Measuring the bend of some formula fins (go directly to the results).


The only purpose of this fin bending test is to show the results of the described loading of fins. It's not the intention at all to try to tell which softness/stiffness or bend curve is "best" for which conditions, for which board, for what kind of rider ... etc. If you have a favorite fin (or kind of fin), and if you can find it among the tested fins, then there's a possibility that you can find other fins with the same kind of softness/stiffness and bend curve. Or if you have a favorite fin (or kind of fin), and if you can find it among the tested fins, then perhaps you can develop your own theory about what kind of flex (absolute stiffness and/or bend curve) is "best".


How important is flex? According to those who know some very good riders prefer soft fins while others prefer stiff fins. It might be a qualified guess that whether you prefer stiff or soft fins is a function of your riding style - but nevertheless it's important to hit the exactly right "amount" of flex (and perhaps even the right distribution of flex along the length of the fin) that suits you best.



The test.


To put it simple this fin test is done by fastening a fin by means of a couple of clamps, after which the distances from the unloaded fin to a fixed stick are measured at the quarter points of the fin. Then the fin is loaded with a 20 kg weight (hung from the tip), and the distances (still at the quarter points) from the now loaded fin to the stick are measured. Subtracting the unloaded distance figures from the loaded distance figures now show the deflection of the fin. Click here to see some early pictures for a better understanding.


Generally, all controlled tests have to make compromises between imitating the conditions the subject have to work in and the need for simple methods. This fin test is definitely biased towards the simple method approach.

Of course, to load the fin only from the tip doesn't reflect the real life of a fin. When sailing, the fin is loaded along its entire length, and probably most from sections with the thickest profile and that's not at the tip (learn more about the real life of a formula fin here). However, there is a good chance that the far-from-ideal loading of the fins in this test at least indicate something about the overall softness/stiffness and the bend curve of the fins (that is: How is the flex spread along the fin at the quarter points).


In the details a couple of compromises are done for instance ...

  • to avoid the strong taper of the tip of some fins (making the measuring pretty delicate) the tip flex is measured 2 cm from the tip.

  • the 20 kg weight is not loading the fin directly from the tip. To avoid the weight to slide off the fin the weight is hung from a webbing that's fastened to the stick around the head of the fin (see the pictures).

But as long as the compromises are the same during all the tests, I think the compromises are acceptable.


Why loading the fins with the 20 kg weight? Of course, loading with 25 or even 30 kg (as in the IMCS test for masts) should highlight the differences among the fins even more. However, one thing is to risk cracking (or breaking) your own fins a far more serious thing is to "risk" destroying fins that you have borrowed for testing from your windsurf mates. To relate the deflection of the formula fins in this test to the deflection in the real life it's interesting to note that the test-deflection is around 50 - 75 % half of the deflection that "pfaffi" documents in his very interesting video sequence (mentioned above). The max. deflection in the video is 30 cm - but the tip of the fin mostly seem to rest around the 20 cm mark (admitted: With a total different bend curve).


Why not measure twist? Well, a lot of words have been said and written about the importance of the torsion stiffness of the fins, and to my (very little!) knowledge most of the fin makers try to avoid this (geometric) twist phenomenon by means of more torsion stiffness (read Sean O'Brian's very good article on the subject). In theory it should be pretty simple to fasten a stick to the tip at right angles to the fin by means of a couple of clamps, and if you support one end of the stick and load the other end with a weight, then it should be relatively straightforward to measure the torsion stiffness. But the torsion stiffness of the fins are not measured here (at least for the time being), and the reason for this is partly genuine laziness and partly a vague (and no doubt very naive) feeling that this twist issue may be a little over-emphasized. I mean, formula fins are pretty vertical, stiff and strong stuff, and it's perhaps a little difficult to imagine that because the line of pull (lift) and the line of resistance of the fin isn't exactly placed at the same position shall make the fin twist in a way that differ considerably from fin to fin. It's easier to imagine a wave fin twisting but a formula fin ...? However, perhaps later some fins shall be controlled for differences in torsion stiffness.

If we make a little comparison with a fancy word from the mast-terminology, perhaps "twist" of the fins reminds a little about the "reflex response" of the masts in the sense, that both qualities aren't quite as easy to control by yourself as tests for stiffness and bend curve, and both characteristics are disputed and perhaps of more value for the marketing guys than for the guys on the water(?).



How to read the figures.


If you from below for compare the Deboichet R13 M 70 cm with the Select Elite R07 Wing Soft 70 cm, you'll see that overall the Select is pretty much softer than the Debo. (flexing 1.6 cm more a the tip (- 2cm) than the Debo). If we calculate the flexes at the tips in % from the lengths of the fins, I think this might be an adequate and easy-to-understand standard for the stiffness of the fins (see the column "Softness: ...").*)

If you want to keep the analogy with masts you should perhaps talk about the "stiffness" of the fins. However as a bigger number in fact express more softness (and less stiffness) the word "softness" is used here.


If you compare the "Flex in % from tip-flex at quarter points..." for the two fins, you'll see that the Debo. fin flexes relatively more from the three first quarter points than the Select fin. You might conclude that although the Select fin is softer overall than the Debo. fin, the Debo. describe a more evenly distributed flex than the Select fin - that conversely describe a more curved arch close to the tip. You might also say that the relative more flex of the Select fin is largely confined to the quarter section towards the tip. If we calculate the differences of the %-flexes between the mid points and the tip points of the fins, I think this might be an adequate and easy-to-understand standard for the bend curve of the fins (see the column "Bend curve: ...").

When (if!) enough fins are tested it should be possible to make a couple of categories that in a verbal sense try to describe the bend curve of the fins - almost similar to the the Hard Top/Constant Curve/Flex Top categories from the IMCS test for masts. Using this kind of terminology we might perhaps at this early stage say that the Select fin is a kind of "Tip Flex" fin relative to the Debo. fin ...


*) Perhaps you should go farther than this simple relation between flex and length and make a kind of imitation of the IMCS way to "neutralize" the effect of length on the stiffness calculation (see the IMCS calculations here).




Fin Length (in cm)


Flex at tip in % from length

Bend curve:

Tip flex (%) minus mid flex (%)

Flex at tip (in cm) Flex in % from tip-flex  at quarter points down the fin
1/4 point 1/2 point 3/4 point 1/1 point
04/08 Deboichet R13 M, +8, 70 cm, approx. 2004 69.50 21.22 73.90 14.75 7.46 26.10 57.29 100
04/08 Select Elite R07 Wing Soft, 3.5 degree, 2008 69.80 23.42 75.54 16.35 6.73 24.46 55.35 100



Click here to go to the results.