Borrowing from old and established design principles
Designing an efficient boat is part art, part science, and part luck. Besides possessing a vast knowledge of engineering principles, construction methods, and boat-building materials, naval architects need to understand the dynamic energy of the sea and its effects on a moving vessel.Few design concepts are so complicated or call for so many compromises, largely because a boat moves in two media at once: air and water. The hull is half in and half out of the water, with the proportions changing all the time as the boat heels or rolls.The aerodynamics and hydrodynamics of a modern sailboat are sometimes compared to a light aircraft on its side, with one wing in the water (the keel) and the other in the air (the sails). Both the keel and the sails have to provide “lift” if the boat is to make any progress against the wind. However, water is hundreds of times denser than air, so the design requirements are vastly different and often at odds with one another. Thus, constant compromises must be made, each one tending to make the design more radical and irreversible.Boats have been designed and

The lines of a 37-foot sailboat designed by Roger Marshall for cruising and racing, showing a deep fin keel with bulb ballast to lower the boat’s center of gravity, and a spade rudder aft.

built for thousands of years, however, so the basic principles have been well established—if not by scientific reasoning and mental skill, then by many trials and many errors. Even though computers have rounded some rough edges and made the mechanics of design easier, and electronics have added information and communication, there is little in modern yacht design that hasn’t been tried before in the history of sailing—including fin keels, wing keels, solid-wing sails, catamaran hulls, forward rudders, hydro-foils, bendy masts, and many other features that we tend to regard as modern innovations.In fact, it’s a general rule that, in any new boat design, nine tenths is 90 percent borrowed from existing plans of boats of about the same size, and 10 percent is adapted to suit. Of the remaining one tenth, 9 percent seems to fit into place by luck, 1 percent is genuine inspiration, and 90 percent is pure trial and error.

Roger Marshall’s 37-foot cruiser-racer sail plan features a fractional rig with the headstay terminating well below the masthead. Running backstays counter the headstay tension.

See also Sails and “Lift”; Seaworthiness.

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