Swim Fin Design
Utilizing Principles
of Marine Animal Locomotion
E. R. Lewis and D. Lorch
Man has been active in the sea for as long as he has walked on the earth,
but only in the past 100 years has technology been used to develop better
devices for underwater exploration and mobility. However, we have not
advanced very far in the area of underwater self-propulsion; even small
fishes can easily outswim man.
Scuba divers rely on their legs for propulsion; they use swim fins to
increase kicking forces and to help resolve the sideways motion into thrust
that is directed parallel to the diver's body. To improve underwater
locomotion, it is necessary to design better swim fins. To accomplish this,
it is first desirable to learn as much as possible about the swim dynamics
of fishes and the dynamics of underwater locomotion.
PURPOSE
The purposes of this investigation were: 1) to determine the mechanical ef-
ficiency of commercially available swim fins in order to evaluate the
feasibility of design improvements, and 2) to make several design changes
to fins, utilizing principles of marine animal locomotion, and to compare
swimming performances with these new fms with performances with com-
mercial fins.
Studies of Fish Locomotion
Studies by Lang and Daybell (1960) have shown that the swimming power
of dolphins, as indicated by hydrodynamic theory, is five to ten times
greater than the capabilities of their muscle. Researchers have discovered
that salmon use far less fat and protein than required by known drag data
(Bret, 1965). Navy tests have shown that sea animals can move through
water with great efficiency because they can maintain laminar flow over
289
290 Lorch and Lewis
most of their body (Rosen, 1959; Lang and Daybell, 1960). It is now
known how this is accomplished. Experiments conducted on the Pacific
barracuda (Rosen and Cornford, 1972) indicate that its capabilities for
high speed swimming may depend a great deal on the thin layer of slime
solution on its skin, which suppresses turbulence and lowers water fric-
tion.
Another possible explanation for the efficient propulsion of a fish is
that the sideways motion of its head sets up alternate vortex centers
against which the fish's sides and tail can push with very little slippage
(Rosen, 1959). The motion of this vortex system is such that the fluid flow
next to the fish's skin may be very low; this would significantly reduce skin
friction, and it may be the reason why a fish can maintain laminar flow
over most of its body. When this author dissected a bluefish, unusual tail
characteristics were noted. The two main branches of the tail are pivoted
at right angles to the plane of the tail oscillations. The water loads on
these two main branches, and on the center membrane, spread the tail
wide apart during the stroke. Perhaps this is why the fin has been observed
to bend less at midstroke, where the thrust forces are greatest, than at the
ends of the stroke. When the fin is spread, the tension field on the
stretched fin membrane offers additional strength to the fin, thus
minimizing bending strain. The bony rays of the tail fin do not seem to
follow Hook's Law after a small deflection. This may result from the ac-
tion of the gelatinous material within the fin rays that acts as a hydraulic
lock. No such measurements were made on the fin, so that this is a fertile
area for further investigation.
Studies done on living fishes to photograph the surrounding flow pat-
terns have always been difficult to conduct (Rosen, 1959; Kent, 1961). At-
tempts were made to perform similar tests using a water tunnel but were
unsuccessful because good pictures could not be obtained. At present, no
one knows the amount of thrust developed by a fish's tail fm or body or the
nature of the interaction between them. It is not certain that the fish's V
tail is the most efficient oscillating propulsion system or whether or not it
must be considered in combination v.rith the head motion.
Factors in Designing Underwater Propulsion Systems Swim fins
have not changed in shape drastically from those first produced in the
early 1940s. The only significant improvements have been the increase in
fin stiffness and surface area. Preliminary experiments and studies were
done to determine the problems of underwater propulsion for the scuba
diver and to investigate possible solutions to these problems. Before
designing new swim fms, several commercial fms were evaluated for their
efficiency. Figure 1 shows three types of commercial fins, types A, B, and
C. Then several types of fins were designed, using information on marine
animal locomotion, and the performances with the new fins were corn-
Swim Fin Design 291
A
c
8
Figure 1. Commercial swim fins, types A, B, and C.
pared with the commercial fins. In designing the new fins, consideration
was given to the fact that stiff fms should be used for high speeds, and
flexible fins should be used for low speeds. The fish's tail seems to have
this ideal self-adjusting stiffness. However, it was recognized that it would
be most difficult to design a fin to function in this manner, even though it
is the most efficient.
SWIM FIN DESIGN
Figure 2 shows an experimental type D swim fin. Its base plate was
designed with two things in mind: 1) to enable several types of fin blades
to be tested with a minimum of reworking, and 2) to provide as large a
separation of the two attachment straps as possible, so that the fin forces
transmitted to the ankle would be minimized and the swimmer would not
get leg cramps.
