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…… So Terry, what do you think of the size of the craft and how it affects the potential top speed? Is there any precedence that would show that a larger or smaller craft is likely to achieve a higher top speed ? ( Going back to post #1) ……
Hmmmm…I’m a little confused since Post #1 in this thread is a video of the 31 mph ride at the Wedge?
But to answer what I assume your question may be regarding the “size of a craft and how it affects potential top speed”, my first reaction would be that I would have to say it depends on the conditions.
All other factors being equal (i.e. same surfer and board weight, same aspect ratio and wetted planform shape, same wave size, same waveface slope along the path line of the craft, etc.) and neglecting air drag, the optimum size board will be when the induced drag (proportional to 1/(V^2) is equal to the other sources of drag (which are proportional to V^2). Any excess wetted area decreases the induced drag, but that is more than offset by an increase in the other forms of drag. Conversely, any lesser area increases the induced drag more than the is offset by the reduction in the other forms of drag.
What the optimum wetted area will be depends on the speed one is going–faster speed, smaller wetted area; slower speed, greater wetted area.
The surfer, of course, can alter the wetted area (as long as some portion of the bottom is initially not wetted) by shifting his weight forward (increasing the wetted area) or backward (decreasing the wetted area)–i.e. by ‘trimming’ the board for max speed). In this sense one might expect that the longer board would have an advantage at slower speeds since the rider can step forward to increase the wetted area) and could step backward to reduce the wetted area to the same as a shorter board when traveling at higher speeds.
But that assumes that ‘all else is equal’–which it generally is not. First of all the greater the unwetted area, the more friction drag that can be expected from the high speed jet of spray shooting forward of the points of contact of the water that first contacts the bottom of the board. It also increases the air drag as the presence of the unwet forward area of the board blocks the flow of air near the sea surface (although the frontal area of the rider is probably a greater factor). In addition, the tails of longer boards tends to have more rocker than the tails of short boards as well as ‘rounder’ rails over a greater length of the board. This reduces the efficiency of the board in generating the lift required to support the rider and board–and these trends generally increases toward the rear of the board.
Then there is also the question of ‘porpoising’ that can occur as the center-of-mass is moved toward the rear of the hull. I’m not very familiar the details of this process, but I suspect that aspect ratio is probably one factor, and the moment-of-inertia about the board/rider systsem about the pitch axis is another. The latter can be substantially larger for a longboard compared with a shortboard.
There are also geometric factors, such as the radii of curvature of the wave face compared with the dimensions of the board (affecting the planing efficiency of the bottom); the height of chop compared with the length of the board, etc… And in the real world, ergonomic factors (such as the ability of the rider to efficiently “pump” the board and how much energy he can put into the pumping process).
In, short, I’d be leery about making generalizations and would recommend considering each case as unique unless many factors are virtually identical in both design and wave environment to a known board, rider, and wave conditions (i.e. the ‘weasel out clause’).
mtb
