Fin Design

Hello all, I need some advice. I’m doing a science project on the shape of surfboard fins in relation to the drag generated. I need to choose on variable to change in the design of the fins (Rake, Hieght, Base length, Thickness, ect.), but this variable needs to affect the drag of the board so that I can get results. So what so you guys think I should do, what single variable greatest affects the drag of the board?

the longer the base of the fin, the more turbulance is created.  That increases drag.  Keel fins have a lot of base.  Some builders are now putting cutaways on the trailing edge of keel fins to shorten up the base of the fin and hopefully reduce turbulenece generated at the base of the fin, without dramitacally changing the template of the fin.  Fin height will also increase drag. There are tons of threads about fins and drag that get pretty involved in the archives.  Many of them talk all about the different kinds of drag and lift that fins generate. Hopefully people with more knowledge on this subject will chime in.  Good luck with your project!

You didn’t list it but the angle for the toe of the fin leads to huge differences in drag

It’s a tough issue to sort out. For one thing, quantitating ‘drag’ in a test tank vs on a surfboard in actual use will likely reveal conflicting data.

With that in mind, my vote is with toe-in - especially when multiple fins are being tested simultaneously.

If evaluating a single fin, I’d have to say that thickness foil is the most likely source of drag. Again, simple test tank vs actual use will not necessarily equate… a thinner foil will create less drag in a test tank but in actual use might eventually lead to tracking if the fin is too thin. A fatter foil might offer preferable performance in actual use in spite of increased drag. Depending on materials used, a ‘too thin’ fin might also have real-use issues regarding rigidity. I.E. it might have low drag in a test tank but be too floppy in actual use.

surface area of the fin will affect drag
Thickness of the fin, will affect drag
When you are riding with an angle of attack on the fin (every time the fin is beeing usefull) the most important factor is the apstec ratio.
aspect ratio = Hieght / lenght
Lower aspect ratio = more drag

 Ok... In regard to the surfboard fin aspect ratio idea. So my wet noodle can grasp it. Lets say one board has a large single fin. The fin is roughly 5 inches base length and the height is about 12 inches. 12 divided by 5 = 2.4. 
 Then we have a tri fin board with all 3 fins the same size. The fins are 4 inch base length and about 6 inches in height. That would be a 1.5 ratio. 
  The 3 small fins have the lower aspect ratio and therefore the higher amount of drag.

Without a way to watch the flow patterns around the fin and in it’s wake a measure of guesstimating is always present.
Fins have many aspects. So many that simple numerical ratios are misleading.
NACA foil studies go on and on with drag charts and flow coefficient variations.
The crossover between fluid dynamics in air and water serve to further confuse the process of understanding.
Beyond performance of the fin configurations there is also flex modulus and where it occurs on the fin.
So If you feel hopeless about sorting out fin performance your not alone; we all are.

However there are some simple things that affect performance.
Wash, that is disengagement, can serve to help directional change, and to many surfers is imperative to performance.
Engagement equals drive but fin can easily overcome or fail board performance.
So the beginning for me is understanding how template and foil variations affect performance,
and for multi-fin boards how fin placement, toe-in, & cant change it.
And this is just the very tip of the iceberg.

Stay Stoked, Rich