The Merk 165 has been a good engine over the years and may deliver the
torque you need to turn a 19 or even a 21 pitch prop. (Depending on a number
of factors). If you can find a used or loaner prop form a friend or marina
with the understanding that you'll purchase it from the dealer if it
accomplishes your goal or if you damage it. Only an live test will answer
the question.

You COULD overload the engine with a higher pitch prop and loose speed, so a
trial is in order in my opinion.

Typically a thinner stainless steel prop pitch for pitch will provide a
minor performance improvement.

Don Dando


"TJF" wrote in message
. ..
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