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
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