R & D Flexible Shaft Coupling f/4" B/W, Hurth - 5HP/100RPM Capacity [910-001]Flexible Shaft Coupling for 4" B W, Hurth 5HP 100RPM Capacity The R&D Flexible Couplings are fabricated from a contoured flexible disc molded in tough yet resilient new type polyurethane. The contoured disc gives clearance for the bolt heads, and is able to flex freely to take up any temporary misalignment of the engine and shaft, due to flexing of the boat structure or the engine moving on its rubber vibration isolation mountings. Forward thrust is