Author Topic: Fins and Flow  (Read 4287 times)

Beasho

  • Cortez Bank Status
  • *****
  • Posts: 3224
    • View Profile
Fins and Flow
« on: November 30, 2016, 02:59:09 PM »
Let's Geek out on Fins:  This video does more to explain how fins interact with the wave than anything I've yet discovered. 

I took the pleasure of edting this down to just the fins (no flying).  I can't stop watching this.

https://www.youtube.com/watch?v=C3z01EF80RE
« Last Edit: November 30, 2016, 03:40:09 PM by Beasho »

Beasho

  • Cortez Bank Status
  • *****
  • Posts: 3224
    • View Profile
Re: Fins and Flow
« Reply #1 on: November 30, 2016, 02:59:27 PM »
Nothing man-made, or in nature, boasts two opposing airfoils other than the bottom of a surfboard. 

This schematic found on Swaylocks does the best job of showing the flow vectors.  Most importantly the 'Stagnation Line' depicts how one fin is eliminated from the flow.  The video above also shows how the down wave fin (going down the line) or upwave fin (on a cutback) is totally compromised. 

Stretch referred to this as 'Deprecated.'  If you watch the video closely you will see the (se) fins spitting and fussing, largely doing nothing but cavitating and slowing things down.
« Last Edit: November 30, 2016, 03:04:52 PM by Beasho »

Beasho

  • Cortez Bank Status
  • *****
  • Posts: 3224
    • View Profile
Re: Fins and Flow
« Reply #2 on: November 30, 2016, 02:59:39 PM »
I am in search of speed.  The challenge is that board designers keep putting 'foiled' or cambered fins on the bottom of the boards.

It is a universally accepted axiom that foiled fins are SLOW and generally used on airplanes like Cessnas to prevent stalling at low speeds.  A symmetric foil (teardrop shaped) is fast and generates lift using angle of attack rather than circulation generated by an asymmetric flow around a cambered fin. 

I had been experimenting with using ALL symmetric fins with NO Toe, or Cant.  I was well into my testing using a TRACE GPS and measuring every wave speed using a variety of fins.  I was pushing towards smaller and smaller fins, essentially getting to the smallest fins I could use at speed that would barely prevent my board from spinning out.  I actually had one fin snap out of the box and then a few days later my board snapped in half.  So much for testing.

The latest theory, if I want to go fast, could be to use a fin more like the Supersonic fin below which would essentially be a waterski fin on the bottom of the board.  Kid tested, mother approved. 
« Last Edit: November 30, 2016, 03:34:14 PM by Beasho »

Beasho

  • Cortez Bank Status
  • *****
  • Posts: 3224
    • View Profile
Re: Fins and Flow
« Reply #3 on: November 30, 2016, 03:19:46 PM »
At exactly 2:34 you will see his fin briefly stall. 

At this point the only fin fully engaged is likely his toe-side Starboard fin.  Because of the Toe-in the Starboard fin has a DECREASED angle of attack compared to the Center Fin while the heel-side fin is in a complete flow-separation mode. 

This decreased angle of attack, combined with the 'Slow Speed' foil on the thruster, helps to ensure that the thruster fin will be the LAST fin to stall and lift is maintained against the force of gravity that would otherwise cause him to tailslide into the barrel.  Thus with just 1 of 3 fins fully engaged he is able to track and elegantly continue down the tube.  Awesome  8)

Click here:
https://youtu.be/C3z01EF80RE?t=150
« Last Edit: November 30, 2016, 04:06:34 PM by Beasho »

addapost

  • Peahi Status
  • *****
  • Posts: 606
    • View Profile
Re: Fins and Flow
« Reply #4 on: November 30, 2016, 04:44:07 PM »
I have absolutely no clue what you are talking about but that video was incredible, thanks!  :)
Bunch of old shit

LaPerouseBay

  • Teahupoo Status
  • ******
  • Posts: 1969
  • downwind dilettante
    • View Profile
    • Email
Re: Fins and Flow
« Reply #5 on: November 30, 2016, 07:36:36 PM »
Wow, that was great.

Shipstern's is scary.  Those guys are the definition of hellmen. 

That wave that pops up out of the bottom reminds me of an alien creature about to devour something.
Support your local shaper

lopezwill

  • Peahi Status
  • *****
  • Posts: 535
    • View Profile
Re: Fins and Flow
« Reply #6 on: November 30, 2016, 07:39:57 PM »

Beasho

  • Cortez Bank Status
  • *****
  • Posts: 3224
    • View Profile
Re: Fins and Flow
« Reply #7 on: November 30, 2016, 07:47:39 PM »
Bad-Ass!  I love the little wheelie stall maneuvre that they apparently used for the thumbnail.
No fins, all rail its part of the theory and worth embedding:
https://www.youtube.com/watch?v=QZpaFM16_vU

PonoBill

  • Cortez Bank Status
  • *****
  • Posts: 25864
    • View Profile
Re: Fins and Flow
« Reply #8 on: December 02, 2016, 10:08:29 AM »
If you knew more precisely how a fin would be used, and under what conditions it would be much easier to design them. For example, if all you are looking to do is go right as fast as you can, then a minimal asymmetric foil on the right side, thin symmetric center fin, thin symmetric left thruster would work fine. Decide to go left or do a cutback and it would be unstable and sticky at the same time.

The "stagnation" line seems poorly named. Any water on that side is hardly stagnant, but it's highly turbulent and often thin.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

Beasho

  • Cortez Bank Status
  • *****
  • Posts: 3224
    • View Profile
Re: Houston We Have a Problem
« Reply #9 on: March 29, 2017, 02:35:01 PM »
Houston we have a problem.

A friend observed that the sum of the width of your Quad fins in your board must be significant.  So I took the liberty of stacking them together.

Here you can see the width of 1.1 inches (or ~ 28 mm) for a set of Controllers.  Considering that a 2 X 4 would be 1.5 inches wide this is almost like plowing through the ocean with a 2 X 4 sticking down underneath.  Add to this the additional surface area, multiple faces, 4 Fin Tips and a streamlined 2 X 4 might offer less resistance.

Aforementioned water-ski fins, utilizing 2 mm aluminum plate, would reduce the frontal cross section by 72% which may be an alternative to reduce drag.
« Last Edit: March 29, 2017, 02:37:43 PM by Beasho »

PonoBill

  • Cortez Bank Status
  • *****
  • Posts: 25864
    • View Profile
Re: Fins and Flow
« Reply #10 on: March 29, 2017, 04:11:16 PM »
Controllers are lined up more or less with one fin behind the other, so there's some benefit. But yes, fins are draggy beasts. People rubbing magic goo on the bottom of their board while they have a fin twice as big and twice as thick as they need should make sure they chant as they rub.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

JEG

  • Teahupoo Status
  • ******
  • Posts: 1016
    • View Profile
    • Email
Re: Fins and Flow
« Reply #11 on: March 29, 2017, 07:39:35 PM »
tis interesting topic though my head  ???

 


SimplePortal 2.3.7 © 2008-2024, SimplePortal