situation: Yanmar 3GM30
, raw water
is mounted amidships, so being a 40' boat, there's about 20' to the transom (direct). Old exhaust
run was roughly 8m (24-25'?) long including turns etc and didn't have a gooseneck - just went uphill, with only the exhaust
fitting in the transom itself facing down. Total lift
from bottom of waterlock was just over 1m (3').
According to what I've read. Everything about this install was Evil(tm) - no gooseneck, too long, too much backpressure etc etc etc.. However, it did work. I have no baseline data like HP lost/temps etc etc.. but it start and ran fine. Only proviso here is that the engine
was new (38 hours) to the boat at the time of purchase
, and I haven't run it for more than about 10 hours total after that when I started the refit
- so since I'm planning on long-term cruising - if this exhaust system is destined to kill the engine in the long-term, but can survive short-term, then that's Really Bad.
As I'm replacing everything in the boat, I'm trying to fix
the exhaust run Evil(tm) as well... however, the geometry of the boat does not allow any flexibility (short of running exhaust hose over mattresses/settees etc...
I can't use a gas/water separator. I can't exhaust closer to the engine above the WL. I can't use a standpipe/drystack. Etc etc.. Not good.
After much thinking - there are two things I can
is to follow the exact old layout and add the gooseneck (say 40cm/just over a foot). (But I'm adding more backpressure/length etc - so I don't want this to be the straw that breaks the camel's back)...
is to cut the exhaust length almost in half, put the gooseneck amidships to loop WELL above the HWL, but
exhaust about 10cm (3") below
the WL (but I can also add another 3" below WL to the opposite side as well so it looks a bit like a North Sea exhaust... but wet and underwater.
I have no idea what the 3GM30's backpressure limit is - however have found this document for the 3YM30
Which states on p.5 that the max allowable backpressure is 1500mmAq, which using an online converter has yielded to be 2.1psi...
Now I have no idea how this relates to water
and gas in an exhaust pipe and am now even more confused. Who knows, maybe the original install was well within spec and the Yanmar 3GM30
can push a 1000' column of water
40' straight up?
If somebody could enlighten me and let me know whether the original was OK and I should just copy and add the gooseneck and be done with it... or offer some other advice, I would GREATLY appreciate it.
Thanks muchly in advance!