There are a lot of different shapes around at the moment and no doubt every fin maker will tell you their fin is the best.

Let’s look through the basics and break it down, so we can understand ourselves the theory of what a fin does.

Different types of windsurfing require different fin characteristics. Image credit PWA/JC

Firstly the foil (the curve of the fin, similar to the shape of an aeroplane wing) has to be good at keeping the water on its surface; if the flow of water over the foil  (technically called the envelope of laminar flow) breaks then you get spin out. Bad, cheaply produced foils can cause this effect, therefore it is vital to consider how a fin is made and the accuracy of the construction.

Most fins these days are made in large facilities in China and many are still produced by hand shaping. These are not necessarily bad fins, but there can be a difference in these when compared to say a CNC (Computer Numerically Controlled) machined fin. CNC fins are more expensive but are a lot more accurate and therefore produce a better performance.

Freestyle fins are a lot smaller with very different outline and foil shapes to other types of fin. Image credit PWA/JC.

The other technology used involves a  mould being produced, the fin is constructed with a layup of material, usually carbon and glass and then pressed under pressure. Another system is where prepreg (a term for “pre-impregnated” composite fibres where a material, such as epoxy is already present) material is used in the moulding process  this contains resin and under heat this bonds the sheets of carbon and glass together. This process is then pressurised under extreme pressure to produce a very compact construction, this is used by Select Fins.

A typical CNC Machine in action.

So, other than construction, what makes a good fin?

The accuracy of the foil and the material used is vital, so is the profile or outline shape. For example the majority of slalom fins use one of two templates.

The first is a trapezoid profile this is a modern looking shape, that has been adopted by many and has the ability to pass through the water efficiently. The second shape is classed as an elliptical plan form. This is thought of as an old shape by many, but in theory is one of the most efficient water passing shapes and it resembles a Spitfire wing. There are many other factors which affect the efficiency of the fin through the water such as the foil shape, the chord width and thickness of the foil.

A elliptical plan form fin

The first is a trapezoid profile this is a modern looking shape, that has been adopted by many and has the ability to pass through the water efficiently.

The second shape is classed as an elliptical plan form. This is thought of as an old shape by many, but in theory is one of the most efficient water passing shapes and it resembles a Spitfire wing.

There are many other factors which affect the efficiency of the fin through the water such as the foil shape, the chord width and thickness of the foil.

The Fin

Definitions    

The fin and foil geometries are described as shown below:

fin parameters

foil parameters

These factors have an affect on the fin efficiency: Fin length, taper ratio, aspect ratio, fin area, relative foil thickness, and position of maximum thickness, foil nose radius and foil trailing edge angle.

Lift and Drag

How is the fin side force created?

As the fin is nothing but a small wing, we can use the same definitions as in aerodynamics. The forces created are lift and drag, which also define the efficiency of the fin. Since the fin is placed vertically and not horizontally like a wing, the lift becomes a side force. Still, the general physics of produced forces is identical to the lift production of a wing.

The lift of a wing is created by inclining the wing foil against the airflow in a specific angle of attack. The flow around the fin is not symmetric. On the upper part of the foil, the flow must pass a longer way than on the lower part. As a result, the flow has to be quicker on the upper part providing a lower pressure compared to the lower part of the wing. The pressure difference produces the lift force. Lift is always combined with drag, which depends on the foil geometry and friction of the water molecules on the surface of the fin.

Streamlines and lift on a foil

More from Dave on page two…

Hot off the press here in Korea, Micah Buzianis has topped the list after a solid fight back at the tail end of the event. It’s his first event win for a while and he looked over the moon.

The young French rider Antoine Questel made a horrible mistake, sailing to the wrong mark while leading his semi final handing Micah the opportunity to take the win. But that said, Questel is a name to watch out for as the PWA slalom season continues. He has shown, mistakes aside, that he’s got the speed and composure to top the rankings.

Micah Buzianis wins in PWA Korea. All images courtesy PWA/JC.

The best news for the UK is, that our very own Ross Williams jumped three spots in the last round and made it to third place on the podium. A great result for Ross who looked sharp all week and laid down a solid foundation to build an assault for the top of the slalom ranking this season.

Ross said: “It’s been a two year hiatus since I’ve been on the PWA slalom podium and it’s good to be back!”

PWA Korea: Ross Williams finishes third. All images courtesy PWA/JC.

Bjorn had a horrible last couple of rounds getting knocked out after hitting something yesterday and then slicing his foot open today, pushing him down to ninth place. Antoine Albeau also didn’t look his usual self and had to settle for fourth here.

The racing for myself got better and better, and by the last couple of rounds I started to at least feel in the fight. I would defiantly like to race with these guys a little more often, it’s so competitive out there on the course and I miss that. But, I have the Maui Race series to look forward to in a couple of weeks and this has been a great warm up for that. The only difference is then it will be back to racing in shorts in steady 25-35 knot trades!!

Thank you Dan for some great reporting all week. Good luck in Maui and we look forward to the next installment!

Reports from every day can be found by clicking through the numbers below…