surfresearch

Introduction
Russell, Robert Christopher Hamlyn:

Page 80

Surf-riding.

It is easy to understand why the sport of surf-riding is confined to very flat beaches.
The sport consists of balancing a surf-board on the front of a steep wave, so that a combination of the forward particle velocity of the wave with the forward movement of the board through the water gives the rider the same speed as the wave motion and enables him to keep in front of the crest.
It is necessary that the wave be on the verge of breaking, in order that the particle velocity should be close to that of the wave itself and leave little to be made up by the board's movement through the water.
The board moves through the water because it is perpetually sliding downhill.

It is very convenient for surf-riders that their position on the face of a wave is a stable one in the sense that, if they move back towards the crest, they are accelerated forwards

Page 81

and if they move forwards they slow down.
This stability is due to two things, in the first place to the shape of an almost breaking wave which is steepest at the crest and secondly to the fact that the forward particle velocity is a maximum at the crest.
If, therefore, the wave remains substantially the same shape for a period, there is one position in which the board will ride such that if it moves away from this position it will automatically return to it.
On the other hand considerations of particle velocity show that the direction of a surf-board is not a stable one, and that in the absence of permanent correctives by the rider the board would "broach to".
Since the forward particle velocity is a maximum at the crest and decreases down towards the trough, the velocity of the board through the water must be greater at the front of the board than at the back.
For example, if the back of the board were exactly at the crest of a wave about to break, the back of the board would be stationary
relative to the water but the front would still be moving through it.
Once the direction of the board became displaced from the normal, however slightly, it would be turned round parallel to the crests in the absence of any human agency to counteract it.
Since the difference in velocity between back and front is least when the length of the board is only a small proportion of the wave-length, the best conditions for directional stability are created by very large waves and by the use of boards that are as short as possible.

Almost flat beaches are necessary for the sport, because only under this condition do waves remain in a state of incipient
breaking for long.
The fact that breakers on these beaches tend to be of the spilling type makes the sport less dangerous than it would otherwise be.

Facing page 88

Plate vii
A perfect plunging breaker
(F.J.T. Kestner)

Plate viii
Surf-riding on spilling breakers.
(Courtesy Australian News Information Bureau)

Russell, R. C. H.,
and
MacMillan, Commander D. H.:
Waves and Tides
Philosophical Library, New York, 1953.

Hathi Trust
https://hdl.handle.net/2027/mdp.39015065511159

WAVES AND TIDES
by
R. C. H. RUSSELL, M.A.
and
Commander D. H. MACMILLAN
R.N.R. (Rtd.), F.R.I.C.S., Assoc.I.N.A.
Hydrographic Surveyor to the Southampton Harbour Board

With a Foreword by
HERBERT CHATLEY, D.Sc. (eng.), M.lnst.C.E.
Officer of the Legion of Honour

Illustrated with over 100 line drawings
in the text and 17 photographs.

PHILOSOPHICAL LIBRARY
NEW YORK

surfresearch.com.au