Re: [omc-boats] where is the red line?

From: Ethan Brodsky <brodskye@...>
Date: Wed, 31 Aug 2005 15:38:09 -0500 (CDT)

On Wed, 31 Aug 2005, Paige wrote:
> So, where is the red line on my tach? Is it ok to be cranking along at
> 4000 rpms?

One thing you should be aware of is that the tach is most likely wrong. My
understanding is that the "return spring" in the tach stretches with age,
so old tachs tend to read high. I'd recommend getting a known-reliable
tachometer to verify if it reads correctly. I used one of these:
(I believe you'll need model TT226NR-4C). Otherwise, some nicer timing
lights have digital tach readouts. Ask a local garage and they might have

My speedometer is completely non-functional - I haven't investigated
whether it's a pitot or paddlewheel or an electrical system yet. I'd like
to fix it this winter, but we'll see if I get around to it. I'd rather
replace it with a oil pressure / water temp gauge, but I've yet to find one
for sale. I've done some crude speed measurements by pacing another boat,
but I mostly run based on the tach as well.

If I recall correctly, the redline on my boat ('70 Explorer 16 with the 155
hp V-6) is 4400 rpm, and the tach on the dashboard reads about 5000 at this
engine speed. At wide-open-throttle, the boat will do about 35 mph with a
14x18 prop at this engine speed. I'll run it at this speed for a while,
but I usually try to avoid it, since I too am afraid of blowing up my
motor. It seems fine, though I have noticed that I smell a bit of burnt
oil if I run WOT for long. I'm not sure if it's just accumulations on the
block burning off or if it's blowby causing oil to come out the crankcase
vents, but I generally avoid running wide open for long.

We'll typically cruise right around 4000 rpm indicated, which would
correspond to around 3500 rpm (assuming the gauge is linear). At this
speed the engine does not seem to be working hard at all, and there are no
bad smells.

For watersports, we'll go wide open for a few seconds until the boat is up
on plane, then pull back to around 3400 (wakeboard) or 3600 (skier) rpm
indicated, which would correspond to about 3000 rpm actual. We've done
this quite a few times this summer (10-15 days so far, I'd guess) and I
haven't noticed the engine overheating or burning oil at all.

Congrats on getting your boat running. Enjoy!


<A HREF=""> Ethan Brodsky </A>
UW FutureCar Team Paradigm: Two-Year FutureCar Challenge Winner
UW-Madison Clean Snowmobile Team: Winner of the 2004 SAE CSC
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Received on Wednesday, 31 August 2005

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