Re: [OMC-Boats] Engine RPM and Boat Speed.

From: Lee Shuster <lee.k.shuster@...>
Date: Tue, 22 Jul 2008 09:22:14 -0600

I have a online website calculator that you can plug in variable data:
the calculator takes several variables into consideration and calculates
a THEORETICAL top speed. It has been verified against factory published
information and in the real world using my 1966 Sportsman with both the
Buick V6 and the Chevy V8.
Here's what I come up with for your boat (making assumptions that may
not be right):
Since 3700 RPM is OVER PITCHED for your power-to-weight ratio, you
either are down on power or up on weight. For this calculation we will
assume a little of both.
HP -- 140 hp
Empty Hull Weight: 1975 lbs
Crew Weight: 2 @... 170 lbs ea
Fuel and Gear: 16 gal Fuel @... 7 lbs/gal = 112 lbs, plus 58 lbs misc
safety gear
Altitude Above Sea Level : 1000' ASL
Hull Eff Factor = 175 (constant factor verified for a clean, smoothly
waxed 1964 - 1968 16ft E/J 16 ft Tri-hull)
The Calculated Result: 34 mph
So to drop another 9 mph you have to add quite a bit more weight, and/or
be at a higher altitude, and/or have a "dirty" hull, and/or being
carrying more passengers, and/or be producing less power.
(Factory specs say your boat, with 1 crew and half a tank of fuel at Sea
Level would do about 39 mph on an AL 14x18 prop at around 4300 RPM.)
Have fun punching in your own data. I suspect weight is your enemy (but
that alone should not bring you down that bar), possibly also look at
your engine and prop combo and then clean and wax that hull.
Lee Shuster
Salt City - Utah


From: omc-boats-bounces@...
[mailto:omc-boats-bounces@...] On Behalf Of Don Mandelas
Sent: Tuesday, July 22, 2008 1:12 AM
To: Evinrude & Johnson Boats of the 1960's and 70's
Subject: Re: [OMC-Boats] Engine RPM and Boat Speed.

I have a 1967 Evinrude Sportsman boat with the V-6 155 hp engine. On
the stern drive unit I have installed the 14 x 18 propeller. When the
engine reaches 3700 RPMs how fast generally should I be traveling across
the water? It seems lately that I am going slower than normal (about 25


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Received on Tuesday, 22 July 2008

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