Re: [Paddlewise] Excessive Weather Cocking (or is it Fairy gliding with the wind)

Matt Broze wrote:
> 
> Peter Rathmann prathman_at_attbi.com wrote:
> 
> Matt Broze wrote:
> > [Lots of good information about hull speed and wave-making.] ...
> > Holding a heavy
> > boat up against gravity means that a lot of energy is being used to do so
> ...
> 
> >>>>>>While I commend you on a good description of hull speed and the
> transition between displacement and planing regimes, I do have a quibble
> with the above sentence.
> It takes energy to initially lift the boat against gravity when it
> starts to plane, but no further work is done against gravity once the
> boat is at a constant level while planing.  In the same way energy is
> required to lift your boat up onto some sawhorses, but the sawhorses do
> not expend any energy while holding the boat up at a constant
> height.<<<<<<<<
> 
> If this is true can we also infer that no energy is being used to keep a
> plane in the air that is flying straight and level (other than that required
> to overcome friction-or the sound wave barrier)?

Yes, that is correct.  If you calculate the power required for level
flight it is just based on the amount needed to overcome the drag forces
on the plane (power of level flight  = speed times horizontal drag
force). There is no extra component for overcoming gravity unless the
plane is rising.

> To remain at a certain height against gravity an equal and opposite force
> must counteract the downward acceleration due to gravity. The ground
> provides this equal an opposite force. So do those sawhorses solidly resting
> on the earth. The water provides an equal and opposite force for a floating
> object only once it has displaced enough water to equal the weight of the
> object.

All of that is correct, but requiring a force does not in itself
constitute a need for energy.  The work, or energy, is calculated by the
vector dot product of the force and the distance through which it acts. 
If the force and the movement are in the same direction then the dot
product is equivalent to simple multiplication, but if they are in
different directions then it gets multiplied by the cosine of the angle
between them.  In the case of a boat moving horizontally, the force to
overcome gravity is acting straight up but the movement is horizontal or
at 90 degrees to the force.  Therefore the dot product of the force and
movement is zero since cosine (90) = 0, and the energy required is also
zero.

> Crawl under the sawhorses and lift the kayak another 6 inches off of
> them and hold it up there for a few hours. The kayak didn't move once you
> lifted it. Are you providing any energy to hold it up?

I would not be providing any energy to the kayak since I'm acting on it
in exactly the same way the sawhorses or the ground did - none of which
imparted any energy to the kayak.  It's true that my muscles will get
tired, but that's because of internal energy losses in my body from
pumping blood around, etc. to keep the muscles functioning; it has
nothing to do with doing work on anything outside my body. Similarly you
can stand and push against a brick wall and get tired doing so, but you
are not imparting any energy to the wall.  All the energy you are
expending is going into internal processes in your body and no external
work is performed. Again, the work performed is determined by the vector
dot product of the force and the distance through which it moves.  You
may be pushing on the wall with great force, but if it doesn't move then
no work has been done *on it*.

> >>>>>>The energy required to move a planing boat goes into wave formation,
> surface friction, air resistance, and some losses to turbulence at the
> propellor (or paddle) to water interface.  Adding up all of these will
> give the total energy with no additional amount needed to hold the boat
> up against gravity.<<<<<<<<<<
> 
> Because the planing boat is not so deep in the water as it was before it was
> planning the waves are much smaller (water waves are also a manifestation of
> gravity but I won't go into that here). Certainly friction and air
> resistance go up with speed although the area of wetted surface goes down
> with increasing speed so the frictional drag is no longer climbing at near
> the square of the speed.

Yes, there is a point just after the boat gets up on plane where the
waves created do get smaller and the boat can be kept on plane at this
point even with a reduction in motor power.  To initially get up on
plane takes considerably more power than to maintain the planing state
after it's been achieved. Conversely, if the motor power is kept
constant after you first get up on plane then the boat will accelerate
to a higher speed until the product of the velocity and horizontal drag
forces equal the available net power from the motor.  Gravity only had
to be overcome when first lifting the boat up on plane.
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