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Lost fins
by Roger Jackson

(from the e-mail posted to WML)

      The fins never fit the finbox correctly. All fins need to have the "fit" of the fin root into the fin box inspected/ checked: Whenever you get a new board...... Whenever you get a new fin...... Whenever the fin "just falls out" when you remove the screw (Powerbox and Trimbox) or screws (2 of them) for Tuttle, Tiga, and Meritex fin systems.

    Your Powerbox fin has tapered sides, and tapered front and rear surfaces. When you put a new fin in a new board, or a new fin in one of your existing boards, you need to ensure that the tapers both on the sides and the front rear of the fin root are actually making good solid contact with the mating surfaces in your fin box. If the fin goes all the way in until the fin root is level with the bottom of the board WITHOUT any tension on the fin screw, then the tapers are not fitting correctly, or the fin root is too deep and the top of the root is "bottoming out" in the top of the finbox under the deck of the board. Either way, if you sail the board without any "draw" on the fin to ensure that all the tapered mating surfaces are making tight intimate contact, then your fin screw will not "draw" the fin in, and the fin will be somewhat loose in the fin box. I suspect this is precisely how you lost all 3 of your fins. There may be a little "moulding imperfection" or sometimes there are actual little crossbars cast into the fin box. You need to file off the top of the fin root until the fin root is very tight in the fin box. You can check where the fin and the box are making contact, if you rub some plain old bar (bath type) soap on the fin root, and then put it solidly in the fin box and tighten the fin screw. The soap will remain "whitish looking" wherever it doesn't make contact. The surfaces that do make contact will have a slightly "shiny" appearance. So now you need to "fit" the fin into the finbox until all the tapered surfaces are solidly engaged. If this results in the fin root going too far into the finbox, and no longer being flush or level with the bottom of the board, then you will need to "build up" the tapered surfaces on the fin root. Hard polyurethane sail repair tape works pretty good, but the best job is obtained by carefully applying some 2 oz. (almost like thin lace) glass cloth to the fin root, and then squeegee in some West Systems epoxy resin. You could also use some sort of paper that the resin will soak into. The idea here is to build up the tapered fin surfaces to make the fin sit higher (toward the bottom of the board) in the fin box. Apply the "build up" layers equally to each side of the fin root to ensure the fin stays centered in the box, and is aligned with the longitudinal axis of the board.

    When the fin only goes in until the fin root surface is about 1/64th" (0.4-0.8 mm) above the bottom surface of the fin box (bottom of the board) then you are ready to start fitting you fin root into that particular fin box. Using a medium cut flat file, and some 180 grit abrasive cloth, file and sand the tapered surfaces of the fin equally on each side until the fin root is seated tightly in the fin box, but is still slightly "proud of" (the flat on the fin root is slightly above) the bottom surface of the board. Now when you tighten up your fin screw, the screw will "draw" the fin root tapers in to tight solid contact with the tapers in the fin box. If you "draw" the fin root in flush, with the screw, you will find the fin will not "fall out" on it's own if you remove the fin screw. You will have to "bump" the fin with your hand, or a soft rubber hammer to get it to come out of the fin box due to the slight "compression" fit (due to the "draw") between the fin root surfaces and the fin box surfaces. Now you have a properly "fit" fin. The screw will never loosen or break, as it's only there to initiate the "compression fit" and ensure that the fin doesn't back out or loosen up if you hit something lightly. If you run the fin aground, or run it into a rock, the screw may break, or the insert may pull out of the fin, but either way you do not end up damaging the fin box in the board. If you can find the fin, just refit, put in a new barrel nut if required, and you are good to go. Yes, "the Manufacturer" could have (should have) made sure the original equipment fin supplied with the board had the proper fit and correct amount of draw, but fitting tapers has always been something of an art, and manufacturing tolerances rarely result in "perfect fits" all the time. Your aftermarket fins were your responsibility to "fit" into your board. My guess is you need to take a flashlight and look for little bars or casting imperfections up in the top of your fin box. If you get a set of swiss pattern files, or perhaps a tiny "burr" on a Dremel tool, you can clean out the imperfections, or file/grind back the little bars so your fin can be fit correctly. Your fins probably have never been tight in the box, and were dependent on the single fin screw to hold them in. That's simply not right. Gotta fit those tapers so the tapered surfaces take all the side load from your fins. The screw is only there to draw them into the box properly and see that your fin doesn't fall out if you strike something lightly.

.... you need to put a rubber washer under the fin screw washer to "seal" the fin screw hole in your board when the fin screw it tightened up. If there's a way for air to be sucked down to your fin, this can result in "spin out" for sure. The rubber washer will also "bind" slightly on the washer and the counterbore in the board, ensuring that your fin screw doesn't loosen up.

    You just have to learn to fit them correctly, and you will never again "lose a fin". You might break it off at the root, if you sail them too long and the little white stress cracks at the junction of the fin blade and fin root start to show up, but you'll never have a fin "fall out" of the box. I've sailed both Powerbox and Tuttle fins in my boards (where the fits are checked, shimmed, and the draw is right) when I've either taken the screw(s) out or forgotten to put them in in the first place, and the fin stayed in the board. I don't recommend this practice (I felt pretty dumb when I discovered I was sailing with no fin screw) but it illustrates how tightly a fin needs to be fit, and when it's done correctly the screw is only there as a safety back up.

(19.01.2001)

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