Square Top Sails - Part 3: Sail Development Story

Technical discussion of ARC products
Bill Roberts
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Square Top Sails - Part 3: Sail Development Story

Post by Bill Roberts »

Part 3: Square Top Sail Development Story

My first experience in attempting to make a square top mainsail (mid 1970s) was with my SuperCat Catamaran partner Steve Edmonds. Steve stirred my interest in catamarans and this took me away from monohull racing/sailing. Steve and I thought we could design and build a better, faster and safer catamaran than what was available in the 1970s. We tried many experiments on all parts of the beach cat design in an effort to build this better, faster and safer beach cat. One of our first new development ideas was the square top mainsail. The first square top sail construction effort was not successful. The sail cloth available then was a soft Dacron cloth and it proved to be too stretchy to make the square top corner stand correctly over a wide range of wind loads/pressures.

When I was developing the RC27 Catamaran, early 1980s, my sail maker and sailing buddy was David Posey. David was a professional sail maker in a town nearby. Around this time a hard and stiff Dacron sail cloth called Yarn Tempered Dacron became available. Again I tried the square top mainsail design. The first effort was to modify an old pin head sail. The top three sail panels of a cross cut sail were replaced with the YT cloth. The leech was straightened out, cut back down lower, and a square top was built into the head of the sail. Total mainsail area was kept constant. After trying two or three slight changes to the sail luff curve in the top region of the sail, Dave had the square top working well. In light and medium winds the square top corner would stand with a small amount of sail twist. When the wind blew hard and the mast bent more, the square top would twist off to leeward, parallel to the wind, and the center of effort of the sail would move down the sail. This results in a greater sail thrust at the same max righting moment limit of the boat. A couple of years later composite sail cloth became available with a Mylar Kevlar sandwich. Dave built us a square top sail with this new very low stretch cloth and the square top design worked well first try..…

My first square top sails were conservative on the tip chord measurement. My initial thought was it would be harder to get a wider square top to stand. A 0.25 taper ratio square top size was selected to get most of the supposed benefit and minimize the structural risk. Total mainsail area was held constant by shaving some area off the curved leech down low. Repeated sail testing over several months sailing mostly to windward showed that the boat was slightly faster with the 25% square top sail relative to my fastest pin head sail. The improvement was small but it was always there.
Attendance at regattas around the state of Florida brought many questions about this first square top mainsail. Some were sincere interest questions and others were “hee-haws” asking, “what happened to the top of your sail” and “who cut your sail off”, “did you leave something at home, ha, ha, ha”. Today square top mainsails have displaced the pin head design in the world of performance beach cats, some performance monohulls and even the Americas Cup Yachts.