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
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