The JB-Welded Propeller
Emergency repairs under offshore conditions. The structural boundaries of epoxies and mechanical ingenuity when there are no spares.
It is a truism of ocean passages that if something can fail, it will do so at the least convenient moment. While motoring through a windless stretch 150 miles south of Nova Scotia, a metallic thud vibration shook the engine compartment of S/V Osprey.
We shut down the engine immediately and went overboard with a diving mask. The diagnosis was discouraging: the keyway key on the propeller shaft had sheared, causing the bronze propeller to spin freely on the shaft, cutting off our propulsion.
The Emergency Epoxy Matrix
We had no spare propeller key on board. Under offshore conditions with rolling swells, we had to get creative. We pulled the propeller, cleaned the shaft and keyway surfaces with wire brushes, and degreased them with acetone.
We machined a crude key replacement out of a bronze bolt shank using hand files. To secure the replacement, we mixed high-strength JB Weld epoxy and packed it into the keyway cavity.
After letting the epoxy cure for eighteen hours under heat lamps powered by our generator, we reassembled the shaft assembly.
Mechanical Boundaries
Epoxy repairs on rotating drive systems are temporary expedients. JB Weld possesses excellent compressive strength (around 3900 PSI), but limited shear strength compared to solid metal.
We ran the engine at low RPM (under 1200) to minimize torque loads on the repaired keyway. The repair held, driving us safely into Halifax Harbor. Safe seamanship requires knowing the physical limits of your materials and acting accordingly.