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John H[_2_]

On Thu, 04 Mar 2010 13:36:01 -0500, wrote:

On Wed, 03 Mar 2010 19:58:58 -0500, Larry wrote:

A fishing buddy of mine is re-powering his early 1976 Boston Whaler
Sport 15 with a new 60hp 4 stoke Mercury. He is replacing a 2 stroke
Mariner that is an 80's model.

He has no gauges and the layout of the console is like a skiff - the
steering wheel is horizontal. He's not concerned about a speedometer,
and shouldn't be, but I think he should have a tach. The 4 stroke will
be heavier and act differently than the 2 stroke. The props are
interchangeable in size and hub configuration so he plans on putting his
SS prop on the new boat. I'm trying to convince him to find, or make, a
spot for a tach so he can make sure the boat is propped properly (say
that 5 times fast!). He's not convinced. The boat is rated for a 75 so
the 60 will still weigh about the same as a 75 2 stroke so the transom
weight will be fine.

Has anyone attempted a similar replacement? Should he need a different
prop? Can you attach a tachometer temporarily for testing purposes?

Thanks in advance,
Larry

I made the exact same repower on my boat (Mariner 75 2S to 60 4S Big
Foot)
The weight was close but the prop had to be changed (does he need a
15p, I have one in SS, make offer)
The tach is a 2 wire hookup from the gray wire to ground (port side of
motor) and you could certainly do this temporarily if he can borrow a
tach.
This actually picks up the unrectified output of the charge windings
but there is also an output from the computer. (there are 2 gray wires
on the motor). They both seem to work the same on my motor.
If you just had a meter with a frequency counter you could interpolate
that with a little simple math. Use the idle speed as a sanity check
(700 or so) then see what WOT works out to. You have a pretty wide
"acceptable" range.5500-6000 with the rated HP right in the middle.
I would shoot for close to 6000 with the boat empty. It will always
get heavier as time goes by. ;-)

A '76 Whaler may already be a heavy boat. Lots of foam and wood under the deck
to hold moisture.
--
John H

All decisions are the result of binary thinking,
which is why conservatives can see in black and white.

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