by Dan Dickison
Recently, I had the good fortune to spend the better part of a Sunday in January on the water under sail. Thanks to global warming, it was a pleasant 77 degrees F in the cockpit as we piloted a Marshall Cat 22 around the harbor in 8 to 10 knots of wind.
Most Marshall Cat 22s carry a gaff rig. Even though Iím not a traditional-rig devotee, I tend to think that any sailor with sufficient time on the water can get almost any sail—even one with four sides—to perform optimally. Despite that outlook, I came away from the afternoonís experience feeling less than confident about the nuances of properly setting the peak and throat halyards on a gaff rig. Even attaining the optimum amount of mainsheet tension seemed a bit allusive aboard this beamy shallow hull. All of that prompted me to consider the many aspects about sails that we all feel we know, but really donít.
If thereís any aspect of sailing and sailboats thatís as involved and complex as hull shape and hydrodynamics, its sail shape and the aerodynamics of foils. Sure, we all share a basic familiarity with how sails function, but Iíd wager that even the most experienced sailors begin to lose their bearings when trying to articulate Bernoulliís theory and its application to sails.
With that in mind, itís appropriate to have at hand some resources that offer information on sail performance and sail shape dynamics. So, the following is a brief inventory of resources that can help us all better understand the function of sails in theory and practice, as well as how to best care for them.
A good introduction to sail theory can be obtained in the work of Arvel Gentry, who began writing about the aerodynamics of sails back in the early Ď70s. Much of Gentryís work is archived on a website that he maintains (www.arvelgentry.com), and several of his papers can be downloaded as PDF files. Gentry served in private industry for 40 years as an aerodynamicist, but found his way into writing about sails through a keen interest in racing. He was troubled at that time because everything he read explaining how sails work seemed to be wrong. After publishing several papers, he was credited by one expert (C.A. Marchaj, see below) as being the first to correctly articulate ďthe jib-mainsail interaction effect.Ē
For those sailors who want to fully immerse themselves in the arcane density of this topic, thereís no better source than C.A. Marchajís series of books, beginning with his classic tome Sailing Theory and Practice, or his more recent Aero and Hydrodynamics of Sailing, and lastly, Sail Performance: Techniques to Maximize Sail Power. Marchaj comes to this topic with rare qualifications. Trained in aerodynamics and hydrodynamics, he has broad experience as a sailor and is an avid glider pilot. His volumes of work contain in depth descriptions of how sails function, backed up by detailed explanations of the underlying mathematical theory. He has also written an exhaustive work on boat design entitled Seaworthiness, The Forgotten Factor.
Perhaps no other person has written so extensively on this topic, nor conducted as much empirical research as Mr. Marchaj, but Frank Bethwaite would definitely be a close second. With a background in aeronautical engineering and meteorology, this Australian also enjoys flying gliders in his spare time. In addition to coaching at every level of the sport, he has also been engaged in research on performance sailing for more than 25 years. He and Marchaj share similar approaches to their work. Both have relied to some degree on wind tunnel testing for investigation, both have bolstered their theories with empirical on-the-water testing, and both support their published findings with mathematical analyses. Bethwaite, however, puts a decided emphasis on speed under sail, which the title of his well-known book High Performance Sailing makes clear. And despite all the formulae and detailed illustrations, his book offers a great deal of pragmatic information.
If youíre in the market for something on this topic thatís both elemental and comprehensive, consider Tom Whidden and Michael Levittís book The Art and Science of Sails. This well-known duo (Whidden is the President of North Sails) begins its work by discussing the various materials that go into modern sails and finishes with a manual on sail care. In between, they sandwich such vital topics as sail design choices and their consequences, and the importance of matching your sail selection to the boatís rig tune. Both Whidden and Jobson draw on decades of hands-on experience, principally in the racing arena, but the information they proffer is readily applicable to most sailboats.
Enterprising sailors who are inclined to do some of their own work on sails will likely want to own Dan Neriís supremely useful book The Complete Guide to Sail Care & Repair. Neri, a very successful racing sailor, longtime sailmaker, and sail designer with North Sails, took the better part of a year off to write a book that serves as the DIY bible for sails. Even if you never pick up a palm and needle, you can glean a lot from this book. Reading just the first chapter will make you much better equipped to negotiate with a sailmaker when purchase time comes. Neri offers sage advice on selecting a repair loft, and important tips on how to best work with that loft. And if you do end up working on your own sails, he counsels his readers on what tools to own and how to use them in deliberate, detailed lessons. Always opinionated, but with well qualified opinions, Neri offers much of his advice through anecdotal narratives that tend to keep you engaged in a topic thatís not inherently scintillating.
Of course there are many other resources out there that can assist sailors interested in obtaining a better understanding of sails and how they function. The Internet is full of articles and digests, and itís hard to overlook Wally Rossí work Sail Power or Brian Hancockís Maximum Sail Power. So consider this short list only a primer, or a point of departure. Pretty soon, you too can start spouting off about Bernoulli, circulation vectors, and Kutta conditions. If not, at least youíll know how to patch a tear in your headsail.
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.