GB2259674A - Sailboard fin - Google Patents

Abstract

A fin for a sailboard which could have a traditional shaped fin body 15 has two built-in tapered channels 32 33 to allow "cross flow" of water from side A 26 to side B 27 for channel 1 33; and from side B 27 to side A 26 for channel 2 19. A common constriction 20 inside the fin body 15 where two channels 32 33 cross creates a "venturi" effect so that the water passing through the constriction 20 is accelerated to a higher speed. A deflector lip 21 and a spreader flat surface 23 formed a sheetlike water jet 24 at the outlet 22 of the channel and is made to flow close to the fin surface. This high speed water jet 24 induces the nearby water 25 to move close to the fin 15. The engulfment of the fin surfaces by the water streams provides good water contact as the fin travels through the water. Therefore, the board directional stability is improved. <IMAGE>

Description

A "SUPER-FIN" FOR WINSEURF BOARDS This invention relates to a "super-fin" design for windsurf boards.

Fins are ed in windsurf boards for the following purposes: 1. To give the board directional stability.

2. To act as a pivoting point in turning.

The fin surface offers lateral resistance to counteract the lateral component of the aerodynamic force which is generated by airflow around the sail. Therefore, its function is to prevent the board from moving sideways due to the lateral component of the aerodynamic force and to ensure that the board forward movement is maximied.

However, there are some problems encountered in realities: 1. Due to changing wind conditions, the board rider has to constantly adjust the sail angle, 6 in order to present the proper angle of attack, , to the wind. This causes the resultant aero-dynamic force, R ,to rotate constsantly. The effect of the rotation of the resultant force, R transmitted to the board via the rider causing the tail end of the board to move sideways, in a back and forth manner.

Since the fin is attached near the tail end of the board, the sideways movement of the fin causes sideways movement of the water near the fin. With sufficient force, the fin movement could create turbulences and bubbles around the fin surfaces.

Turbulences and bubbles around the fin surfaces disrupt good water contact to the fin surfaces. The Is of good water cortot of the fin could lessen the board its directional stability.

2. In lower wind speed, a larger fin is used to give Ire lateral resistance to ensure more power is converted for the forward board movelilerlt. A larger fin surface ared makers the board more difficult to turn.

3. Most of the time, the board and, therefore, the fin does not travel parallel to the water current. Smooth water flow around the fin is disrupted as the angle of attack of the fin becones larger. This causes turbulence around the fin dis rupting the fin surface contact with the surounding water.

This results in directional instability of the board and reduces the effective forward drive of the sail.

According 'wh the present invention, a precisely calculated channel to allow "cross flow" of water fro side A of the fin to side B of the fin via the body of the fin. A similar channel is provided to allow "cross flow" of water from side B of the fin to side A of the fin.

Each channel is comprised of an intake, an input channel tapered into a constriction at the center thickness of the fin. and a tapered channel section widening to the outlet of the channel. A deflector lip and a flat spreader surface are formed at the outlet of the channel to create a sheet-like water jet stream running close to the surface of the fin. A small constriction( or na3:rowing) is provided where the two channels cross to create "Venturi" effects in which the water stream is accelerated to a higher speed when passing through the constriction.

Specifics regarding the invention can be described by the accompanying drawings in which: Figure 1 shows in perspective. the top view of the board-fin assembly 16,15 , sail gosition(or sail angle 13)with respect to the agìparent wind, VR 10, and the resultant aerodynamic force w R, 12.

Figure 2 shows in perspective, the fin 15 moves (as shown by arrow 41) closer to the sail 14 as the sail angle, e , 13 is reduced( as shown by arrow 42).

Fix 3 shows in perspective, the fin 15 moxtes(as shown b arrow 43) away from the sail 14 as the sail angle, 6 , 13 is increased( as shown by arrow 44).

Figure 4 shows in perspective, the water flow pattern 30 around the fin 15 at small angle of attack, , 17.

Figure 5 shows in rerspective, the water flow pattern 31 around the fin 15 at larger angle of attack, , 17.

Turbulence 16 occurred near the trailing edge of fin 15 Figure 6 illustrates he water stream moving through one of the chantiel. 32 when the fin 15 is travelling In one direction Figure 7 illustrates the water stream moving through the other channel 33 when the fin 15 is travelling in the other direction.

Figure 6 shows both channels 32, 33 in the fin 15.

