Many of my fins have toe-in twist where the bases point in towards the nose and the tips run straighter up and down the board. You can see how this looks on the fin below.
The reason for the twist is it aligns your fins better to the water-flow under your board, giving you a little more speed down the line.
Images, like the one below, are the key to understanding how this works.
They are from an experiment by Harry Knight.
Harry's an epic individual, he presently works as a surf-instructor in Costa Rica and writes for Surf Simply.
In this experiment he drew black lines along the bottom of his board and then taped black and pink plastic streamers on it (tell-tales).
The tell-tales follow the water-flow and let you see what is happening. If you look closely you can see where the streamers have been pulled in a diagonal direction by the water-flow.
About two years ago, Harry sent me every image and video from this experiment and consented to me using them on my website - a great example of the shared-stoke that's the real core of surf-design.
Water-photographer Dan Mullins held his breath under the water and shot photos and videos of Harry surfing over the top.
What the images show is the water rarely runs straight under our boards.
Instead, it curls out towards the rails - especially in the area in front of your fins.
If you look closely at the images you'll see this is going on.
Essentially, the water gets pressurized by the board planing over the top.
It then wants to escape, running towards the area with the lowest pressure, near your side fins that is the rail.
Probably the easiest way to spot this is to look closely at the image above.
The side-fins on Harry's board aren't set straight, they're toed-in about 3.6 degrees.
But if you look at the black tell-tales you'll see they're running at more or less the same angle as the fins - instead of straight up and down the board.
In the early 2000s FCS spent a $1 million Commonwealth Grant and another $1 million of the company's own money testing fins in the water tunnel at the Australian Maritime College.
But as the image below, from the patent application for the fin which came out of the tests shows, all of their research assumed the water flowed straight under your board, when it was running straight down-the-line.
As Harry's experiment proves, it doesn't.
By understanding the fluid-dynamics of fins and how much lift they generate at different angles, we've been able to work out the effect of this change in angle.
And by experimenting with different fins at different toe-in angles, we've dialed into what it does.
The problem Harry's experiment highlights is the left fin is generating lift that pushes your board to the left, while the right fin is pushing against that force and pulling right.
Meanwhile you're just wanting the board to run straight as fast as possible.
This is where the toe-in twist gives you more glide and trim-speed.
CFD gives you a general indication of the forces in play.
The CFD image above was generated using Nablaflow which is software designed by a company based in Norway.
Many of the guys at Nablaflow are surfers and they've been hugely supportive of my work.
Dr Sam Crameri from Deakin University ran the experiments which tested my fins.
Sam's a surfer, an engineer and all-round legend with an impressive understanding of CFD and fluid-dynamics. He will be publishing an academic journal article on the results from our experiments together in the next few months.
The CFD gives a preliminary estimate of the forces in play.
For example, if we shoot the water flow straight at the fin we get estimates of how much lift and drag it is creating - this is close to what is going on when the board is running across the wave - as shown in Harry's pictures.
For this particular fin the amount of lift being generated, when the water is running straight at it, is just under two kilos (about 4.5 pounds).
The drag forces at low angles are tiny - only about 170 grams per fin (about 0.4 pounds).
But the two fins are generating an estimated four kilos of lift-force in opposing directions and this was only with the board moving at 5.5 metres a second (about 20km/h).
That's two kilos of force from the left fin - pulling in one direction - and two kilos from the right, pulling in the opposite direction.
This slows your board down.
As surfers we tend to love thinking about ways to reduce drag from our fins, but in reality when you're moving down the line it's really unhelpful lift that's slowing you down.
This is why there is extra toe-in at the bases of my fins but less at the tips. I am aligning them to the angle of the water-flow, reducing how much they pull your board in opposite directions.
As you move away from the board, the water flow is less and less disturbed.
Out there the water is running straighter.
So at the tips we use foils which align to the angles of your toe-in settings.
The result is fins that feel a little faster on take-off or when you're moving down the line.
If you actually watch surfing closely you'll notice it's pretty rare for the outside fin to be out of the water.
And as much as we like to think we spend all our time on big rail turns - a lot of surfing is trying to get a board through a long section.
This is where the twist helps.
The trade-off, and there is always a trade-off with fluid dynamics and design, is if you twist the base of the fins in too far the board begins to feel too loose and unstable.
And if you go way too far with it, your board begins to feel directionless, requiring you to pump and force the fins to start engaging the rails in order to get moving.
Too much twist can also mean your fins release a little earlier through turns (something that is managed by twisting the fins out and improving the foils and templates) but is still a factor we take into account.
Lastly, and probably most importantly, if you use a fin with more toe-in twist than your fins are set at then the tips end up running straight up and down the board.
Or worse pointing outwards - something you really should check before buying any twisted fin.
I am not the first fin-designer to use toe-in twist in this way.
It was pioneered by German windsurf shaper Bouke Becker in the early 2000s - you can read about how Bouke used CFD and other water-flow studies to establish this effect here.
Belgian windsurfer, surfer, engineer Hans Robeers then put me onto it about two years ago.
The twist is very different to the one on Vince Longo's Vector 3/2 fins from 2006 which actually use more toe-in and a more cambered (or curved) foil at the tip.
However, you will notice that Vince's latest twins - the Rasta Reverse 3/2 - twist the other way with less toe-in at the tips than the base.
The difference with my designs is the twist is gradual, we aren't trying to take lift out of the fin like Rasta and Vince were - we're stopping them creating lift when you don't want or need it.
Hans and Vince are both featured in my series for Swellnet where I explore the history of real fluid dynamics in fin design. You can read the history of the toe-in angle insight I am using here.
Ultimately, surfing is really only just at the start of exploring water-flow under our boards and the real fluid-dynamics of our fins.
But twisting our fins to align them to the water-flow is a big step in the direction of making better fins.
As Bob McTavish said to me when we started talking and emailing back and forth about it: "As far as I know no designers have previously come to grips with this dispersion factor."
"Maybe the outcome is ALL outside fins will be twisted from now on."
If you don't know what toe-in setting your fins are at, ask your shaper.
In my opinion this is something everyone should be told when they buy a new board and it should be written on the stringer alongside the board's dimensions.
It's probably as important, if not more important, than volume when it comes to performance - especially when compared to adding or subtracting one or two litres of foam, or taking out at 1/4" of width.
As an indication of what you're working with:
1/8" of toe-in sets your fins at about 1.6 degrees (this is standard for quad rear fins, many modern Lis-style fish and mid length twins).
3/16" of toe-in means your fins are set at about 2.4 degrees (this is standard on most classic twins).
1/4" of toe-in means your fins are set at about 3.6 degrees (this is standard for thruster and quad front-fins and many performance twins).
If the amount of twist you have in the fins is too high for your toe-in setting then the tips end up pointing too straight up and down the board, slowing it down.
This is worth taking into account, not just for my fins, but any fin that has a twist in it.
If your fins are set at 1/8" (1.6 degrees) and your fins have two degrees of twist, the tips of your fins are pointing outwards and cupping water, slowing your board down - a lot.
It is also worth taking into account when you are looking at fins that have a lot of inside concave or camber in the foil - if you set fins like that too straight they are both cupping water on that inside foil and really slowing things down.
It's quite surprising how noticeable this is in the surf - it even seems to make paddling onto waves challenging.
If you're not sure what toe-in your fins are at, reach out and I can help you work it out.