[Paddlewise] Bad Forward Sweep

 I'm going to stick to my guns here, please don't think this is flippant
or sarcastic.  I tried hard to eliminate any trace of these types of
phrasings in my reply.  FYI, I'm a Naval Architect out of Michigan and
got an A in model testing  :-)

Jed wrote:    While it's my belief that the boat actually turns around
the boat/paddler's center or gravity when at rest, descibing this as it's
center of floatation is close enough for me. With regard to turning the
boat by turning both ends, this is true *as long as the boat is not
moving*

The COG and COF are the same the same for longitudinal and transverse
axes, (not vertical though) otherwise the boat will trim/list until they
are, however the boat turns about the point where the volume of water
forward and aft of the COF are equal.  The effort (force x lever arm)
required to rotate the hull is the same, no matter which end or anywhere
along the hull it is applied.  Also the COG/COF changes during the sweep
as your body moves forward and back.

but different forces come into play when the boat is moving.

I agree that the dynamic forces added to the system by the fact that the
boat is moving complicate the model, but your statement "as long as the
boat is not moving"  is wrong.  When the boat is moving the hull still
rotates around the COF.  What is happening and I don't think you realize
it, is that the amount of effort (the push away from the bow) the paddler
is able to impart on the outward sweep is noticeably less than the amount
of effort they are able to bring to bear in pulling in at the end of the
sweep.  I'm not sure if this is a result of the lever arm becoming
shorter quicker (the COF moving towards the point where the paddle is
planted in the water) as the boat moves forward (it follows that the
lever arm gets longer faster as the COF passes the paddle for the finish
of the draw) or the body can't push out as hard when you are leaning
forward (leading arm sorta' over the shoulder) as pull in, when finishing
the sweep and the arms are below your shoulders and pulling in.  Probably
a combination of both.  Another thought; the amount of time that you
spend in the forward part of the sweep is less since the boat is moving
forward (less time = less work, since work = force x time)

When the boat is *moving forward* there exists a bow wave that has a
similar effect as pushing the bow deeper into the water, retarding any
tendency for the bow to move left or right appreciably.

The bow wave is a result of water being displaced to the side.  Forward
motion of the hull through a fluid actually creates lift!  This is why we
need bow lines when motoring down the highway.  :-)  Makes no difference
though, since the hull has to rotate around the COF.  The bow wave does
not anchor the bow.  Your forward lean does move the COF/COG forward and
increases the trim by the bow (and decreases the lever arm), then as you
sit up during the sweep the bow rises to a more even trim.  I expect that
the amount the lever arm is shortened during the initial part of the
sweep is greater than the shorting of the lever arm during the end since
we typically can lean forward farther than leaning back.  Again another
example of why the first part of the sweep doesn't work as well as the
last half.

The first part of a forward sweep is pushing against this bow wave and
having little effect. The middle part of the sweep cannot turn the boat
either because the bow wave again fights any effort to move the bow left
or right. So the middle part of the sweep tends to move you forward. But
the last part of the sweep is acting against the stern that is not
"anchored" by a bow wave and may in fact be in a wave trough. This makes
it possible to pull the stern  towards the paddle during the last phase
of the forward sweep, in effect performing a stern draw.

This is wrong, if true, your thesis would have the hull turning around
the stern at the beginning and middle of the sweep and around the bow at
the end.  Just isn't so.  Even a stern draw rotates the hull around the
COF.

A clear example of this can be seen by taking your boat up to speed, say
about 4 knots. Then try to turn the boat by doing only the first 1/2 of a
forward sweep. Count the number of 1/2 sweeps that are required to turn
the boat 180 degrees. Then perform the same test only this time use only
the last 1/2 of the sweep stroke. To keep things easy, try to do the
turns with no edging of the boat and be sure to take the last 1/2 of the
sweep all the way to the stern. Think hit the boat with the paddle. Most
people find that the boats turn with significantly fewer strokes when
they use the last 1/2 of the sweeps  ompared to the first 1/2 of the
sweep stroke.

A perfect example of being able to input less work into the system with
the initial weaker outward push than with the final inward pull.  Putting
the boat up on edge significantly reduces the amount of overall work
required, and allows the momentum of the hull to assist the turn due to
the asymmetry of the hull when on edge.  I'd sure like other Naval
Architects to weigh in here, am I that far off base?

John Blackburn

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