SPINNAKER DESIGN TECHNICAL BRIEF IN SEAHORSE MAGAZINE

UK Sailmakers is excited to once again feature a sail design technical brief in Seahorse Magazine, known for its coverage of elite sailboat racing and cutting-edge sailing technologies. You can explore it in the October edition of Seahorse Magazine, available in print or online here.

Our team of writers, led by Buttons Padin, unravels the mystique behind designing asymmetrical spinnakers by tapping into the expertise of three UK Sailmakers’ top designers: Pat Considine, Geoff Bishop, and Stuart Dahlgren. They emphasize that spinnaker design hinges on two key factors: the boat’s characteristics and the intended style of sailing.

The specifics of the boat, including its type (high-performance, displacement, one-design), precise measurements, and the planned sailing style (IRC, ORC, offshore, coastal and professional vs. Corinthian), form the foundation of asymmetrical spinnaker design.

The article spotlights three essential data points that wield significant influence over a sail’s performance and rating:

1. The point-to-point distance between where the spinnaker will be tacked (on a sprit or a lowered pole) and the max height for the halyard.

2. The mid-girth (SMG) luff to leech measurement, expressed as a percentage of the foot length.

3. The trim position for the clew in terms of height off the deck and where the sheet will lead.

Seven key sail measurement points for an asymmetric spinnaker. It’s crucial to understand how these parameters affect a sail’s performance when asking a sailmaker to design a new A-sail.
Seven key sail measurement points for an asymmetric spinnaker. It’s crucial to understand how these parameters affect a sail’s performance when asking a sailmaker to design a new A-sail.

Spinnaker designs are fine-tuned using these measurements and more to optimize performance for either offshore or inshore racing. Our experts delve into explanations about how SMG impacts a sail’s ideal wind angle and how luff length is a critical design element influencing performance and sail projection.

This digital rendering illustrates how an asymmetric spinnaker rotates as the sheet is eased. A wider SMG allows it to rotate further in front and to windward, which is ideal for inshore racing in a displacement boat. Sails for offshore use and for fast planing boats tend to have a narrower SMG which is more efficient at tighter wind angles.
This digital rendering illustrates how an asymmetric spinnaker rotates as the sheet is eased. A wider SMG allows it to rotate further in front and to windward, which is ideal for inshore racing in a displacement boat. Sails for offshore use and for fast planing boats tend to have a narrower SMG which is more efficient at tighter wind angles.

The article underscores the importance of leech length and sheeting angle, highlighting differences between reaching and running spinnakers, and the role of tweakers. Additionally, it briefly touches on sail panel layout and fabric considerations, highlighting the importance of structural integrity.

J/111 CAG races downwind towards the Alma Grund Lighthouse in the 2022 Gotland Runt. Photography credit: Henrik Trygg.
J/111 CAG races downwind towards the Alma Grund Lighthouse in the 2022 Gotland Runt. Photography credit: Henrik Trygg.

This year’s UK Sailmakers technical brief offers valuable insights into the complexities of asymmetrical spinnaker design, underscoring the need to comprehend boat characteristics and intended sailing conditions. Understanding these principles empowers you to actively participate in the design process, optimizing your sails for your unique sailing needs. 

For those interested in sail design and seeking to enhance their racing performance, this article is a must-read. Check it out in your print copy of the October edition of Seahorse Magazine, or find it available online here.

Heather Mahady
Heather Mahady

Heather Mahady is the General Manager of UK Sailmakers International. She is based on Vancouver Island in the Pacific Northwest, and is a passionate sailboat racer, sailmaker, and sustainability advocate.

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