What could be easier than stability? Just make the boat wide and it will be stable … right? Yet, there are kayaks out there from 20" to 32" wide, all of which the manufacturers say are stable. After all, what manufacturer is going to say, "you need to be born in a kayak to keep this sucker upright"? How can they all get away with this? And what is "secondary stability" anyway? I know from personal experience that this question will provoke a discussion that can go on for days.

Stability is almost always the first concern of the beginning kayaker. Stability is the first thing an experienced paddler will notice about a kayak, and improper stability performance will immediately disqualify a boat for them. Everything people want to know about the stability of a kayak design is contained in their "stability curve".

Sea Kayaker Magazine has been publishing stability curves with their kayak reviews for some time now. Novices look at the curves and are baffled, assuming that the information must be over their head. Skilled paddlers look at the assumptions involved in creating the curve and feel that they are irrelevant to someone who really knows how to paddle. A little experience can make the curves informative regardless of your paddling skills.

What is Stability

The definition of stability seems pretty clear to most people. A boat that keeps them out of the water is stable, one that dumps them in is not. Although that seems pretty clear cut, two people trying the same boat will still have different opinions about its stability. It is useful to start by agreeing on what it means to be "stable". The dictionary definition that applies to boats in water is probably: "designed so as to develop forces that restore the original condition when disturbed from a condition of equilibrium or steady motion." In a kayak we want to return to an upright position after being "disturbed" by tipping to the side. So a "stable" kayak will develop forces that restore the boat to an upright condition after being leaned or tipped.

A ruler balanced on your finger is unstable. You need to constantly correct the balance by moving your hand. As the ruler tips to the right you must move your hand to the right to catch the ruler before it falls. Because the weight of the ruler immediately moves out beyond the support of your finger, there is nothing to stop it from toppling over unless you move your hand.

What Forces are Involved

There are two major forces at work on a kayak at rest in the water. The weight of the paddler, his gear and the boat all add up to a force pushing down towards the center of the earth. This weight is supported by an equal and opposite force from the buoyancy of the water, which pushes up. It is the interaction of these two forces that are involved in stability. The relative distribution of the forces will determine whether a kayak is stable or not.

The buoyancy force of the water is distributed over the whole submerged part of the boat. The water pressure pushing on the outer surface of the boat adds together to support all the combined weight in the kayak. Instead of trying to keep track of a bunch of distributed forces engineers generally find a "centroid" or center of force. If you add together all the distributed forces and apply the result through the center of force, this one force would cause the same reaction as all the little forces acting at once. This technique lets a kayak designer combine all the weights in a kayak into a "center of gravity" (CG) or "center of mass" (CM) and all the buoyancy forces into a "center of buoyancy" (CB). Since the force of buoyancy is equal and opposite to the force of gravity, the designer does not even need to pay much attention to what the actual value of the force is. Instead, they can just remember that on flat water the force of gravity is straight down and the force of buoyancy is straight up, and just look at the relative horizontal locations of the CG and CB.

With a boat in equilibrium, the centers of force will be aligned one directly above the other. In a kayak the center of buoyancy will be directly below the center of gravity. This way the buoyancy is pushing straight up towards the weight that pushes straight down.

If some new condition comes along to disrupt the equilibrium, such as wind, a wave or the paddler reaching for an escaped water bottle, the kayak will start to tip. As you tip, your CG moves in the direction you're tipping. Unless the CB moves in response, your weight will be hanging out beyond the buoyancy forces supporting you and you will capsize. In a stable kayak design, the action of tipping the boat rearranges the buoyancy forces to move the CB in the direction of the tilt beyond the CG, thus forcing the kayak upright again. In a stable boat the center of buoyancy moves side to side faster than the center of gravity.

How Stability Works

"Weebles woble but they don't fall down." Weebles are stable. If they are sitting on their wide end, their shape makes the point of support (S) move out beyond the center of mass (W). This causes them to rotate back towards an upright position. If you try to stand them on end any shift in weight will cause them to roll away from the support until they are upright again. Because there center of mass is weighted to their fat end, they always want to return to a position that makes the mass lowest.

For a kayak to be stable it should either apply a force to push you back to the upright equilibrium condition, or if you want to lean, it should apply force such that the boat finds a new equilibrium condition before it tips you over. The kayak designer controls this by manipulating the cross sectional shape of the kayak and the height of the seat.

Remember that the goal is to keep the CG vertically in line with the CB. Unfortunately, the most stable position is always going to be with the CG hanging below the CB like a rock hanging from a string. But, since you want to breathe, the CG needs to stay directly above the CB. When you move your body to one side, the CG is going to move to that side, away from the CB. To keep you from hanging upside down, the CB now needs to move under you before you rotate all the way over. As the boat rotates in the direction you are tipping, the hull pushes down into the water on that side while the other side lifts out of the water. This action of adding volume (buoyancy) on the side you are tipping and subtracting volume on the other side will cause the center of buoyancy to move toward the side you are tipping. If the boat is shaped to be stable, the CB will move out to the side faster than the CG.

As a boat tips the buoyancy is moved. In the picture above, the blue line is the original "even-keel" waterline. As the boat tips to the right the wedge shaped green volume (b) lifts out of the water and the other wedge of purple (c) sinks into the water. The original center of buoyancy (Ba+b) is moved to the point Ba by the subtraction of volume (b) and then moved even more by the addition of volume (c). It is this motion of the buoyancy which creates stability.

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