Shaping the Cruising Spinnaker

Posted by Darrell Nicholson at 11:21AM - Comments: (2)

The North G3 is a versatile sail designated for wind angles of 80 to 120 degrees and truewind speeds of 8 to 20 knots.

In designing an asymmetrical cruising spinnaker, most sailmakers begin with the boat’s fore-triangle rig dimensions (I and J), and combine those with information about the intended use of the sail (tight reaching, reaching, or running) and information regarding where the sail will be used.

Shapes for cruising A-sails generally range toward the fuller, deep-reaching sail (in red). Racers often employ flatter "code" sails (in blue) to maintain speed in light air without significant penalty to a boat's ratings.

In general, close reaching sails are smaller and flatter, and sails meant for broad reaching and running are larger and deeper. For a flat, tight reaching sail, like UK Sailmakers' Code 0, North’s G-0, Quantum’s V-0, or Doyle’s UPS (a generic equivalent is shaded in blue at left), a sailmaker would likely start by prescribing the A-sail’s luff to be anywhere from 90 percent to 100 percent of the distance from the stem to the spinnaker halyard exit box on the mast. That will give the sail a relatively flat luff for tighter sailing angles, but also preserve some latitude for the user so that he could ease the tack line and develop a slightly fuller shape for sailing somewhat deeper angles.

The foot dimension speaks to the overall breadth of the sail. Generally, sailmakers take their cue from the boat’s J dimension to determine this. A flat reaching A-sail would have a foot dimension around 150 percent to 160 percent of the J measurement. A deeper-reaching sail would likely have a foot dimension closer to 180 percent.

The mid-girth measurement is ordinarily described as a percentage of the sail’s foot length. For instance, a large, versatile, reaching A-sail like UK’s A-Star, North’s G-2, Quantum’s V-3, or Doyle’s APC (a generic equivalent is shaded in red in the image), would have a slightly longer luff than its flatter counterpart, and would be designed with a mid-girth that’s closer to 95 percent of the J. The mid-girth measurement on the flatter sail (the G-0, V-0, or UPS, etc.) might be smaller, say 85 percent to 90 percent of the J.

Three other concepts are instrumental in determining an A-sail’s shape: Aspect ratio—how wide the sail is relative to its height—the twist profile of the leech, and the distribution of vertical camber. Keep in mind, a sail’s fullness will almost always be in flux due to uncontrollable factors: The tack and clew can and do move about, and their relative positions affect the sail’s depth. Also, the wind is rarely constant; when it’s stronger, Nylon tends to stretch more, increasing the sail’s depth.

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