Where there are leaps forward, there are inevitably steps backward..

Failing Hose Nipple

Mobert is at the boat yard currently for a few last-minute major projects and in the course of that work the coolant hoses on our Yanmar engine are being replaced.  One of the hoses between the thermostat and the heat exchanger has been leaking a small amount and since the other projects necessitated draining the coolant from the engine it was a good time to replace them.   I was also considering opening up the heat exchanger and removing the bronze core to have it checked and cleaned since it’s 10 years old, but I haven’t gotten to that yet. (more on the projects in another post)

As is often the case, opening things up sometimes enables you to see the hidden problems you hoped were not there, and one such problem was discovered today.   It appears that the hose nipple on the heat exchanger is rotting or corroding and already has a piece of metal missing, seemingly broken or crushed somehow.

2 pass heat exchanger courtesy of Wikipedia

Quick digression for anyone who doesn’t quite know what the heat exchanger does.   First, to level set, on a typical car engine you have a radiator which contains the antifreeze+water mix that circulates through the engine to cool it.   The radiator has small hollow fins that the antifreeze flows through on the inside, while air flows through the outside.  The relatively cool air flowing through the radiator cools the antifreeze which then circulates through the engine to cool the engine.   By comparison, in a typical boat, the engine is not cooled by air, it’s cooled by the sea/lake water.  The water outside the boat is pumped into a heat exchanger, which is similar to the radiator but enclosed inside a box or tube.   The cool sea water flows through tubes in the heat exchanger while antifreeze is flowing around the outside of the tubes.   The sea water cools the antifreeze which then cools the engine.  The antifreeze portion of the system is basically the same as a car, but the sea water takes the place of the ambient air.   Sea water is awesome at cooling engines but it comes at the cost of corrosion.  The salt and other deposits in the sea water clogs the bronze tubes inside the heat exchanger over time and sort of gums up the works.  You can usually clean the bronze core at a radiator shop though so it’s not all that horrible.


Okay, back to our situation.   The hose nipple that looks bad is part of the housing (the outer box) which is cast aluminum and fairly large.  Repairing the damage essentially requires replacing the whole housing, which incidentally is also the exhaust manifold.

Engine Looks good for 10 years old right? Keeping it clean helps detect problems earlier.

This is a costly part of the engine unfortunately plus all the marine service businesses around here are backed up so it delays the completion of our boat projects past the date when I wanted to have it back at home port for moving aboard.

There is some possible good news.  First, if we hadn’t discovered it, there’s a chance it could have failed entirely at an inconvenient time.  Had the hose connection failed it would have promptly pumped all of the antifreeze out of the engine and overheated.   Second, which is sort of good news/bad news, is that it appears Yanmar no longer sells the housing by itself, they sell the entire assembly with a new internal core, gaskets, end caps, etc (about 20 individual parts) which is theoretically more expensive but probably less labor intensive to replace.   If we replace the entire assembly, there’s no need to clean the core since we’d have a new one.

Tomorrow the diesel tech will take a look and give an official assessment, estimate, and timeline.  We have contingency budget, but timeline is something I’m worried about, it’s just too close to our move-aboard date for comfort.  Online the price for the heat exchanger assembly appears to be about $1200.  My guess is we will have at least that much in labor costs as well.  So 2 to 4 boat-buck$ to resolve this issue.


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