Exhausting all Possibilities for Engine Water Ingress

Stainless steel ball valve installed between the exhaust mixing elbow and the water lock muffler.

I have received lots of questions on my recent post about repairing my engine after filling it with sea water. The most common question has been how will you stop this from happening again? I will attempt to explain as best can.

The Problem

Emigre’s exhaust system is very simple. This simplicity is part of the problem. The exhaust mixing elbow is bolted on the back of the exhaust manifold of the three cylinder 27hp Kubota . It allows the seawater used to cool the engine to be combined with the hot exhaust gasses and sent out the Exhaust Pipe.

In a typical exhaust system, there is a water lock muffler between the exhaust mixing elbow and the exhaust outlet on the stern or side of a boat. This is the case for Emigre.

When the stern of the boat is engulfed by a wave, water is forced down the exhaust pipe, filling the water lock muffler. This can be prevented by the exhaust pipe running vertically down the transom and connecting to the exhaust outlet by a 90° elbow. The higher  that the exhaust pipe rises above the outlet, the more pressure is required for a wave to fill the water lock muffler.

A potential issue with having a large rise and fall in an exhaust system is that any water within the exhaust pipe between the highest point and the engine will flow downhill into the water lock muffler when the engine is shut down.

Due to the shape of Emigre’s stern, the lowest point that the water lock muffler can be installed is right above the stern tube where the propeller shaft exits the hull.

With the engine mounted low in the boat, well below the waterline, there is only about 250mm of height between the water lock muffler and the exhaust mixing elbow. This means that any water that flows into the muffler, or has not been pushed out the transom when the engine is shut down, has a high potential of flowing back into the engine’s exhaust manifold if the boat surfs down a wave at a steep enough angle.

When water gets into the exhaust manifold, it has a direct path to enter one of the cylinders that had its exhaust valve open when the engine was shut down. The salt water fills the cylinder, slowly bypasses the piston rings before draining into the oil sump.

If the engine is started with water in the cylinders, it has the potential to bend connecting rods or damage the pistons as the water cannot be compressed unlike diesel.

If the engine does start without breaking something, as it did in my case, the water in the oil quickly turns it to a milkshake consistency, causing a lot of grey smoke.

Ball valve and blue water lock muffler.

The Solution

Due to the design of the Duncanson 35 and the location of the engine. It’s difficult to follow the traditional ‘rules’ for exhaust system installations.

To have suitable fall between the engine and the water lock muffler, I would have to route the exhaust hose forward to the main bilge and mount the muffler at the lowest point inside the hull. The outlet hose would have to rise from the muffler to the highest point below the deck head before falling sharply and discharging through side of the hull close to the waterline. This is not a project I want to undertake in Esperance.

My solution for the time being was to install a second ball valve between the water lock muffler and the exhaust mixing elbow. This is in addition to the valve I installed near the exhaust discharge in the aft locker a few years ago.

Ball valve near exhaust discharge.

Once the engine has been shut down, I can close both valves preventing water from filling the exhaust pipe and preventing standing water in the water lock muffler from draining back into the engine when surfing down steep waves.

The downside is the additional steps required when stopping and starting the engine. For the distances I need to cover, it shouldn’t be a problem.

This certainly isn’t my area of expertise and I appreciate the guidance I have received from my mechanic friends.

Fingers crossed it solved the problem.