Cruising RPM vs. WOT
 

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

Cruising RPM vs. WOT
 

You can't "govern" an engine with the carburator. The rpm at wide open
throttle is controlled by the pitch of the prop. You can't go to a 21 pitch
since you're already at 4200 with a 17 pitch. You might be able to go to a
19 pitch but the 17 is probably good, particularly if you did not have the
boat fully loaded when you tested the wot rpm. Boat rpm is high compared to
cars. Best cruising is probably around 3400.

"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

Cruising RPM vs. WOT
 

d

Cruising RPM vs. WOT
 

If anything I'd suggest that your truck needs an overdrive to get its
rpm down on the highway unless its towing or a 4 cyl.

Cruising RPM vs. WOT
 

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

Cruising RPM vs. WOT
 

On Mon, 14 Nov 2005 16:57:51 -0700, "TJF"
wrote:
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?

=================================

For a gas engine with WOT in the normal range (as yours is),
recommended cruising RPM is about 70% of WOT, typically 3200 to 3400.

At 70% you will be developing about 50% of the engines rated power
since horsepower varies as the square of RPMs (.7 x .7 = .49).

Cruising RPM vs. WOT
 

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

Cruising RPM vs. WOT
 

On Mon, 14 Nov 2005 16:57:51 -0700, "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


About 3400 for me!

5.7L GMC

--
John H.

"Divide each difficulty into as many parts as is feasible and necessary to resolve it."
Rene Descartes