Primary stability

Also called initial stability. The is the tendancy of a boat that has been heeled a little bit from the level position to return to the level position. This is what makes a boat feel either stable or twitchy. Two factors that significantly affect initial stability are:

1. . Height of the center of gravity (CG),
2. . The width and shape of the waterline

The height of the CG is often called the vertical center of gravity (VCG) determines the moment arm of the heeling moment. A higher VCG will create a greater heeling force and thus lower primary stability. The waterline determines the momement arm of the righting moment. Generally a wider waterline will create a greater righting force and thus greater primary stability. However, since the actual shape of the waterplane varies from boat to boat, the overall width at the waterline is only an a rough indicator of the righting capability of the kayak. A boat with a longer waterline will be more stable because it has more area out at a wider point than a shorter boat might have. The most accurate predictor of the righting force is to look at the centroid of half the waterplane or the moment of inertia of the waterplane.

Typically, the shapes that lead to low initial stability are rounded or deep Vee cross sections. Flatter or shallow Vee sections tend to provide greater initial stability.

When looking at the stability curve the primary stability is the slope of the curve at zero degrees of heel. This is also proportional to the metacentric height.

It is possible for the slope of the curve to be negative at zero degrees of heel. This represents a hull that is inherently unstable. If you place such a hull in the water unloaded, it will tip on its side immediately. Some racing hulls exhibit this behavior. They require active balance on the part of the paddler. This characteristic is a result of making a hull with minimal wetted surface and thus less frictional resistance.

High primary stability is often valued by beginner kayakers, since it give them a feeling of comfort. It is less valued by more experienced kayakers who prefer more maneouverable kayaks. However, paddlers who prefer activities like photography or fishing will appreciate higher initial stability. The choice of primary stability level should be based on experience and intended use.

See also: Secondary stability