Originally Posted by Boracay
The outlet is tucked under the stern on the port side, about 285mm/11" above the waterline. I can hear it going underwater as we motor
The muffler manufacturer says that I need only bring the exhaust pipe 450mm/18" above the waterline but it feels like I need to make it higher.
Personally, I would not be happy. With the measurements you give you will only have 7" rise in the exhaust from the discharge to its highest point. If the discharge is exposed to following or quartering seas I would be concerned that water
would be driven over the 7" rise.
We have cockpit
drains approx 4"x2" out the back of our cockpit
through the transom approx 19" above the waterline and we get plenty of water
from following seas driven solidly in through them so I would expect plenty of water to be driven into an exhaust that is considerably lower (especially if it is 4" diameter). Our exhaust discharge is out the transom approx 12" above the waterline (so about the same height as yours) and the highest point in the exhaust run (at bottom of inside pipe diameter) from the muffler is approx 36" above the waterline - the exhaust runs level several metres back from the transom and the rise to the highest point is then dead vertical with a 90 degree bend to provide head
Our exhaust discharge is often underwater and while I doubt there is any chance of water getting far while the engine
is running. I think the worst case for a transom discharge is probably sailing down the face of a wave (so boat is pitched forward lowering the effective height of the rise in the exhaust) and a wave breaking against the transom. I could easily see that if the 7" suggested rise was not close to the discharge much of it could be eaten up by the pitched attitude of the boat so an almost straight run for encroaching water through to the muffler.
All assuming a valve is not fitted at the discharge (and am not a fan of valves there).