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SHAPING THE SHIP

Copyright 2009 Michael Kasten

The Process

Boat designers, builders, aficionados and dockside admirals are always looking for the best way to think about boats... Among the following thoughts are several yacht designers' gems to be remembered...

"The sense of proportion and form, best described as 'boat-sense' is obtained by observation and comparison of lines and details of successful craft, and is of far more importance than either the ability to make difficult calculations, or to draw well." "The goal in Yacht Design is to turn out a boat that is suitable in every possible respect for the intended purpose."
- Chapelle, Yacht Designing & Planning.

"The core of training in Boat Design involves the capacity to visualize a form in three dimensions - and to translate that object to a graphic medium."
- Gilmer, Introduction to Naval Architecture.

"A significant element of boat design is artful compromise, since the number of conflicting factors in a vessel is awesome. The indeterminate nature of naval architecture requires that the design process be done in a number of iterations. This can be thought of as taking the ordinary engineer's linear task-line and bending it into a spiral. The resulting 'design spiral' provides a clear concept of the task path for creating a vessel design. Starting with the original concept, a designer works his way around the design spiral. Each time a task line is crossed, the concept has become further refined since the last round, bringing it into closer harmony with the other requirements. When finally a round is made with no further changes, we can assume the design is finished.
- Hamlin, Preliminary Design of Boats and Ships
 

General Observations

In the search for speed under sail, several observations come to light... Among them, the following:

"The use of a straight quarter beam buttock at the intersection with the after load line appears quite often... The value of the straight quarter beam buttock is recognized by designers now more than formerly... The angle such a buttock takes to the after load line is an important and related factor. The [speed producing] components to be sought in the run are straight flow, at a small angle to the load line. Generally, high speed-length ratios require softness in the sweeps [of the buttocks] in the entrance at the load line of sailing craft. The conditions which produce speed are found when the quarter beam buttock-bow line camber is relatively shallow, producing small angles at the forward and after intersections with the load line."

Fineness of entrance is considered a benefit in terms of speed, within limits, especially regarding windward performance, provided that the entrance is not so fine as to produce a "shoulder" as the waterlines move toward amidships. Rather than diminish it, such a shoulder serves only to move the bow-wave farther aft.
- Chapelle, The Search for Speed Under Sail

"A particularly significant line is the bilge diagonal. More than any other single line in a yacht's drawings, this diagonal is revealing of hull shape, and there is a system of drafting hull lines in which the bilge diagonal is one of the first lines to be drawn. The bilge diagonal of a balanced yacht will be found to have its greatest breadth near 55% of its length from forward, and the line will curve symmetrically about this point for more than 50% of its length."

The amount of wetted surface, and thereby surface frictional resistance, has a large effect on performance at slow speeds, before wave making resistance begins to become dominant. The shape containing the most volume within the least surface area is the sphere. Thus, rounded sectional shapes tend to produce less wetted surface than angular shapes with the same displacement. The trend toward fin-appendages, rather than true keels, is driven in large part by reduction in drag.

Skin friction is largely a matter of skin roughness. Fouling, even a small amount, produces far more skin friction or drag than the mere area of wetted surface. The importance of the amount of roughness to skin friction and therefore the light air sailing performance should always be kept in mind.

Directional stability when heeled is enhanced by balanced lines, fore and aft. Directional stability in general is also enhanced by the length of the keel, and there being some amount of "drag" or aftward slope to the bottom of the keel. A fine bow is a detriment to directional stability.
- Philips Birt, Sailing Yacht Design