Every designer has a group or types (families) of vessel with which he frequently works and on which he has developed data that allows assumptions to be made during the preliminary design stages, which include displacement, prismatic coefficient, hull volume, scantlings, powering, sail area, cost, etc. This information allows him to determine feasibility of possible modifications of an existing design or the need to do a custom design that will fulfill the client’s needs. Custom design work is recommended only when the owner requires a vessel larger or smaller than an existing design of a particular family; or when there are special requirements such as rig, styling, mechanical innovations and complications; or when cruising grounds are so exacting that it is impossible to satisfactorily alter an existing design. In commercial vessels, increased capacity, shallower draft, different types of cargo, or a specialized use may suggest that the only logical method for the best solution is to start from scratch.

How many families a designer has depends upon his interests and how diversified he must be to earn a living. Some have only one or two; other designers may have six, and some as many as twenty.  The first hull of any family is always the parent. The development of a new family is time-consuming and, therefore, costly to a client. When a designer is confronted with a set of parameters that will not fit any of his families, he will usually suggest and refer the client to another designer who specializes in that type of hull.

The parent of a family can have many mutations, but the hull always has a direct relation to the parent. This is one of the reasons that the “rubber band” hull with all dimensions multiplied by a factor approach must be avoided. Except for more or less stability, the designer has not made a hydrodynamic improvement on the original. Sooner or later the hulls become decadent and lose most of the virtues of the parent. It is only by continual refinement that designs will improve upon the parent. I have never known of a parent being modified or abandoned, as there must always be a known base from which to start.

In the past, there were many small yards that built a stock model (design) and wanted exclusive rights to the design. They paid for the custom design and then paid a royalty on every hull that was built, which also entitled them to my continuing services on modifications, rigs, and restyling. I would also design most of their jigs and design special tools to decrease their man-hours per hull.  

Occasionally, ego is the reason for an exclusive custom design. The owner never wants to look at another hull exactly like his and takes pride in owning “the only one ever built”.

Within a given family, there will be a size and model that appeals to many, and these designs eventually become stock designs. When major changes are required in stock designs to suit an owner, then the only feasible avenue is to do a custom design. The cost of stock plans or custom plans is just a small fraction of the total cost of the vessel.  The owner is ill advised to start off with the cheapest possible set of drawings just to "save money."