Six different designs for the Baja-Board have been built and tested to date. For clarity, however, only one of these will be described in this section. Advantages and disadvantages of the other five will be discussed in a later section. The design described here (Fig. 2) - named Big Red because of its large size and its red striped sail - is the fastest of the six and has been used as a trial horse against which to judge improvements in speed and handling.
Big Red is 15 ft. long, 2-1/2 ft. wide and draws 3 inches with hull alone or 2-1/2 feet with the center board extended. A sail of 56 sq. ft. is carried on a 14 foot mast. The entire unit weighs about 70 lbs. and can be carried with two hands, one for board, the other for the sail assembly in its storage bag. Though it has not yet entered formal competition, Big Red has easily outraced both Sailfish and Sunfish.
The double-ended board has fine lines fore and aft. The 1-1/2 sq. ft. center board is located six
inches aft of the center of buoyancy; the mast is stepped six inches forward of the center of buoyancy. A detachable skeg is mounted near the stern to give the needed amount of directional stability. The board is constructed of 3 lb./cu. ft. foam with two red wood longitudinal stiffeners and covered with one to two layers of fiberglass. The construction technique is similar to that used is the manufacture of contemporary surfboards. The weight of the board alone is about 50 lbs.
The mast is hollow and tapers from about two inches O.D. at the base to one inch O.D. at the tip. It is constructed of fiberglass with a technique similar to that used for vaulting poles. The twin booms (often called a "wishbone" boom) are made of laminated fir or pine shaped so as not to interfere with the sail's longitudinal curve. They are joined to the mast with heavy nylon tape to prevent stress concentration on the fiberglass mast. A heavy-duty, stainless steel universal joint, attached to the base of the mast, permits 360deg mast rotation around a vertical axis and 180deg around orthogonal horizontal axes. The base of the universal joint is attached to a wooden plug that fits a longitudinal slot in the board. The sail is cut from 4 oz. dacron sailcloth and attached to the mast with a nylon sock. Battens are used to obtain a fair degree of roach on the sail's leach and foot. When the outhaul is released the sail assembly folds together and is stored in a bag 14 ft. long, 1-1/2 ft. wide. The weight of the sail in its bag is 20 lbs.
An understanding of how the articulated sail assembly is able to control the direction of the board can be gained by considering relation between the principal forces that act on the board/sail combination (Fig. 3). These forces are the heeling and driving forces of the wind on the sail. (FH and FD respectively) and the lateral and longitudinal resistance of hull/centerboard/skeg combination (RL and RD respectively). Sails of the proper chamber have their center of effort near the geometric area mean; the center of hull resistance can be approximated by the geometric are mean of the skeg/centerboard combination. When these forces and their moments are balanced in the horizontal plane there is zero turning moment and the board will continue forward in a straight line. Turning the board may be accomplished then by purposely upsetting this balance. The most efficient way to do this is to move FH with respect to RL since these are normally the strongest forces. This may be done by "dipping" the mast forward or aft depending on the desired change of direction. Dipping the mast forward moves FH forward with respect to RL producing a downwind turning moment and the board will fall off; dipping the mast aft moves FH aft with respect to RL producing an upwind turning moment and the board will come up into the wind.
Actually, its a good deal more complicated than merely adjusting FH with respect to RL. These and the other forces all pass through the rider in the form of twisting, pulling and pushing with arms and legs. It is not always possible to isolate FH and RL from FD and RD since he must also maintain his balance, adjust his position and keep the sail trimmed at the same time. He also needs to maintain some margin of stability to account for unexpected upsetting forces such as wind gusts and waves. With practice, though, these forces may all be brought into balance and the rider will utilize FH and RL as the primary means of control.
