Beyond Static Stability
By Farwell Forrest
May 11, 2010
When folks who've never held a paddle in their hands think about canoes — if they think about them at all, that is — they conjure up an image of a dangerously tippy boat that's pointy on both ends, with a big hole on top to let the waves wash in. And as broad‑brush portraits go, this isn't too far off the mark. But it's not the whole story, is it? Experienced canoeists take their "tippy" craft out onto turbulent waters again and again without misadventure. Of course, even old hands sometimes swim when they least expect to, and on these rare occasions they often curse their boats' poor stability. I've done it, and I'm not alone, as a recent letter from reader Deane Blazie proves:
I read your article about rocker in canoe design. I have paddled the Ocean‑to‑Ocean Cayuco Race through the Panama Canal for the past 10 years. This year we paddled in a new boat and swamped nine times.
We are trying to find out what makes a canoe (Cayuco in Panama) "tippy." Can you suggest a good source of reading about canoe design? Any help would be appreciated.
This enquiry, coming out of the blue, certainly got me thinking. First off, the Ocean‑to‑Ocean Cayuco Race sounds like a hell of a contest. I'd like to know more about it. But business comes before pleasure, right? And our business here is the relationship between canoe design and the quality of "tippiness." It's something I should have given a lot more thought to, even before I got Deane's letter. Yet while I have half a dozen books on naval architecture on my shelves, I've never read a treatise on canoe design. I'm sure they exist, but if I've ever come across one, I overlooked it. A Bing or Google search would probably turn up a long list of candidates, but in the meantime, here's…
My Take on Stability
I'd better begin with a note about rocker, however, since it was my short piece on that subject that caught Deane's eye. Put simply, rocker is the measure of the lift at the ends of a boat's keel. Why "rocker"? Easy. When you put a boat with a lot of rocker on level ground and push down on the bow, the boat rocks back and forth on its keel. Boats with the most rocker were once called banana boats, and this picture shows why:
Now it's true that rocker does influence a canoe's stability — especially the pronounced rocker seen in banana boats. But other considerations usually weigh more heavily, and one of these is the boat's beam, or width. Wide boats feel stable when you first get in. Narrow boats feel tippy. Simple, yes? That said, racers don't often choose beamy boats. They like 'em lean and mean. Luckily, beam is only one part of the story. The cross‑sectional geometry of a canoe's hull matters, too. Flat‑bottomed boats feel stable. Round‑bottom boats feel tippy. And V‑bottom boats fall somewhere in‑between, with shallow‑Vs tending toward the "feels stable" end of the spectrum, while deep‑Vs favor the "feels tippy" extreme.
But — there's always a "but," isn't there? — boats that feel tippy when you get into them often firm up reassuringly as you lean them over. This is particularly true of boats with flaring topsides and shallow‑V bottoms. On the other hand, a flat‑bottom boat quickly becomes a deep‑V when you lean it. So it feels tippier when heeled over than it did at the dock. Round‑bottom boats are the worst of all. They're like logs rolling in the water. They don't care which bit is up. That's why very few boats have truly round bottoms. The closest that most get is what some makers call a "shallow arch" — sort of like a shallow‑V, but without the V, of course. Which brings us to…
Primary v Secondary Stability
These two terms often crop up in product literature and forum posts. "Primary stability" tells you how a boat feels when you get into it at the dock; "secondary stability," how it behaves when you lean it over, bringing the gunwale closer to the water. As we've already seen, these two are frequently at odds. Boats with high primary stability (e.g., flat‑bottomed canoes) often exhibit low secondary stability, and vice versa. But this is something of an oversimplification. Primary and secondary stability are just two snapshots taken from a boat's stability curve, a continuous plot of righting force against angle of heel, beginning at zero degrees (upright) and continuing all the way round to 180, or upside down in the water — a cautionary phrase that's acquired especial poignancy in recent years, though in fields far removed from canoeing.
