Wait a minute, something does not add up. This is a 23' hull with an
80HP Nissan diesel getting 7.5 knots? Assuming a full displacement 20'
LWL and 6000 lb displacement it should theoretically take only about 23
HP to get 7.5 Kn.
As far as the exhaust is concerend what you describe is asking for
problems. I assume a relatively short run from the injection point to
the transom which works in your favor but if the outlet is level with
the exhaust manifold there is defilitely a risk of flooding the engine.
You probably don't have to worry about seawater getting in from a
following wave but while cranking that engine will probably need to put
3 or 4 gallons of water someplace. That plug of water you see coming
out when you start up is what was pumped into the hose during cranking.
If the engine is a little slow about waking up and the exhaust is
trapped there is a very real danger that the hose will fill up. When
you stop cranking to rest the battery at least one exhaust valve will
remain open. The water will flow back and fill that cylinder.
David Flew wrote:
Jim
The installation is fairly typical of small wooden boats in my area - at 23
ft this is at the upper end of small. From the exhaust manifold piping
angles down. Water injection point is in this downward sloping metal
section. At some convenient point the system becomes rubber hose, which
then connects to more metal components where the system penetrates the
transom. The outlet is "about" the same level as the exhaust manifold, and
the centrifugal cooling water pump is directly driven by the engine. A few
boats have water lifts or water locks.
I've not heard of any backing up problems as you describe, I'd guess that
even during extended periods of cranking and failing to start the engine,
there is enough air going out of the engine to keep the exhaust from filling
with water - and there is little out put from the cooling pump at cranking
speeds anyway.
I suspect the hose acts a water lock, if it's big enough it certainly acts a
crude silencer. I went from 1.75 inch to 2.5 inch hose on a 10 HP diesel in
my "old" boat ( hope it will be my "former" boat rather than my "old" boat
in a day or two ) and whilst I have no figures, subjectively the engine
could be run to much higher revs before it became unpleasant to sit on the
stern thwart. In the quieter mode, there seem to be discrete plugs of water
ejected from the exhaust, along with continual spray. Once the RPM and
exhaust velocity goes up, it's just a gas/water mix. So I suspect that
water accumulates in the low points of the hose, and it's a water baffled
silencer, if you get my drift. The water locks I've seen advertised suggest
this is part of their function - but don't give much data to support the
claim.
I measured the hose on the "new" boat today - it's 2". I'm really looking
forward to replacing it with 3.5". I think minimum bend radius goes up
something like the cube of the diameter ... I disagree with Glen, it's not a
job for a circus contortionist, but for an animal trainer. I'm really
looking forward to taming the new hose.
I can see the attraction of just piping the exhaust straight from the
engine through the sides - even if you burnt some dangly bits every time you
walked past the engine ....
This time I'm going to take some noise readings as I go along, also
performance details. Although the difference between 7.5 and 7.6 knots at
full throttle is not exactly important!
Regards
David
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at:
http://www.rutuonline.com
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