Figure 9 illustrates the channelling eFfects at the intake 19 of channel(1) 33, and the sheet like water jet 24 at the outlet 22 of channnel(1) 33.

Referring to figures 1 and 2, the resultant aerodynamic force vector, R, 12, which acts on the sail surface 14 to provide the necessary forward drive for the board-sail assembly 15 16, rotates with change in sail angle 13 (or sail position). As the sail angle, ##, 13 is reduced as shown by arrow 42 in figure 2, the force vector, R 12 ,is turned away from the forward direction. This in turn increases the lateral force acting on the sail 14. And is transmitted to the board-fin area 15 16 via the rider.

Part of this lateral force is applied onto the fin 15 via the rider's feet which push against the tail end of the board 16. This increased in lateral force tends to move the tail end of the board 16 towards the sail 14 as shown by arrow 41 in figure 2. An reaction force is created by the water on the fin surface to resist sideways motion 41 of the fin 15. Therefore, the forward motion of the board 16 could be maintained.

As the sa.il angle, 13 is increased as shown by arrow 44 > the force vector, R 12 is rotated closer to the forward direction reducing the lateral force acting on the fin surface 15. Because of inertia of the water flow around the fin 15, a reduction in lateral force on the fin 15 due to an increase in sail angle, 13, makes the fin 15 move sideways in the opposite direction as shown by arrow 43 in figure 3. Again, a reaction for:ȯ ss set up arounci the fin 15 to resist sideway motion 43 of the fin 15.

As the board 16 is reaching it: maximum speed at steady state, the lateral force on the fin surface 15 becomes very great because the final sail an'-'1e. # 13 is very small. At this stage, trimming of the sail 14 to the apparent wind 10 in order to maintain the proper forward drive could create more vigorous sideway motions of the fin 15 in a bac and forth manner 41 43. In the course of action of the fin in water, turbulences and water bubbles are created around the fin 15 reducing the good water contact between the fin and the surrounding water.

This in turn could lead to the board directional instabilities.

A larger fin area will offer more resistance to limit excessive sideway motion of the fin 15 and therefore, the forward motion of the board 16 is maximized. But a larger fin area 15 makes the board 16 more difficul.t to turn.

The angle of attack, # 17 ,of the fin 15 could be sufficient large so that the smooth water flow near the fin surface is disrupted as shown in figure 5. Turbulences 16 could cause excessive drag. The detachment of smooth water layer near the fin surface could ead t directional instabilities.

In order to provide good contact between the fin 15 and the water, a sheet of high speed water jet 24 is made to flow near the surfaces 26 27 of the fin 15 as shown in figures 6-9. This sheetlike high speed water jet 24 will in turn induce a larger volume of nearby water 25 to flow with the high speed water sheet 24. Two tapered channels 32 33 as shown in figures 6-8 allow "cross-flow" of water from one side of the fin to the other. A constriction 20 at the center of the fin 15 creates a "venturi" effect which accelerates the water pasaing through the constriction 20 to a higher speed coming out of the channel.A deflector lip 21 and a spreader surface 23 creates the higher speed water sheet 24 to flow close to the fin surface The engulfment of tLie fin surfaces 26 27 ty the higher speed water sheet 24 nd t1fr?: induced water flow 25 next; to the water sheet 24will.

provide a better water contact of the fin 15, thereby enhancing the board directionnal stability.

The accelerated ontgoing water sheet 24 at the outlet 22 of the channel has a higher speed than the incoming water stream at the intake 19 of the chunnel giving additional propulsion to the fin 15.

This provides adc tional forward drive to the board allowing a.

smaller fin ariba to be used. A smaller fin makes turns easier.

Claims (3)

1. A "super-fin" comprising a traditional shape fin body 15, two tapered channels 32 33 which allow "cross flow" of water streams from side A 26 of the fin to side B 27 of the fin for channel 1 33. And from side B 27 of the fin to side A 26 for channel 2 32.

2. A "super-fin" as claimed in Claim 1 wherein two tapered channels 32 33 sharing a common constriction 20 at the center, each has a deflector lip 21 and a spreader flat surface 23 at the outlet 22 of each channel.

3. A "venturi" effect is created at the constriction 20 as claimed in Claim 2 to generate a higher speed water stream when passing the constriction. A deflector lip 21 and a spreader 23 t the oulet 22 of the channel creates a sheetlike water jet 24 running close to the surface of the fin.