Not surprisingly, naval architects and yacht designers look beyond primary and secondary stability. They can't afford to rely solely on a couple of glib generalizations. Millions of dollars and thousands of lives are sometimes at stake here. (Think America's Cup yachts and cruise ships.) So the pros pay close attention to the total stability curve when analyzing competing designs. They want to know exactly how a vessel will behave when she heels over in response to a hurricane‑force wind gust or suffers a knock‑down from a storm‑generated rogue wave. That's why you'll find exhaustive discussions of all matters affecting stability in any good book on yacht design. But despite the fact that the canoe is sometimes called "the poor man's yacht," nobody could mistake one for a Concordia yawl, let alone an Oasis‑class cruise ship. So we also need to look at…
The Human Factor
The cargo — the paddler(s) and his (or her, or their) gear — in a canoe or kayak almost always weighs more than the craft itself. Usually much more, in fact. And the combined center of mass frequently lies above the waterline, sometimes well above it. This is an inherently unstable condition. The upshot? The ultimate determinant of stability in paddlecraft has less to do with hull design than with the paddler's (or paddlers') skill. A strong, well‑executed brace can keep the tippiest craft upright under almost all conditions, whereas a single clumsy misstep can capsize even a 20‑foot, flat‑bottom freighter with a 40‑inch beam. (I know this for a fact. I've done it. And more than once.)
So here's my bottom line on stability: Specifications and labels (shallow‑V, high secondary stability, and the like) are important, but they're only a start. You judge a boat like you judge a car. You take it out for a spin. First, though, get a feel for it by giving it a good looking‑over while it's out of the water. When the boat's on the rack, stand at one end or the other and sight down the keel‑line. Are the sides straight or flared, or is there a lot of tumblehome? Check the bottom shape. Is it flat, or does it have a shallow‑ or deep‑V? Or is it gently rounded? Does the boat seem narrow or wide? And how about the keel? Is it straight as a die, or does it lift at the ends — and if so, how much?
Consider a for‑instance:
What do we see? Well, the canoe has straight sides, no noticeable tumblehome or flare, a shallow‑V cross‑section, and only a little bit of lift at the ends. It also has a pronounced keel and molded chines (they're the little steps near the waterline). And what can we tell about this boat just by looking at it? Well, it will probably track competently, and — unlike many plastic craft — the bottom shouldn't "oil‑can" noticeably when moving through rough water. (The keel and molded chines will stiffen the hull.) Moreover, while this canoe may feel a bit tippy before you settle into the seat, it should stiffen up nicely as you lean it, though it won't be as forgiving as a boat with flared topsides if you push it all the way to the gunwales.
Now, while we're looking at canoes that are out of the water, let's come back to the subject of rocker for a minute. Here's the flip side — a hogged keel:
It's not a desirable characteristic. I've only come across one new boat offered for sale with such a profile, in fact, and that was many years ago. The builder — a climbing‑shop owner who wanted to branch out into paddlesport — was trying for a straight keel, but he'd made a botch of his first mold, and he was hoping to sell enough boats to pay for a second attempt. So he invented a catchy phrase ("negative rocker") and launched a modest marketing campaign. It worked, too. He sold a few boats. (I wasn't among the buyers.) Nowadays, though, most hogged canoes are aging plastic boats that have simply been left out in the sun too long. Prevention is pretty simple: store your boat in the shade.
OK. You've eye‑balled the boat of your dreams. It's time to take it out for a paddle. Pick a warm day in a place where the water is also warm. A small pond or sheltered bay is ideal. Wear your life jacket. Dress for swimming. Bring a couple of friends along in another boat to pull you out if you bang your head. Then go to it. Put the boat through its paces. Lean it over until the water laps at the gunwale, keeping it from going under with a sculling brace. Next, throw your weight from side to side by sliding across the seat(s). Still feeling good? Then — if it's a tandem canoe, that is — stand up in the boat and change places with your partner. Now repeat these exercises till you're convinced you've tested the boat to its limits. Better yet, try several boats and compare them. You'll quickly suss which boats feel too tippy for your taste. This is all that really matters. And that's my bottom line.
Is a tippy canoe a problem? Yes and no. Canoes and kayaks aren't barges, and they're not yachts. Viewed from the perspective of a naval architect, they're inherently unstable. But a stability curve doesn't tell the whole story, and the paddler's perspective is unique. He (or she) isn't just "live ballast." He's not a passive passenger. He and his boat are one. And more often than not, it's his skill that determines whether he goes for an unplanned swim. Does this mean you should ignore a boat's lines and specifications? Certainly not. There's a lot to be learned by looking. But the real test of any paddlecraft comes when you take it out on the water. Only then can you see what lies beyond static stability.
For more on this and related subjects, check out the following articles. All of them are archived with other In the Same Boat columns at Paddling.net:
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