When do you need a waterlock muffler?

[CassidyNZ]

Originally Posted by sailingunity

My boat has a walk in engine room as well, but flanked on both sides by cabin space which is why the exhaust hose has to dip back down to go under the floor. Our exhaust originally exited right at the waterline, the previous owner actually relocated the exhaust higher on the side for fear of the fitting below the waterline springing a leak. I don't blame him for doing this, but now we get a lot of "overspray" in the cockpit anytime the wind is coming from the port side (I know, we should be sailing) this is the main reason I'm relocating it.

From the waterlift, it runs roughly 9 feet below the floor before rising about 4 feet to the gooseneck. Image attached with the ideal new route.

Honestly, I’d be nervous of the volume of water in the piping of your proposed layout. Firstly, the exhaust gases have to have enough pressure to send the water all the way to the top of the U section before it free-falls to the skin fitting. That’s quite a big ask or at least I think it is. I suspect that layout will give you excessive exhaust back pressure which may limit engine RPM at upper throttle settings. The more RPM, the more gasses but also proportionally more water

Secondly, the volume of water in the pipe to the top of the U all flows back the the waterlift when the engine shuts down. What you need to calculate is the volume of the waterlift and ensure adequate “storage” because if there is already a lot of water in the waterlift and you for some reason crank for extended periods, you could fill the system to a point where water backs up the the mixer.

My exhaust exits the boat right alongside the cockpit but a few inches under water. We get some splashing noise and in exactly the wrong conditions we sometimes get stray exhaust fumes in the cockpit when running the engine at anchor. But I’ve never had water splashing onto the boat.

[sailingunity]

The proposed exhaust run is actually no longer than the existing run - so I'm not too worried about backpressure affecting performance. Also my 75 HP engine *should* have 3.5 inch hose from what I've found online, but the manifold outlet as well as all the current hose is only 2-3/8. Maybe this has had an affect on performance, maybe not, but its been running for 45 years like this.

[GoAway]

My exhaust hose also goes up and down, up and down each time about 12" between the waterlift muffler and the thru-hull on the transom. The pipe is quite old and rigid, likely original. I don't know if it started out rigid, but can you buy exhaust hose this way? I want to replace the hose this summer and since the system has worked well presumably since the boat was new in 1984, I'd like to duplicate the shape of the hose with the bends.

[CassidyNZ]

Originally Posted by sailingunity

The proposed exhaust run is actually no longer than the existing run - so I'm not too worried about backpressure affecting performance. Also my 75 HP engine *should* have 3.5 inch hose from what I've found online, but the manifold outlet as well as all the current hose is only 2-3/8. Maybe this has had an affect on performance, maybe not, but its been running for 45 years like this.

Well, practically speaking, a smaller diameter pipe makes some sense over that length. Think of a pea-shooter - a long thin tube will work well, a fat tube any length, not so much.

Geez, I have so much trouble with imperial measurements. I can instantly imagine 100mm but 2-3/8” has me reaching for a calculator