A gennaker’s behaviour comes from its unsupported luff, three-dimensional seam shape and the way its width and volume are distributed. The customer specifies the job; the sailmaker turns it into a stable flying shape.
The unsupported luff
A free-flying gennaker has no stay, mast or permanent cable defining its leading edge. The finished luff is longer than the straight tack-to-head distance, and its cut curve lets the head and shoulders rotate out to windward into clear air.
Too little curve leaves the sail flat and unable to project. Too much leaves the luff soft and prone to folding. The correct distribution depends on the boat, tack position, rig proportions, target angles and sail role.
Half width and shoulders
Half Width compared with Foot describes how much width the sail carries through its middle. A high ratio gives volume and projected area for running; a lower ratio produces a flatter reaching shape.
The shoulders express the same decision higher in the sail. Full shoulders rotate farther into clear air at deep angles. Narrow shoulders keep the sail closer to the centreline and reduce heeling force as the apparent wind moves forward.
These relationships are what distinguish the A1-A5 roles more reliably than area alone.
Handling changes geometry
A Stasher leaves the sail free to be cut for its aerodynamic job. Top-down furling requires a flatter shape, a luff built around the cable and controlled bulk through the reinforcement and edges.
A retrieval patch adds a concentrated load in the body of the sail and is positioned from the geometry and recovery path. A release system changes how the sail is bundled for the hoist but is designed to release completely before the sail does its aerodynamic work.
Panels build shape and strength
The nylon panels do two jobs. Near the corners they radiate along the main load paths. Across the body, the curve cut into each seam builds the sail’s three-dimensional fullness.
The panel layout is therefore structural and aerodynamic rather than a separate styling choice. Colour schemes follow the production layout, not the other way round.
Corners and reinforcement
The head carries the halyard and, on a furling sail, the swivel. The tack carries the sail’s rotational and vertical loads through the tack line or drum. The clew carries the sheets and crosses the foredeck on every gybe.
Corner fittings, webbing and reinforcement scale with the sail’s area and intended use. Chafe protection is added where the sail contacts the forestay, pulpit, bow gear or spreaders.
Lines and tapes
On a free-flying sail, the luff line is structural. Leech and foot lines control flutter and curl. Tapes carry those lines, spread edge load and resist abrasion.
All of them scale with area. On a top-down sail they must also roll cleanly without creating hard local bulk.
Sudden loads
A gennaker is lightly built but can be loaded sharply when it collapses and refills. The corner patches, radial structure and edge finishes are designed for that transient load as well as the lower steady load of a full sail.
For the configuration sequence, return to Boat & Usage.