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Fin Depth : Length Relationship Question


rimodico
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Am I looking this the wong way?... For a 66" Radar Vapor (or any ski for that matter) there are two stock fin settings below. How can a shorter fin be deeper, and a longer fin be shallower?

 

Beign the fin a ~semi-circle, as you place it deeper the length will increase as a result.

 

 

Long & Shallow:

*Fin Depth: 2.450

*Fin Length: 6.975

 

Deep & Short

*Fin Depth: 2.510

*Fin Length: 6.840

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Everyone wants this to be complicated but it is not.

 

It is just about surface area. You can have a long fin that is shallow or a short fin that is deep and have the same total square inches of fin in the water. The outcomes are not exactly the same but primary feel of the ski will be similar-ish.

 

Long shallow results in a faster ski that has more automatic on side turns and more slide at Off Side. Short deep settings slow the ski down a bit and give a more carving off side turn. ( these are broad generalizations - this is the super short version)

 Goode HO Syndicate   KD Skis ★ MasterCraft ★ PerfSki  

Radar ★ Reflex ★ S Lines ★ Stokes

Drop a dime in the can

 

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Opps I may have misunderstood the question. The fin blade can move in all directions (2 dimensions) Pull the back down to make it deeper. Pull the front down to make it longer. Or anywhere in between - it is infinite.

 Goode HO Syndicate   KD Skis ★ MasterCraft ★ PerfSki  

Radar ★ Reflex ★ S Lines ★ Stokes

Drop a dime in the can

 

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@Horton 95% sure you've still misunderstood the question. I had the same confusion as @rimodico before the first time I ever actually adjusted a fin, because I made some (incorrect) assumptions about the shape of a fin.

 

The answer is: The fin is NOT approximately a semi-circle. One side is dramatically steeper than the other, so as you rotate it the exposed length can change independently from the depth.

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61vYLZ05E8L._SX425_.jpg

 

 

With the key being that length is shortened by bringing the forwards edge of the fin up. While overall fin depth is made by lowering the rear while raising the front.

 

That of course becomes the unique aspect of the whisper fin where Tip Adjustment is not recommended.

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I want some math whiz (@than bogan?) to come up with a quick calc to show where the center of mass (?), center of resistance (?), center of effort (?) is on a fin given length and depth measurements and the outline of that particular fin. A long shallow fin and a short deep fin with the same area could have that point in different places. Not to mention DFT. Where is that point and which position of that point is perfect for me, my ski, my bindings, my skiing, my toughest pass?
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@jimbrake It turns out that any decent CAD tool can be co-opted into making area and c.o.m. computations for a given set of measurements, assuming that you can correctly model the fin. I'm 99% sure there is no closed-form solution for these things, and thus no hope for a simple spreadsheet. But CAD programs can use numerical computation methods that get however much precision you desire.

 

I set this up for a simple model fin a while back just to prove to myself that it conceptually would work.

 

But I think you'll find that is 100% useless toward your actual goals, stated in your last sentence!!

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I think your assumption is flawed as: a, the “center of resistance “ would never be the same place, and b, the leading edge, trailing adge, and depth all have effect on differing phases of skiing
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