Here is a good article that pretty well explains it
Symmetry: Sym, Fish, or Swede?
"Symmetry" describes the shape of the kayak from bow to stern, looking down from overhead. Imagine yourself looking straight down on the kayak, and pretend that the kayak has been cut in two equal halves: the front half (bow) and the rear half (stern). If the shape of the front half matches the shape of the rear half, the kayak is symmetrical. If the two halves differ in shape, the kayak is asymmetrical. Generally speaking, symmetrical kayaks are more maneuverable than their asymmetrical counterparts because, on a symmetrical kayak, the cockpit is very closely aligned with the true middle of the kayak (equally spaced from both ends). This means that you (and your paddle) are positioned as close to the true pivot point as possible, making it easier to turn and maneuver the kayak.
Asymmetrical kayaks are divided into two categories: Fish-form and Swede-form. These terms describe the location of the kayak's widest point in relation to its true middle. If the kayak is widest forward of true middle (closer to the bow), it is a Fish-form kayak. If the the kayak is widest to the rear of true middle (closer to the stern), it is a Swede-form kayak. Fish-form kayaks have roomier cockpits and greater directional stability, meaning that they tend to hold a straight course better. Swede-form kayaks will tend to ride drier, cruise at slightly faster speeds, and feel as though they have slightly better stability, but they will require more effort to turn. Having said that, applying a slight lean to "carve" a turn in a Swede-form kayak can compensate for its turning disadvantages. If a wave alters your course in a Fish-form kayak, you may actually find it harder to correct back to the correct course than you would if you applied some corrective lean in a Swede-form kayak. Of the two asymmetrial types, I consider Swede-form to be the more desirable performer.
Fish-Form Facts and Fictions
Interestingly, the appeal of Fish-form kayaks stems from the fact that a Fish-form shape generates less resistance in the water or in the air. This has led some designers to conclude (incorrectly) that a Fish-form kayak will paddle faster and more efficiently than other shapes. But since a kayak actually occupies the space between two elements (the air and the water), the advantages of the Fish-form shape are lost to the unique surface effects of wave action and drag. For a submarine (fully under the water) or an airplane (fully in the air), the Fish-form shape offers real hydrodynamic and aerodynamic advantages, but for a kayak traveling on the surface of the water, a symmetrical or Swede-form design offers superior advantages. This is not to say that you can't be happy with a Fish-form kayak, or that every Fish-form kayak will paddle less enjoyably than its symmetrical or Swede-form counterparts. In the end, the real deciding factor is your own personal preference and subjective experience. Try before you buy, and buy the kayak that feels best to you.
Hull Shape and Performance
Hull shape has some bearing on stability as well. A kayak with a deep V-shaped hull will generally feel very tippy in terms of its initial stability, but will actually seem to get more stable as it begins to lean. A kayak with a rounded, U-shaped hull (called "rounded chines") will generally possess a mixture of good initial and good secondary stability, but it will take some practice to get a good feel for how far the kayak can be leaned before it capsizes. A Greenland-style hull, which consists of a series of flat surfaces joined by hard, distinct angles (called "hard chines"), can offer superb initial stability and surprisingly comfortable secondary stability. As you lean to one side, the flat surfaces are laid into the water and feel as though they are "locking in" to become more stable at specific angles of lean.
Opinions vary widely, but there is probably no "single best design" for a hull shape. Every hull shape has its specific advantages. In extremely general terms, V-shaped hulls tend to slice waves more efficiently, resulting in a slightly higher crusing speed and better tracking in choppy conditions; they also tend to have superb secondary stability, even though they will feel the most tippy initially. U-shaped hulls offer smoother transition from initial to secondary stability, turn relatively easily, and are more forgiving when waves or surf break against the sides (a hard-chined boat gives a beam-breaking wave a flatter surface to strike, thereby generating a bit more "tipping" force compared to rounded chines, which allow the wave to slip under more smoothly). Hard-chined, "Greenland" hulls track slightly better and offer a clearer, more stable feel for leaning and carving tighter turns, but their hard edges generate slightly more friction, theoretically resulting in a slightly slower cruising speed than kayaks with rounded chines. In each of these cases, we're talking extremely small, if not negligible differences. The issues of initial stability versus secondary stability, and tracking versus maneuverability, are what really set these hull designs apart. As with most things, these factors always involve some sort of trade-off. Decide what's best for you and stick with it. Whatever you choose, I'm confident you will end up with a wonderful kayak. Stressing too much over the hull-shape decision will ultimately do you little good. It's an issue you will develop your own preferences about over time.
Rocker and Waterline: Turning vs. Tracking
Two other terms you should be acquainted with are "rocker" and "waterline." "Rocker" refers to the curve of the hull from front to rear. In other words, as you look at the profile (side view) of the kayak, is the hull curved from bow to stern like a banana (lots of rocker), or is it fairly flat and straight (little or no rocker)? The more rocker a kayak possesses, the easier it will be to turn, but the harder it will be to keep going in a straight line. In other words, as rocker increases, turning-ability increases, but tracking decreases. Since good tracking is so crucial to a kayak's performance, most kayaks on the market (with the exception of white-water kayaks) have very little rocker built into them. Even in a kayak with no rocker, the benefits of rocker can still be exploited by simply leaning the kayak to one side as you turn. This places the curvature of the sides into the water and creates the effect of rocker to assist in turning the kayak (called "carving" a turn). For this reason, little to no rocker is desirable in most cases. The dramatic improvement in tracking is well-worth the minimal extra effort required to turn an unrockered kayak, and the benefits of rocker will become available to you (even in an "unrockered" kayak) when you learn to "carve" turns properly. Regardless, it's rare to find a kayak with much rocker in the first place, so this is a minor issue. Just be aware of it as you shop around, and avoid any oddball kayaks that look as curvy as a banana.
"Waterline" refers to the length of the hull at the waterline (where the hull touches the water). The longer the waterline, the better a kayak will track and (theoretically, at least) the faster it will cruise. Since waterline can be radically affected by design, I need to qualify the earlier rule that longer kayaks track better. Consider this: A kayak that is 16'5" long may only have a waterline of 14 feet. Why? Because kayaks usually taper and flare dramatically at the ends. Consequently, although the kayak may indeed measure 16'5" long from tip to tip on deck, the taper of the nose and tail may reduce the hull to a noticeably shorter distance (say, 14 feet) where it actually touches the water. Why is this important? Because when comparing two different kayaks to decide which one will track better and cruise more efficiently (assuming you don't have an opportunity to actually test each one out), it is not necessarily true that the longer kayak will track better (at least not if their lengths are within a foot or so of each other). What you need to compare is the waterline of each kayak. Depending on how it is designed, a 16'5" kayak could have a waterline of approximately 14 to 16 feet, and a 15'5" kayak could have a waterline of approximately 13 to 15 feet. It is possible, then, that the longer kayak could actually have a shorter waterline. Of course, in most cases, the difference will be negligible if the lengths do not differ by at least a foot or more. As I said earlier, it takes about one full foot (possibly two feet) of added length to make a noticeable difference in a kayak's tracking ability.
This article pretty well sums up why my fastest boat THE UGLY DUCK had the dimensions it has ,16 ft long with a 15ft 11 inch water line ,very little rocker ,v bottom , asymmetrical swede
form 20 inches at the widest point on the bottom, 22 inches at the four inch water line.It is fast stable and tracks like it is tied to a track
I have yet to have a 16 ft boat that can come close to her and that is with a 65 year old motor . So it works
Ron
