Shaping the Cruising Spinnaker

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No sailor likes to switch on the motor if they don’t have to, but the northern hemisphere summer brings fickle light winds to some of the world’s most beautiful cruising areas. In response, sailmakers have turned to the power of computer assisted design (CAD) to develop custom light air sails that can keep us sailing faster in summer zephyrs.

For cruising sailors, the light-air furling code-zero has made it easy to keep moving when the winds go light. In a two-part series on light air furlers, we reviewed several furling systems designed to handle these sails. Prior to that report, at the the height of the cruising code-zero craze, PS surveyed sailmakers to find out their views on light air sail design, materials and construction. Whether you want a sail strictly for downwind running, for close-reaching in light winds, or one that you can carry while reaching through a wide range of apparent wind angles, every major sailmaker will be able to custom design a sail that fits your general needs. Here’s a brief summary of the design parameters that a sailmaker will consider when designing a custom light air headsail for your boat.

In designing an asymmetrical cruising spinnaker, most sailmakers begin with the boats 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.

Shaping the Cruising Spinnaker

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 Zero, Quantum’s Code Zero, or Doyle’s UPS (a generic equivalent is shaded in blue in the image above), a sailmaker would likely start by prescribing the A-sails 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 boats 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 sails foot length. For instance, a large, versatile, reaching A-sail like UK’s Cruising Code Zero, North’s G-2, Quantum’s A-2, or Doyle’s APC (a generic equivalent is shaded in red in the image above), would have a slightly longer luff than its flatter counterpart, and would be designed with a mid-girth thats 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-sails 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 sails depth. Also, the wind is rarely constant; when the wind is stronger, Nylon tends to stretch more, increasing the sail’s depth.

Practical Sailor has been independently testing and reporting on marine products for serious sailors for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising or any form of compensation from manufacturers whose products we test. Testing is carried out by a team of experts from a wide range of fields including marine electronics, marine safety, marine surveying, sailboat rigging, sailmaking, engineering, ocean sailing, sailboat racing, and sailboat construction and design. This diversity of expertise allows us to carry out in-depth, objective evaluation of virtually every product available to serious sailors. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser with more than three decades of experience as a marine writer, photographer, boat captain, and product tester. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.

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