Yachts - Detailed Explanation Of The Design
( Originally Published 1911 )
THEORY, practice, experience, actual tests, lifelong study, all based on common sense, are the requisites that help to make the successful design. As the saying goes, a yacht cannot sail faster than the slowest part of the hull can be driven. In other words, a beautiful bow, showing speed lines in every curve, accompanied by a poor stern, means that the boat as a unit cannot go faster than the stern, and thus it is the combination of bow, stern, and middle body that goes to make up the yacht as a fast sailing unit. Symmetry in design is one of the greatest factors of success, the bow must fit the stern, the rig, the body, and the centers must be in correct relationship to one another at all angles of heel to make a good all-round boat.
The centers that I mention above are the center of gravity of the lead ballast on the keel, the center of gravity of the entire craft, the center of buoyancy of the hull, the center of lateral resistance or actual plane, this being in some cases simply the center of the fin or keel in flat bottom boats, the rudder some-times being taken into consideration, and being the center of the entire submersed body as a plane in "V" shaped bottom boats. The center of effort is the center of efficiency of the sail spread, as a plane.
Designers vary in their ideas in regard to the relationship of these centers to one an-other, and, of course, the relationship changes a great deal in the different types of yachts, the combination depending on the form, whether she be scow or wedge bottom.
In the scow form, the center of buoyancy quickly shifts to leeward depending upon the angle of keel, and as the yacht is water-borne practically from stem to stern on a long straight side or sailing line, she does not have the tendency to twist and turn but sails straight ahead on her form and does not take hold on her rudder. In her upright position she is a flat, square plank, short on the water-line for measurement purposes, but heeled over to her proper sailing angle, she at once becomes a long, narrow cigar-shaped craft, getting the benefit of her huge overhangs, her sailing length being practically that of her over-all length.
An excellent example of boats of this sort are the great scows that recently raced for the Quincy Cup in Massachusetts Bay, their dimensions being fifty-five feet over-all, and twenty-one feet on the water-line, but their actual sailing length was fifty feet, a tremendous increase.
The Seawanhaka International Cup boats are another good example of this sort of thing. This type, forty feet over-all, twenty-five feet water-line, eight feet beam, and five inches draft, weighing twenty-five hundred pounds, without ballast except for their steel or bronze bilge boards (which weigh about one hundred and twenty-five pounds each), are the. fastest yacht, of their sail spread in the world, the area being five hundred feet.
In the actual drafting of a design, the first step after you have decided on the class is a study of the restrictions under which the boat will be built, determining by these and by the previous perfor ances of the yachts in the class what corn 'nation of dimensions will turn out the fastest boat, taking everything into consideration. This being decided upon, a brown paper sketch is drawn of the profile, deck line, and midship section. When these suit your ideas lay same down on the final drafting paper and work in the body plan.
Then comes the sail and construction plan, and cabin last of all, if there happens to be one. From the finished lines a table of offsets is taken in feet, inches, and eighths for the use of the builder in laying the boat down, full size, on the floor of the drafting loft; and from this the builder makes his moulds for setting the boat up in the shop.
This seems simple enough, but requires an infinite amount of care and perseverance.
It was very long ago that the factors in the table never thought of in the design of the small racer, and the boat was built from a wooden model whittled out of a block of wood to suit the eye of the builder, guessed at, the centers, displacement, etc., being simply guessed at, and the builder trusted to luck that they come right. But today instruments are used to get the different areas, etc., the designer is much more sure of himself than under the old rule of thumb system the weights of the wood used in construction and the ability of the builder to follow the plans being the only real uncertainties.
In the racing knockabout of today we have a small boat that comes nearer being perfection for salt water purposes than any other type of small yacht in the world yet produced. This boat is first of all very safe and seaworthy, being practically non-capsizable, very easy to handle and with small sails. She moves through the water with very little resistance, either in light winds or heavy, is quick to turn, and certainly a pleasure to steer. Her wake is smooth, well ironed out, she opens the water nicely, going partially through and partially over it; the bow wave being practically spray with no heavy curl of solid water to be thrown aside ; and in the bow wave there is enough lifting force, as the overhang scoops out over the surface or through a sea, to carry her head well clear and free from solid water, so different from the old style of straight-stemmed boat that became very dangerous when being driven hard in a breeze. The deep forefoot and sharp entrance would grip the water, slue around and bury, often capsizing the yacht, and this danger has been absolutely done away with.
Another very important point in the present design is the balancing of the sails with the hull so that the boat shall steer, no matter on what angle of heel.
The center of effort of the working sails comes slightly forward of the center of the mainsail, and the placing of the mast and entire rig depends on where the center of the lateral plane is. It is necessary, in wide shoal boats, to place the center of effort well forward of the center of L. P. as the center of buoyancy quickly shifts to one side, and to make them steer properly on this heel the lead of the sails should be large, especially where the rudders are small, as for instance on the unballasted scow.
On narrow wedge bottom boats the lead is not so great, as the center of buoyancy stays more nearly in the same place and the boat consequently is much easier to steer as the pressure on the rudder is not so great, because when the rudder is hauled well to one side it quickly kills the headway and is a great detriment to speed, and should be kept as near the center line of the hull as possible, and still have the boat go to windward.