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Consider the paddler sitting or kneeling in his boat. The combined weight,W, acting at the center of gravity, CG, pulls the boat down into the water until the buoyancy force, BB, generated by the submerged part of the boat, increases sufficiently just to balance the weight. The buoyancy force can be assumed to act at a point at the center of the submerged part called the center of buoyancy, CB.
If the boat is tipped to one side the shape of the submerged part of the hull changes and the center of buoyancy moves as shown. The effect of the two forces, now out of line, is to create a net righting force which attempts to return the boat to the upright position, and so for small angles of tip the boat is stable.
If the angle of tip is increased further, then the movement of the center of buoyancy is insufficient to compensate for the movement of the center of gravity and the boat will capsize.
When fully capsized the boat again becomes stable with the center of buoyancy and the center of gravity in line. An additional buoyancy force also acts on the torso from the moment it becomes immersed and this is shown as BT.
The Stiff-Body Roll
So far we have considered the paddler as being rigidly fixed in his boat, with no movement at the waist or hips. The roll starts with the submerged paddler reaching for the surface with the paddle. A force, P, is generated which attempts to lift the body. As the boat begins to roll, the center of buoyancy moves away from the side nearest the surface and increases the resistance of the boat to being righted. By the time the rigidly held body is clearing the surface the buoyancy force generated by the torso, which has until then been assisting the roll, rapidly decreases to zero. The weight of the body is then too far from the roll axis and unless the paddle stroke is exceptionally strong, the combination of these two effects causes the roll to fail. They must be minimized by improvements in technique - the first by a movement known as the 'hip flick', and the second by bringing the center of gravity of the upper body much closer to the roll axis.
The Hip Flick
The principal aim of the hip flick is to roll the boat far enough upright that its buoyancy force begins to assist the righting action before the body is lifted out of the water. The body is twisted at the waist, facing up towards the surface and bending forward for Pawlata-type rolls, and towards the bottom, with the back arched, for Steyr types. It is allowed to remain there, supported by the water, while the boat is righted with a rolling action of the hips.
This is the hip flick. One knee pulls the boat towards the body while the opposite hip pushes it away. In the rolls involving a sweep-type stroke, the waist behaves like a universal joint, converting the twisting action of the upper body into a rolling action of the pelvis and boat. The net result is to roll the boat almost upright while the body remains in the water. During this action the body is pushed further under the surface and so this tendency is resisted by the buoyancy force that acts on the torso, assisted by the paddle if necessary.
The 'flick' part of the hip flick is a refinement of the basic technique, in which the boat is turned upright with a fast driving action of the lower body. Momentum is thereby gained, so that at the end of the flick the boat pulls the body out of the water and helps to restore it to the upright position.
Retracting the center of gravity - the follow through
When the boat has been rolled up far enough with the hips so that the action of the boat's buoyancy force changes from resisting the roll to helping it, the center of gravity must be pulled close to the boat to enable the body to be lifted out of the water.
As it does so, the action of the body must change from resisting the downward force caused by the rolling action of the boat (when the center of buoyancy of the torso needs to be as far away from boat as possible) to that of reducing the capsizing moment by bringing the center of gravity towards the roll axis. This can be done in a number of ways: by lying along the back or front decks, or by folding the body over the side of the boat.
If the body lies along the front or rear decks too early then the hip flick is curtailed and the benefit of keeping the buoyancy force acting on the torso is lost.
Here the torso is supported by the water and so it is easier to roll the kayak upright.