A repost of an article I wrote years ago.
When you walk into your favorite surf shop and get all google eyed over the masterpieces in the racks, it’s not the fins you’re drooling over. Think about it for a minute though. What is a surfboard, but a summation of hydrodynamic surfaces. Just as an airplane generates lift from its wings and control from its tail; your board’s bottom generates lift and affects speed. But, it’s your fins working together with rail and bottom contour that most influence the feel of your board when turning. And let’s face it how many short boarders straight-line the waves they ride.
Few surfers really understand how fins affect the way their board rides and leave all the specifics to someone else. Rail and bottom contour assist fins in performing their function. But, what really influences the way your board turns is the combination of several important fin factors. No wonder so few surfers pay attention to the "Fin Affect". It’s f’n complicated!
But, today, as the form shape of surfboards goes through finer and finer adjustments, the biggest gains you can make to your board’s performance have to do with your fins. The predominant factors that influence your "Fin Affect" are:
1) Foil Shape- the curvature from leading edge to trailing edge as it changes from base to tip.
2) Template shape- The combination of depth, width, and rake that make up the profile outline of the fin.
3) Placement- which incorporates how far the fins are from the back of the board, how far apart they are from one another, toe and camber.
4) Stability and flex.
Let’s discuss each of these and how changing them will affect the way your board performs.
Foils are surfaces that affect lift and drag. If you notice an airplane wing has a flat side on the bottom and curved side on the top. The path of least resistance is the bottom side. It takes more effort (drag) to flow around the curved top surface, so more air flows under the wing than over. This creates high pressure under the wing. The air that does flow over the wing separates from the wing at the apex of the leading edge of the wing and creates a low-pressure area above the foil and a higher pressure area below. This difference in pressure forms the force (lift) that allows the wing to go up. The more curve a foil has the more drag it induces over the curved surface; which means that a foil with greater curvature will have more lift at lower speeds. The problem is that at higher speeds that additional drag will develop turbulence and stall the flow across the foil. The exact same scenario occurs under water with side fins. The big difference is that instead of lifting your tail out under water; Side fins orient the curved surface so that they actually pull your boards fin and rail down into the water. This gives you hold when cranking a nasty slash. Consequently, thicker more curvy foils for slow waves and flatter more fine foils for high-speed waves.
Template shape has to do with how the fin looks in profile. An over simplification would be deeper, rakier & wider fins provide more control. But, the more profile you have the more fin you drag around. So, you have to optimize the combination of the three so that it is loose enough for your conditions. Yet, it is also tight enough to not get too squirrelly on you. Other factors that figure into requiring more or less of these three variables are:
1) Type of wave: steep and heavy or slopey and fun.
2) Surface conditions: Choppy and irregular or clean and smooth.
3) Rider Size: Big and heavy or small and featherweight.
4) Rider Style: Subtle and flowing or Extreme and radical.
Each of the first considerations requires more fin template area and each of the second work better with less.
Placement has traditionally been left to convention. Simon Anderson set a benchmark twenty years ago for approximate location. Each shaper has their own personal preference for each of the "models" they make. But, there are subtle differences in most boards and in all riders. Otherwise why would custom boards be in such demand?
And remarkably as little as an 1/8" movement fore or aft in either or both the center fin or the side fins can have almost as much effect as going from a rakey 4 ¾" fin to a vertical 4 5/8" fin. If you move your fins closer together they act looser and if you spread them further apart they get tighter.
Toe is the amount of angle the base of your side fins are pointed in towards the center of the board relative to the leading edge and trailing edge at the base. Camber is the amount of angle the body of your fin is set at relative to an imaginary horizontal plane perpendicular to your stringer. Both affect the angle of attack that your fin foils experience as they flow through the water. More angle forces more water flow around the outside plane at lower speeds. The net affect is that it becomes easier to initiate turns on slower waves. To much angle at higher speeds increases turbulence and drag.
Finally, stability and flex are crucial to making this all click. If you have a deeper fin you can get away with more tip-flex and not wash out. The benefit of tip flex is that it dampens or smoothes out some of the bite in direction changes. The down side of tip flex is that if you get too much tip flex it will wash out. Base stability is crucial to a good set of fins. If a fin moves around at the base it will set up turbulence. Turbulence generates drag and disturbs the lift, which keeps you fins holding. So it is slow and out of control. If you like a more pivoty board you would do well to try a stiffer set of smaller more vertical fins.
Prior to eight years ago the only options a surfer had to muck with all these variables was to grind his glass-on fins off and install a slightly different set in a slightly different location and record the differences until they found the optimum. Not very likely. So, with the exception of a few elite pros, we all lived with what we were given. Now with the advent of removable fins (i.e. FCS, Future, Lock Box, OAM and O’Fish’l) you can at least muck with the first two "Fin affect factors". But, if you want the ability to dial in your board with all four factors you have got to try the newest competitor to the fin system market Red X.
Tom O’Keefe
but I was wondering if your Probox works with hollow boards because i have built a surf board which uses a ribbing structure (I have attached pic) but is hollow I was thinking of glassing on fin but I would like to fit a system like your:) any toughs on the subject/project