Aaron Perry is a San Francisco-based surfer who makes boats. Not the kind of boats that T-Pain and friends used to so thoroughly enjoy, but the kind that use cutting edge research and technology to move at incredible speeds through water. As the Design Engineer for the ORACLE boat racing team as well as an avid Ocean Beach surfer, Perry knows a thing or two about going fast. While working on our September issue's cover story, "Unlimited Speed", about searching for the uppermost limits of surfboard speed, I decided to ask Perry for a crash course in hydrodynamics. Here are the CliffsNotes:
Fins Give Control, But Create Drag
"When it comes to the drag of fins, or any body moving through water, there are different categories. First you've got form drag, which is the shape of the foil itself. Some shapes create more drag than others, but there is a generic set of foils developed by NASA called NACA airfoil, which have been shown to produce minimum drag. Right now I doubt that surfboard fins are matching NACA airfoil shapes, so drag could probably be reduced there. On the other hand, with surfboard fins the goal isn't always maximum speed in a straight line. The most drag resistant foil might allow more speed, but could negatively impact maneuverability. Another factor for surfboard fins is fin friction, which comes from how polished the surface is, but the biggest factor on a surfboard fin is ventilation. That's when you see underwater shots of guys doing big turns and they have a huge bubble of air that's trailing on a fin, which is creating drag."
Finless Is Faster
"If you took the fins completely out of the equation on a surfboard, you would get less drag for sure. I've seen some videos of guys like Derek Hynd on those boards moving very fast, but you give up so much in terms of maneuverability. Many kite boards are finless but have a side cut [symmetrical indentations in the plan line] much like those surfboards, and with that you can get a lot of grip on the water even without fins. Your ability to do abrupt turns will obviously be compromised though."
What Works for Golf Balls Does Not Apply to Surfboards
"I've heard people talk about how textured surfaces could increase speed through water, much like the surface of a golf ball through air, but there isn't any hard evidence to suggest that is true. I was involved with a program where we placed plates of steel sanded to varying degrees into a towing tank to see the differences in drag, but we didn't see any change. I've heard many conflicting views on surface texture, but we've never seen any evidence of it working better. In boat racing, we polish everything until it's pretty much a mirror."
If You Go Really, Really, Ridiculously Fast, Cavitation May Occur
"Cavitation happens at really high speeds when the pressure is low enough in the fluid around a foil for it to vaporize, which in surfing would mean the water vaporizing around the fins. In the sailing world, it's pretty rare until you get up to 50 miles per hour. When this happens, it causes vibration that slows you down, but it would be very difficult for a surfer to reach speeds that would actually cause cavitation in the fins. Perhaps if you are a big-wave surfer, you might find a wave that would allow you to reach those speeds."
There Is a Speed Ceiling
"You can minimize drag but you can't escape it. Think of a skydiver in free fall: at a certain point, the air resistance is going to cause them to reach terminal velocity, and further acceleration is not possible. In sailing, or surfing, when you calculate the drag of any surfaces above the water, plus any surfaces below the water, that dictates your maximum speed. You can try to reduce the dragging components above and below the water to push that threshold higher by getting rid of the leash, changing the fins, manipulating the bottom surface of the board, but there will always be some drag working against you."
We're Probably a Lot Slower Than We Think
"I was talking to someone who did speed sailing with wind surfboards. He said he was in an event and reached the fastest he'd ever gone, but when he got out of the water and looked at the charts he actually came in last place going thirty miles per hour. In surfing or sailing it ends up feeling like you are going much faster than you are because you're exposed to the wind and water. When our guys are going 50 miles per hour in a racing boat, it always looks much faster than that if you are comparing it to something moving on land."