If you don't understand how foils work, you'll never understand how fins work.
But when you read the marketing spiels on most fin-brand websites, you get the impression fin performance is decided by templates.
This is because most fins are designed with no input from people who know about foils.
And the shapes they use are essentially limited to this:
And this:
The truth is because the fin-industry has become dominated by big businesses we've only really seen a fraction of what's possible.
And even this:
Is just the tiniest glimpse of the infinite number of different foil shapes that are out there; just waiting to be tried.
Each of them has their own benefits, drawbacks and sensations to offer.
Even symmetrical foils, for single and centre fins, have hundreds of variations like this:
The big brands have never been able to say it because they've always had millions of dollars worth of poorly foiled stock sitting on shelves in surf-shops and dozens of pros and shapers who want to collect a royalty.
But even within those big-brands all the people who really understand fins will tell you this:
Isn't a good foil.
Firstly, the leading edge on your fins should never be sharp.
The reason for this is the foil on your fins is designed to wrap the water-flow around the leading edge.
You can see this in basic images from our computational fluid dynamics (CFD) experiments:
If we zoom in you can see how when the water is angled towards your fin (like through a turn) some of it is redirected back towards the middle of your board, towards the stringer, while some wraps around the curved surface on the outside.
See how the flow is wrapping around the leading edge? It is also being coloured orange to show that it is accelerating - or moving faster.
From above this looks like this:
The water wraps around the leading edge - and is accelerated.
The water that collides with inside surface gets pushed back towards the centre of your board - so it runs straighter.
In 2D this looks like this:
This is what makes the inside of your fins hold an area of high pressure and the outside low pressure.
And that is how your fins help you engage your rail and initiate a turn.
It has everything to do with the foil and nothing to do with the template.
Don't misunderstand - the template has a big impact - but a great outline without a good foil is just a nice idea, a good foil makes it a great performer.
A sharp leading edge makes no sense because it means the flow stops wrapping around the foil earlier.
If the flow can't wrap smoothly, your fins start to lose lift early in your turns, get really draggy and slow your board down.
The weird part about all this is, the big fin brands have had this knowledge for more than 30 years.
That became really clear in my series for Swellnet where I talked to the top fin-designers from around the world (including the big brands).
But this is the leading edge on a set of thrusters I bought at a surf-shop for more than $150:
Ironically, that foil is on a template that's marketed as having great hold through turns.
That leading edge will mean they slip out earlier than a well-foiled fin.
Our CFD shows sharp leading edged foils can take a well-designed template from a great performer to a poor performing fin.
Next time you go to buy a set of fins, take a long look at the leading edge. Run your thumb over it or hold the fin up and look at the shape.
If the person who designed your fins knows about foils, it should be smooth and rounded - made to keep the water-flow attached as long as possible.
Like this:
Which is a close-up of the rounded leading edge on a set of our TMCs - something we do by hand using low-grit sandpaper.
If it's sharp or has a flat-spot, then ask yourself, are these good fins? Or just a cheaply made set from a big-fin brand?