Re: [omc-boats]Top Speed variables

RE: Subject of Top Speed.

You didn't say where you live, or which motor is in your Sportsman, but
let's assume it's a 155 and you live 1000 ft ASL.

1) First start by getting the boat as light as possible. The 40 mph factory
claim was with no extra gear and a 170 pound average weight pilot. You can
even go weigh the boat and trailer on a public scale (disconnected from your
tow vehicle). If you have the Evinrude trailer and the fuel tank is less
than half full you should see something around 1850 - 1900 lbs on the boat
and 400 to 500 on the trailer for a total boat, v6 motor, and trailer of
around 2200 to 2400 lbs. The boat may have gained weight from water below
the floor, but most likely you're lugging several hundred pounds of "stuff"
around. Weight is the most common speed killer.

2) Next be sure and check the condition of the hull bottom for excess drag.
Wax and buff it out if possible. Always do you top speed runs in calm water
with no winds. Try to make your runs in opposite directions, to cancel out
wind or currents. Dirty bottoms are also a speed killer. You 69 hull is
slightly (5 percent) more efficient that the 64 - 68 Sportsmans, but that
can get quickly offset with any crap on the bottom or deformation of the
hull due to trailer "hooking."

3) What is your altitude above sea level? Your horsepower is reduced 4 % for
every 1000 ft above sea level. For example, at 1500 ft you lose 6 percent,
no way around it. At 6000 ft ASL where I boat, I lose 24 percent of the
available hp. But where I live , we understand the effects of air density,
most people never take it into consideration, unless they are an aircraft
pilot flying out West.

4) Check your speedometer accuracy against a GPS. The old pitot
speedometers are usually pretty accurate when held at a steady, on-plane,
speed in smooth waters. Check your tach accuracy against another just for
good measure, as well. We used to measure off a known distance and clock
timed runs for determining accurate speed, but now with GPS that's a bit old
fashioned.

5) If your engine is truly only able to turn a true 3800 rpm, then you
should consider swinging a smaller pitch prop. The 16-footers usually came
with a 16-inch-pitch prop, verify the pitch by getting the number off of the
prop. They range from 12 to 20-inch pitch. Your engine, if properly tuned
should run WOT (wide open throttle) at 4300 to 4500 rpm, with 4400 the
factory recommended redline. By only turning 3800 you are probably losing 15
% of your power. A two-inch decrease in prop pitch will generally yield
another 500 rpm at WOT. Also, a stainless steel prop will generally pickup 1
to 2 mph over the aluminum. See Prop part number info at:
http://hhscott.com/evinrude/choosing_a_prop.htm

6) Check your gear ratio. It's not unheard of for a previous owner swapping
the sterndrive. It's easy, you count how many revolutions the engine makes
for one turn of the prop. On a V6 the engine turns 1.5 times for one turn of
the prop. A V8 is close to 1-to-1. IF somehow your V6 is now turning V8
gears, you won't have the torque to turn "tall" of gearing. And you won't be
able to decrease prop pitch enough. (Of all the variables I've listed, this
last one is least likely.)

If you are interested in seeing how these variables all can cause your
boat's top speed to vary greatly, play with the online speed calculator at:

http://hhscott.com/evinrude/docs/BSLAC.htm

I plugged in the numbers and 30 - 31 mph is about right for a V6 @... 3800 RPM
in a Sportsman, at 1000 ft ASL, with 4 adults on board and a couple hundred
pounds of fuel and gear. My recommendation is try spinning a smaller-pitch
prop and/or put the boat on a diet.

----- Original Message -----
From: <ruddfamily4@...>
To: <omc-boats@...>
Sent: Monday, September 25, 2006 5:14 PM
Subject: [omc-boats] MIME-Version: 1.0

>I have a 1969 sportsman, i believe i am the 3rd owner and have owned the
>boat for over 20 years with very little trouble, The only problem i seem to
>have that re-occurs is the tilt mech. breaks, the clultch doesn't work or
>doensn't work properly.
> If i hit anything, the unit raises up but in the process it pushes the
> worm gear and cap out of the end of the thilt mech. and into the bottom of
> the lake. Last time it broke some of the alum casting around the retaining
> ring.
> I checked some web pages and the worm gear, o ring and brass plug are
> pretty expensive. This old classic runs well, however i only get about
> 30mph at 3800 rpm with the v6.
> I am looking for suggestions to remedy the tilt problem, and suggestions
> on increaasing the speed to closer to the factory spec of 40mph.
> thanks for any advice.
> Keith
> -----
> To get off this list send mail to omc-boats-unsubscribe@...
>

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Received on Monday, 25 September 2006