Between Two Worlds
By Farwell Forrest
Whether you're a paddler in a pack canoe or the master of a supertanker, you've got many of the same worries. You want your vessel to stay afloat, for one thing. You want it to stay right side up, for another. And you want to reach your destination on schedule. Buoyancy, stability, and speed, in other wordsthese matter to every skipper, whatever the size of his (or her) command.
Of course, there's nothing saying you need to spend hours with textbooks on physics and naval architecture to be a good paddler. Just watch a family of otters at work or play. They're mammals, of course, and therefore air-breathers, but they're also superb swimmers, entirely at home in the water. Yet none has ever opened a textbook or even read a primer on how to fish. The foundations of their skills are hard-wired into their genes. Their subsequent perfection is the result of long hours of study in a harsh school, where the penalty for failure is often hunger and sometimes death.
We humans are both more fortunate than the otter and less. The otter is a specialist from birth. We, on the other hand, are generalists, bred to no one particular tradeas, for example, the otter is born to the life of a fisherbut nonetheless capable of turning our hands to most things with some hope of success. In consequence, our physical adaptation to particular tasks is often less than ideal, and we rely on tools to do what other creatures do as a matter of course.
This is certainly true when we set out on the water. We can swim, but even the best swimmers among us can never approach the skill and stamina of an otter pup. However much we might wish otherwise, we're always at a disadvantage. We're creatures of air and not of water.
So we turn to our tools to help us out. Modern canoes and kayaks are tools, after allthe end-products of a development process that probably began more than ten thousand years ago. From the beginning, boat-builders and paddlers have wrestled with the constraints that still govern their modern descendents: buoyancy, stability, and speed. So let's take a brief look at some of what we've learned in the course of this ten-thousand-year-long experiment.
Buoyancy first. After all, if your boat doesn't float, you're not going anywhere, are you? So just why do boats float, anyway?
As is so often the case, this problem was solved by artisans long before it was addressed by science. Take the example of the Hjortspring boat, a magnificent 20-man canoe made of lime-wood and built in what is now Denmark around 350 BC. Its builders designed their craft by eye and rule of thumb, confident that their boat would float when the last plank was stitched in place. They worked as hundreds of generations of boat-builders before them had done, without benefit of calculations and without any theoretical understanding of what held a boat up on the surface of the water. Not until a hundred years later did a Greek mathematician named Archimedes finally understand the physics of buoyancy. When he did, he leapt naked from his bath and ran home through the streets of Sicilian Syracuse, shouting Eureka!, "I have found it!"
And so he had. Watching the water run over as he settled down in the bath, Archimedes grasped the principle that has been known by his name ever since. The force that holds a floating body up in the water is equal to the weight of the water it displaces. When an objecta boat, say, or a cast-iron anchoris placed in water, it immediately begins to sink, displacing water as it does. If the weight of the water it displaces before it goes completely under equals its own weight, as is the case with the boat, then it floats. If not, it sinks to the bottom like the anchor.
Happily, as I noted last week, water is heavy. Not only are many materials lighter than water, volume for volume, and therefore certain to float in their natural state, but a watertight shell made of even the heaviest substances (steel, for example, or concrete) can still be made to ride high above the surface. The trick is to make the shell so large that it displaces enough water to offset both its own weight and the weight of any projected cargo. It's a boat's volume, therefore, and not the stuff it's made of, that largely determines its buoyancy.
Now, to be sure, paddlers seldom have to worry about whether or not their boats will float, but there's still a practical application of Archimedes' principle to paddlesport. In selecting a canoe or kayak, it's important to choose one whose size matches the job it will be asked to do. Select too large a boat, and it will float too high. Such a boat will be at the mercy of every gust of wind, and hard to control in even moderately breezy conditions. If you've ever paddled a big tandem canoe like an Old Town Tripper solo on a windy day, you know what I'm talking about.
Get too small a boat, on the other hand, and you risk overloading it. Even if you don't send it to the bottomand this is most unlikely, unless you're planning to carry a load of rock samples or scrap ironyou'll find that an overburdened boat is sluggish and unresponsive. Worse yet, if your boat is an open canoe instead of a kayak, it will start taking on water immediately, even in mild rapids or the slightest chop. And every pint of water you ship will add another pound to your load, settling you that much deeper. Since all open boats leak at the top, canoeists have only their freeboard (the distance their boat rises out of the water) to keep them afloat. The less freeboard, the less margin for error. If you plan to freight heavy loads of gear down difficult rapids or across large lakes, therefore, and take the weather as it comes, you'll find you need a big, deep boat. Writer Robert Ruark was in the habit of cautioning novice big-game hunters to "use enough gun." If he'd been a canoeist, he'd have written "use enough boat," instead. Both are good advice.
