We have a call in to the owner of this particular boat for more details on the conditions that led up to this blockage.
In general, scaling most often builds up at the point where water enters the hot section of the exhaust system. The sharp temperature increase at that point apparently causes salt and other minerals (iron, etc.) to precipitate out of the engine cooling water and collect within the hot section. Since this encrustation is in the aft end of the manifold itself, it's not clear to me where the water would have originated to cause this particular blockage.
I don't know how this happened, but the photo is actually upside-down. What appears to be a drain on the bottom of the photo is actually the standard early model water outlet on the top (rear) of the manifold.
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