Old Manifold
That looks very thin for cast iron to me but I don't know how thin they usually are.
Maybe small internal changes to a manifold design / casting could cause some designs to crack in a particular area?
Still can't see how any engine issue could cause manifolds to repeatedly crack. There must be many boat owners with same engine who have had head gasket / valve / block / fuel / ignition problems, do such problems often also lead to manifolds cracking?
But the cracking doesn't occur unless you've run the boat at speed... So worth checking coolant flow to manifolds while someone drives the boat? If coolant flow is inconsistent at high throttle loads could understand that cracking manifolds.
Was a picture of a pre-sledgehammer crack posted? Sorry if I missed it.
Already suggested taking a good manifold off the engine that doesn't have a problem and putting it on the engine that does have a problem, fit the new manifold on the engine that doesn't have a problem. See if the new manifold cracks on the good engine / if the good old manifold does crack on the bad engine.
However, last time I did not see the crack anymore and build a small testsetup in my back yard - video is added. Also got some pipe and will be replacing the hose going to the bottom of the manifold with clear one on both engines, will get the IR thermal gun back and compare between temps between the engines - it has to be issue with water getting to the right points to cool it down or too much cold water getting there when it is still hot.
