In flat water, this never happened, it worked just as expected.

This might have been due to an aging powerplant (she’s up to 8200 hours) or some other quirk unique to our boat, but just in case anybody else is left with one of these engine/propeller combinations and has had some of the same issues, here is what we did.

It seemed the engine needed to be un-loaded just a tiny bit.  The next lower setting on the MaxProp is 18 degrees with gives a 13.5” Pitch.  All other things being equal, I calculated this as bringing the engine speed at any given boat speed up by about 8%.  I knew from observations if the engine reached 2100 that was more than enough to get the boost pressure up and let it generate full power.  After consulting people who know a lot more about turbochargers than I do, we made the change.

Maximum engine RPM is now about 3350RPM (still way below the engine makers recommended maximum) Highest boat speed is still just about 8.0 knots, maybe just a hair faster.  So that was all as expected.  The performance against headwinds and seas has dramatically improved.  We haven’t yet found any conditions where the boat gets “stuck.” Unless we find something dramatic, I expect this to be a change we will stick with.

Bill Kinney
SM160, Harmonie
Eleuthera, BS