IMHO your cutlass bearing is worn. That allows the shaft to move around and you can hear a thumping sound as it hits the strut.
If you have a fixed blade prop you can test this. pick a day when can sail quickly, say 5 kts or so. Turn the engine off and leave the trans in neutral. check the shaft to make sure it's still turning. If you hear the thumping, place the trans in reverse. When the shaft stops turning the thumping will stop. Now place the trans into neutral. Do you hear the thumping when the shaft is turning? If so, it's the cutlass bearing that is worn out.
Deep Blue C, 2002 C&C 110
Mt. Sinai, NY
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