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Kayak Rolling - The Principles of Kayak Rolling
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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.
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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.
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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. |
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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. |
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The Stiff-Body Roll
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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. |
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| 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.
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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. |
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Here the torso is supported by the
water and so it is easier to roll the kayak upright. |
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Kayak Rolling