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The Thruster and Mr. Murphy

“Safety Moments, Presented at CCA Stations and Posts”

By Brian Guck, Essex Station

April in New England is normally wet, cold and windy.  Working in the boat yard is challenging.  We had made the most of the nice days to get our single screw motor boat, Ada Hall, ready for launching.

Upon launching Ada Hall my wife and I tested all systems at the dock before bringing her around from the boat yard to her mooring.  Every system checked out, which is always a good feeling after a maintenance-heavy spring. Little did we know that Mr. Murphy was planning on making a visit at just the most awkward moment.

When we were ready to bring Ada Hall around to her mooring in the Warren River, we got to the boatyard early with a thermos of coffee and fresh cherry scones from my wife’s Sea Witch Bakery, thinking that the wind would not yet be blowing too hard.  By the time we were ready to leave the slip we had a strong NW wind.  We were being blown onto the dock and had to make a hard turn to starboard into the wind to get out of the boatyard.  This was not really a problem, but just when I went to use the bow thruster that we had tested days earlier, it failed.  A little single screw backing and filling got us safely out of the slip and underway to our mooring.

Two days later I went into the bow and found the thruster’s internal shear pin broken into four small pieces.  Replacing the pin was straightforward, but in the process I realized that the two lower hex motor mounting bolts were not tight.  This caused a misalignment between the motor drive shaft and the drive gear which eventually resulted in the shear pin doing its thing.

In talking with the Side Power product specialist at Imtra, I learned that a tight and clean motor mounting was essential to preventing shear pin damage.  Even a small amount of debris between the motor surface and the mounting surface can cause misalignment.

I ordered spare shear pins and am going to put a shear pin replacement kit in the bow compartment which will include spare pins and the correct wrenches to do this job again.  That should guarantee that the pin will never shear again. Mr. Murphy will have to look elsewhere to do his damage.  Beware!

Lessons Learned

  • Check the mounting bolts on the bow thruster every spring as part of normal thruster maintenance
  • Make sure you have the correct tools on board, some of them are specialized.  Do you always have a 3mm punch, a thin 17 mm open ended wrench, and a 6 mm hex key onboard?
  • Carry spare shear pins.  Side Power thoughtfully provides a set of spare pins attached to the motor cover which gives you a head start.
    Tools Safely in Place
    Tools taped safely in place. Ready for "next time."


The Cruising Club of America is a collection of accomplished ocean sailors having extensive boat handling, seamanship, and command experience honed over many years. “Safety Moments” are written by the Club’s Safety Officers from CCA Stations across North America and Bermuda, as well as CCA members at large.  They are published by the CCA Safety and Seamanship Committee and are intended to advance seamanship and safety by highlighting new technologies, suggestions for safe operation and reports of maritime disasters around the world.