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
"Jim Woodward" jameslwoodward at attbi dot com wrote in message
...
Question. You say, "aside from not having a water lock", which concerns
me.
I assume Glenn and you mean what I call a waterlift -- a pot at a low
point
from which the exhaust leads out near the bottom so that water in the pot
is
blown out after the engine starts.
The primary function of these is to hold the water that is pumped through
the engine before it starts. Something to perform that function is
essential unless the engine is mounted high enough so that the exhaust
drains by gravity without help. Without it, you run the risk of water
backing up through the exhaust valves. If that happens and the engine
then
starts firing in another cylinder, you may break something in the cylinder
with water in it -- certainly it's not good.
This may be a low risk event, particularly if the engine and batteries are
in good shape and it starts quickly every time, but most of us try to
avoid
even low risks of putting a rod through the block.
--
Jim Woodward
www.mvFintry.com
.
"David Flew" wrote in message
...
Glen
I was hoping for a response from you - always seem to give a good
practical
approach. Aside from not having a water lock, I think you are spot on
in
both the required sizing, the original sizing, and the potential for
damage. I suspect that damage was avoided by the previous owner by
limiting
RPM to 2200 -2500. If 3.5 " is OK for 3500 RPM., then 2.5 " is OK for
roughly 2200. Either way the exhaust is clearly too small.
As to the "new" engine. It's "new" for this boat. In Australia it's
very
economical to buy a low mileage second hand Japanese diesel, which is
what
I
have.
David
"Glenn Ashmore" wrote in message
news:lcsqb.7087$62.5549@lakeread04...
Acording to the Yanmar JH installation manual the exhaust should be
the
size of the mixing elbow outlet up to the water lock and then if the
run
is "long" (they don't say how long long is) to increase by 25%.
From the Centrek site a 3.5" exhaust can handle 85 HP max so you
might
consider using 4" to have a little margin.
The exhaust size for the original 80 HP gasoline engine was probably
2.5". If that system is still installed the back pressure is probably
doing some damage to your new diesel.
David Flew wrote:
Can anyone provide sizing info for wet exhausts on diesel engine.
My
recent
purchase originally had 80 HP petrol engine, then 30 HP Lister, now
80
HP
converted Nissan diesel. The Nissan is OK up to about 2200 RPM,
but
over
that the exhaust has both a little smoke and what looks like steam
to
me.
And the exhaust if absolutely rocketing out the pipe ! I've seen
somewhere that diesels have twice the exhaust volume of similar
sized
petrol
engines, which suggests the exhaust may be half the required area
( I
bet
it's the original diameter ...) but I'd love either rule of thumb or
rigorous methods for sizing exhausts.
Thanks
David
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or
lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com