Motor-sailing: the most under-rated gear your boat has

The Mediterranean fleet figured this out years ago: when summer wind dies, you motor-sail — main up, engine at low revs, apparent wind doing half the work. Australia's east coast summers ask for exactly the same seamanship. The catch is that motor-sailing is where a fixed propeller is most wrong.

The moving target

Motor-sailing continuously changes the load on the propeller: a puff fills the main and the hull needs less thrust; the breeze dies and it needs more. A fixed pitch is tuned for one load. As sail power varies, the engine wanders between lugging and free-revving — you hear it hunting, and you pay for it at the bowser.

Blades that follow the wind

The Autoprop's blades re-pitch with the load, automatically, every rotation. As the sails take up work, pitch coarsens and revs drop for the same speed; when the breeze fails, it fines off and keeps the engine loaded correctly. Owners consistently report motor-sailing as the single biggest difference — long light-air passages at a fraction of the fuel and noise.

Doing it well

• Keep enough sheet tension to steady the boat — the prop rewards a settled hull.
• Think in fuel-per-mile, not per-hour: a knot of apparent wind assistance at the same burn is free distance.
• On catamarans, one engine plus sails is the classic long-passage economy mode — see catamaran propellers.