I'm not going to quote you, as the following isn't a dissection of what you've posted, but my take on what you've suggested or described in your post. My interpretation my be
way off... if it is please let me know... but I have to admit, this is a fun one if it is a misinterpretation.
... shedding flow
You have suggested that under certain
conditions the flow will tend to move off towards the rails once is
has impacted that the bottom, or perhaps interacted with flow that has
impacted the bottom. That from the
interaction of this shedding flow with the toed/canted fin, there can be a
component of the lift generated having a direction similar to that
of the motion of the surfer/surfboard as a whole. More precisely,
because the incident flow experienced by the surfboard's bottom
surface is slightly different than that experience by the fins due to
this 'shedding', the lift produced from the fins interacting with
this shedding flow will have a component in the direction of the
motion of the surfboard.
In fact, following this argument, the
presence of cant would suggest that the lift produced under these
conditions would have a component directed upwards, at least in the
direction towards the deck of the board.
You've also seem to have suggested that
this shedding, at least the direction of some of it, tends to depend
on that upward component of the flow in the face of the wave, and the
displacement water by the board/surfer. That is, the water has to go
somewhere and not all of it is going to proceed in a straight line
off to the other edge of the board once it has impacted the board. At
least the more upward flow that the bottom surface experiences the
more shedding there would likely be.
If fact, if I've interpreted you post
correctly, the deeper a board seems to ride in the water the more
likely this degree of shedding from any upward flow, will occur and hence, the more
effective toe/cant can be, if you can get the toe/cant right.
That's definitely an argument, albeit
one addressed solely to the operation of fins without consideration of their
respective drag, but more so, to the system of surfer/surfboard as a
whole. Nevertheless, perhaps when a surfer finds himself in a
position that encourages this kind of shedding, this kind of mechanism may
be possible, make that likely.
...that is, lets say it is.
What is interesting, is that what
you've suggested can operate as a sort of negative feed-back system.
As conditions slow and displacement increases, this kind of shedding
will increase and provide both a component of force in the direction
upward towards the deck and one in the direction of motion. The one
in the direction of motion tending to counter balance the increase in
drag of the surfboard as a whole due to the increase in displacement
(which produces increased shedding), and the other assisting to raise
the board out of the water (reducing displacement and hence
shedding.)
As the board is raised out the water,
the shedding is likely reduced and the effects on whole reduced. But
once the flow moving under the board is not shedding to any
significant degree, the toed/canted configuration reverses its net
effect and produces a net down and back force. What you then have is a dynamic feed-back system. During conditions where the board is experiencing non-shedding
flows, say when its speed is sufficient to minimize displacement, the fins produce a net force of down and back which
ultimately encourages more displacement and the kind of shedding you've suggested, but as displacement and shedding increases the toe/canted configuration operate to minimize
that down and back force, if not reversing it. Ultimately a dynamic balance or more appropriately tension is produced, allowing for a tighter connection with or sensitivity to changes in the flow, at least ideally, given my take.
.. so what's up with toed/canted multi-fin configurations?
I'm guessing this may not be exactly
what you had in mind. But it is kind of a nice interpretation. I have
listened to people speak of the “thrust” they have experienced from these kinds of
configurations since the late sixties, at least not much later(?) (Or whenever tri-fins started
showing up on Long Island in large enough numbers to catch my eye. I'm not very sure about the dates here, I'm not much of an historian. I am sure that the first shortboad I actually owned, was a single fin.) That there was something
special about them was never in any doubt, but I've never felt that 'thrust' properly captured my experience with them. But I have to admit, given the model described
above, I can see how that could be interpreted as a sensation of
'thrust', compared to the way one surfed a single
fin configuration, where such a feed back mechanism is not operating. Of course this may not be the only way they are operating, as once again I've failed to address turning etc.
Nevertheless, the nature of the experiment to find out whether or not this kind of mechanism is operating seems simple enough.
Of course there's alway just going out and seeing if it works, as opposed to knowing why. I seem to be interested in doing both... people are what they are.
Thanks, that was fun.
kc