RedactedNarrative: Vessel 1 was in 28 feet of water when they thought they hit a submerged item. A marine surveyor found that three of the four bolts and nuts basically came unscrewed over time and were ejected from the holes in the couplers, the fourth broke off after the other three backed out.  Bolt holes in the coupler appear elongated in the photos and the shoulders on the bolts marked.  The stainless steel bolts and nuts are of insufficient strength for this application and furthermore are of inferior quality as the nuts can be "rocked" on the bolt threads, indication a poor/loose fit between the bolt threads and nut threads.  Substantially stronger grade 8 hardened steel bolts, washers, and steel nuts are customarily used to fasten the coupler halves and that is considered accepted marine repair practice. Likely the repair vendor that completed the 2006 repair installed the stainless steel fasteners.
  
  The surveyor states the damage was clearly no impact damage.
  
  On the starboard side the shaft is broken at the end of the keyway, the prop is gone and the rudder is nicked.  A closer look shows the bolts holding the coupler halves on this side appear to be steel however close examination is not possible at the time to determine if they are correct. Flat washers under the bolt heads is also inconsistent with accepted marine repair practice.
  
  Failure analysis: Inspection reveals the break topography on the shaft is indicative of a fatigue failure/crevice corrosion propagating from the keyway, this shaft failure was already in progress due to improper instillation of the woodruff key too far forward in the key shaft keyway, creating a stress riser often referred to as being "high Keyed." Because the shift was already in failure mode it cold not accept the load imposes upon it and broke off when the port side shaft disconnected from the transmission and lost thrust at 26mph boat speed. The prop stuck and nicked the rudder when the shaft let go.