When buying or making a surfboard fin, most people just want something which works ok and looks the part. If however one wants a perfect or as near perfect a fin as possible, there is one bad foiling fault which should be avoided.This foiling error is the Flat Spot. Here is an example of the flat spot on a fin:
As can be seen from the photo above, there is a large triangular area at the base of the fin which is flat. In this area the cross section of the fin will not follow a smooth underwater foil shape, but will instead have a flat sided shape with hard corners or bumps. Water flowing around this area of the fin will become turbulent, causing drag, slowing the board down and possibly even causing cavitation and 'spinning out' when the fin stalls due to the water flow separating from the finThe fin above is from a major well respected Californian fin manufacturer. This might seem surprising, but the fact is that the vast majority of surfboard fins, including those from the world's 'best' fin makers, all have the hydrodynamically incorrect flat spot. It is almost impossible to find a correctly foiled fin, although they do exist.The flat spot syndrome has become so entrenched that surfboard buyer's guides even show fin foil diagrams which supposedly show the buyer how a fin should look, but which are grossly incorrect. Here's one such example from a popular surfing website, diligently pointing newbie surfers in the wrong direction:
The truly ghastly foil shown above actually represents a rectangular foil section at the base of the fin. Rectangular fin sections are basically unfoiled pieces of fin panel, and are like towing a bucket in terms of drag.Here's the same diagram, corrected somewhat in the fin base area by ourselves:
Here's another example of a poorly foiled fin from a supposedly leading USA fin retailer, the contour lines in the glass panel reveal quite clearly that a nearly flat spot extends half way up the fin:
And here's another example from one of the ( currently) most revered surfboard manufacturers of the moment. This fin is very nicely foiled, except in the base area, where the contour lines reveal a bad flat area with associated lumps and corners. . . this time the error has been kept to the base of the fin and hasn't gone halfway up the fin as it usually does. It seems likely that the fin maker is aware of the problem and has tried to avoid it to some extent:
So, why do fin manufacturers make these inferior fins?There are several answers:1) Because that's the way they have always done it, and if the fins sell, then the manufacturer has accomplished his goal.2) They don't know any better, and don't want to improve.3) Because in the case of fins with tabs for fin boxes ( which make up most of the fin market these days) it is tedious and difficult to foil the fin correctly when near the fin tabs. . . . so they simply don't bother, knowing that the marketing will sell the fin rather than the quality of the foil section.4) There is a race to the bottom in terms of low price and cheapness of manufacture, and the market won't support ( they assume) the cost of a well foiled fin, which can take 3 or 4 times as long to make, at least. It is more effective to market the fin as the best and then fail to deliver. .. most people won't know the difference5) The Gumby with a rotary disc grinder factor. Most fin foilers use a rotary grinder or disc sander, and armed with these tools they make a few passes alomng the leading and trailing edges of the fins, this makes a parallel band and inevitably leaves an unfoiled piece at the widening base of the fin. . . . the rotary sander doesn't reach the middle of the fin, and so instead of using another tool to finish the job properly the fin is left unfinished. . . if the job takes longer than 20 minutes it's a no go.Here are badly foiled quad fins from a foiler with over 40 years experience, it wouldn't pass in the Americas Cup, so why does it in the surf industry ?
It is often assumed that those who have been doing it the longest have learnt the most and that therefore they do the best job. In fact, repeating the same mistake for decades does not improve anything.To be continued. . . . ..
