Fins and speed

New article from surfermag:

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 October 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.”

 

Interesting to hear from someone who knows actual 50 mph speed on water. So according to him, dimples and textured fins are not helping b/c boards aren't going fast enough. Back to fin templates and foils. I think you can get more performance from fin configuration and placement (e.g. twinzers, quads, etc.). 

For better understanding of sail aerodynamics, check images below:

Artz is right, you are wrong.

A sail acts like a wing. What you describe is partially true when sailing straight down wind.

In all other courses, a sail works exactly like a wing or a fin. Sails allow you to travel faster than the wind, wich is impossible with your theory.

Sails on boats actually create directional movement and speed by a vacum; it gets sucked into the dead air space on the leward side of the sail; more of a pull than a push. I’m not really sure how that relates to fins under water.

I wouldn’t care too much about cutting the fin if you’re not a windsurfer.

Racing equipment in windsurfing is like a Ferrari.

It more than halves in value the moment you buy it, no matter how good it may look.

In the amateur Racing scene your status is mostly determined by your equipment, these guys always have the newest equipment.

It is different in the wave and freestyle scene, there status is more determined by your surfing capabilities. And most guys ride equipment that had undergone major surgeries.

Hans, you are correct about the bulb on sailboat keels. They load them up with extra weight, but that’s not the way the surfboard fins were. The bulbs were not heavy.

I bought an old windsurfboard that had one of those super long straight fins. I bought the board for my daughter to use as a flat water SUP, it’s 30 inches wide, fairly light, and only cost me $20. The only problem is that it has that windsurfer box, and I need a new fin because the place where she would do the paddling is also full of swimmers, and no one abides by the channel markers. Swimmers will swim where paddle boards are supposed to be and paddlers will go across the bouys into the swimming side. That long fin will be a huge problem.

That’s where I think that fin I made will work. I just need to make an adapter for a normal surfboard fin. I think that fin I got with the windsurfer costs over $200. It’s made from Carbon fiber and is in very good condition. I don’t want to cut it.

As far as rake on a fin for surfboards, I like more rake. The fins I like the most seem to have more rake but also have a fuller or wider base.

not to forget that you collect the seaweed without rake

Rake does not reduce drag.

Rake makes the fin more resistant to stalling (spin out).

 

In windsurfing you see that speed and race fins have low rake and high aspect ratios (like glider wings).

Wave fins have a higher rake because a spin out on a wave is not really acceptable. But high efficiency is not that important.

What is the benefit of adding rake to your fins?

Is it only to reduce the drag (and shed seaweed)?

Or is it done to move the center of the surface area aft to change the leverage on the fin box?

Does a fin NEED to be foiled to work properly?

For instance if you could get the same amount of stiffness or flex from a material 1/4 the thickness of a conventional fin and left it parallel, while keeping the surface area the same,  would it work as well or better ?

 

 

 

I’m sure that the bulb reduces drag a little bit, by reducing the tip vortex.

However, in sailboats the bulb is there mainly as a counterwait and the reduced tip vortex is a secondary benefit.

 

Don’t get me wrong, I fully support your fin experiments! I just couldn’t resist to correct the cavitation statement.

My recollection is that they both affect the flow coming off the wing or keel or bow, and they help reduce drag. I don’t know the scientific data or terms, and don’t care. I can say that the affect of the small bulb on the fin helps the board hold more speed through turns. I was able to make turns beyond 180 degrees and it was very smooth. Most noticeably was when coming off the top, I could go well beyond 180 degrees without a lot of effort. This was in 1986, by today’s standards it’s not a big thing, but for me at that time I was quite happy to be turning as much as 270 degrees off the top, and not losing speed, or having the board release and slip out from under me.

The shape of this fin was based on the keels that racing sailboats were doing about 5 or 6 years ago. I have no idea whether it will work as a skeg. I think it will be fine as a flat water SUP fin. Maybe this fin might work as part of a multi-finned in the center board like Wildy did.

