Part of my responsibilities as a designer is to sort out all the information that a client provides and then narrow all of it down so that he or she can make an intelligent decision regarding one or more design families and finally make a choice. In my mind at all times is selecting a design family that will accomplish all the basic requirements, retaining the elements of seakeeping and safety for the crew. Perhaps the most difficult item to isolate is the real purpose for which the vessel will be used. If the vessel is to be used extensively for ocean voyaging, the requirements are such that it would be a poor choice for day sailing, some coastwise cruising, and at most a fortnight’s cruise. By the same token, it is possible to design a coastwise cruiser that, with slight modifications, later in life would convert her into a passagemaker. In practice, this is not a clear-cut case of this or that, but numerous other requirements which often begin to contradict each other. At times when all the information is complied, I find that three or four vessels are needed, and it is then up to the client to decide which one is to be built first or adjust his or her requirements to fit all into one vessel.
A tentative sketch of an interior arrangement and deck layout determines the overall length of the vessel. Upon selecting the overall length, it is then possible to determine the waterline length, beam and draft, which are related for any family. This, in turn, determines the displacement. This method works fine for sailing vessels and there is enough information gleaned to work out a tentative sail plan. Motor vessel design is somewhat different in that the first question must be: which engine? Knowing the horsepower of the engine, it is then possible to determine the maximum displacement that will utilize the selected engine. Length on the WL determines the relative speed of a displacement vessel and, with the same hp and displacement, the longer it is, the faster she will go. The comparison however, is based on the Ö WL x the speed length ratio. For example, the Ö 25’ WL = 5, which is a speed length ratio of 1.0 or 5 knots, the same ratio for 36’ is 6.0, and for 49’ is 7.0. A vessel conforming to these dimensions would be fat, normal, and skinny. Displacement hulls have an upper limit of a S/L of 1.34, which is the length of a trochoidal ocean wave. Thus the maximum speed in knots for the lengths given are 6.7, 8.04, and 9.38. To gain this maximum requires 3.8 times the hp that is required at S/L =1. The S/L opted for in the majority of ocean voyaging motor vessels, which designers refer to as their service speed, is near S/L = 1.2. The speed in knots is then 6, 7.2, and 8.4. The hp required is then 2.16 times that at S/L =1. All displacement vessels should be able to operate for extended periods of time at a S/L =1 without carboning up the engine.
The good method of planning is to think in tons of displacement. We have a known finite mass that can be molded to infinite number of shapes. The manipulation of the variables, which are length, breadth, and depth, will indicate that the alteration of one automatically alters one or both of the others. All of the shapes derived may be seaworthy and sea kindly or unbuildable and/or treacherous.
The shape of the hull should be the major contributor to stability, and not the ballast. I have designed many commercial sailing vessels that required no ballast when light. Some yachts need some ballast; however, if the hull is doing most of the work it is just a belt and suspenders form of safety. Passagemaking motor vessels often add the weight of one-half the fuel weight as ballast, especially if they have high superstructures, which can cause an excessive wind heel in gales.
Many long-time sailors moving into power feel that the ideal vessel should be less than 12 meters (39’5”) over the rails with a displacement of 10 or 11 tons. Accommodations are normally for two with the possibility of using a settee for extra guests on rare occasions. None seem to mind a 6 to 7 knot average as it is better than they did under sail (none were much on motorsailing since I only gave them a maximum of 40 hours of fuel). The upper limit seems to be around 16 meters, or 52’. The standard equipment for them is an autopilot, compass, small watermaker, and VHF radio. Some say they might like to try GPS.