Archive for October, 2010

Venturis and dimples

October 9, 2010

I was thinking of calling this post “Carburetors and golf balls” but I figured that would attract gear-head golfers. Over the years I’ve noticed that surfers often talk about how fast they go (or want to go) on their surfboards. At the same time I have observed that they choose slower board designs in preference to faster designs. Why is this?

Let me re-state here that the best surfboard design is the one that a surfer likes the best; there is no other standard of “best”. Surfboard manufacturers will tell you theirs is the best (they want you to buy it) but only if you try it and like it, does it become “best” for you. Surfers usually prefer control above speed so fast is normally only a marketing term.

I previously posted about dimpled surfboards and a lot of surfers have been searching for that post, so I thought I’d add a bit. With golf balls and surfboards we have a fluid dynamics problem which will require some study if you wish to learn more. A golf ball is not a good shape for moving through the air rapidly. The best shape for greatest distance (speed) would be a streamline (“teardrop”) shape that was very smooth (no dimples). We can see that this would not make a good golf ball since it wouldn’t roll. I suspect that dimples are not the best surface treatment for a golf ball, but it may be that roughed-up surface (scratches) would be. But try selling an ugly scratched ball against a cute dimpled one.

So why does the dimpled ball have less drag than the smooth one (at certain speeds)? The smooth ball is a poor shape and air flowing around it forms eddies behind the ball increasing drag. The dimpled ball enturbulates the air next to the ball which reduces the eddies, resulting in less overall drag. A surfboard has a different fluid dynamic problem and dimples are not the way to increase speed but they might be useful for other reasons (which I don’t have time to discuss here). In any event, if you want to try dimples, I suggest a piece of course sandpaper; scuff the surface (of the finished board) where you want the dimples and you should get the same effect.

Now a word about venturis: a venturi is actually a tube with a narrowed section near its center. It is designed to create a pressure drop in the narrow section for various purposes. The most well known use is in a carburetor where the venturi is used to “suck” fuel into the air flowing through the carb. Note that the air-flow speeds up through the narrow section but this speed-up comes at the expense of increased overall drag. The auto racers know this and (rules permitting) will choose fuel injection (no venturi) over carburetion every time, because a fuel injected engine will flow air in faster (more power) than a carbureted engine (the venturi slows the flow).

What does this mean for surfers? I’ve never seen evidence of an actual venturi on the bottom of a surfboard. A venturi will add drag (slow the board); yes the flow does speed-up at the narrow section but you have to look at the whole picture. You only get a localized speed-up by forcing the fluid through the restriction and that force takes energy (equals more drag). This does not mean that “venturi-bottom” surfboards are not good or even that they are slow. It just means that “venturi-bottoms” can only be seen as a marketing term and are based on a misunderstanding of what a venturi is and how it works.

There is one fin for surfboards that does appear to have a venturi. That is the Turbo Tunnel (TM) fin. While we can see from the above discussion (or by testing it in a water flow) that it will have more drag than the same fin without the tunnel pod, that does not mean it’s not a good fin. It appears that a lot of surfers like it so that means it is a good fin.

Slowing a surfboard a bit will often enhance performance and control and that is usually more important than speed. Check my earlier posts on this (“Fast or slow” and “Slow or fast” both posted March 24 2009) for more. My advice is to look for a surfboard with the best control; then start looking at ways to get more speed without sacrificing control.

Bob

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En Espanol?

October 2, 2010

Attention Spanish speaking surfers: I recently signed a publishing contract with a publisher in Barcelona Spain. This publisher will be translating “The Basics of Surfboard Design” into Spanish. A whole new area of the surfing world will soon have access to surfboard design theory. Additionally, the publisher requested an option to publish in French and Portuguese so if the Spanish version sells well, French and Portuguese speaking surfers will get their chance too.

Bob