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Preventing exhaust system damage

Aug 28, 2014

During our six-year circumnavigation we had two raw water impeller failures and, when we had the engine overhauled recently in Mazatlán, we found three partially torn blades on the impeller that we had installed two years ago in Australia. We were close to having a third impeller failure in about 5,000 hours of engine running time.

Failure of the raw water impeller can be inconvenient, but replacing an impeller is not that difficult, even on our engine, where the pump faces aft and getting to it involves standing on your head in the locker under the pilot berth and peering through a tiny door the boat’s builders were thoughtful enough to provide.

The bigger problems we have had have been because of the downstream damage that occurs, very quickly, when an impeller fails or the raw water flow is interrupted for other reasons, like a plastic bag getting sucked up against the water intake through-hull.

Once the raw water flow is cut off, the temperature of the circulating coolant in the engine will slowly rise and set off the engine over-temp alarm. This protects the engine itself from damage caused by inadequate cooling, but long before the engine’s temperature alarm goes off, the effects of hot exhaust gasses passing into the rubber exhaust hose, and on to the plastic water-lift muffler, can do a lot of damage.

Prior to installing our exhaust temperature alarm, we had two incidents in which the exhaust system suffered thermal damage. In both cases, we had to jury rig repairs to get the exhaust system working well enough to let us move on. It’s a very desolate feeling to have your muffler start leaking water and exhaust gases in a remote anchorage in Fiji or the Galápagos.

At some point in the 1970s, someone came up with the now ubiquitous water-lift system that most modern sailboats use. In these water-lift systems, the raw cooling water is injected into the exhaust system immediately after the exhaust gases leave the outlet manifold on the engine. This approach offers many benefits.

The primary benefit of this system is that the water cools the exhaust gases enough that they can be conducted to the exhaust outlet on the hull through flexible rubber hose without heat damage to the hose or surrounding materials. That is important because the asbestos option is no longer on the table, the cost is much less than a custom-made double-jacketed exhaust pipe and the yacht designers have a lot more flexibility in routing the exhaust line to an appropriate location high on the stern. Another benefit is that the raw water leaves the boat through the same through-hull fitting as the exhaust, and fewer holes in the boat is always better.

The maximum exhaust gas temperature specification for the Yanmar diesel on our boat (4JH3E) is 600° C, yet the temperature of the water that leaves the boat with the exhaust is not as warm as a comfortable bath. Put your hand in the exhaust water behind a diesel-powered boat and you will be surprised at how it is barely warm. If you are apprehensive about sticking your hand in the stream, catch some of the water in a bucket and take its temperature.

The water-lift systems work very well as long as the raw seawater keeps flowing. But if the water flow stops, the dry exhaust gases raise the temperature of everything downstream, and damage occurs quickly.

The muffler is usually located below the engine and is designed so that the pressure from the exhaust gases blow the water up the outlet hose and out of the boat. The water ejection process becomes a constantly recurring cycle where water accumulates in the muffler until the level is high enough that the exhaust gases must push the water up the hose so the gasses can escape from the exhaust line. We observe this as intermittent blasts of water coming out with the exhaust.

After thermal damage occurred to our muffler, we learned that the manufacturer had changed the design of the inlet spigot on our muffler and the version of the replacement part that would fit our muffler was no longer available. It was a small change in the design, and probably an improvement, but it meant that I had to shell out close to $800 for a new muffler in New Zealand, just because a part that probably cost less than 50 cents to manufacture was no longer available eight years after the boat was built and the original muffler installed.

We paid through the nose for a new muffler, so we wanted to do something to help prevent the same sort of damage from occurring again even though we hoped replacement parts might continue to be available long enough for us to complete our circumnavigation. Ultimately we built our own temperature sensor system.
 

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