hearts and minds

August 6, 2006

IV. Buoyancy and Uprightness – by clyde winter

Filed under: Paddlesports — Hearts & Minds @ 1:40 pm

The cumulative weight of boat and everything in it, including you, can be considered as a force acting at a point called the center of gravity, directed vertically downward towards the center of the earth. If things move around inside the boat, the center of gravity also moves. If everything inside stays in the same place relative to the hull, then the center of gravity stays at the same point, regardless of whether the boat heels left or right, or stays on an even keel. You can control the center of gravity by keeping weight as low as feasible in the boat, and by keeping it from shifting and lurching about. The worst kind of weight to carry in your boat is anything that is free to uncontrollably shift or slosh around. It’s bad because when the boat heels to one side, this kind of weight moves, thus shifting the center of gravity to the low side. From the standpoint of stability and seaworthiness, the best way to carry your outfit is as secured ballast that doubles as secured flotation.

To keep a boat both afloat and stable we depend on buoyancy, which is a force equal to the entire weight of the boat and outfit but directed vertically up. (If the buoyant force and the weight were not equal in strength and opposite in direction, the boat would either levitate or sink.) The center of buoyancy of a boat is at the geometric center of the volume bounded by the submerged or “wetted” portion of the hull and the projection of the surface of the water. The location of this center of buoyancy at any instant depends entirely on the hull design and on what part of the hull is below water level. It’s right in the center of a canoe or kayak floating on an even keel, several centimeters above the bottom of the hull. Our boats are designed so that if the boat leans, the center of buoyancy moves to the low side of the lean. Be aware that the center of buoyancy of a displacement hull designed strictly for speed will not move very much to the side when the boat tips to that side and you will have to be more alert and reactive to keep it upright. If the center of buoyancy did not move when the hull heels to one side, you would find that boat extremely unstable. Have you ever tried floating in a barrel cut in half lengthwise, or riding on a straight, limbed log?

When the center of buoyancy, the center of gravity, and the center of the earth are all aligned, the boat is in equilibrium. If at any moment the center of buoyancy and the center of gravity are not in line with the center of the earth, the boat is out of equilibrium. At such time the weight and the buoyancy act as a couple and tend to heel (lean) the boat one way or the other. If the buoyant force (acting straight up) lines up outside of (i.e. farther from the keel than) the gravity force (acting straight down) the couple of forces tend to right the boat. If, on the other hand, the gravity force lines up outside the buoyant force, they tend to capsize. The farther apart horizontally these two vertical forces are separated, the stronger the righting or capsizing effect.

The center of gravity of a boat with paddlers in it is always higher than the center of buoyancy, so any paddle craft when leaning at a large enough heeling angle will be subject to capsizing forces rather than righting forces, and it will be necessary for you to brace with your paddles &/or adjust your body position. The critical angle is the heeling angle of your loaded boat at which the weight/buoyant force couple changes from righting forces to capsizing forces. The higher your center of gravity is, the smaller will be this critical angle. The strength of the righting or capsizing couple is proportional to the horizontal distance between the buoyant force and the force of gravity. With a high center of gravity, once you have passed the critical angle these two forces separate quicker and farther. In simple terms, when your center of gravity is higher you start tipping over sooner and you tip over harder and faster. When you understand the principles of buoyancy, your focus will be on your ability to read the water, to anticipate and respond to hydraulic forces, and to use boat-body english and those incredibly ingenious and versatile dynamic stabilization and propulsion devices called paddles.

When a boat is aground, much or all of the weight is supported by the point of contact rather than the water. This point is a couple of inches lower than the center of buoyancy (which is bad enough), but what’s worse, it does not automatically move correctively if the canoe or kayak leans, like the center of buoyancy did when afloat. The result is that often, a boat that goes aground (on a low head dam, a rock, a log near the surface, ice) suddenly becomes less stable. The critical angle instantly decreases to zero degrees. Any angle of heel now results in the force of gravity being either directly above or outside of the supporting force. So you have no righting couple available, and you’re either precariously balanced or you’re going over. To make matters worse, the canoe is likely simultaneously experiencing a sudden change of momentum and perhaps a strong current pushing against the boat’s side. But the sudden deterioration of stability caused by the change from a buoyant force to a force applied at the point of hull contact is the least understood and usually the most important cause of unplanned swimming when the hull sticks on the hard place. Don’t blame your partner!

Go HERE to see the table of contents and other articles in this series. I appreciate any comment you care to make.

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2 Comments »

  1. Yikes!

    Comment by Howard H — March 23, 2007 @ 11:02 am | Reply

  2. Your description of the sudden deterioration of stability caused by a shift from bouyant force supporting the hull to point contact supporting the hull explains very well the astounding rapidity with which a boat flowing sideways with the current goes over upon contact with a barely submerged rock. I am very familiar with this phenomenon, but always considered it a rather simple matter of the hull “tripping over” a rock, but I see now there is a bit more to it.

    You touched upon it when you mentioned “boat-body english”, but it might bear elaborating that the center of gravity of the boat-body is not fixed, but is instantly adjustable. The best way to stay upright in a kayak or canoe is to keep your body mass centered over your hips by executing a “J lean”. For those unfamiliar, the “J” describes the curvature your lower spine makes as you execute this maneuver.

    Especially in whitewater, when unforseen forces act suddenly and sometimes unpredictably on the boat, it is important to remain supple in your hips and trunk, allowing the hull to heel underneath your upper body so as to keep your center of gravity as far inboard as possible. “Loose hips save ships”.

    Beginning paddlers often tense up when they feel the boat heel unpredictably, or worse, grab the gunwales with both hands so as to fix their body mass center of gravity to that of the canoe. As a result, paddler and boat execute a “bell buoy lean” with predictable results.

    Comment by pblanc — June 30, 2009 @ 1:10 pm | Reply


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