Shaft, coupling, prop, stern gland

The drivetrain carries engine torque to the water. On a shaft-drive boat, the transmission turns a flexible or rigid coupling, which joins it to a stainless propeller shaft. The shaft passes through a stern tube and is supported near the propeller by a cutless bearing: a water-lubricated rubber-lined sleeve. Where the shaft enters the hull, a traditional stuffing box compresses packing around it and normally drips slightly underway; a dripless face seal such as a PSS uses polished sealing faces and a rubber bellows. The propeller may be fixed, folding or feathering.

A saildrive instead bolts the engine to a geared leg that passes through a large hull diaphragm. It has no long inboard shaft or stuffing box, but adds gear oil, seals, corrosion anodes and a manufacturer-specified diaphragm replacement question. These configurations must never share the same line items.

Misalignment destroys bearings, seals and transmissions. A failed coupling removes propulsion. A loose or lost propeller does the same. A failed shaft seal or saildrive diaphragm opens a flooding path; a bent shaft or worn bearing causes vibration and collateral damage. Rope around the prop can bend hardware or pull a shaft aft.

Service a stuffing box by adjusting or repacking it and inspecting the hose/clamps. Service a dripless seal according to its manual, including bellows and O-rings where due. Replace a cutless bearing when shaft play exceeds acceptable clearance or rubber is damaged. Align only after the boat has settled afloat; alignment can expose failed mounts or a bent shaft.

Prop repair can dress minor damage and correct pitch/balance. Replace a shaft when bent, deeply crevice-corroded, undersized or worn at the seal/bearing. A folding/feathering propeller is an upgrade, not a required repair when a correct fixed prop is sound. Saildrive work follows its own manual: oil, anodes, prop seals and diaphragm.

• Covered by a standard survey — external prop damage, shaft play at the bearing, visible corrosion, accessible coupling, seal leaks, hose/clamps and vibration during sea trial. Haul-out is essential for the underwater observations.
• Not covered — dial-indicator shaft runout, shaft removal, coupling bore fit, prop scan/balance, internal transmission inspection and saildrive diaphragm disassembly. Specialist measurement is roughly $500–$1,500 before repair. Do it before purchase when vibration, a bent blade, impact history, oil/water contamination or severe play appears; otherwise a haul survey plus sea trial is enough.

• The broker or owner — “Is this shaft drive or saildrive? Send invoices for cutless bearing, shaft seal/bellows or diaphragm, alignment, coupling, shaft and propeller work.” “Has it struck rope, a log or the bottom?” “For a dripless seal or saildrive, send the exact model and dated service record.”
• The surveyor — “Measure and report radial shaft play, inspect prop edges/hub and visible shaft for impact/corrosion, identify seal type and hose/clamps, observe vibration through the RPM range, and note whether the engine reaches rated RPM.”

At haul-out, grab the prop firmly and lift it sideways; visible clunk or large movement means bearing investigation. Look along blade edges for pink dezincification, bends, cracks and mismatched pitch. Inside, run a dry tissue under the coupling and seal. Rust fretting at coupling bolts, black rubber dust, a hose with one clamp, a compressed or cracked bellows and salt trails are clues. Under power, note whether vibration is tied to shaft RPM and whether it worsens sharply at one range.