My hypothesis in Brief
The resultant force developed by the flow of water in a wave impacting upon the bottom of a surfboard is the primary force that makes the surfboard go (-i.e. changes its motion.) The more detailed version is contained in the thread ‘Dynamics – The Trim Equation’
But here’s the important design point, the flow that makes the surfboard go is impacting the bottom of the surfboard at an angle which is often close to being perpendicular to the bottom of the surfboard.
The Classic ‘Spoon Under the Running Faucet’ Experiment
There’s a example floating around (no pun) which is often used to explain how concaves (and convexes) work; the ‘spoon under the running faucet’ experiment. To run the experiment you take a spoon by the handle and place it under a running faucet so that the water flows more or less parallel to the handle or stem of the spoon. Basically if the concave side is presented to the flow, the spoon wants to move out of the way of the flow, if the convex side is presented the spoon is less inclined to move out of the way of the flow. The conclusions are obvious - concaves lift, convexes not so much – well, sort of.
The New ‘Spoon Under the Running Faucet’ Experiment
Here’s my new spoon experiment, let the water hit the spoon right in the middle of its spoony part, its bowl, concave side up. (The stem of the spoon is now at right angles to the flow of water.) Or turn the spoon over and let it hit directly on its convex side. In both cases the pressure on the spoon makes it go down. The conclusions are obvious, first, you didn’t really need to run this experiment to know what was going to happen, but if you did you now know that – impacting water (a flow) can make things move. If you think the spoon moves because gravity was pulling it down, you be partially right, so hold the spoon in the running water, then hold when its not in the running water, and ask yourself if it took the same amount of effort in each case.
But while you’re at the sink, start to angle the spoon a bit and see how you can get the water to shoot off it, this way and that. Here is the rough principle; if you can get the water to shoot of the spoon in ‘this’ general direction, a component of the force produced on your hand will be in the opposite direction. Admittedly, it’s rough, but generally in the broadest sense it’s true. (That force being produced on your hand is the propulsive force that I’ve been referring to in all these threads. Also, the experiment would make more sense if you were able to perform it upside, -i.e. with the water shooting up at the spoon.)
If you hang around the sink long enough, you will be able to start to sense the difference in force required to hold the spoon at different angles, whether convex or concave up towards the flow. You will also begin to sense the difference that a given angle or contour has with respect to downward, sideward, backward, or forward forces produced.
This is the nature of the flow that makes surfboards go. The classic spoon experiment still has value, not because it explains why concaves lift, but because it tends to address the kind of interactions that the once propelled surfboard will have with the ‘apparent flow’ or the flow of water the surfer sees from the standpoint of his surfboard. (But I’d be cautious in drawing any conclusions, such as the one in the classical spoon experiment.)
In Closing
My original final piece in this series was on the order of twelve pages and had all sorts of equations and diagrams, and grew a page or two every time I re-read it. I don’t think people would have read it, hell I probably wouldn’t have read it if I didn’t write it. The above spoon illustration seemed to be the solution.
The new spoon experiment will not tell you exactly how to design, but should make it clear as to how the propulsive flow is hitting your board (and your contours, and fins for that matter… think cant and toe-in, ‘which way does the water go when struck at this perpendicular angle?’) and hopefully serve as some reference as to what might be accomplished by using some contour or design element.
(Edit: 08/27/06. Corrected misspelling, kant to cant. KC)
It would be nice to run the experiment upside-down. If fact, I have. Worse, I made little balsa models of the contours that I was interested in. (Tip: Don’t do this in front of women, they will leave you, almost immediately.)
Finally, I am not an academic, nor a professional, nor anyone of note, nor likely to be in the future. Though I have a background in Mathematics (undergraduate), my knowledge, if any, of fluid mechanics has come through self-study. Which probably served me well, as I felt no need to memorize arcane terms, but only to understand concepts, appreciate when they might be applied, and move on.
Finally, if you’ve made it to here, thanks.
Kevin
(Edit: 08/23/06 Couldn’t resist, had to add the picture. KC)
