Swim fin

An improved swim fin includes a foot pocket, first and second flexible beams supported by the pocket and projecting forwardly from it, first and second struts attached securely to outer ends of the first and second flexible beams, respectively, a narrow, rigid, streamlined blade having a large span rigidly attached to outer ends of the first and second struts. The first and second flexible struts are composed of flexible plastic and have elliptical cross-sections to reduce drag and to prevent twisting. The first and second struts form legs of a U-shaped member, the bottom of which forms a cross-member maintaining spacing between the first and second flexible beams at a fixed value.

The present invention relates to improvements in swim fins. 
A principal objective of the invention is to provide a swim fin with 
greater efficiency. 
Swim fins that are currently being manufactured comprise three basic 
components; namely, a foot pocket, two flexible beams that project from 
the sides of the foot pocket and a flexible blade. Each lateral side of 
the blade is attached to a flexible beam and the side of the blade that is 
adjacent to the foot pocket is attached to the toe area of the foot 
pocket. 
Supporting the blade in this manner creates an impediment to the flow of 
water to the blade. It is an object of this invention to support the blade 
in a way that minimizes entry losses of water flowing to the blade 
surface. 
Since the blade angle of a swim fin must reverse itself at the end of each 
stroke, a considerable amount of energy is expended by the conventional 
swim fin because of the drag generated by the blade surface near the foot 
pocket. It is an object of my invention to eliminate this surface. 
Another feature of the conventional swim fin that wastes energy during 
blade reversal is the long blade. It is an object of my invention to 
provide a swim fin with a rigid blade that is narrow, has a large blade 
span and has a comparatively small blade area thereby reducing to a 
minimum both the quantity and the velocity of the water that is moved 
during blade angle reversal. 
A further object of this invention is to provide a suspension system; which 
can support a rigid blade that is positioned so that a large space exists 
between the foot pocket and the blade; which confines the flexing of the 
flexible beams to an area that is located near the foot pocket; which 
permits the blade to be angled to a greater extent with respect to the 
foot pocket than the flexible beams; which permits the blade to reverse 
its blade angle easily at the end of each stroke; which resists excessive 
deflection on a hard kick by the swimmer; which provides a simple means 
for attaching a blade having a streamlined cross section; and finally, 
which does all of the above with parts having a low drag profile. 
Further objects and advantages of my invention will appear as the 
specification proceeds.

