Spin-out

 

The factor causing spin-out is the peak of low pressure at the leading edge, which can initiate separation of the water flow from the fin surface and a sudden “loss of grip” .

 

Separation will happen at a certain “spin-out pressure”, which depends of a number of factors. However, the presence of AIR in the water is usually killing, as air can expand at low pressure and form bubbles immediately. And... air is always present, especially in the top-layer of (choppy) water, and it can also be sucked down by the low pressure of the fin itself, e.g. even through the bolts of the fin-box (if not sealed properly by rings).

 

Until now, the only solution to avoid spin-out has been to use a larger fin, such that the low pressure extreme is reduced.

 

Smartfins however generate less sharper peaks in the pressure distribution, as shown in the graphs. Therefore, spin-out will not occur, despite a smaller fin-size, even if you really step full weight on it !

 

The absolute load limit is reached at vacuum (pressure 0 bar). At that moment a spin-out will occur, in practice probably a bit earlier. The theoretical limit can be computed accurately and is an indication for the sensitiveness of fins for spin-out. In the graph on the left below shows the theoretical “spin-out” load for a SmartFin of 32 cm and a conventional slalom fin of 37 cm at variable speed. Both fins have been designed for the speed range up to 60 km/hour, typical for slalom races. The graph shows that, although the SmartFin is 5 cm shorter, it can take the same load at low speed, but at high speed it can develop much more force.

 

Interesting is also that at extreme speed over 70 km/hour the load curve will decrease and even fully collapses at 90-95 km/hour (ca 50 knots). This speed limit also occurs for smaller fins and is the reason why the speed record of 50 knots can only be reached by special fins. A speed-fin with an asymmetrical profile is less sensitive to spin-out, but it is difficult to sail back up-wind for a new speed run down-wind. SmartFins do not have this set-back; a special speed-version of Smartfins is under development.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Efficiency

 

The graph on the right shows the efficiency at variable speed for fin loads of 15 kg and 30 kg; the efficiency is expressed in the proportion of side load and drag (ratio lift/drag). The computed drag comprises 2 components: the friction at the surface of the fin and the induced drag, which is caused by the tip vortex. The latter is directly related to the proportions of length and width of the fin (aspect ratio).

 

Up to a certain speed (left of the green dots) the efficiency of the 32 cm SmartFin (red curves) is about the same as a standard fin of 37 cm (blue curves). Once a certain speed is exceeded (right hand side of the green dots) the efficiency of SmartFins becomes much higher due to the fact that the drag in the water is smaller. High efficiency ratio Lift/Drag simply means that you will go faster or will sail higher up-wind.

 

If more force is put on the fin in wind gust, the speed will increase, if the force does not exceed the spin-out limit. At speed below 30 km/hour the board will accelerate, despite the fact that the efficiency reduces; e.g. the blue curve at 25 km/hour and 30 kg load is below the curve for 15 kg load, therefore the efficiency of the fin is lower, but the speed will increase if you can put more pressure on the fin.

 

However, at speed higher than 30 km/hour an increase of load (15 to 30 kg) also results in higher efficiency; on the water this gives extra acceleration in wind gusts, and it feels like a “turbo-effect”.

As the efficiency of SmartFins increases more than a standard fin (red curves are above the blue curves) the effect is also much stronger with SmartFins.

 

The pressure that a surfer can put on the fin depends of his weight and surfing technique. On average this will be about 30 kg. The fins in the above example can produce this pressure if speed is higher than 25 km/h.

 

In typical slalom conditions at about 55 km/hour and fin load of about 30 kg the efficiency ratio of a standard fin of 37 cm is about 15; the efficiency of SmartFins 32 cm at that speed is is still about 17, which means that the drag of SmartFins is 12% smaller. The speed can increase by 5 km/hour before drag is the same as the 37 cm fin. The board and the sail also determine to what extent this potential advantage can be transferred into speed - in practice the advantage is between 3 and 4 km/hour.

 

The right fin length depends on wind speed, and on your weight and skills.

 

 

Influence of wind speed

 

In lighter wind conditions a larger fin is preferred; the curves will shift to the left for larger fins, which means that performance at low speed increases, but decreases at high speeds. The right fin length therefore always should balance the qualities at low speed and high speed. Less wind would mean a larger sail area and lower speed, therefore a larger fin will perform better.

 

 

Influence of weight

 

The heavier you are, the more pressure you can put on the fin. As a consequence, the curves will shift to the right. The efficiency increases at high speed, but reduces at low speed; therefore a larger fin suits better for a heavier surfer in the same conditions.

 

A new Smartfins of 37 cm is under development, which will enable heavier surfers to take full advantage of the fantastic qualities of SmartFins, not only in strong wind but then also in lighter wind conditions and larger sail area.

 

                                                                                                       

 

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