Hi Bill,
At zero velocity, I'd agree, its all
about hydrostatic pressure, or if you like buoyancy, but it should be
understood 'static' is implied. I mention this because it's not
uncommon to read or hear the term dynamic buoyancy in reference to
the forces developed by moving fluids. In this case, hydrostatic
pressure is still very much present, but once the water relative to
the object starts to move other force are developed.
Regarding the direction of the flow, I
agree. I've said as much elsewhere, most recently in my thread “Notes
on the flow below”. As I pointed out to blakstah in my reply. Here, in this thread, I was presenting the most general of cases in order to suggest
another possible, and somewhat unnoticed benefit of modern
toed/canted fin configurations.
While we are on the topic however, the
following three diagrams I believe represent the general case regarding the direction of the flow, and the forces developed, see below.
I won't bother with detailed
explanations, as they can be found elsewhere, but if you like, I
will.
Planing is actually a name given to a
very common phenomena, in fact arguably the biggest, if not the most
important class of phenomena in fluid mechanics.
All forces developed by moving fluids
are about changing momentum and the force that is required to do so.
Consider some solid object which is impacted by a fluid flow. The
momentum of the fluid particles are changed as a result. Formally,
momentum is mass times velocity, conceptually it's just mass in
motion. To change mass in motion, a force must be applied. So the
object must apply a force to the fluid particles. But the same holds
true for the object - the momentum of the object being impacted is
also changed, and the fluid is applying the force to do so. Think of
it as a collision – the fluid colliding with a solid object, both
are going to both subject the other to a force, and of course, both
will receive a force from the other in the process. Planing, actually hydroplaning
refers to a specific instance of this momentum exchange, where the
fluid is water and the object is a watercraft of some sort.
In the most common form of planing
encountered, planing itself, is not the means of propulsion. For
example, an outboard motorboat which is planing most often has some
external source of power, like an outboard , which is used to propel
the boat. The planing here is associated with the rise of the hull
out of the water. But in surfing planing itself is a means of
propulsion.
It's not the only means, you can also
simply slide down the face. Oddly enough, during such a slide, you
will also likely be planing, but like in the motorboat your
“propulsion” is coming from an external source, which is dropping
through a gravitational field, and not necessarily from planing
itself. By the way, both dropping through a gravitational field and
using planing as a means propulsion can also occur simultaneously.
Forgive me for that explanation, but
when they coined the term, the only form of planing people had in
mind was the hydroplaning exhibited by high speed watercraft. I'm
never quite sure if people quite understand its application in
surfing.
In a nutshell, it is that upward and
forward flow of the wave face impacting the board which gives rise to
forces developed -i.e. commonly referred to as planing. And in
surfing the forces can resolve in such a manner as to provide a means of
moving the surfer/surfboard forward or laterally, depending on the orientation of the
board. Of course, the same force will also try and lift the surfer/surfboard up and over
the top of the wave, and here's where gravity comes in again. It [gravity] is used
by the surfer to literally hold himself and board in the flow, so
he can then take advantage of the forward and lateral components of
the force of planing.
So here the way the moving fluid is
developing a force is referred to as planing but the actual
mechanism, is the same during a turn of otherwise. The displace of
water requires changes in motion, or momentum, even Bernoulli's
principle can be brought back to this fundamental notion.
On turning however...
You can't increase mass by turning.
Perhaps you meant weight, or apparent weight, or just something akin
to pushing in the right direction, which ever that direction happened
to be. Turning is possible when a force makes it possible. You may
think of this force as a centripetal force (center seeking force). In
surfing you turn by applying a centrifugal force to the the water
(center fleeing force), the reaction force from the water is the
centripetal force, which makes you turn. The actual mechanism, again
being a matter of changing momentum of the fluid particles, and they
in return changing your momentum.
Pumping is actually a interesting
application of, well, planing. During a power stroke on a pump or
pumping cycle, the surfer momentarily forces his board downward at
least against the direction of the incident flow, which results in an
increase in the flow under the board, and hence the force of planing.
The same thing happens at the bottom of a turn, or even before. It's
the same dynamic phenomena, being applied, fairly creatively, in
different ways.
I don't know why you think the force
developed on fins is upward and out. They can be during particular
orientations or maneuvers during surfing, but in general unless you
are pulling off some particular maneuver, the toed/canted
configuration tends to produce a force which is directed roughly
downward and backward. It may be small, it depends on the orientation
of the configuration, but its there. You may choose to resolve
this force into lift and drag components, but the net force or
resultant force, is nevertheless the same. Perhaps you might consider explaining why you think the forces resolve the way you've suggested. Perhaps it's just a matter of language and I've simply misunderstood.
In the post, I don't argue that this is
always the case, in fact I begin by stating the case that I'm
addressing, no turns or maneuvers. The benefits of modern toed/canted
multi-fin configurations go well beyond what I've addressed in my post.
Sorry for the lengthy reply, but it
seemed important to point out that a lot of the things people seem to
be suggesting here and elsewhere, as being unique, are really just
the same thing presented in a slightly different form. Planing, in
the end, is just another term for 'force developed by a moving
fluid'. As are the the way fins operate at the most basic level.
At some point, if you want to get back
into pumping I'm game. There has been some suggestion, more explicitly
in other threads, that modern fin configurations enhance pumping via
another king of mechanism. tomatdatum termed his interpretation as "kinetic propulsion" which is tied to the asymmetrical nature of
the paired lateral fins. Sounds interesting and I'd love to read more
about it.
But the thread wasn't really about all
this,.... but it does seem to have become so.
kc
ps
I'm inclined to open any post on fins with a reference to a video like Bill "Beaker" Byran surfing on his "One" (of Tom Morey's design) - a finless piece of less than 0.5 meter sq. of bottom surface - surfing's own 'quark' if you like, see http://www.youtube.com/watch?v=Bg-0ibLZrgg&feature=player_embedded ... it's likely to help to give any discussion of fins perspective.. let alone the forces developed between the flow of water on the face of a wave and a surfer/surfboard.... pumping too.