While I have shown the preferred form of my invention, I wish to have it 
understood that various changes and modifications may be made within the 
scope of the claims hereto attached without departing from the spirit of 
the invention. 
Referring to the drawings in detail, my swim fin comprises a foot pocket 1 
with two flexible beams 2 projecting from it, a narrow rigid blade 3 
having a streamlined cross-section and a large blade span. 
The rigid blade 3 has two grooves, one of which is illustrated in the 
partial section view of FIG. 3, in which two struts 4 which project from 
the blade 3 are clamped to the blade by hook-shaped fasteners 5. The 
struts are joined at their extremities by a cross-member 6. The struts and 
cross-member are made as a one-piece U-shaped wire form, and the ends of 
the struts are bent 90.degree. so that they can be inserted into holes 7 
in the blade. 
The wire form is made of corrosion-resistant high strength wire such as 
stainless steel, spring wire having a diameter of approximately 5/32 of an 
inch in diameter. 
Each of the flexible beams 2 has a groove to accept part of the wire form. 
The groove is illustrated in the section view of FIG. 4. Because of their 
length and flexibility, designing the flexible beams 2 to provide a 
suitable means to attach the U-shaped wire form while at the same time to 
provide a flexible beam which generates a minimum amount of drag is a 
primary object of this invention. In my swim fin the flexible beam 2 is 
given an elliptical cross-section having a low drag profile, illustrated 
in the section view of FIG. 4. This configuration generates a minimum 
amount of drag on both the up stroke as well as the down stroke. 
The dimensions of the elliptical cross-section of the flexible beams at a 
point near the foot pocket is approximately 1.500 by 0.640. A larger 
cross-section would be required for stiffer models. The flexible beams are 
made of flexible plastic and graphite or glass fibers may be added to the 
plastic to increase stiffness and strength. The flexible beams have to be 
stiff enough to prevent excessive deflection of the blade on a hard kick 
by the swimmer otherwise a loss of thrust will result. 
Placing the strut in a groove which positions the strut on the center line 
of the flexible beam 2 reduces the tendency of the flexible beam to twist. 
To further reduce the tendency of the flexible beams 2 to twist, each of 
the flexible beams 2 has a boss 8, illustrated in the section view of FIG. 
4, projecting from the flexible beam 2 to which the cross-member 6 of the 
wire form is clamped by hook-shaped fasteners 9. Also, each of the 
flexible beams 2 has a boss 10 located at its extremity to which a strut 4 
is clamped by hook-shaped fasteners 11. 
The operation of my swim fin is as follows: Referring to FIG. 2, during 
swimming, the trailing end of the blade 3 deflects alternately in one 
direction and then the other describing an arc-shaped path 17 illustrated 
in FIG. 2. Because the struts 4 of the wire form deflect only a little, 
the parts 18 of the flexible beams 2 located between the cross-member 6 
and the foot pocket 1 are forced to flex; thus, giving the blade 3 a 
hinging action which is located far enough away from the blade 3 to 
provide good finning performance. 
The struts 4 are bent so as to angle the blade 3 to a greater extent with 
respect to the foot pocket 1 than the flexible beams 2. This is an 
important point for efficient operation of the blade 3 because it 
positions the blade 3 closer to the desired blade angle in which, during 
swimming, the blade deflects about as much on the down stroke as on the up 
stroke. 
One of the areas of this swim fin design which is responsible for a 
substantial gain in efficiency is the design of the blade 3. The leading 
edge of the blade 3 is swept back at the tip area to the trailing edge, as 
indicated by reference numeral 19. This is done primarily to remove the 
90.degree. corners. The two inside corners or tip areas tend to strike 
each other during swimming unless removed. Also, tapering the tip area 
reduces tip losses. The blade 3 has a streamlined cross-section, as 
indicated in FIG. 2, which reduces drag to a minimum. Lift and drag for 
this type of blade have been well documented over the years. But in order 
to develop a high degree of efficiency with this type of blade, it is 
necessary to have a narrow blade with a large blade span. In the drawing 
is illustrated a blade measuring only 3 inches from the leading edge to 
the trailing edge, and having a blade span of 18 inches. The blade area is 
51 square inches, and yet develops adequate thrust. Also the blade is 
stable and requires no stabilizer surfaces. Using a blade of these 
dimensions runs contrary to current swim fin design practice in which the 
blade measures about 11 inches measured from the toe of the foot pocket to 
the end of the blade and about 9 inches measured across the blade. 
To achieve maximum efficiency, the blade 3 is positioned so that there is a 
large open space 20 between the foot pocket 1 and the blade 3. In my swim 
fin, this distance is 11 inches. There are two suspension systems capable 
of supporting and providing good finning action for a rigid blade such as 
blade 3 that is located at a point so far from the foot pocket. One is the 
suspension system described in this specification and the other is one for 
which I recently applied for a patent, Ser. No. 798,676, entitled "SWIM 
FIN", filed on Nov. 15, 1985, and incorporated herein by reference. Both 
systems require a U-shaped wire form. They differ mainly in two points: 
1. The U-shaped wire form described in this specification is fixedly 
attached at two points to each flexible beam, whereas the cross-member of 
the U-shaped wire form in the design of my prior patent application acts 
as a hinge pin and is pivotally attached to each flexible beam. 
2. The flexible beams described in this specification are short whereas the 
flexible beams in the design of my prior patent application extend all the 
way to the blade to which they are attached. The suspension system in this 
specification generates substantially less drag. 
Another area of the swim fin that has been improved is the heel strap 16 
and the means to attach it to the foot pocket. To reduce the drag profile 
of the heel strap and the bosses on the foot pocket to which it is 
attached, the heel strap 16 has a long hole in one end and a series of 
long holes 12 in the other end. The foot pooket has two bosses 13 on each 
side having a hole to accept a fastener 15 which has a threaded end. A 
fastener 15, on each side of the foot pocket is inserted into a hole in 
the heel strap. A stop nut 14 completes the means to attach the heel strap 
to the foot pocket. 
A long strap is supplied when a new pair of fins is purchased. After the 
swimmer has adjusted the strap length for his foot size, the excess strap 
length is cut off.