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

From: Lee Shuster <lee.k.shuster@...>
Date: Tue, 22 Jul 2008 16:33:35 -0600

That's all great data, thanks for sharing. I plugged your data into my
online calc and got 36 mph - just about what others have reported. To
drop to 26 mph means the efficiency is dropping from roughly 67% to
roughly 33%.
PS -- I appreciate all the data. Loved the waterline marking technique.
Weighing it is also great benchmark data. Great work.
I hadn't realized you were apparently the one recently asking me about
sources of potential "slippage." Gee, this sure sounds like what this
The prop (as other have noted) does have a rubber clutch inside the hub.
These rarely fail, but are designed to absorb the shock of hitting an
object and shearing or bending the so called "shear" pin. (They also
rarely prevent prop damage.) Could be an unlikely source of slippage,
but you've recently installed a new prop and it still does it. The other
source could be the forward clutch hub assembly. This part interfaces
between the electrical coil and the spring. Others on the list might be
able to shed more light on this, but I have heard they can fail in this
manner, ie., allowing so much extra slippage.
Feel free to call my favorite OMC electric stringer guru: Ask for Roy
Hansen at: 801-466-4731, I'm sure he'll shed some light on this. He runs
a certified OMC repair shop and has grown up on electric stringers.
PSS -- Where are you located? Just curious...


From: omc-boats-bounces@...
[mailto:omc-boats-bounces@...] On Behalf Of Don Mandelas
Sent: Tuesday, July 22, 2008 2:12 PM
To: Evinrude & Johnson Boats of the 1960's and 70's
Subject: Re: [OMC-Boats] Engine RPM and Boat Speed.

Thank you everyone for your help. I appreciate it greatly!
Glad to know I'm not alone in this boat restoration project.
Here is what I have done so far in hopes of correcting the problem.
I have checked the Ohms resistance going to the forward shift coil in
the lower unit, the Ohms resistance is 6.1.
I have Installed a new OMC Aluminium Propeller 14 x 18 pitch.
I have plenty of power to reach 4300 - 4500 Rpm's on the tackomater with
the 14 x 18 propeller.
I have compared the boats speedometer to a GPS and was amazed at how
accurate the boats speedometer is. (When I'm going 17 mph on the
speedometer the GPS reads 18 mph)
The bottom of the boat hull has absolutely no barnicles, it has been run
in fresh water lakes only.
This Morning I re-waxed the bottom of the boat and obtain only about 2
additional MPH's (26 MPH).
The motor runs very smooth, all 6 cylinders.
We have 2 people in the boat (me and my father).
Fuel tank reads 3/4 tank.
My location is 1200 feet above sea level.
I have launched my boat in the lake, tied it to the dock, and let it
float for 8 hours unused.
When I returned 8 hours later the boat had not sunk any deeper into the
water then when I first launched it. I placed several water line marks
on the outside hull with a felt tip marker to tell if any changes
This morning after our test run the Boat and Trailer weighed in at 3,040
lbs (by themselves on the public scale).
The Boat Trailer I guesstimate is about 700 lbs.
Accordingly My boat weights in at about 2,340 lbs.
I see no water inside the boat.
Question, What RPM's should I be reaching to comfortably cruise down
the lake without causing damage to my 155hp engine.. and whys so slow
at 4,000 rpm. (26 MPH)
Help, Anyboady....
Thanks Don.
Tue, 22 Jul 2008 12:30:33 -0600
From: lee.k.shuster@...
To: omc-boats@...
Subject: Re: [OMC-Boats] Engine RPM and Boat Speed.

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.

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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