I've got the same bow thruster and the same problem. Mine is on my 2006 38' Silverton Sport Bridge. Thruster has hardly been used at all when I got the boat. Boat has extremely low hours.
When it happened to me last year, I had 2 new shafts custom fabricated. Stronger material. Over the winter when my boat was out of the water
, my techs put in one new shaft. This season, after just a couple of uses, it stopped working again. This time the shear pin failed. That didn't require the boat to come out of the water and it was easily replaced. But then this week it stopped working again! Took off the motor
unit and discovered that the new shaft broke, AND while the shear pin was intact, there were shear pin shavings everywhere. This QL bow thruster is a POS. No wonder QL went out of business.
I'm not going to replace the shaft because apart from the expense (even though I have another shaft), the shaft and shear pin will fail again. The mechanical and electrical
design of the QL unit is so poor it's amateurish. The motor needs a better coupling to the shaft, and more importantly, it needs an electrical
solution that allows the motor to accelerate and decelerate rather that hit the shaft instantaneously with raw full power.
At this point, I'm searching for another solution to this problem, and open to suggestions. One thing I'll try is giving SidePower a call and seeing if they have any suggestions short of replacing the entire system.