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

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)?
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. 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? Damn right, you had
to take over the roll of the sawhorses to provide the equal and opposite
force to the acceleration downward due to gravity and you will expend energy
to do so just not as much as in lifting it higher. A planning boat is no
longer displacing enough water to totally do that job. The energy that
powers the boat is providing the difference it takes to hold itself up (the
inclined plane--due to the density of water and the speed of the boat)
against gravity.


>>>>>>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. Certainly there was extra energy used to lift the
boat up on to the plane (just like when you first lift the boat off the
sawhorses) but you have to keep providing plenty of energy to it to keep it
up there.

Matt Broze
http://www.marinerkayaks.com


***************************************************************************
PaddleWise Paddling Mailing List - Any opinions or suggestions expressed
here are solely those of the writer(s). You must assume the entire
responsibility for reliance upon them. All postings copyright the author.
Submissions:     PaddleWise_at_PaddleWise.net
Subscriptions:   PaddleWise-request_at_PaddleWise.net
Website:         http://www.paddlewise.net/
***************************************************************************