Friday, August 17, 2007

On raking fins


A while back, I wrote about race prep and raking some fins forward a bit. Michael (of the excellent Peconic Puffin) asked in a comment:

"When you tilt the fin in the box, do you worry at all about the fin base on the "shallow" side (the front, in this case) leaving space for air bubbles to catch and induce spinout?"

So I thought I'd clarify a bit...
Tuttle box/base systems control the rake of the fin, as well as how deeply it goes into the box, via the taper at the front and back of the base. Luckily, converting a fin from a +6 to a +8 rake only involves a very minor 'tilt'. The +x cm notation, btw, is the somewhat arcane way in which Deboichet measure fin rake - it's forward relative to the 'original' rake of their fins from a few years back, measured at the thickest part of the foil 68 cm from the base; that measurement probably originated with sailors playing with fin rake and looking for a way to quantify those experiments. But I digress...

So you're only looking for a wee bit of change in the rake of the fin (large changes, like taking an original R13, say, and making it into a +8, would be better accomplished by getting the fin repotted completely). In my case, I played with rake by putting a bit of a shim in the top rear part of the box before tightening the fin. That resulted in a bit of rake forward, and I tested different settings. Once I found one, I wanted to fit the fin base flush in the box - neither because of the less than 1/4" that was sticking out the back (as that drag would be fairly negligible), nor the potential for air getting sucked down through any cavity (as the washers on the fin bolts tend to keep things pretty tight), but because anytime you have a sloppy fit inside a Tuttle box, you can generate all kinds of point-loading, which will eventually compromise the box.

The pivot point being the front top edge of the fin base, you are then faced with needing to file down the corner of the base diametrically opposed to that, and to fill in the other two corners. Filing down is easy; filling in isn't rocket science either, though. All it takes is a bit of epoxy and some four-ounce glass. For this project, it took four layers of glass to build it up enough at the corners, and then about twenty minutes of sanding with the palm sander to get the tapers all nice and straight and smooth.

Now you may ask why go through all that work - it's a bunch of work, there's some risk you might damage the blade as you do it. Proven fast race fins have a good resale market, so I could have just sold mine and gotten new ones that were already potted to the precise angle I wanted. The answer is that fins that come out of a mold in wet layup construction are not super-consistent. You can take two 'identical' Deboichets, and you'll likely find some difference in testing on the water. So getting rid of a blade you know works and taking the risk that the replacement won't work as well wasn't something I was prepared to do. Of course, the answer to that is higher-precision fin construction with a highly repeatable, low tolerance manufacturing process. Dave Lassila of Finworks has been working on just such a Formula fin, and I'll be writing about that soon. Suffice it to say, he's really onto something.