Join Date: Oct 2006
Location: Callao, VA
Thanked 18 Times in 18 Posts
Rep Power: 11
It appears that your boat has a water lift style muffler/mixer...if so...you may be looking at a broken baffle as you indicate the periodic thump gets worse when the thru-hull is submerged...that would put more back pressure on your exhaust system and perhaps cause something loose in the waterlift/mixer to do the thumping...I think that is what I see in your rear engine segment...
Contrary to the others, I do not think it is shaft or bearing, as it does not happen at the dock...were something out of whack there, one would think you would see it any time the engine is running