Scroll down for stability part 1. This is stability part 2. The stability of kayaks is not just an attribute of the boat, but an attribute of a conglomerate of factors. Let me list them:
1) Water conditions
2) Boat shape
3) Boat loading
4) Skill of the paddler
5) The paddle
About the only factors that people typically take into consideration when trying to measure boat stability are factors 2 and 3.
Stability is considered to be a characteristic of the boat and the paddler is victim or beneficiary as the case may be of the boat's inherent stability. But realistically, the paddler has to be considered a part of the boat since he/she not only affects how the boat sits in the water but is also an integral part of its stabilizing system which may include ballast and cargo, but will also include a paddle.
The paddle of a kayak in the hands of a skilled paddler acts like an outrigger and increases the effective width of the kayak from around 2 feet to about 4 to 5 feet, making the boat considerably more stable, so stable in fact that the boat need not capsize except under the most extreme conditions, and if it does, the paddle in the hands of the skilled paddler makes the kayak self righting.
Aside from the skill of the paddler, the second biggest factor that impacts stability of the kayak is its cargo and the weight distribution of the paddler. A kayak by itself is fairly stable. However, a paddler will move the center of gravity upward and make the boat less stable. The heavier and taller the paddler, the worse the effect on stability. The easiest way to counter the weight of the paddler is to add ballast in the bilge of the kayak. According to stability curves published in Sea Kayaker Magazine, ballast has a much greater positive effect on kayak stability than does hull form. But recreational kayaks on day trips are mostly paddled empty so that stability becomes primarily a result of hull form.
The only down side of ballast is that it make the boat harder to propel. Oh well.
Finally, there is the matter of the paddler's skill I will return to this topic at another time, along with the influence of water conditions and wind.