Select Hydrofoils

THE FIN: What you need to know

A fin is a combination of a contour, a profile, materials and dynamic characteristics such as the twist and the flex. Each of these parameters influences performance and determines the type of use.

• the contour
• the profile
• the materials
• the twist and flex

The Contour

A straight narrow elongated fin transfers maximum surface power just like a more compact fin. The greater the elongation the more power is developed. The power or lift effect is created by the profile extending throughout the fin's length - a 500 cm2 fin of this shape (draw diagram) is immeasurably more efficient (draw diagram); the leading edge is part of the contour - the straighter it is the greater the directional stability and the more stable the trim of the float for generating the highest accelerations, holding a good course and reaching maximum speeds! A curved leading edge (draw diagram) makes the float faster and more manageable - the float is less stable in terms of direction but more fun in rough conditions and when manoeuvring.

The Profile

The key element! This should be selected with a great deal of care - without a high-quality profile, good fins cannot be produced! In the 1980s the good old NACA catalogue was part of the bible for fin manufacturers - the NACA 009 was used in every situation! Between 1990 and 95 the baton was passed to the 63 010 and 6410 and real gains in performance were achieved. For the last 8 years at Select we have been producing our own profiles based on the work of EPPLER and TOM SPEER, 2 eminent experts in aerodynamics, to achieve our best products.

Why not use the raw data of these 2 sizes?
It is both simple and complex at the same time. Our fins change in the turbulent environment between water and air, and variations in the trim of the float are transmitted directly to the fin in a liquid that is between 800 and 1000 times denser than air (as a function of temperature) and is particularly incompressible!
It is precisely the incompressibility of the fluid which causes us to redefine many parameters for these profiles! - radius of the leading edge, main beam, trailing edge!

The Materials

There are a number of these: the first materials used in the 80s to make high-performance fins were of laminated polyester. This product satisfied the specifications of that period when robustness prevailed over hydrodynamic qualities. The hydrodynamic know-how of manufacturers at that time was rather basic and making sturdy fins was a big step forward compared with the thermoplastic fins with which floats were fitted during those years!
Nowadays those fins have more or less disappeared. The work done on fins is much more significant. Above 32 cm they become really fragile - only some 'wave-type' and free-style fins can still be made from these materials.

The 2nd material known to the world of industrial laminates, the G10 (plate used for making printed circuits and other items in electrical engineering), came to the rescue of polyester. We manufactured our first fin from g10 in 1989 and until 1995 this was the best material for making fins that were both robust and high-performing. Up to 44 cm fins can take quite a lot of stress without weakening significantly! Then came developments to the g10, g11 and the carbon g11. By the end of 1996 we had reached the limits of this material and machined manufacture!

The 3rd material is in fact more of a production method than a simple monolithic material! This is the moulding technique using modern technology:
Hot 'prepreg' compression reserved hitherto for aeronautics!

This was a revolution as the availability of materials was pretty extensive - carbon, Kevlar, glass fibre offering us enormous development opportunities. The only major constraint was the production of metal moulds that can withstand the high pressures and temperatures. This was a technological breakthrough which today enables us to design and constantly improve our products.

This technological leap forward has transformed our perception of a what a good fin should be - we could make fins more flexible or rigid without altering the geometry thanks to the mould which we could fill according to the desired objective (the monolithic g10 restricted this approach - the only realistic way of obtaining more or less rigidity was to alter the profile by machining it). Hence the best products in 2007 are moulded products! Moulded but respecting the true principles proper to good fins, respecting the hydrodynamic specifications and respecting the geometry determined by R&D ! And by respecting safety!!!

The Twist and Flex

Two rather barbarous words for the uninitiated. At Select we should make our excuses for not having taken the time to explain these oh-so-important terms and which summarize within them the qualities of the fin!

Like a sail, a fin must have dynamic qualities. Making fins that are much too rigid is simplicity itself but making sensitive, robust fins that can transmit power for unlimited accelerations is a long-term undertaking. We realized that the famous twist was crucial for accelerating and obtaining high speeds - a degree of twist is tolerable and must be as precise as possible to obtain the best performance (see the tables on the influence of the twist).

The flex (or fin flexion) is the ability of the fin to flex under stress. It's a sort of damping effect, as the flexing of the fin absorbs the variations experienced by the float - the change in transverse and longitudinal trim. The larger the fins the more precise should be the flex. A harmonious flexion curve generates vertical components of force which free the underside: the sense of speed increases and the float glides easily over the surface - a balance between these two values of flex and twist is essential. Many thanks to moulding technology for having shown us the way forward!

Flex (above) and Twist (below) control system - click pictures to enlarge.