Generic designs take advantage of the fact that a kayak can be fairly accurately described with only three measurements: length, beam and depth amidships. These dimensions are surprisingly uniform within the confines of a given type, which means that you can create an "idealized" design with only three concrete numbers. Study a few drawings from the books listed in bibliography and you'll quickly learn the numbers that work for a given type.
For example, a King Island kayak, from the Bering Strait, seems to run from about 14 1/2 to 15 1/2 feet LOA in virtually all examples. This makes 15 feet a pretty safe guess for a generic LOA. If you know the anthropometric rules you can refine that number to your liking. Even if you don't know the rules, you could use common sense, reducing the length for a shorter, smaller paddler. These boats were meant to carry a load; I don't think I would increase the length beyond the maximum no matter what, certainly not without cutting back on overall volume to compensate for the extra length. If you had some practical experience with a specific boat and paddler you can get even more information to work with - such as this boat should be 6 inches shorter to feel right for me!
King Islanders are beamy, 25" +/-, with a usual depth amidships of about 12". Beam and depth should be treated with care. They are taken in relation to the length, but are proportional. If you add 6 inches to the length, you DO NOT want to add 6 inches to the beam. In fact, you may decide that no changes are needed here at all. Changes to beam and depth amidships need to be undertaken in small amounts for best results. An inch or so of change is quite large for these parameters.
We haven't mentioned anything about a midships section yet, and this is a major part of any design. In most cases you will probably use the original midsection, or modify it only slightly. This will keep you out of really serious trouble.
Radical changes to the section can have far reaching effects. Not much can be said here except to proceed with caution based on experience. An additional posting on the midsection is available.
Details about rocker, profile and cockpit location are about all that is needed to turn this into a full-fledged design. Such details can be gotten from drawings or actual boats. An important point is that this type of building, like other traditional methods, relies to some extent on the builder's knowledge. The system assumes that he has seen other boats of the type, and that the construction details and scantlings are carried over from those boats to this one.
Culling relevant details from other boats, and drawings is a skill that any boatbuilder will find useful. Once you realize that details can migrate from boat to boat, you have provided yourself with a useful and powerful tool. A rib size that works for one 14 footer can certainly be applied to another on. The same can be said for details of joinery in many cases, especially if the boats have a similar size and function.
Cockpit location can be hard to pin down. Say you make the boat 6 inches shorter, or longer. Obviously, you need to move the cockpit, but how much? The quick answer is not very much, an inch or two. You can eyeball a new location based on hull volume. The cockpit is usually centered in the area of maximum volume/beam. You can also express the original location as a percentage of the length from the bow or stern. Then use this percentage to find the new location. Sometimes, to play it safe, I'll use all these methods to check on one another.
Boat is 174 inches LOA,
cockpit at 58 inches from the stern... or 33% of the distance fwd.
One last point about generic designs. There will always be more than one boat that fits into any generic category - say 15 feet LOA, 24 inch beam with a depth amidships of 8 inches. There is no way all these boats can be called identical, so what is that makes them different - you know - less generic?
It's all in the details, which is what makes the idea of type really important. That's where the subtle differences in section, profile and rocker really make themselves known. Where you get your details is up to you. Other boats, books, drawings, are all legitimate sources, full of infinite possiblity.
Skin boats are not like planked boats - they have their own quirks. For planked hulls, and hardshelled kayaks, the shape of the actual midsection - ribs or hull - are what dictate sectional shape. In a skin boat the arrangement of the stringers on the ribs is the most important factor. You can have two boats with ribs bent on the same mold, and they can have different sections due to stringer arrangement. As we said above, it's all in the details.
Building a generic design is no different than any other kayak. The only difference is you may fool with it more getting it set up so you like its' looks. Making a full-size mock-up has always been the best, and easiest way to fine tune the sheer line or keel rocker, or to eyeball a cockpit location. An additional posting which covers prototyping is also available.