Kayak Materials & Hull Design
Roto Molded Polyethylene is the most common material on the market. Pellets of virgin plastic are poured into a mold, then heated and rotated in a big oven until the material melts against the walls of the mold. The advantages of this material are to be found in impact resistance and price: poly boats can be bashed up pretty hard without suffering significant damage and the price will be in the $350-1500 range, depending on the style of boat. On the flip-side, poly boats are heavier than the other materials, and their flexibility also makes them slower than stiffer materials.
Thermoformed ABS is also a plastic, but stiffer and lighter than poly. Big sheets of plastic are heated in an oven and then vacuum formed into the deck and hull of the boat, and joined together with a special cement. The performance of the thermoformed boats is closer to fiberglass/aramid boats, while being close in price to roto poly boats. All of our Eddyline boats are built from this material as well as some of our Wilderness Systems boats.
Composite is the generic term for boats made from fiberglass, aramid (Kevlar®), or carbon fabric. These are the stiffest and lightest boats on the market, and also the fastest and (surprisingly) the most durable, since even major cuts or holes in the hull can be repaired for as long as the boat lives - which can be decades. They have gelcoat color exteriors that allow for you to be imaginative in terms of your color choices - come up with your colors, and it can be built to order.
It's All About the Lines
If you lay a boat on the floor and look at it from the side, you'll notice that there will be some curvature of the hull from end to end. That line is called the "rocker line" of the hull, and will have a big impact on both speed and maneuverability of the boat. More rocker will make a boat easier to turn in tight situations, but will be less likely to track straight. A boat with a flat rocker line will be very fast and track well, but will be stiffer to turn. Most boats will employ some rocker in the ends and a flat mid-section; the amount of rocker you see in the ends will tell you whether it's more of a wave/play boat or more of a straight line cruiser.
The cross section of the boat will employ a range of angles, ranging from very round to very sharp. Collectively, the bottom shape and sidewalls of the boat determine stability when the boat is heeled (at an angle) in the water (this is called the boat's "secondary stability") and the amount of "vee" in the keel line of the boat will either help it track, in the case of a defined vee, or allow it to be more easily maneuvered, in the case of a shallower vee. In general, you want to avoid short boats with perfectly flat bottoms, since they have a tendency to be both slow and hard to keep on track - the recipe for a meh afternoon.