PS Advisor: 05/01/04
Sailplan Balancing Act
Your February 1 editorial on the benefits of small, self-tending headsails is thought-provoking. As I enter my golden years, I like the concept of converting my C&C 24 to self-tacking. But, since it now has the small, high-aspect main with the big (150%) genoa, what happens to the performance when the jib is reduced to working size? I am guessing that the center-of-effort would move forward and a lee helm would be the outcome. Or would one try to get a redesigned main to increase its power to compensate for the new smaller jib, and thus maintain a balanced helm?
There are some boats from the IOR-inspired design era that would be troublesome to retrofit with self-tacking jibs (as primary, all-weather headsails) because their mainsails are "abnormally" small—designed to take advantage of IOR ratings. We forwarded your question to naval architect Steve Killing, who designed for C&C Yachts in the 1970s when the C&C24 was created. Here's his answer:
"I have spent many fine days sailing on the C&C 24, and it is a good little boat, but as you have noticed, the genoa overlap is large and tacking can still be, well…a chore.
"I have included a small sailplan of the C&C 24. It shows the typical style of sailplan we designed in that era—a small but very high-aspect ratio mainsail and a large overlapping jib. As noted, some of that was promoted by the rating rule of the day, the International Offshore Rule. For comparison, I have shown a current design fashioned with a self-tacking jib from the outset. This is a dinghy design, but the principles of balance and weather helm are just the same.
"If the goal on your C&C 24 is to put on a small self-tacking jib that stays forward of the mast, you can simulate the feel by sailing your boat as-is with a small headsail—like a working jib or No. 3 genoa. That small sail will probably be just fine in heavy air, heeled over, but in a medium breeze with not so much heel, you will have lee helm and not much power to drive the boat.
"On the sailplan drawings I have noted the aerodynamic centers of effort (dots and lines on the sails) and the centers of lift of the keel and daggerboard. If the sailplan center of effort is aft the keel's center of lift, you will have weather helm. The darker-colored dot and line on the C&C sailplan represents the center of effort with a small self-tacking jib. It is well forward of the keel center of lift, resulting in lee helm.
"If you compare the C&C with the Fusion 15, which I have scaled to the same size, you can see that the Fusion has a small self-tacking jib and a huge mainsail set well back on the boat to give the desired weather helm.
"So what can you do to make a self-tacker work on your C&C 24? Putting a larger mainsail on the boat would help solve the problem—but we can only lengthen the boom by about 8" until we will risk hitting the backstay on a jibe, and my calculations say that it would not move the center of effort of the sailplan back enough to balance the boat. You could change the rigging to swept-back spreaders, so that the backstay could be eliminated, and then you would have a free-rein on the size of the mainsail. That one might be a reasonable solution. We could also increase weather helm by increasing the mast rake a few more degrees—that helps again, but still not enough.
"A solution that would work, but that I am cautious to mention, is to move the keel forward. The keel's position is most influential in determining where the sailplan should be to balance the boat. Ignoring how the boat floats in the water for a moment, if you moved the keel forward by, say, 8", the weather helm would increase significantly.
"I think you could install a self- tacking jib for use in breezier conditions, when the boat will balance well. For light- to medium-air sailing, I think you are stuck with winch-grinding."
-Steve Killing is the author, with Doug Hunter, of "Yacht Design Explained," a book to which we often refer here at PS. The Fusion 15 dinghy was Sailing World's 2003 Boat-of the-Year.