A Conceptual Design for CSC's Primary Trainer

By Paul Kamen, August 31 2000

Operational requirements

Some design requirements are conspicuous in their absence, and these enabled other requirements to be met with relatively little compromise. These are:

Concept summary:

This design represents an unusual approach to combining some of the essential features of centerboards and fixed keels. The keels are shallow and fixed, with ballast bulbs. But the retractable rudder is large and deep. The rig is centered further aft than usual to take advantage of the rudder's ability to generate side force more efficiently than the shallow keels.

The design rational:

The high live-load capacity and light-skipper self-rescuing requirements work against each other. They can only be met simultaneously by a design that has some ballast at the end of a fin. This can be achieved with a small amount of ballast at the end of a deep centerboard, but it still leaves the boat vulnerable when the board is up. It also adds considerable complication, especially the gear required to raise and lower a ballasted board. The easy repair and maintenance requirements call for an easily removable deck, and this is also complicated by a centerboard case.

These constraints suggest fixed ballast. But instead of a single ballast keel, this design uses twin biplane keels. The twin keels have the following advantages:

The strategy for recovering some of the efficiency lost due to the shallow keels is to use a deep and efficient rudder. To make this work, the rig center of effort is well aft. This would cause excessive weather helm on a boat with a conventional rudder, but this rudder can be fine-trimmed to get just the right balance area to give the helm the right feel even when it's providing most of the side force.

Note that other designs use the same strategy with success. The Cal-20 is one of the older examples, and modern sailboards use single fins for side force resistance.

The low-aspect-ratio rig and long boom help get the sail forces well aft, and are also consistent with other design requirements. Because high pointing is not a high priority, the low rig and higher aerodynamic drag associated with it are acceptable. The low rig also allows a large enough sail plan for the boat to feel lively in light air, but still manageable in heavy air.

The Bird and Golden Gate classes are the best examples of successful low-aspect rigs. They sail decently in all wind speeds, and almost never have to reef on SF bay.

Design Specifics:

The CSC-15 is a few inches longer than 15 feet (depending on bow and stern deck flange details) and six feet wide at the hull (about two inches wider than a Lido 14). Including the deck flanges, it will be about 6.5 feet wide. Design keel draft with a light crew is 20 inches, slightly more with a full crew.

The mast shown is the existing mast from the current CSC Lido 14 fleet. The foretriangle base (J measurement) is almost two feet longer, and the boom is one foot longer. There is also a lot more roach on the mainsail. The result is that the rig is considerably more powerful in terms of effective sail area, but no taller than on the existing boats.

The cockpit seat geometry is similar to the current version of the Lido 14, but the foot well extends further forward for a little more space.

The aft two feet of the cockpit are decked over. This prevents the common student error of sitting too far aft in the boat and getting in the way of the tiller. The decked stern provides a good place to install a large block of flotation foam. It also helps keep the capsized center of buoyancy further away from the ballast, increasing the capsized righting moment.

There is no centerboard case, so crossing the cockpit under the boom should be easy, especially for large people.

The rudder has to do a lot of work on this boat, and will have to be well engineered and very robust. Controls for pivoting the blade up and down will have to be positive and reliable. The rudder is a potential maintenance problem, but it can be separated from the boat for overhaul or replacement.

The hull-deck joint involves no adhesives or sealant. The seal is provided by a double O-ring, shown here as 1/4 inch diameter circular-section rings of neoprene or other appropriate O-ring material. The mechanical clamping force is provided by through-bolts spaced about every six inches along the rail. An unskilled volunteer with a socket wrench should be able to remove or replace a deck in about 30 minutes.

The rub rail is wood, in 12-inch long segments. Each segment of wood is held in place by two of the through-bolts. These wood segments of rub rail are designed to be easily fabricated by anyone with access to a table saw and a file. They can be cut from 1" x 1-3/4" stock.

The amount of ballast required to achieve hands-off self-righting in most wind conditions is about 150 lb. total, in the form of a 75 lb. bulb along the bottom of each keel. This will probably eliminate the need for masthead flotation.