Lessons from Square Top Mainsails
by Dan Dickison
Not too many months ago, sailboat racing fans the world over gathered around television sets to watch as the 2007 America’s Cup competition played out in Valencia, Spain. Shortly thereafter, sailing magazines were awash with full-page photos of the 75-foot International America’s Cup Class machines going about in the Mediterranean. In the photos that depicted entire rigs, you could easily see that every sail designer in this vaunted realm strongly favors square-top mainsails.
Not long after those images appeared, 89 21-foot, single-handed racing machines departed the Atlantic Coast of France, bound for the Funchal in the Madera archipelago and then finally to Brazil as entries in the 30th anniversary of the TransAt 6.50. Though these boats are over 50 feet shorter than the IACC steeds, their mainsails are eerily similar because of the exceedingly broad dimensions of the sails at the headboard.
Square top mainsails—that’s the common terminology for this profile, though some sailors prefer the term fathead—aren’t new, by any means. Some of the first iterations of this mainsail configuration appeared on multihulls almost 15 years ago, and similar designs were evident on sailboard sails years earlier. In fact, some sailors would argue that this design is simply a contemporary adaptation of the gaff-rigged mainsail typically found on traditional vessels. Whatever the case, it’s not difficult to understand why this shape is so favored in the competitive arena. Essentially, increasing the size of the sail in the area where the wind is apt to be the strongest and the most consistent is simply logical if enhanced performance is your objective.
Certainly more technical explanations could be offered for the advantages presented by increased area at the head of a mainsail. Those more familiar with the science underlying these designs would tell you that a broader head tends to stabilize the air flow across that section of the sail by diminishing the tip vortices. Ultimately, the effect is that the foil becomes more efficient.
That’s interesting, you say, but ‘why would I want to spend money in order to increase the area on my mainsail when racing isn’t that important to me?’ Actually, the point I’d like to make isn’t about adding additional sail area, it’s about using the sail area you have more effectively.
Almost every modern Marconi-rigged mainsail has a series of battens built into it. Battens essentially help to support the leech of a mainsail. In recent years, many sailmakers have favored the use of full-length battens, depending upon the particular needs of the boat owner and the boat’s rig design. (Full length battens tend to make a sail easier to handle and last longer, but the shape of the sail won’t be as easy to adjust.) Regardless of whether your mainsail has full length or partial length battens, you can still use the battens, particularly the uppermost ones, to gauge and affect your sail trim and thus achieve better efficiency under sail.
When sailing upwind in light to moderate conditions, you can sight up your sail from beneath the boom to check the position of your uppermost battens. Some sailmakers counsel using the top batten as a reference on those sails with three battens, and using the second batten from the top if the sail has four or more battens. Either way, in these conditions, you want to ensure that the batten you’re using as a reference is parallel to the boom. Most sailors already know to gauge their mainsail trim by way of the telltales on the sail (both those attached to the body and those on the leech), but far too few take the trouble to look up the sail and check the position of the upper battens. It’s these more lazy sailors who characteristically sail their boats less efficiently.
Understanding how the angle of these battens relative to the boom can help you trim your sail more efficiently is a pretty simple concept. Slightly more complex is the idea that sail shape is affected by the tension on the battens. The battens in most mainsails can be individually tensioned, but of course, it’s something that you have to do when the sail isn’t raised. Generally, sailors increase tension to deepen the shape of the sail around the batten, and lessen the tension to make the sail flatter in that area. Most battens used these days are tapered, so to some degree they adjust on their own (effectively moving the draft in that section of the sail) when the pressure applied to them increases. Taking this practice to the extreme, many racing crews keep a selection of battens on board—some more firm and some softer—to use depending on the anticipated wind and wave conditions. Of course, it’s not necessary for recreational or cruising sailors to bother with such extensive equipment options, but knowing how to properly adjust the mainsheet, traveler, vang and backstay to produce the desired affect in the upper sections of your mainsail can truly increase efficiency under sail.
To get more proficient in making adjustments like these, there’s no substitute for empirical experience. Just get out sailing and allow yourself the luxury of really looking at your sail when you make these adjustments. Spend time doing that, and you’ll being to recognize the subtle but effective differences in shape that you can produce in the upper sections of your mainsail. You may not own a square-top main, but you might as well use what area you do have up there to its best advantage.
About the Author: Dan Dickison is known throughout the sailing community for his in-depth articles on a variety of sailing topics. His resume includes stints as a staff editor at Sailing World, Editorial Director of SailNet, and Editor of Practical Sailor. In those capacities he has written principally about racing, sail handling, and maintenance. He has also written over 50 freelance articles that have appeared in major sailing publications around the world.
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