Why do we need so much fin area for multi- fin wave boards ?
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Discussed is how much fin area is needed for multi-fin boards compared to a single fin board.
Multi-fin boards have been developed by utilizing shorter fins than one single fin in order to improve agility for wave riding.
The required fin area can be illustrated by a simple sketch showing different multi fin configurations from the rear in a tight turn- example the graph of 45 degrees or more of bank angle.
For different fin configurations some of the fins are partially or completely out of the water and therefore not performing under those conditions. That means fin area is reduced in a tight turn.
In order to be stable and to have no spin out the remaining fin area must be equal to the single fin area.
The result is that the total fin area must exceed the single fin area.
If we set the single fin area to one it can be illustrated how much fin area must be provided by assuming that the remaining fins in the water must provide the same fin area as a single fin.
The results are :
Single fin area = 1
Twin fin area = 1,33 (33% more area than single fin configuration)
Tri-fin area = 1,33 -1,5 (33% to 50% more area than single fin configuration)
Quad-fin area = 2,0 (100% more area than single fin configuration)
Twin-Quad fin area = 1,66 (66% more area than single fin configuration)
As an example : assuming a single wave fin of 22 cm with 200 sq cm is adequate for a single fin wave board then :
the Twin fin board should have an area of 266 cm sq cm
the Tri-fin board 266 - 300
the Quad fin board 400
the Twin-Quad board 332
Twin-Quad = Twin fin board with thruster
We should have in mind that 400 sq cm fin area is identical to a 48 cm slalom fin!!
That means that there is twice as much fin area installed in a Quad fin board than necessary in order to improve the turning response.
The additional fin area of multi-fin boards help to improve upwind performance but provide more drag compared to a single fin configuration. Further flow interaction between short distant fins reduces their efficiencies.
Further there is a mode switching, in tight turns some fins are ‘out’ while at normal sailing all fins are ‘in’.
Source : U-Ride