Definition
A saildrive is a geared drive unit that bolts to the engine and turns the drive through approximately ninety degrees, passing down a leg through the hull to a horizontal propeller shaft, sealed at the hull by a model-specific diaphragm.
Saildrives package a yacht's propulsion compactly, remove the long shaft line a conventional installation needs, and present the propeller at a favourable angle. They are extremely common in production yachts and catamarans, and dominant in the Australian cruising fleet.
The two things that define ownership
Corrosion control. The underwater housing is commonly aluminium, so anode choice, the approved coating system and the drive's electrical relationship with the vessel's bonding are a single protection system. Copper-bearing antifouling applied directly to an aluminium leg causes serious corrosion.
The diaphragm. The hull seal is a safety-critical, time-limited component with a replacement interval set by the drive maker — and one maker's interval must never be applied to another's.
Assessing one
Judge the drive separately from the engine, and ask for dated evidence of diaphragm work. See is a saildrive less reliable than a shaft and how to check saildrive oil for water.