Winglets reduce the tip vortex, not cavitation.

The bulb does the same as the winglets on the wavegrinder fin. It helps to reduce cavitation. When I did my research I found that the bulbs on the keels were for the same reason, reduce cavitation. They act the same as the bulbs on the bows of large ships.

Winglets are very popular on fins these days, but I want to go back to what Downing did. Downing put these on his fins in the 80s and it worked. The only down side was leashes can get caught and then it causes a big problem.

I had a board from Downing that had his bong fin and I really liked it. Rode the board to death, then gave it to a friend’s kid.

 

Hydrodynamics is the study of fluid flow - from a technical point of view I don’t understand what else is surfboard design about?

The bulb on yacht keels is to keep weight as low as possible to couteract the heeling moment from the sails - I can’t see that it would be of any benefit to surfboard fin?

  Guys, you are forgetting one very important consideration!

  When you surf, you TURN, you don’t go straight.

  So, every surfboard has TAIL ROCKER.  What is the point of talking about fast fins when you add tail rocker to a board to increase wetted surface?

  And 400-800 is the fastest, if you keep it clean.

  AC-72’s below waterline is sanded to just beyond 800…but their foils allow them to go over 36mph.

Yeah, I hear you. When I drew this up, I was going off what the racing sailboat designers were doing, and adding the bulb tip. We all know that it’s apples and oranges, so the straight down shape of a boat keel may not be what works well as a surfboard fin. I haven’t tried it, but I’m also thinking of sticking it into a flat water SUP. It should be OK for that purpose if it doesn’t work as a surfboard fin. I still have more lexan somewhere, and a set of templates for that fin. I could easily add rake. I’m not so sure that lexan is the right material though, I think it will eventually snap off. I just like sanding it because it’s not as itchy as fiberglass.

Shark I would like to see a fin like that witha bit more rack like if you took the whole thing and tilted it back by 10 to 14 degrees.  

I designed this fin after the wave grinder fin came out, and reading a little about what Cheyne Horan did to make the star fin. I used what boat designers were doing with Keels and added the little “bong” tip that Downing Hawaii was doing in the 80’s. This fin was cut from a sheet of lexan and could be cleaned up more. I sanded it down to 600 and stopped. It’s about 8" long.

I believe there is another aspect to how fast a surfboard is that is not acceleration, but how well the board maintains speed versus losing it or slowing down. When I say I want my boards to be as fast as I can make them, I want them to run without the slowing down. We all know that you can generate speed by pumping, or turning, but I also want a board that you can set into the high line and get it running fast, or one that doesn’t slow down quickly if you’re not turning. I think this is where the single fins have an advantage, they have one fin pointed straight on. You can get a symetrical foil very close to a good naca foil on that center fin. The down side of the single is when your tail is too wide, the fin disengages and you spin out. That’s where the thin, hard, sharp tail rails come in to play.

A good quad seems to go faster than a thruster, and Greg Griffin’s 5 fin design is another unique design that seems to carry speed through turns better than thrusters. Alexander’s Gemini is a fast board that carries speed well.

There are apsects to the board that help make it a fast board that go beyond the fins. The rocker, bottom shape and rails. The fins are control devices that help steer the board, but they also add drag. I think the term I see a lot is controlled drag.

greg griffin’s got the formula down for flat bottoms hard edges

so does McCoy with his Gullwing for hull bottoms and soft edges

for me speed comes from leverage + release

the more leverage and quicker/smoother release the fin has

the faster I can “propel” myself forward with a turn

the other aspect regrading fins is control

I find many fin setups lose control at higher speeds

kind of like a wide tailed quad trimming at high speeds

the tend to get all wiggly

where as a single fin wouldn’t

never seems to happen on my wide tailed griffin 5 fin fishes

less fins = less movement needed (Gerry Lopez Surfing)

more fins = more movement needed to prevent bogging

I call it the thruster hippy hippy shake business