I want to give a full open
contribution about a issue that, after some years from their appearance, is still unclear; this isssue regards cut-outs devices and will be treat with a close analysis of windsurfing board behaviour and using detailed schematic (in scale) drawings.
Following analysis is based on accurate surveys crossed with personal engineering / scientific background; windsurfing board shown is a generic free-formula board (270 cm lenght x 87cm width with 61 cm fin) with a 8 m. sail.
First of all we should distinguish between: a )
a sub-planing (full displacement) and b )
a high-speed planing condition where a complete windsurfig system (board, rig and rider) assumes different peculiarities.
As visible in Fig. 1
, at low sub-planing speeds board bottom surface in contact with water (wetted area), is larger than same area in planing condition, of a size of approximatively three times.
To develop a more efficient windsurfing board in terms of hydrodynamic efficience, one of ways followed by manufacturers has been to reduce wetted area as shown in Fig. 1
This can be achieved using a so called "cut-out device" that has taken geometrical forms different from manufacturer to manufacturer: thin cut-outs are used by Starboard while deeper cut-outs are used by F2 or Fanatic; anyway, that is the simpler way to reduce drag resistance and increase speed performances, above all when board is running free from wind.
Just in this running (free from wind), I think F2 or Fanatic approach should be more efficient than Starboard but this involes some problem that makes Starboard solution preferable (above all in low wind condition), as we can see further on.
A closer view of board reveals interesting details about board dynamics in sub-planing (full displacement) condition (Fig. 2
and Fig. 3
shows a detailed view of board tail: it is clearly visible that in full displacement running, thin cut-outs are in contact with water flow and contribute to increase overall board wetted area while deeper cut-outs (Fig. 3a
), as used in F2 and Fanatic Formula boards, do not allow a contact between board bottom surface and water flow, so reducing overall wetted area (and early planing qualities in low-wind condition).
On the contrary, in planing hi-speed condition, when board finally rises on tail, wetted area is naturally reduced of three times, as seen above.
and Fig. 5
show what happened in this condition (hi-speed): with a wetted area strictly limited by board contact surface with water flow (Fig. 5
), cut-outs (Starboard like) top surface are now finally freed from water allowing higher performances (higher top speed) when board is running free from wind (compared to board without cut-outs).
Horizontal plates placed on cut-outs, as introduced by F2 and Exocet in last season board (FX-100 iv and Warp 2006), in my opinion are the attemp to gain a better board control on longitudinal plane but, substantially, even a larger wetted surface on board with good qualities in mid to hi wind condition but not so good in terms of early planing.
So, returning to starting question: Cutouts - fact or fashion?
, what I feel to answer is that this is a good solution for increase speed performances in particular conditions (i.e. when board is running free from wind), but that they have to be carefully designed if one has to do with "early planing" questions too.
In other words (a issue as old as the hills) I think it's really difficult to combine different qualities in a board: for this reason Exocet approach in tail design of new Warp 2007 (see image shown at the beginning of this thread) is really interesting because it seems to combine both deep and thin cut-outs positive characteristics.