In fig. 1 the wetted surface of the
bottom of a board is illustrated. Obviously the wetted area changes
constantly, the illustration is meant to be a example.
In fig. 2 illustrates the force of
planing developed for a flat plank (ref. Savistky). Also, in fig. 2,
I've overlaid rectangles on the Savitsky's force curve [which is
actually force per unit area] for eventual comparison with fig. 3.
In fig. 3 I've indicated what I suspect
is the actual velocity of the flow under the board with the
orientation as given. You can see its not completely nose-to-tail,
and that's important. Also, as illustrated, the force curve is not
the same as for a flat plank if you put a concave wall in place.
(I've only put in the one side, see below.) The contribution to the
overall force of planing due to its increased angle of attack
relative to the flow is shown as an increase in the force located on the wave-side
section of the board, or rail. (Sure it's all guessing at this point
as to the particular form of the force profile, but I believe what's shown is fairly reasonable, at least with respect to the big
picture.)
In fig 3 I've only drawn in the
wave-side concave wall contribution. I suspect you could argue that
the beach-side concave wall contribution reduces the angle of attack.
Whether or not this reduces the force of planing along that side of
the board in any significant way, I don't know. But it would be
interesting if it did.
Of course this is without consideration
of fins. In particular, slap some laterals, toe/canted or otherwise
on this puppy and things change.
Where's this going?
In a recent thread 'Drive Defined' by
LeeV, most of the definitions that were offered provided some real
insight into how the term is currently used by surfers. It was
interesting. Though it would appear that my interpretation, that it
relayed an impression or sensation of a smooth and uninterrupted
acceleration didn't go over well. Nevertheless I believe that claims
as to 'lift'* and 'control' being made in this thread suffer in the
same way as claims of 'drive' do – they don't relate to the
physics, but to a sensation or impression as expressed by the
individual. It's that old problem - what you want, and how to go
about getting it being two different things.
Anything that gets a surfer closer to a
feel of control is likely to invoke a positive response. In
describing the response the surfer is likely to use words that come
closest to reflecting the quality of his experience. For instance,
if you feel that you no longer have to lean into the wave, or put a
lot of your weight on the inside rail as compared to what you did on
some other board, the overall sensation or impression may very well
be one of enhanced 'control' and 'lift', and again I'm referring to
lift's more common usage, not the more precise engineering usage.
After reading through the posts here,
what is clear is that some obviously like concaves more than others,
for whatever reasons. For me, the lesson is that personal choice in
how something gets done, matters.
I'm not too sure about nowadays, but
way back when, you could buy a stock GM car and modify into something
with a little more power. But unless you modified a lot more than the
engine, you usually wound up with a car that felt a lot different all
around, and to some degree awkward, relative to the original product.
GM spent a lot of money on research getting the 'feel' of their cars
right, or right for the market they wished to sell into, right down
to the foam in their seats or the texture of the plastic dashboard.
The point being that some people might prefer leaning into the wave
or on a rail, it may provide them with a more of a sense of being 'in
touch' with the wave, or maybe they just don't want the help a
concave might offer.
If concaves truly were a solution to
'lift' – again I'm using the more common usage of the term – then
you'd likely see it used as an alternative to the 'fish' solution,
and its not. As indicated in some of the posts, its [concave's]
effect would seem to be roughly proportional to the conditions under
which it is surfed – the faster the flows, the more impact it's
likely to have. But the faster the flows, the more powerful the force
of planing, and arguably the less likely you'd need the additional
'lift' (again, used in the more common sense.) In fact the faster the
flow, the narrower the tail section of the boards that are likely to
be used to surf it.
One additional bit of nonsense.
Though it was sort of refreshing that
none of the posts evoked Bernoulli or the Venturi effect (the
exception being crafty's link to the Nick Carrol's RealSurf post),
for the better part of the last century, and well into this one,
people have nevertheless felt the need to – to evoke Bernoulli,
that is.
The Bernoulli principle or Venturi
effect is a principle or effect that comes out of an energy argument,
which also incorporates a continuity argument (as in conservation of
mass, or in the case of an incompressible liquid like water,
conservation of volume – basically a 'what goes in, must come out'
argument.)
As a flow with a given speed moves into
an expanse, its speed will decline and its hydrostatic pressure will
increase. The converse also being the case, move into a constriction
and the speed of the flow will increase and the hydrostatic pressure
will decrease. (That's not all there is to it, but that's the form of
the principle which is commonly evoked, call it Bernoulli Lite.)
So as the flow encounters the concave
(an expanse) it slows, and the hydrostatic pressure increases, hence
'lift'? It might. Bernoulli's principle has been borne out time and
time again. But is the effect operating here to any significant
degree, let alone the cause of the sensation of 'lift'? In my
opinion it's unlikely. Remember, the speed of the flow must slow,
hence the force of planing is reduced? Not to mention there are a lot
of other things going on which are likely to contribute far more
significantly.
...bonzer's have it both ways?
In fact many of the proponents of the
Bonzer design evoke Mr Bernoulli both coming and going. That is the
flow enters the expanse (concave) and then it's restricted again by
other contours or fins. The latter effect causing the speed of the
flow to increase providing a bit of a jet or rocket like effect?. But
I guess that makes the whole 'lift' as applied to bonzers argument a
zero-sum design element, if that's what concaves and convexes did.
(I've assumed that most would see the 'rocket' or 'jet' argument as
kind of ridiculous.) Actually, less than zero, for to restrict the
flow would increase the drag dramatically. Which may very well be
apart of their [bonzers] charm, but who would want to admit that
slowing a board down and enhancing the sense of 'control' under those
conditions in which the bonzer runs best, is just what they were
looking for?
Whatever turns your screws
All this relates back to personal
preference, or perception. Some don't mind sitting a little
more on the tail of a board to slow it down, some the opposite. Some
don't mind leaning on a rail, some prefer it not to so much. Or maybe
its just a matter of sensitivity, as in some when they lean, they
like to lean hard, others prefer a lighter touch. Not everyone liked
the smooth ride GM cars offered, among other things about GM cars.
Out of the trees, on two feet and a
little increase in brain power... okay, so what's next?
I'm inclined to believe that the big
steps in surfboard evolution have been made, and now it's all small
increments. And those increments are about fine tuning a product for
a given market. Like GM's worrying about whether or not the foam in
their seats gave male riders (of their chosen market) the right
sensation in their balls. It may sound funny, but why not, if the
portion of the market you're after wants it. Surfing is about fun –
it's pure pleasure, and pleasure comes in many forms... find the form
it takes for your chosen market and give it to them.
kc
*Note on Lift and Drag. A given force
can be resolved into any number of components. The orthogonal
components of lift and drag are simply a useful convenience. How a
force can be resolved depends on the reference frame being used to
analyze it. Hard to believe but if you give me a lift and a drag and
I can find a reference frame in which they 'disappear' and your once
again left with original singular force, the direction of which being
invariant.
Sorry to get all mathy on you, but it's
just not my experience that when some guy is going on about his
sensation of 'lift' in the line-up or in the parking lot, he's
strictly adhering to the engineering definition – my guess would be
he's just trying to relay some sensation and 'lift' as in it's common
usage either captures it, or comes as close to capturing it as he
able to at that moment.
