A conventional shaft-drive installation places a marine gearbox behind the engine. A coupling connects the gearbox output flange to a stainless-steel or alloy shaft. The shaft passes through a seal or stern gland, runs through one or more bearings, and turns the propeller.
Principal components
- Gearbox and output flange
- Flexible or rigid coupling
- Propeller shaft
- Shaft seal, stuffing box or mechanical face seal
- Stern tube
- Cutless bearing or other water-lubricated bearing
- Strut or P-bracket where fitted
- Propeller, nut, locking device and shaft anode where specified
Why owners choose shaft drive
There is no large drive leg through the hull and fewer aluminium components underwater. External parts are comparatively simple, and many yards understand the system. Major repairs can still require haul-out, shaft withdrawal and careful measurement.
Common faults
Misalignment produces vibration and loads the gearbox, coupling, seal and bearings. Worn cutless bearings allow shaft movement. A bent shaft or damaged propeller creates cyclic vibration. Packing that is too tight can overheat the shaft; packing that is too loose leaks excessively. Mechanical seals have different operating and venting requirements, so generic advice can be dangerous.
Alignment should be checked in the water after the hull and machinery have settled, following the coupling and engine manufacturer’s method. Simply making the flange bolts pull two misaligned faces together does not align the system.
Read shaft seals, bearings and alignment and marine propellers.