TJF wrote:
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



That's a big boat for the 165. If your holeshot is good, you are
already in the "sweet spot" prop wise. A good 17" SS cupped prop with
adequate surface area will give a better bite and let you trim out more
before blowing out, resulting in more boat out of the water & a slightly
higher top end. It's a real common prop - you may be able to borrow one
for a test run.

A 19" or 21" will most likely cause a drop in holeshot & top speed, but
stranger things have happened. The 165 straight six is a "torque-y"
engine with good grunt, but it likes to run 4400 too. A very smooth
running engine due to the inherent balance of the I-6 configuration.

Best cruising range is almost always 3000-3500 rpm when propped right &
on plane. The 165 is not too bad on gas w/ the 2bbl Rochester anyway.

Rob