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Attempts to analayse surfboard dynamics
in the early 20th century have not always greatly advanced our understanding.
For example, continuing the above
narrative Blake goes on to suggest a, not altogether satisfactory, solution
...
The answer is relatively simple.
Gravity does the trick.
The front slope of the wave on which
one slides presents a down-hill path, while the friction of the
slippery board against the water
is very small. (7)
It's the same as skiing on a snow-covered
hill, and there is no doubt as to what makes one slide down
a hill on skis.
However, in skiing, one can start
down hill from a stationary position, while in surfriding some
momentum must first be attained
, to catch up with the incoming swell.
This is accomplished by paddling
the board with the hands and arms. (8)
- Blake(1935)
page 43.
7. "the friction of the slippery
board against the water is very small"
My physics is a bit rusty, but I think
that the friction on the board is significant - overwise the board would
sink.
More work/thought required.
8. "to catch up with the incoming
swell ... by paddling the board with the hands and arms."
One of the most common misunderstandings
by surfriders - technically the wave "catches" the rider.
1. As a wave approaches the shore and enters shallow
water...
1.1 The wave speed decreases.
Ws > 0
1.2 The angle of the wave face
increases.
Wf > 90 degrees
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Wave of Translation |
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Shoulder in Deep Water |
Although surfboard design is not studied in this book, they are planing hulls and severval principles expounded by Lindsay Lord appear applicable...
7.1. "hydrostatic
naval architecture is not applicable to the planning hull...The fundamental
hydraulic laws upon which standard naval architecture proceedures are based
simply do not apply to a hull skimmimg the surface."
Preface, Page vii
7.2. "the
submerged body, moving with sufficient rapidity, increases turbulence and
the resulting suction drag, soon reaching a speed at which the viscosity
of the liquid prevents further increase in speed regardless of practical
increases in power.
With the plate
(surfboard)
moving
in its own plane, this type of suction drag due to the viscosity of the
liquid is not a factor in the performance.
Rather, the
resistance, aside from skin friction, is largely due to the simple transfer
of kinetic energy at the leading edge.
Thus it becomes
apparent that the leading edge of the plane at once accounts for a major
portion of both drag and lift.
But since
lift rises as the square of the speed, and drag increases at less than
the square of the speed, every proportionate increase in leading edge increment
becomes successively more and more worthwhile.
In other words,
while incresasing speeds require the displacement hull to become progressively
narrower, the planning hull moving at high speed requires the widest possible
beam.
To simplify
still further, the displacement hull can improve its speed only with added
length; the planning hull requires added beam."
Pages 12 - 13.
7.3.''With
planning hulls, then, there is no theoretically sound proceedure by which
the total resistances of one hull can be directly compared to the total
resistances of another hull radically different in size."
Page 25, follows
analysis of Froude's Law of Comparison and the Reynolds number.
7.4 ''Unfortunately,
airfoil or hydrofoil data is of limited value as an approach to this problem
(of bottom loading).
The boat's
(surfboard's)
bottom operating at the boundary between two mediums, one of which is approximately
800 times as dense as the other, allows but for one working face of the
plane.
Furthermore,
while this one face should ideally be subjected only to positive pressures,
certain configurations of the average bottom lead to varying degrees of
transient negative pressures which may detract from the net dynamic lift
of the plane."
Page 31.
Note that this does
not apply to fins, which are true hydrofoils.
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Board C
6 ft x 22'' Wf = f g |
Board B
8 ft x 22' Wf = f b |
10 ft x 22'' Wf = f a |
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i. The board accelerates down the face
towards the trough.
This will ultimately result in rapid deceleration
as the wave face angle approaches 0.
ii. The rider stalls the board (applies drag) and maintains board speed at wave speed.
iii. The board travels at an angle to the
wave face, the resultant vector being at wave speed.
In this instance potential board speed
is further increased because...
"When the
board cuts or angles across the wave face, natural wave dynamics cause
the leading edge of the board to extend longitudinally, thereby greatly
increasing the board's speed." (#10. above)
2. Three riders on similar boards take off on a wave at a fixed point (T/O).
3. No board's wake effects the other riders.
4. Rider #1 takes off (T/O) behind the
peak and turns into the tube at a second fixed point (T1).
With minor adjustments the surfer stays
inside the tube in a straight line till a third fixed point (T2).
5. Rider #2 takes off (T/O) on the shoulder
and sets up a bottom turn at the second fixed point (T1).
With major adjustments the surfer climbs
and drops on the wave face till a third fixed point (T2).
6. Rider #3 takes off (T/O) on the wall
and walks to the board's nose at the second fixed point (T1).
With minor adjustments the surfer noserides
in a straight line on the till a third fixed point (T2).
This is represented graphically...
Observations...
1. Rider #2 rode the longest distance.
2. Riders 1# and #3 rode a shorter
and equal distance.
Therefore...
Rider #2 has the highest velocity (v
= d/t).
Riders #2 and #3 have the same velocity
!!! - the Analytical Dilemma.
Weirdo...
Tube riding surfers report that inside
the tube "time slows down".
Possibly related to ...
1. Extreme board speed, see above.
2. Visual "tunneling" as predicted
by Al Einstein when approaching the speed of light.
| Lord, Linsay :
Naval Architecture of Planing Hulls Cornell Maritime Press 241West 23rd Street New York 11, N.Y.1946 Hard cover, 305 pages, 21 black and white photographs, 118 black and white diagrams and graphs, Index Review Although surfboard design is not studied in this book, they are planing hulls and severval principles expounded by Lindsay Lord appear applicable. The book was treasured by seminal Californian board builder, Bob Simmons. Many of the models and diagrams appear similar to Simmons' famous wide tailed Spoons of the early 1950's, Lord emphasizing the increase in lift by incorprating parallel running lines, page 71. |
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| Kinstle, James :
Surfboard Design and Construction Natural High Express Co. Long Beach, California.1975 Soft cover, 139 pages, extensive black and white illustrations and diagrams. Image This is not an original, but a photocopied version contributed by A. -thanks to A. Review 1. Probably the most technically detailed work on surfboard design ever published, many sections are unique. It would be unfair to attempt a critical review at this point since I don't feel that I fully understand much of the work, in particular Chapter 2 Surfboard Dynamics. 2. October 2004 |
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|
Kuhio Pier, Waikiki, circa 1962 Photograph by Val Valentine Kelly, facing page 192. |
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