There’s a great little (well known) experiment using a spoon and running water from a faucet. Basically, you turn on the water, and holding the spoon by its handle you alternately present its convex and concave sides to the flow, observing the results each time. It is definitely a neat experiment, and I recommend it to all. Upon performing it you are likely to come away with some conclusions regarding bottom contours, but before you decide to scoop out the bottom of your next board, or leave some great hump of foam somewhere, lets perform another simple experiment.
Get a business card or something similar and bend into a half pipe. Now present the convex and concave surfaces to the flow in the same way that you did with the spoon. (The presentation of the bent business card should be such as to have the flow of fluid be alone long axis of the card or the pipe curve.)
If your result leads you utter something like, “Huh?” or maybe event “This can’t be right?” or at least causes you pause, than the chances are good that you’ve performed the experiment correctly. (The bent business card does not behave like the spoon!)
So what’s going on? Surely it isn’t just concavity (or convexity) that is operating here. Lets take a closer look at the spoon, especially when the water is flowing over it - lets follow the momentum.
Convex presentation.
The general result seems to be that the spoon is actually drawn into the flow and, it is (at least every time I’ve performed the experiment.) But first take a good look at the spoon and the way the way flows off it. Lets try an account for some of the forces which are at play here, remembering of all the while that to change momentum requires force.
Some of the water appears to follow the surface contour of the spoon. Notice how some seems to be directed away from the line of flow leaving the faucet, it goes off at an angle (at the bottom or side of the spoon) - the presence of the spoon surface changes the momentum of some of the flow, sending some of it off in another direction. The direction in which that flow is sent off is key to understanding why the spoon moves in the opposite direction. (Its that trade-off stuff, Newton’s Third Law, whatever you want to call it.)
Now, aside from the fact that the spoon is actually able to perform the way it does because your holding it, (i.e. you are actually supply the force necessary to change the flow by keeping the spoon in place!) a cursory application of conservation of momentum dictates that if water is being shot off in one direction, some other part(s) of the system will be shot off in the opposite direction (with equal momentum.) Again its a cost/effect thing or trade-off (or Newton’s Third law, even Conservation of Energy, or Conservation of Momentum, all of which are basically telling you the same thing, nothing is for free… and only nothing is free.)
Now, contrast this with the business card version. Because the surface doesn’t have the same kind of lip (as the spoon did which tended to curl the water off in another direction) the water tends to flow off the card almost parallel to the flow leaving the faucet and there’s very little or no real pull into the flowing water. But the presence of the card still manages to change the momentum of some of the flow, spreading it out a bit. Again all the momentum can still be accounted for. But what is the net effect? You’ve slowed down and redirected some flow to achieve what? Or putting it another way, you manage to burn up some energy, and what have you got to show for it? (Good question, huh?)
Concave presentation.
Once again, check out the water leaving the surface of the spoon. Once again you will likely be able to match the motion of the spoon to ‘opposite’ that observed by the redirected flow. Again, contrast this with the business card presentation results.
Aside from the fact that the shape or contours of the spoon seem to make all the difference, if you’re at the point were you’re beginning to wonder, whether concaves or convexes may or may be doing what you think they are, I will have made my point. Don’t get me wrong, I think they’re quite useful, but exactly how they work (or how I think they work) will be left for another post.
In closing.
The most important point I hope to make is that the things you do to the bottom of a surfboard may have some benefit, but all will have a cost (and more often than not, the cost is to slow the board down.) Disturbing, huh?
