Canoe and kayak design is incredibly complex. It is difficult to discuss the subject in a way that conveys usable information to help the boat buyer make a final decision. Most articles on design break down hull shapes into a series of simple forms and suggest that each of us will understand the fundamentals of boat design by absorbing the information. This is like suggesting that the flavor of a dessert can be appreciated by reading a list of ingredients. Although we claim to know a great deal about boat shapes we approach canoes and kayaks as paddlers. If it doesn't paddle well who cares what the designers credentials are? If the designer can't paddle well, who do they think they can design for? Canoes & kayaks should be designed for paddlers.
It is true that we can share a common language by studying these articles and they should be read at some time by a serious buyer. The problem is that a limited number of boat companies and people have contributed to writing them. It is only natural that their corporate bias or egos will show. It is easy to be convinced that there are a few limited truths that should be incorporated into all boat shapes. The more you study however, the more options you'll realize and the more simplistic or even false each tidbit of information can appear if taken by itself.
The proof is always in the paddling. Contrary to the belief of those designers who create boat shapes by mathematical formula, design remains an art form.
There are a few concepts that are true across boat design.
Overall speed and efficiency is effected by changing the length and width of the waterline. By changing the shape of the wetted surface the speed and efficiency can be fine tuned to the designers wishes. The longer and thinner the faster the boat. Where's the art in this portion of boat design? A boats finite hull shape determines at what speed it is most efficient and what weight it can hold at its most efficient hull speed. That is up to the designer to determine. It is also something that you as the paddler needs to go out and feel for yourself. A good salesmen may be able to give you a rough idea of a boats optimal speed, but you will find that some boats will match your paddling better then others and no salesmen can determine which boat will match you exactly. So go test paddle.
Canoes and kayaks, for the most part are thin enough that you can flip them over without too much effort or even by accident. When considering the stability you want in a boat you need to consider what you are going to be doing. In general the wider a boat is the more stable it will be. However, there are two kinds of stability that must be taken into consideration. Even once you know all about the two kinds of stability, a pure measure of width is not enough to tell you much of anything about a boats stability.
Lets do a little mental experiment. Envision a loaf of bread and a salad bowl. Now start tipping them both up on edge. At forty five degrees the bread is ready to fall over; but the salad bowl will still settle back flat. How hard was it to push the bread from flat to 45 degrees? harder then the salad bowl right? The salad bowl left flat very easily while the bread wanted to sit flat much more. Coming up to 45 degrees the bread was getting easier to tip and the salad bowl was getting harder. Those are the two extremes of stability. All boats fall somewhere in between.
A combination of initial stability "loaf of bread" and secondary stability "salad bowl" is needed to make any boat comfortable and safe. How much do you want your boat to rock around? None? Look for a flat bottomed boat that is fairly wide. You want to feel every wave and flow with the water as if you were part of your kayak? You should look for something for your intended purpose with a more rounded bottom. Whatever you do, get in as many boats as you can and rock them from flat to as far as you are willing to go to feel what they will do. Test paddling is the key to finding a boat that fits you well.
A boats turning ability is affected mostly by the shape the ends of the boat present to the water when looking at the boat from the side. A directionally stable boat will present and even amount from end to end and will have a fair amount of surface under the water near the ends of the boat. A directionally unstable boat will have less boat under the water at the ends and often be very round in the ends allowing the boat to slide of the water when being turned. The only way to determine what a boat will do is to paddle in a straight line, take a strong final stroke, wait and see what happens. If the boat keeps going straight and maybe only turns a couple of degrees before coming to a rest, it is a very directionally stable boat. If it spins a 180 before stopping it is not very directionally stable. There are boats out there that claim to be easy to turn and also very directionally stable. They do this by putting "runners" like a keel into the hull. This concept works quite well and is something to look into when you are buying a boat.