I concur. For belt and suspenders, you could go to several more engine
manufacturers' sites -- most of them have specs, but 4" sounds about right
to me.

Another risk with a exhaust that's too small is that when you crank the
engine you're pumping water into the waterlift. Until the engine starts, it
doesn't go up to the exit. A smaller exhaust has a smaller waterlift and
therefore is more likely to drown the engine if it doesn't start easily.

The appearance of "steam" is not unusual -- after all, the exhaust stream is
gases fully saturated with water at an elevated temperature. When it hits
the cooler air outside the boat, some of the moisture condenses.

On the other hand, smoke, after the engine is warm, , is not desirable.
Color will usually tell you what it is. Roughly, if it's blue, you're
burning oil; black is overloading; and white is antifreeze leaking into the
exhaust.

--
Jim Woodward
www.mvFintry.com



"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