Re: [Paddlewise] Stability

Dave wrote:

(SNIP)


> > Once the wave breaks, the situation alters as the rotational motion of
the
> > water molecules changes to translational motion which can cause a
capsizing
> > moment proportional to the righting arm of the boat. Note the term
righting
> > arm rather than righting moment. Righting moment  is the product of the
> > righting arm and  displacement and acts to orient the boat to the
surface.
> > The righting arm is a function of the boat's shape and center of gravity
> > without consideration for displacement. Of two boats with the same
righting
> > arm, the heavier will have more stability or righting moment.
>
> Not following you here, John.  Are you thinking of a **fully broken**
wave,
> which pushes a wall of soup horizontally towards the paddler?  I don't
think
> that's what you mean.

As soon as the wave breaks the rotational motion in the top of the wave
breaks down and becomes translational. Once the motion of the boat becomes
translational gravity acts "normally" (i.e. as on a surfer) and the boat
begins to plunge towards the trough. At that point the action on the boat
becomes rotational and one leans into the wave to remain upright. The forces
vary with the type of breaking wave. For example, a plunging breaker has
higher velocities and more volume in the breaking portion than a spilling
breaker. We most often see plunging breakers breaking on shore and spilling
breakers in open water. Open water plunging breakers may create the most
danger for paddlers possibly because paddlers don't expect them.  I suspect
most paddlers have little trouble with spilling breakers on small waves up
to four feet or so. Once they start getting big even a spilling breaker can
cause problems for inexperienced paddlers.

In an ad for a safety device the picture shows the boat in a two foot
spilling breaker. Sort of like those pictures of rescues done in calm water.
Everything seems easy under the right circumstances.

>
> I am thinking you mean a wave **in the act of breaking** -- the basis for
the
> following comments:  In my limited experience, the breaking part of the
wave
> seems to hammer my up-wave side downward -- "catching" that edge of the
boat,
> as it were -- and attempting to heel the boat into the breaking wave face.
Of
> course, once this happens, my upper body leans toward the breaking face,
and my
> paddle brace goes out, forming a stable configuration as I slide shoreward
> along with the wave, in the wall of soup of a fully broken wave.

Yes. Once the wave has broken the water begins to fall down the face. at
that point you would experience a downward component. One could  or should
differentiate between "breaking" and "broken" waves. In open water as
opposed to surf the broken wave doesn't seem to cause much aggravation. In
suspect that surf soup becomes a whole 'nother problem due to the
interaction with the bottom.

>
> > This difference between righting arm and righting moment leads to
another
> > interesting and sometimes confusing aspect of stability. Due to the
> > centrifugal force, the apparent gravitational force varies with location
on
> > the wave. For example, the apparent gravitational force on the trough
> > exceeds that of the force at the crest. Ocean sailors may have some
> > familiarity with this for boats heel more on the crests than in the
troughs
> > (more than the variable wind force causes).
>
> Wait.  Isn't this difference in "apparent" gravitational force more due to
the
> "free fall" nature of the boat's motion as the crest passes underneath,
> contrasted with the wave's acceleration of the boat upwards as the hull
passes
> the trough?  In other words, this is similar to the loss of apparent
gravity I
> experienced as a kid when my dad would take our old '48 Ford over a bump,
and
> as the car fell underneath my young tush, I "felt" less upward force from
the
> car seat -- a not yet free fall situation, but less force between me and
the
> car seat.  Or, perhaps the effect I describe supplements the one you
describe?

Supplements it. Elevators, swings,  etc. produce the same effect. Froude
made a device to test this using something called a "Scotch yoke" (I had to
look this up in a book on mechanical linkages) that mimics the motion on a
wave. Damned clever those Victorians. If I recall correctly NASA mimicked
zero gravity this way but in airplanes. Flew them up high and then let the
bottom drop out. Makes my stomach turn and perhaps may help to explain why
we get sea sick on big smooth waves.

Cheers,

John Winters
Web site address http://home.ican.net/~735769






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