Once your boat is afloat, you'll need to keep it right side up. Here's where stability comes in. As it happens, there are three different types of stability, one for each axis around which a boat can pivot. Most paddlers are concerned only with lateral (or roll) stability, however. We want to know if a boat is likely to tip over and dump us into the water. Boats which capsize easily are said to be "tender" or "tippy." Those which resist upsetting are "stiff" or simply "stable." By and large, most paddlersthough not allthink that stiff boats are good and tender boats bad.
Perhaps for this reason, stability gets a lot of space in the paddling press. While you can read dozens of catalogs without finding any discussion of buoyancy beyond an indication of a boat's volume or some more or less arbitrary load capacity figures, you'll find stability mentioned on almost every page. You'll even find that much is made of a distinction between "primary" and "secondary" stability. Boats with high primary stability are those which feel solid when you step aboard (or sit down in the cockpit). Those with high secondary stability, on the other hand, may feel a bit tender at first, but will "firm up" when leaned a little to one side.
Well, maybe. It makes good catalog copy, at any rate. But it's not a distinction that figures in the discussions of many boat designers or naval architects. The pros look at the whole of a boat's stability
curvethe plot of righting force against angle of heel, from 0 degrees (sitting upright) to 180 degrees (upside down)before deciding whether to accept or modify a design. In practice, I've never noticed much difference between boats that I attributed solely to differing degrees of primary or secondary stability. The ultimate determinant of stability in paddlecraft is always the paddler's skill. Even a novice can soon learn how to lean a canoe over until water laps along the length of the almost-submerged gunwale, and then hold it there with a sculling brace. And a kayak, of course, can be kept up on edge almost indefinitely. On the other hand, every boat I've ever paddled could be capsized by some combination of clumsiness and determinationeven 20-foot-long, 4-foot-wide freight canoes ballasted with half a ton of gear and supplies!
Still, stability is important, particularly for less athletic paddlers and those who plan to use their boats as working platforms for fishing or photography. Such folks are probably best advised to choose relatively beamy, reasonably flat-bottomed boatsboats described in the catalogs as having high primary
Lastly, there's the matter of speed. And speed is important, even if, like me, you usually take it easy and never intend to race. A fast canoe can help a paddler get off the water before a storm hits, or reach a planned campsite before dark falls, or just make the day's quota of miles while still leaving time for fishing or photography. Unfortunately, though, under most conditions no kayak or canoe can be paddled faster than a speed set by its waterline length. That's length, not beamwhile skinny boats are (usually) easier to paddle than fat boats, a boat's maximum speed is determined by its length alone. I discussed this last week, but I didn't give a full explanation. Here's the missing link:
As you paddle your boat faster, the bow wave* builds up and the trough behind it deepens and moves back. Once the trough moves aft of midships (the mid-point of the hull), the stern settles back into the hollow of the growing trough and your boat starts to "squat down". This drag-tail squat is what determines your boat's ultimate speed. In effect, trying to paddle your boat any faster is like trying to paddle up and out of a hole in the water. It's hard worktoo hard, no matter how strong a paddler you are.
Why does boat lengthand length alonedetermine your maximum speed? Simple. The longer your boat's waterline, the greater the distance from bow to midships, and the faster you can go before the bow-wave trough moves far enough aft to impede forward motion. This is the relationship I summarized last week. Think of a boat's length as the setting on an engine governor and you won't go far wrong. Knowing only a boat's waterline won't tell you much about how easy it is to paddle, but it will give you a pretty good idea what its top speed will be. And that's something you may someday want to know.
Supertanker or pack canoe, all vessels navigate the same shifting, fluid plane marking the boundary between two very different worldsthe world of water and the world of air. Our boats inhabit one world, we another. Buoyancy, stability, and speed define the terms on which our two worlds meet.
* Strictly speaking, a wave is made up of two partsa mound of water (the "crest" of the wave) and a hollow (the "trough"). In day-to-day speech, however, we usually say "wave" when we mean "crest," and that's what I'm doing here.
But that's a story for another day.
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