When I started this blog I said that my plan was for a cat ketch, with a balanced lug fore (main) sail, and a standing lug sail with a sprit boom at the mizzen (aft) mast.
Upon reflection, I decided that it would be simpler and better to have the same type of sail both fore and aft. I like several things about sprit booms, and B&B Yacht Designs expresses them eloquently. One of the best is that, when the boom goes about, at worst you get slapped by a bit of canvas, instead of a length of wood or metal which can knock you senseless and/or overboard.
The second reason was a balanced helm. The placement of the mizzen mast partner (at the rowing thwart, just forward of frame 5) and the centerboard (between frames 3 and 5) was largely dictated by the boat’s basic design (the placement of the boat’s frames), so I had to design the sail plan around them. I did, but at first I neglected to take into account the effect of the skeg and rudder on the Center of Lateral Resistance (CLR). Adding these to the calculations moves the CLR aft, which means that the original sail plan would have resulted in a serious lee helm. A lee helm can be dangerous: the moment you let go of the tiller, the boat turns away from the wind and it’s off to the races. It’s best to have a slight weather helm, where the boat will naturally turn into the wind in an emergency and stop dead in its tracks. I therefore had to move the Center of Effort (COE) aft too.
To accomplish this, first I moved the mainmast position aft to just forward of the foremost bulkhead, about 22 inches from the bow. This led to a reduction in the mainsail size, which also moves the COE aft. Then I changed the sail type to the standing lug/sprit boom combination, which added to the effect, since the tack of this sail is further aft than a balanced lug’s. Finally, I opted for a smaller main mast/sail and a larger mizzen mast/sail.
Taking all that into account, the sail plan now looks as you see it below. It took me a while to make good on my promise for a full hand-drawn sketch, but better to be late and accurate than early and wrong. The centerboard did not fit on a regular sheet of paper at an inch-to-millimeter scale, so it's truncated. COE marks the combined center of effort.
The mainsail has an 84-inch (2134 mm) foot and a 96-inch (2438 mm) yard. The sail area is 58.5 square feet (5.44 m2). The mast will be 142 inches (3607 mm) long and 2 ¼ inches (57 mm) in diameter at the partners (click on "spars" label on right to learn more). That’s a short mast, which has many virtues: it fits easily inside the boat (with the fore locker sliding door open), it’s short enough to be unstayed (no standing rigging!), it has a lower COE and center of gravity (which makes the boat more stable), it’s cheap to make and simple to work with. The clew of the sail will have about 6 inches (152 mm) of clearance forward of the mizzen mast, and 36 inches (914 mm) of clearance from the seats, leaving plenty of room to avoid the swinging boom-end with no ducking for most people.
The mizzen sail’s dimensions are proportional to the mainsails, and about 93% the size. Thus we have a foot of 78 inches, an 89-inch yard, and an area of 49.5 square feet (4.6 m2). The boom-end at the clew will be a little aft of the transom. Clearance from the seats is the same as with the mainsail. Mast length is 139 inches (3531 mm), and diameter the same as the main’s. Given the dimensions of the two masts, it is possible to make both with a total of nine 8-foot boards. That assumes no errors: hope springs eternal.
You may wonder why the two masts do not conform to the 93% ratio. First of all, the mainmast step is a full 5 inches further above the baseline than the mizzen one, so the total length can be correspondingly less. Secondly, what matters for the proportions is not total mast length, but the length between the tack and the yard, which is only 97 inches in the mainmast, and consequently 90 inches in the mizzen.
Under the final sail plan, the COE is well aft of the middle of the lowered centerboard, which should make for a balanced helm (fingers crossed). The total sail area will be 108 sf (10 m2), which should make the boat reasonably zippy.
Balanced helm, zippiness and how close and how well the boat will sail upwind all remain to be seen, and I may need to make more adjustments after the boat’s launch. I am hoping for 90-degree tacks but will settle for 100. Heck, I won’t be racing against any tall-aluminum-masted affairs with complicated standing rigging, genoas, fancy boom-vangs, jibsheet capstans and the rest. My whole boat design and building effort is based on a principled rebellion against high-tech complexity, uniformity and the racing rules that spawned them. In any case, if other traditional sail aficionados are to be believed, my Aerie should be able to leave behind any comparable Bermuda-rigged boat on a beam or broad reach, where lug sails excel.
By now almost all of the hull work is done. As you can see, though, there’s plenty more, so keep reading 176inches!
boatbuilder
No comments:
Post a Comment