Creative Drag: Revisiting the Ancient Hawaiians

The original title of this thread was ‘Creative Drag’. In his post below, JohnMellor points out that the notions presented here are not unique. In particular he references George Downing’s interpretation called ‘Calculated Drag’. The reference given in my post below John’s is highly recommended.

Creative Drag

Assertion: Drag is critical, not as something to be minimized, but actively controlled by the surfer through good design.

WATER FLOW direction test resultsFigure 1 below is a quick and dirty analysis of the forces operating during planing as it applies to the diagram I posted in thread. The model is very simplistic, however it does serve to illustrate the relationship between the forces of drag and lift.

The physics make be a bit thick for some. The important point is the relationship between the angle (greek letter phi in the diagram) and the ratio of the coefficients of drag to lift.

A beside on coefficients

Savitsky diagram Coefficients like those used here - CL for lift and CD for drag, see figure 1 – are very useful in this type of analysis. They can be viewed as ‘lumps’ of all the variables that are likely to have a bearing on determining a given quantity, in this case the forces of lift and drag. Here they take into account parameters like fluid density, template shape, tail shape, rails, board orientation with respect to wave face, wetted surface, etc… For a given body, they are generally determined experimentally. Often in lift and drag analyses the coefficients themselves are given as functions of angle-of-attack, which in this case would be somewhat analogous to the tau angle (see in the WATER FLOW direction test thread mentioned above.)

Please note: The phi angle given here is not the tau angle (i.e. angle-of-attack) but is just used to locate the board on the face of the wave.

As suggested by the analysis in the figure, the position of the plank on the wave face is a function of the lift/drag coefficient ratio. The board will ride higher on the wave face for a greater drag to lift coefficient ratio, or to put it another way, all else remaining the same, increasing the drag coefficient will tend to make the board ride higher on the face.

In surfing the surfer has some control of the value of these coefficients e.g. he can change board orientation. In fact a surfer is constantly adjusting the value of these coefficients, at least with respect to those parameters that lend themselves to adjustment by his actions. It could be argued that surfing is about the creative use of this relationship – that good design provides the surfer with a large degree of creative control of these coefficients.

burnsie’s postConsider ‘shape-drag’ –i.e. the shape of the template, particularly the rear and tail end of the template. Take a look at the picture in the in the thread reference above. The surfer isn’t exactly moving as suggested in the above diagram, he’s got some lateral motion, but it’s close enough – he’s likely finless, and what he’s on is pretty close to a plank. Obviously the shape of his tail and rear rails contribute to drag. Consider what the difference it would make if his rear rail line and tail were parabolic -i.e. a curved template.

Extrapolate this a little further. The surfer in figure 3 (click for the video)is leaning on his inside rail –i.e. dragging it, in order to maintain his position on the face.

Here there will be a large drag component both in the lateral direction and in the direction ‘up the face’, as in figure 1. This latter drag component will be operating in a similar manner as in the simple case presented in figure 1. The formal relationship however will be more complex.

… continued in next post.