Flex and twist

A more detailed explanation is as follows. Fins will flex and twist when dynamic load peaks occur, e.g. in choppy conditions. Flex will dampen wave motions and it helps to smoothen the loads, which contributes to comfortable sailing and it also reduces the chances of spin-out.

Smartfins are shorter and have less flex and twist than standard fins, but this is compensated by hydro-dynamics. The graph shows the lift force of a standard fin and for Smartfins for variable drift angle (or angle of attack aoa). The curves are typical for a Smartfin of 32cm and a standard fin of 38cm at a speed of 50 km/hour.

A standard fin will not generate side force at an angle of attack of zero degrees (the blue line goes through the origin of the axes), whereas Smartfins does (about 15 kg). The slope of the curve is directly related to the fin area, therefore the curve of the 38cm fin is about 30% steeper than the 32cm Smartfin.

At a constant speed in choppy conditions the aoa will not be constant, but it will vary in a range as indicated by the arrows (typically 0.5-1 deg) because of the transverse water movements of the waves. For a fully rigid fin sailing in a straight course this would result in load changes according to the slope of the curves. The fin will absorb some of the load changes by flex and twist. Smartfins have less flex but dynamic load variations are smaller.

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