-- Last Updated: Mar-27-13 4:21 PM EST --
The longer the boat, the faster it can go before becoming "trapped" between two waves of its own making, one at the bow and another at the stern. Essentially this is because the longer the wavelength, the faster the waves travel (and the boat travels with them). This is called hull speed, and it's not an ultimate speed limit but for practical purposes it acts that way. Very slender boats can be more easily forced to go faster than this, but most boats won't go faster than hull speed without Herculean effort on the paddler's part. You can calculate hull speed pretty closely (in miles per hour) for most boats as the square root of the waterline length x 1.54 (the formula you'll find most places gives the speed in knots, but most people find mph easier to deal with). Note that waterline length is generally less than the total length.
If you are more interested in ease of paddling at "practical" cruising speeds, other factors come into play, and other types of advice will be helpful, but if you are willing to paddle hard and make the boat go as fast as possible, get the longest one you can.
Since you probably want ease of paddling too, look for sharper entries (how "pointed" are the bow and stern - more pointed is better, but plastic boats won't usually have a "knife edge" splitting the water), narrower width, and a rounder bottom (as seen in cross section). All of these (especially narrower width and rounder bottom) decrease stability, but no rec boat will be too unstable for comfort. Any hull that has multiple "keels" or various kinds of grooves and ridges onthe bottom will require more effort to paddle, because these shapes increase wetted surface area for a given amount of displacement. The least wetted surface area (and thus the least resistance to being pushed through the water) will be provided by a hull who's bottom is semi-circular in cross section, but only a pro can keep such a boat upright. The faster boats among your likely choices will only "trend" toward having a rounder bottom instead of a flat bottom.
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