So what can we deduce from these physical facts? First: a kayak hull has only has a fixed amount of stabilization reserves. If they are spent early providing primary stability, we can expect the kayak hull to capitulate earlier. Also, as a long wrench with more leverage can apply more torque than a shorter wrench, a wider kayak will have more leverage to apply more counteracting torque against a leaning torque. But widening the beam will dramatically sacrifice speed and increase water drag when the kayak moves. A tradeoff that must be considered wisely.
So what are these features that work for us? What does a featureless hull look like? Lets examine a featureless hull which is simply a floating cylinder. Since it is round and featureless, its center of buoyancy will always be in the center and cannot move to either side. Its perfectly round shape does not allow any more volume to be added to one side or taken from another. It is the same on both sides all the time. Consequently, any offset in the center of gravity will generate torque on the cylinder, opposed only by the small forces of the cylinder's inertia, and friction of the water. Picture yourself standing on a perfectly round floating tree trunk.
A kayak facing rough seas will need to minimize the instability side effect from its primary stability, and reserve its stability budget for secondary stability. Unlike primary stability, secondary stability will not respond instantaneously but apply stability further into the lean. Secondary stability also exhibits less of the destabilizing behavior in waves since the hull will not react until much further into the lean. Unlike primary stability, secondary stability assets are in the dry volume of the hull above the waterline. In the first illustration above, notice how the "V" concentrates most of the flotation in the center, while the flotation at the extremities is pushed out of the water into the dry area of the hull. This is the secondary stability area in reserve. Since the dominate flotation force is in the center, the kayak will pivot about it and feel initially unstable until the secondary stability is deployed. In the second illustration, when the kayak rotates about its axis, dry volume is deployed into the water bolstering flotation at the edge of the kayak, which in turn moves the center of buoyancy to counteract the leaning force. Since secondary stability assets are stored above the waterline, these kayaks enjoy an added advantage of a more streamlined hull with much less wetted hull surface resulting in far less drag from water friction when the secondary stability is not deployed. Secondary stability kayaks cater to more advanced paddlers seeking performance. In many models, manufacturers will further narrow the beam (width) considerably stripping much of its righting force leverage. And by this action, delegate much of task of stability to the bracing skills of the paddler in exchange for a considerable increase in performance. Manufactures may also choose a more rounded hull without a "V". But the stability principles are the same with more rounded surfaces offering less primary and more secondary stability, with flatter rounded bottoms offering a higher degree of primary stability. Novice paddlers will find secondary stability kayaks deceptively unstable and unsettling. With a much more narrow beam, these kayaks will have a much smaller stability budget, but will store most of this tighter stability in reserve for a time when it is really needed.
To illustrate primary stability and secondary stability I presented two mutually exclusive theoretical kayaks. But in reality, no kayak will have all of one and none of the other. All hundred or so kayak models will fall somewhere in between catering to many skill levels and a wide range of venues and conditions. When a paddler chooses where they want to spend their stability budget, they should deliberate long and hard to find the kayak that best suits their needs in the near term and longer term. Also consider where you are going to paddle and where you want to paddle. They must also assess their skills and allow room for improvement. A kayak designed for calm conditions can also perform well in challenging conditions if used with proper skills. When I purchase a kayak, I am initially a little unstable and grow into its characteristics as my skills improve. Paddlers for whom the kayak is a vehicle for another purpose or activity may want a lot of primary stability so they can focus on their secondary activity. Kayakers wanting performance with the intention of piling up a lot of distance will want a performance kayak with a low drag. Paddling a considerable distance with a higher drag hull can feel like towing a second boat. A day on the water with a prospective kayak is better than a short test paddle. When shopping for a kayak, try a lot boats. You may just fall in love or learn a little more about who your are on the water.
In the next article of this series, we will apply some of our new found knowledge to examine the stability characteristics of a number of actual hull shapes.
Copyright 2012 Lyman A Copps