I’ve been corresponding with an old schoolmate of mine about kayak stability and the graphs that we include with our kayak reviews. We’ve included a typical graph here, see Figure 1. You’ll find more stability graphs with the technical specification (Tech Spec) sheets that we post on our website with each review we’ve done for the past several years.
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| Figure 1 |
When you get into a kayak for the first time, especially if you are new to kayaking, stability is the first thing you notice. Some kayaks, usually the narrower ones, feel more tippy than others. Our stability graphs provide you with a good way of comparing the stability of boats.
The graphs have curves representing a 150-pound paddler and a 200-pound paddler in a particular kayak, first without cargo—the lower pair of curves—and then with 100 pounds of cargo aboard.
We calculate the stability by setting a standard height for the paddler’s center of gravity. We set that at 10 inches above the seat and 10 inches forward of the seat back. If you’re a 150-pound paddler with an average build sitting in a kayak, your center of gravity would be a little in front of your belly button. Yes, your center of gravity with your legs out in front of you is in mid air, just as it is with a boomerang.
In a nutshell, the higher the center of gravity (and the heavier the paddler), the less stable the boat is. Try putting a thick cushion on the seat and you’ll immediately notice the difference. With cargo aboard, the weight placed in the boat pulls the combined center of gravity of paddler, boat and cargo downward, making the kayak more stable.
The graphs represent the stability of the kayak if the kayaker were immobile in the kayak. That doesn’t actually happen in real life. People flex at the waist in response to the tipping of the kayak. The flexibility of the kayaker is actually what gives sea kayaks their extraordinary seaworthiness. It is also what makes rolling possible. When we measured stability in a tank of water we used a 150-pound block of concrete as a paddler. He was a lousy kayaker, but allowed us to quantify the stability characteristics of the kayak independent of any live paddler distractions.
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| Figure 2 |
On to reading the graphs. The initial stability, or how the boat feels moving from an even keel (level) to set at an angle, can be gauged by the steepness of the curve from the zero point upward. A steep rise is like leaning against a wall, a flatter curve is like leaning against a low railing. The secondary stability, or how steady a kayak feels when put well up on its edge, is gauged by the upper part of the curve on the left side of the apex. At the apex the stability “softens” and provides less resistance to leaning.
Once the kayak tilts past the apex of the stability curve it becomes unstable. This is where my schoolmate had trouble interpreting the curve. Fortunately, I once had to gather the data for these curves from a kayak in a tank full of water, so I know what that right side of the curve feels like. It’s not what it looks like. The righting moment on the far side of the stability curve is really tenuous. An increase in the angle of heel takes it to a lesser righting force, so any minute acceleration toward a greater angle of heel instantly overcomes the successive and rapidly diminishing righting forces. That’s quite unlike the other side of the curve where an increase in heel is met with greater resistance. So a righting force of say 20 foot pounds on the right of the peak is nothing like the 20 foot pounds on the left. The force creates equilibrium, but on the left it is like holding a broom right side up from the tip of its handle, the right is like balancing the broom upside down on your fingertip. In doing the measurements in the tank we could measure one or two points high up on the right side of the curve. It was impossible to get the kayak to be still enough to balance farther down on the curve. In the tank, "static"—to the degree that would make measuring the entire curve possible—simply doesn't exist. With a human paddler on open water, “static” is also more concept than reality.
One good way to read the graph is to imagine the angle of a tangent to the curve. At the origin the angle is steep. The resistance is low, but successive angles of heel are met with quickly rising stability. Toward the peak the tangent approaches horizontal and increases in heel are met with a small increase in the righting force. You feel secondary stability diminishing prior to the apex, in spite of the righting force being quite high, because from an equilibrium at, say, 50° on curve #4 in Figure 1, the increase in resistance at 51° is smaller than the increase on the steeper portion of the graph at lower angles of heel. At the peak the tangent is horizontal and there is no increase, and while you are at the maximum static righting force —the real-world instability has begun.
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| Figure 3 |
Leaning (and the paddle) is actually what gives the kayak its functional stability. Novices tend to lean the wrong way, away from the water, and the resultant reaction increases the degree of heel and decreases the stability. A skilled paddler leans toward the water, with the opposite effect—making the kayak more stable—so the paddle bracing required to regain balance is minimal. Skilled paddlers, by the way, know how to use technique to keep their balance and prefer kayaks that have lower initial stability. It makes them easier to edge and to lean. When the water gets rough a kayak with less stability can be much more at ease since it is not as likely to roll with the waves as a kayak with high stability can.
There is a second dynamic aspect of stability that is not reflected by the graphs. A hull with a sharply creased keel line or chines will resist the flow of water laterally across the hull. The flow of water isn’t present in a static state, but you can feel it. The resistance to flow can slow the rolling motion of the kayak enough to give your reflexes time to make subtle adjustments. The Eddyline Falcon (reviewed in the June ’96 issue) has a pronounced edge the length of its keel. See Figure 2. On the graph, its stability curves showing paddlers without cargo barely rise above zero. See Figure 3. In the water, the Falcon didn’t feel quite so unstable. It wasn’t twitchy at all.
When you take a look at the stability graphs in the reviews, keep in mind that they are not a measure of how a kayak feels with a paddler in it. They are a tool to allow you to make comparisons between kayaks and to target boats that have the degree of stability that works best for you.
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