Because kayaks are long and thin they often look "tippy" which of course is a real concern especially to those who have only paddled in small boats or canoes. If you speak to boat designers when they discuss stability there are at least two terms you will hear: center of mass (CM) and center of buoyancy (CB) among others. Very simply put stability depends on the interaction of the center of mass (CM)of the kayak and the kayaker and the center of buoyancy (CB) of the kayak. As long as they stay lined up vertically, the kayak will remain stable.
So what happens when the CM of the kayak and the kayaker, and the CB of the kayak are not lined up? If the CM is closer to the center line of the kayak than the CB the kayak will tend to rotate upright and stay stable. If the CM is farther from the center line of the kayak than the CB the kayak will tip and feel unstable. What helps to keep CM and CB lined up? In simple terms the height of the CM in the kayak and the shape of the kayak's hull.
CM (center of mass) refers to your weight and the weight of the kayak. With a cedar strip kayak it will be mostly your weight. A CM that is closer to the bottom and the center line of the kayak the more stable the kayak will be. You certainly notice this when you stand up in a small boat or a canoe. It becomes very unstable often with predicable results. Try this with a kayak and you will have an opportunity to test your life jacket. Thus the lower your seat the more stability you will have. You will not see kayaks with seats "up in the air" like you see in a canoe though commercial seat heights will vary. The middle of the seats I carve is one inch or 1 1/2 centimetres deep, thus your CM will be very low. With a narrower kayak you often have an even lower CM because it will sink lower in the water thus your mass will be lowered as well. Because of this a narrower kayak will not be as unstable as you would think.
If two hulls have the same cross sectional shape the wider one will tend to be more stable. This measure of a kayak's resistance to being tipped from the upright position is referred to as initial stability. Thus recreational kayaks (often the ones you rent) tend to be wider than touring or sea kayaks and thus more stable in the water. As a kayak starts to lean the shape of the bottom and the sides where they meet the sheer and deck will begin to have an affect on stability because this changed shape of the kayak in the water will change the amount of water displaced and will move the center of buoyancy. This measure of a kayak's resistance to steadily tipping throughout a roll is often referred to as secondary stability.
Initial and Secondary Stability Illustration
For an illustration of initial and secondary stability, consider the differences between a regular chair and a rocking chair. The regular chair feels very stable while you sit in it. Due to high initial stability, it has a very high resistance to tipping a small amount in any direction. However, once the chair is tipped onto two legs, it has very little resistance to further tipping. Due to low secondary stability, it can very easily be pushed the rest of the way over. Now imagine this chair not on a flat floor but on a steep wave and it stays perpendicular to the water on the wave face. You can see how you would easily capsize.
The rocking chair, on the other hand, feels somewhat unstable when sitting at rest in it. A person who has never sat in a rocking chair would find it "tippy" and might tend to think that it will roll over backwards. This is an example of low initial stability. However, as the person begins to tilt the chair back, they will discover there is more resistance to the tilting the farther back they go. In fact, the rocking chair is very difficult to tip over backwards. This is due to high secondary stability. Now imagine this chair not on a flat floor but on a wave face. As the wave rolls under, it will not stay perpendicular to the water on the wave face but will roll towards the wave moving the CM towards the center line reducing a potential capsize.
In summary the effects of initial stability are felt at little or no tilt in the kayak while the effects of secondary stability are felt at greater angles of tilt.
What Stability Is Best
High initial stability has obvious advantages for beginner kayakers who intend to paddle in calm waters. Unfortunately, if the waters are not calm, that high initial stability will cause the kayak to become more sensitive to waves and choppy water causing the kayak to become very difficult to handle. Consider the factors that make a kayak stable and what those factors do to your speed and efficiency. Kayaks do not have a weighted keel like a sailboat so they get practically all their stability from their form (beam and shape of the hull) and a low center of mass of the kayaker. Thus wider kayaks are generally more stable than narrow ones, but the wider a kayak is, the more work it takes to paddle it at any speed. Another reason why you don't really want an excessively stable kayak is that the kayak's width can cause the waves to capsize it. This may not make sense but the width doesn't necessarily act to keep a kayak right side up, rather it acts to keep a kayak perpendicular to the surface of the water. If a steep wave hits a kayak side on and you stay perpendicular to the water on the wave face you can easily capsize. As a kayaker, you need to lean into waves that hit you from the side, so you can use your body weight to overcome your kayak's stability in order to prevent a capsize. Thus in rough water, the more stable your kayak the harder you have to lean to cancel out your kayak's stability. A kayak with less initial stability will be much easier to lean into the wave. Another aspect of kayaking to consider in relation to initial stability is turning. When turning a kayak, advanced paddlers will tilt the kayak to the side to make turning easier. If the kayak has too much initial stability this will be more difficult to do and will result in a kayak that is difficult to maneuvre.
One thing to keep in might about all the above "theory" is that normally no one sits in a kayak up right with no movement of their arms, or their body or the paddle. Paddlers are always adjusting all of these things to keep their center of mass over the center of buoyancy of the kayak. The more you paddle the more this will come to you automatically as it has in life. When you walk across a steep grade you do not walk perpendicular to the ground (because you would fall down) but you stay vertical with your center of mass straight above you feet. You do this without thinking and it is the same in the kayak the more you paddle.
So you have seen that stability has a lot to do with the center of mass, the center of buoyancy, initial stability and secondary stability of the kayak. In addition it also has just as much to do with you and you comfort level and experience. Can you test a kayaks stability to see if it is right for you? Here are a couple of suggestions.
Simple Tests of Stability
As a quick test of stability, put a kayak in the water, and if you can easily balance while sitting on the rear deck with your feet on the seat (with no assistance from your paddle) and it does not make you jittery it is probably too stable.
To test whether a kayak is stable enough to allow you to relax, eat a sandwich, study a map, take photos, look through binoculars etc. sit in the kayak in calm water with both hands off the paddle. If you feel jittery then it may not be stable enough for you but you do not want it more stable than required for these tasks. The kayak may be fine though where you'll have your hands on your paddle as the paddle will increase you balance and help maintain stability. (Think of a tight rope walker and his balance pole.) If a kayak doesn't feel stable even in calm water, then it is probably too tippy for your level of experience (though balance will improve the more you kayak.) In rough water you need to use your paddle for stability at all times as no kayak is stable enough without the paddle in these conditions. So try to find a kayak that is just stable enough to make you feel relaxed in calm water - no more, no less. In rough water use you're paddling skill for balance or head for shore and get off the water.