Re: [OMC-Boats] timing/carb adjustment

I believe the 17' ft hulls had vents at the top (high) of the stern
molded into the top deck. These are not visible in the photos provided.

Another point to add to Ethan's: In order for raw (liquid) gasoline to
ignite or explode the combustible mixture must be at the proper Air to
Fuel ratio ( in other words in a gaseous state). Chemists call this gas
stoichmetry. For gasoline, by weight the ratio is 14.7:1 or roughly 7
percent fuel by weight.

So just introducing more air into the equation isn't necessarily a safe
thing, contrary to common sense. Matter of fact it could be a very bad
thing, if the vapors or gases are allowed to travel near any open
ignition source (as in an electrical arc or spark). Resulting in Boom!
Followed typically by fire.

This safety principle is used in oil refinery gasoline storage tanks
which feature floating roofs. The idea is you don't want evaporation
vapors collecting between the roof of the tank and the floating level of
the gasoline. So the tank roof moves up and down with the tank's content
eliminating most of the volume where dangerous vapors could build up.

Lee

-----Original Message-----
From: omc-boats-bounces@...
[mailto:omc-boats-bounces@...] On Behalf Of Ethan Brodsky
Sent: Thursday, July 31, 2008 8:35 AM
To: Evinrude & Johnson Boats of the 1960's and 70's
Subject: Re: [OMC-Boats] timing/carb adjustment

On Wed, 30 Jul 2008, JEFF DOOD wrote:
> and in my case, (and probably all the other people who have my exact
> same boat) there are no vents at the bottom or directing things to the
> bottom.) anyone with a Johnson Deluxe wanna chime in? my
> cowling is a big bubble of fiberglass that doesn't touch the boat
> sides. i have noticed the vents on the top back rails on other
> boats the past couple days. pretty common. nothing on mine.
> makes me a little nervous. will i screw up anything to put a pair of
> the standard fin vents along the bottoms sides?

It's rare to actually see vents at the bottom - vents are place that
water can come in. Vents are usually positioned high, with pickup ducts
going down into the bilge for ventilation. The duct should end fairly
low, but above the level that the bilge pump will empty to, so they're
at a point where they can never be underwater during normal operation.

I would tend presume that the manufacturer knew what they were doing
when they designed the engine compartment. Bilge ventilation has been
taken seriously for a longtime and the Coast Guard has some pretty
strict regulations about it, though many of them were not yet in effect
when our boats were designed.

My boat (Evinrude Explorer 16) has two inlets (about 1" high by 6" wide)
in the front, just behind the rear seats, and two outlets high on the
transom, and two "vent plates" also mounted on the transom (the port one
is attached to a fuel tank overflow, the right is attached to the
blower).
The inlets can be seen here:
  http://homepages.cae.wisc.edu/~brodskye/boat/boat3.jpg
and the outlets here:
 
http://images.craigslist.org/010204010407010304200807291d0a6f44e66445ff5
2008e4f.jpg

All of these originally had duct hose connected to it, but the ducts had
all rotted out I'm uncertain as to the exact original configuration. I
recently repaired it all (dryer hose) and choose to do the following.
The bilge blower sucks from the bottom of the bilge, near the aft end
(I'm fairly sure that it originally was mounted forward). The two
exhaust outlets also suck from the bottom of the bilge using duct. The
port inlet is ducted to right next to the engine air intake, and the
starboard inlet has no duct on it at all. I've been running for several
years though with no ducting in the inlet and it didn't seem to make any
difference, but I thought I'd try to make it look nice since since I was
working on it.

Anyway, I just looked at your photo and I'm not qualified to really
recommend any chances. It's difficult to believe there are no vents at
all on the compartment - the engine likely wouldn't run then. Is the
floor of the compartment recessed so it is the lowest point of the
bilge?
It looks nice so I'd be hesitant to start cutting anything.

Incidentally, somebody in New Hampshire is selling a boat exactly like
mine on craigslist for $800, including a trailer.
  http://nh.craigslist.org/boa/775226896.html

Ethan

--
<a href="http://www.engr.wisc.edu/~brodskye/"> Ethan Brodsky </a>
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