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

From: Lee Shuster <lee.k.shuster@...>
Date: Tue, 22 Jul 2008 12:30:33 -0600

I left out a couple of important points:
1) Assumption A: I assume you have accurately measured your "true"
speed. Using a GPS or an accurately measured and marked-off course?
Verify the Pitot is accurate or not partially obstructed. (My experience
is they are surprisingly accurate - but still check/verify.)
2) Observation B: Something is way off here. Your boat with a 90 hp
engine would top 30+ mph new, lightly loaded. Either your V6 is only
hitting on 3 cylinders, or there is literally a ton of weight or maybe a
ton of barnacles or some combination of all three?
3) Observation C: Your observed engine RPM of 3700 is not allowing the
engine to deliver it's peak horsepower. Could be many reasons for that.
I'd start by incrementally dropping your prop pitch, 18", 16" 14" 12"
until your can achieve 4300-4500 RPM at WOT (Wide Open Throttle). The
SST (Stainless Steel props are more efficient (less flex) and are
available in add pitches (19, 17, 15, 13). The best running props are
in the middle pitch ranges, not the extremes.
4) Probably not a bad idea to weigh the boat/trailer on a public scale,
take out as much junk as possible, and unhook from tow vehicle.


From: omc-boats-bounces@...
[mailto:omc-boats-bounces@...] On Behalf Of Lee Shuster
Sent: Tuesday, July 22, 2008 9:22 AM
To: Evinrude & Johnson Boats of the 1960's and 70's
Subject: Re: [OMC-Boats] Engine RPM and Boat Speed.

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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