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Old 23rd February 2007, 03:59 PM   #1
utthita
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Default use of weed fins on evo and acid

The initial ramp and sweep of a weed fin, even those that are not that radical has the effect of moving the fin center of hydrodynamic pressure back by 3 or more centimeters.

Small wave fins with sweep when powered up do okay at Punta San Carlos. but, otherwise some days are difficult. (Otherwise: bigger board and fin, less powered up, etc)

Evo's come with short fin boxes allowing only a bit of ajustment and the Drake fins have very little rake at the base, suggesting that the board characteristics would change a lot when using a weed fin. I perceive a big difference and so have made my own weed fin that does not shift the center of pressure so far back.

One trick is to reverse the two attachment points of the fin, so that the tab does not prevent one from moving the fin further forward. This requires some filing and drilling.

Powerbox weed fins come with an offset that compensates.

How should the shifting of the fin so far back affect the handling of an Evo 83, say?
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Old 25th February 2007, 04:19 AM   #2
Ola_H
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Default RE: use of weed fins on evo and acid

I never use weed fins, so I can't really comment on that issue. Neither did I try what happens if you move a normal fin all they way back to try to emulate the weed fin feel. Generally your board gets a bit more directional when you move the fin back, but if I go too far back on an EVO, I don't think I get the balance I want, so I never do that.

Your "reverse mounting" trick sounds like it must help.

The expert on weed type wave fins must be Wardog who post here sometimes. You might try to page him at the Hot Sails Maui forum too (haven't seen him here for a while, as far as I can remember).
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Old 25th February 2007, 09:32 AM   #3
steveC
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Default RE: use of weed fins on evo and acid

Hi utthita,

While I can't comment specifically about Evos and what works and doesn't with them in particular, I can say that there is an older True Ames wave fin that does quite well in the weeds and kelp without the normal 40-45 degree rake associated with classic weedfins. The great thing about the fin is that it has a remarkably shallow ramp (quite viable in a US Box), yet still has a narrow chord because of the notch at the base of the fin. The fin is a special order fin called the "Wave Ramp", and it's manufactured in G10 material, so its quite tough but very precise in shape and flex, and it's clearly a wave design. This is the fin that I normally use for waves in really tough kelp beds, and its remarkable how well it sheds weeds and kelp, yet its still quite loose and maneuverable in nature.

Although other newer designs exhibiting a longer lower aspect keel-type base are certainly available, I wouldn't discount this older concept, as the ramp is very minimal in nature and it might better suit your idea of the right design concept. Unquestionably, its a personal call.
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Old 28th February 2007, 01:56 PM   #4
Lessacher
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Default RE: use of weed fins on evo and acid

I worked for such boards the weedfin Chamäleon 24cm in US box.
I worked something, that I can give the fin 3-5cm forward the box.
Where the box in front stops, the fin looks 3-5cm more in front.
So I have the same feeling how with a wavefin. The 24 cm can hold
over 7m².The screw is not in front, she is at the end of the fin.
Wolfgang
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Old 28th February 2007, 09:01 PM   #5
Roger
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Default RE: use of weed fins on evo and acid

Hi Utthiita,
While I'm sure there is some "effect" on the feel of the board with a weed fin, I think for smaller boards, like the Evo, there are some "options".
Other than the OEM Drake fins, many wave and surf type fins already have significant ramps to them, and with the quite curvy and swept back planforms, I'm not sure that the COR (Center Of Resistance) is
moved as far back as you suggest.
Many wave and B&J fins have alot of their chord in the "swept back" lower portion of the fin, and these fins are thinned out and have significant bend or twist similar to the "flexy foil" surfboard fins.
Wolfgang's Duo fins shed weeds (eel grass and similar "leafy or round stock small diameter weeds) by a different principle than simply using the Leading Edge (LE) angle. Since Wolfgang's fins are "assymetrical" the weeds "flutter off" the concave side of the fin.
You might be able to get away with a much steeper LE angle and use the assymetry to shed your weeds.
This won't work with larger diameter "weeds" like kelp stalks, but as with all "weed fins" you need to match the fin style (and it's operating principles) to the types of weeds you are actually encountering.
A number of fin producers make "surf weed" designs, and it's pretty easy to put the screw at the back of the fin vs in the front for this type of fin.
Biggest problem with the Evo's is the very short fin box, that does not allow much additional chord length (at the base of the fin) beyond the OEM supplied fin.
Since you are making your own fins, you could easily extend the ramp
beyond the front of the fin box (this will require some close tolerancing between the overhang and the pin, but it can be done and you would getdecent gasket compression on the overhang gasket, or you could "fill and fit" to your particular board). 3 cm (1.18") is not too much of an "overhang" to deal with.
I'm not familiar with what sort of weeds you get at San Carlos, but if they aren't of the thin flat "eel grass" variety, you may get away without a ramp and perhaps with a bit steeper LE angle.
Yes, it will effect the "feel" of an Evo, but in many cases the use of a weed fin with a significantly longer chord and alot more "chordal twist" (like the flexy foil surfboard fins) it may make your board feel better and stick better in your bottom turns. So, the "change" in the feel could be a positive rather than a negtative.
Hope this helps,
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