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First--I got a couple of File Error! messages before I could get to this message.
Now on to the real business. A great source of information on boat design and stability is John Winter's page (see below)
You're part way there on understanding stability. Primary stability is the resistance to rolling motion when the kayak is upright or nearly so. Most people describe primary stability as the feeling of security you have within 10-15 degrees of vertical.
Secondary stability is the resistance to rolling at the point where the stability curve "goes over the top." As you heel a boat resistance to rolling increases with heel angle until you get to the point of no return, where resistance starts to decrease. Unless you take corrective action, like a high brace, the boat will continue to roll until it achieves a new stable position---upside down.
As you suspected, secondary stability is more important for keeping upright in rough conditions. Primary stability mostly makes you feel good when sitting on calm water.
Hard-chine boats tend to show a more abrupt increase in rolling restistance at maximum angles of recoverable heeling. The total amount of rolling reststance may be no more than in a rounded hull or multi-chine boat, but it is easier to feel the approach of capsizing in a hard-chine boat
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Guillemot Kayak - High
Performance Wooden Sea Kayak Designs
