Re: [Paddlewise] Bad Forward Sweep

Yes, as John Blackburn wrote, the Center of Gravity and the Center of
Flotation are on the same vertical axis (as opposed to close to it as Jed
suggested). However, contrary to his hypothesis, the hull does not pivot
around the COF unless the hull is both completely symmetrical and the COF is
also not moving.

A hull pivots around its Center of Lateral Resistance. To provide an extreme
example (that should serve to make this distinction obvious) imagine a light
round flying saucer shape floating on the water. At one point on the
saucer's edge imagine attaching a large fixed fin that radiates out from the
edge (is oriented to line up with the center of the saucer). This big fin
penetrates the water deeply and in effect becomes a fixed vertical skeg to
the smooth flat shallow saucer. To keep the saucer level we will
counterweight it opposite the skeg side (to keep the center of flotation in
the exact center of the saucer shape for this example). Now who would like
to get in this skegged floating saucer and try to make it spin it around its
COF (which is still the center of the saucer).
Note: the center of Lateral Resistance of this "skegged saucer boat" would
be directly down the center of the saucer only if it were pushed from
directly behind the skeg or 180 degrees around the saucer from the skeg.
However, if you pushed this "boat" from the "side" you would probably find
that you would have to push somewhere out on the skeg to get this object to
move directly away from your push without rotating. So in this instance the
Center of Lateral Resistance is no where near the Center of Flotation/Center
of Gravity axis even when it is not in motion.

Now for a more moving imagination example, lets consider a more traditional
hull shape that is symmetrical on each end so both the COF/COG and COLR are
in the middle of the hull when it is stationary.

When pushed sideways from the center it will move evenly sideways without
rotating. Now if we put this hull in motion along the long axis (in either
direction) the end that is pushing the water apart will experience greater
water pressure (than when not moving) and the end that is withdrawing from
the water will experience less (than when standing) water pressure on that
end. This will be true at all speeds (but the relationship will vary with
different speeds). If you apply a side force directly to the center of this
now moving hull you will find that the stern is easier to push over than the
bow and the hull will rotate about the new Center of Lateral Resistance that
has shifted towards the leading end. If you move the point of the center of
your sideways force forward to the point that pushes the hull sideways
without rotating it you have found the new Center of Lateral resistance for
that particular hull (at that speed, in that direction). As you can, I hope,
see the pressure changes around a moving hull move the Center of Lateral
Resistance (the pivot point) but not the Center of Flotation/Center of
Gravity.

Next think of the wind as the force moving a kayak sideways and you should
understand why most hulls weathercock when moving even though they might be
neutral to a side wind when stationary. As was discussed by others, shifting
the center of gravity by leaning forward or backward will shift the COF/COG
in the direction the weight is shifted. This change in trim will also add
more lateral resistance in the weight shifted direction and less lateral
resistance in the end that rises some out of the water. So the Center of
Lateral Resistance is shifted by moving the COF/COG.

Gee, maybe we could invent some device to shift the COF/COG (and therefore
the COLR) to help compensate for the conditions we find ourselves in. Any
suggestions as to how we might do this?

Matt Broze (who flunked out of college)
http://www.marinerkayaks.com


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