All,
I have a 1971, 21' Fiberform that has a 165 Mercruiser (250 cid inline
Chevy 6 cylinder). WOT on the tachometer registers @ 4200 RPM. I believe
that the Rochester 2 barrel carburetor is governed for that RPM. I am
running a 17 pitch aluminum prop which came with the boat, but have
contemplated a more aggressive pitch as well as stainless steel. The
outdrive is geared @ 1.68 to 1. This all equals to 32 mph according to the
fish finder/gps unit @ WOT.
The original owners manual states that "3900-4300 RPM lets the engine
turn at the recommended RPM and gives best performance". It also states that
I could go up to a 21 pitch prop and still be in this window. The boat
currently has very good hole shot. I am more about getting to the fishing
hole than pulling a skier.
My question is, what is a good RPM for cruising without thrashing on the
motor and leading to premature engine failure? The recommended RPM seems a
little high compared to what my truck's RPM is cruising down the interstate
(2800 RPM). Any suggestions?

Tim