A spinnaker is the large, lightweight, deeply curved sail set forward of the mast when a boat is sailing off the wind. Unlike a genoa, which is flat and attached along its luff to the headstay, a spinnaker flies free: it is held only at its three corners and fills like a parachute, catching wind that a headsail would spill. Beginners know it as the colourful sail that appears on the run; experienced sailors know it as two quite different families of sail – the symmetrical spinnaker flown from a pole, whose two lower corners swap roles on every gybe, and the asymmetric spinnaker or gennaker, which has a fixed tack and clew and is sheeted more like an oversized genoa.
Anatomy and construction
A spinnaker has a head, two lower corners, and three edges. On a symmetrical kite the two lower edges are identical, so the “luff” is simply whichever edge is currently to windward and attached to the pole; the other is the leech. The key dimensions a sailmaker works from are the luff length, the foot length, and the mid-girth, which is the width of the sail halfway up. A running spinnaker has a wide mid-girth to project area outboard; a reaching spinnaker is narrower with a straighter luff.
Almost every spinnaker is built from woven nylon, usually between 0.5 and 2.2 ounces per sailmaker’s yard, because nylon combines low weight with the stretch and tear resistance needed to survive a collapse and refill. The panels are laid out radially from the corners – tri-radial construction – so the warp yarns follow the load lines that fan out from head and clews, with a cross-cut or bi-radial centre section in heavier cloth where loads are lower. UK Sailmakers match the profile and depth of each spinnaker to the boat’s stability and target wind angles, because a kite that suits a heavy displacement cruiser will be a handful on a light planing hull.
Symmetrical versus asymmetric
- Symmetrical spinnaker: set on a spinnaker pole with a guy controlling the windward clew and a sheet on the leeward clew. It projects area to windward of the centreline, which is why it remains the fastest choice for sailing deep angles, and it is the sail meant when a rating certificate lists a spinnaker.
- Asymmetric spinnaker: tacked to the bow, a bowsprit or a short pole, with a sheet on the single clew. It is trimmed by easing until the luff curls and needs no pole gybe; the sheet is simply released and the sail floats round in front of the headstay. Modern sportboats and most cruising boats now favour it.
- Cruising chute: an asymmetric cut fuller and smaller, often set from a snuffer or furler so a short-handed crew can douse it without drama.
How to trim a spinnaker
The basic rule is the same for every kite: ease the sheet until the luff just begins to curl, then trim a fraction until the curl disappears, and repeat continuously. A spinnaker that never curls is oversheeted, heeling the boat without driving it. On a symmetrical spinnaker the pole is squared aft as the boat bears away so it stays roughly perpendicular to the apparent wind, and the pole height is adjusted so that both clews fly at the same height. The tack should be lifted or lowered until the luff breaks evenly along its whole length rather than only at the top or bottom.
Cruising versus racing
Racers carry a wardrobe: a light runner, a heavy runner, a reacher, perhaps a code zero for tight angles, each with its own weight and cut, and they change sails to match the leg. They will fly a kite to the ragged edge of control because the gains downwind are enormous. Cruisers want one sail that goes up easily, stays up without constant attention and comes down before it becomes a problem. For them UK Sailmakers usually recommends an asymmetric on a snuffer, built a little heavier than a racing sail, flown at 100 to 150 degrees apparent and doused early.
Frequently Asked Questions about Spinnaker
When should I fly a spinnaker instead of poling out the genoa?
Once the apparent wind is aft of about 90 degrees the genoa collapses behind the main and loses drive, while a spinnaker keeps projecting area. In moderate breeze a kite can add a knot or more to a cruising boat; in heavy air a poled-out genoa is the safer option.
Why does my spinnaker collapse and refill with a bang?
Usually the sheet is either eased too far, so the luff folds well past a curl, or the pole is too far forward for the wind angle. Trim so the luff curls steadily, square the pole, and steer a slightly hotter angle in lumpy water.
How long does a spinnaker last?
Nylon suffers from ultraviolet exposure and from stretch when the sail is overloaded. A racing spinnaker may be tired after two seasons of hard use; a cruising sail that is kept out of the sun and flown within its wind range can last a decade.
A spinnaker rewards attention. Set it, keep the luff curling, adjust the pole as the wind moves, and take it down while you still have the choice. Sailed that way it turns a slow, rolling run into the most satisfying leg of the day.