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The Masthead
Question: Say, you’ve been digging around in an engine room and you noticed something that seems weird on the
engine’s muffler. You can see in the photo that the muffler is the black cylinder in the lower left side of the picture. It has
what looks like an engine-sending unit screwed into the muffler with two wires coming out of it. The engine is a diesel so
it’s not part of an emission control system. What is this and what does it do?
Answer: What you are looking at is an important safety feature that most boat builders unfortunately leave out. You are
indeed looking at a sending unit for an exhaust-overheat alarm system. This overheat sensor is designed to sense excess
heat build-up in the exhaust system in the event that the seawater pump that supplies cooling water to the exhaust
system fails, which is a fairly common occurrence.
Many boat builders have felt, quite incorrectly, that the engine temperature gauge is adequate for monitoring this. This is
simply wrong thinking as the response time for the engine coolant to run too hot in the event of pump failure is much
longer than for the exhaust system to overheat in the event of a loss of cooling water. The temperature difference is quite
significant as well. Engine exhaust can run in excess of 1000°F as it leaves the engine. Coolant inside your engine is only
running in the 150 to 220°F range under normal conditions. It’s easy to understand that at 1000 degrees, things can and
have historically caught fire on boats losing their exhaust cooling water, even for a brief period of time. Early warning to
shut down the affected engine and fix the pump is important to prevent a fire, which is why the ABYC incorporated the
requirement for exhaust temperature sensing alarm systems into their standards many years ago. The only wish is that
more boat builders would incorporate this feature into their engine installations.
Electric Engine Exhaust? What’s Up With This?
By Ed Sherman
Sept. 2014 Page 14
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