From Nellie D. 37VT63 Crusing Blog
Dave Howell
Posted: 13 Sep 2016 08:19 PM PDT
 The Waterlift Muffler's View When Looking Back, Through the Exhaust Hose, at the Engine's Raw Water Injection Elbow
The hose, pictured above, is rated at 250°F. A few weeks ago, when cooling water didn't emerge from any of those 20 little holes, thanks to my bout of forgetfulness, 600°F engine exhaust, emerged from the large center hole and scorched the hose's interior.
As dramatic as the failed hose looks, I doubt it's the gremlin causing the engine to currently overheat. Needless to say, that scorched hose was replaced today.
 The Stern Exhaust Hose, Removed from its Thruhull, with the 'Chimney Sweep' that's About to be Pulled Through it
My working theory for the overheating problem was that the exhaust hose was partially blocked by pieces of PVC which had melted out of the muffler. Two other Lord Nelson tugs had this very thing happen. The theory was shot down, however, when a 'chimney sweep' went through the 21' long hose unobstructed. Darn.
When everything's ruled out and the problem remains, something has been overlooked.
Perhaps the gremlin is in one of the very first places we looked: the raw water thruhull. In this case what was overlooked, or at least discounted, was the thruhull's low flow rate. Using a waterhose and forcing water backwards through the thruhull improved the flow rate dramatically.
So, it appears that the over temp problem was a simple matter of marine growth or debris blocking the thruhull. Darn, my melted PVC theory was so much more elegant...
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