The aluminum fish blade could flex only at the attachment to the
base plate; otherwise it was rigid over its entire length. The blade was
made as wide as possible without permitting the fms to strike each other
when the swimmer's legs crossed. Since the fins did not flex much, the
swimmer had to adjust the fin's angle of attack by allowing his ankle to
bend just enough to get maximum thrust. The angle extrusions along the
side provided stiffness and acted as end plates, which would reduce tur-
bulence near the edges of the fin and, therefore, improve the thrust effi-
292 Lorch and Lewis
Figure 2. Experimental swim fin, type D.
ciency. The V cutout was made to resemble the tail of a fin, so that enough
water would move toward this cutout to reduce the tip turbulence further.
The fin blade of type E (Figure 3) was designed to approximate the
shape of a fish's tail. The two fiberglass rods on both sides of the blade pro-
vided stiffness for the vinyl plastic sheet. Because the rods were J.!lounted
rigidly, they did not perfectly simulate the action of the bony rays in a
fish's tail, which spread under load. The fins were also made as wide as
possible without permitting them to interfere when the swimmer's legs
crossed.
These fms were constructed to test whether or not it would be possible
to improve efficiency by reducing fm-tip vortices (induced drag) by allow-
ing water to flow through the center of the V. Squared-off fins, with no V
cutout, have a low aspect ratio and, therefore, a large induced drag. The
resultant force on these fm blades is developed near the base rather than
at the ends. Likewise, the resultant thrust of a fish's tail would also be
close to the base of the tail, enabling the fish to get maximum thrust, but
the bending moment arm would be smalL When this principle is applied
to swim fins, a small moment arm results in minimum ankle-bending
loads and, therefore, less chance of leg cramps.
The type F leg fin shown in Figure 4 was designed to enable the swim-
mer to handle very high fin loads without concern for ankle flexing. These
fms were attached to the calf of the swimmer rather than to his foot.
Figure 3. Type E swim fin design.
Swim Fin Design 293
.()40",ft VM.IN'UA;f
LEG ST~APS'
Figure 4. Type F swim fin design.
Because alternate kicking was used, the width of the fin blades was
limited to avoid interference, but the length and stiffness of the blade were
unrestricted. If the rectangular blade had functioned successfully, other
blade shapes using this support system would have been tried.
PROCEDURES
Drag Test
A spring scale was calibrated and then connected to a 5-m long rope. One
end of the rope was attached to the back pack of the scuba diver, and the
other end, with the spring scale attached, was pulled by an assistant. A
distance of 4 m was provided to enable the assistant who pulled the tow
line to stabilize the force applied. The assistant maintained this force
while he towed the subject 10 m. The plot of drag force versus speed2 is
shown in Figure 5.
Leg Forces
To measure the leg forces during swimming, the same spring scale used
for towing was attached to the diver's ankle at 90° with his leg. The test
subject lay on his side and applied leg forces similar to what he thought he
applied while swimming at 0.86 m/sec. A plot of fin force versus leg
distance is given in Figure 6.
To obtain a meaningful comparison of the overall performance of the
various ftns, it was necessary to devise a fin performance factor, P, where:
p = distance/kick cycle
(change of tank pressure) (time)
The distance/kick cycle was determined by having the subject swim
15m with skin diving equipment (no scuba tank) while an assistant
counted the kick cycles. Air consumption and time were determined for
the subject who used scuba equipment while swimming underwater 229
294 Lorch and Lewis
V\!)
/):::: /08 V1 /
tr
/
,../
V
V
20
V
/0 ..-... V
~
() ./ .s- .6 .7
("}1ec)'-
Figure 5. Drag versus speed2• Subject, 160 !b, 5 ft, 10 inches tall, with 72 ft3 scuba rocket jet
fins, wet suit top, and 6-lb weight belt.
m. The air consumed was proportional to the change of pressure in the
tank. Before the initial pressure was measured, the tank was placed in the
pool for 30 min to permit the air temperature inside the tank to reach
water temperature. The test subject maintained an uncomfortable water
speed to keep from holding his breath and going into oxygen debt.
This technique provided a reasonably accurate measurement of air
consumption.
RESULTS
Mechanical Efficiency of Commercial Fins (Type B)
The conditions were 0.82 leg cycle/sec with a water speed of 0.86 m/sec.
The Leg power input (see Figure 6) was:
Total work for one leg cycle = 88 J/cycle
Workforbothlegs = 2 X 88·= 176J/cycle
The power input was 176 J/cycle X 0.82 cycle/sec = 144 W.
Swim Fin Design 295
0 .+ .8 /.2 1.6
01-S"rANt:~ t'J,C ,#'tJDr r~AV&L (M)
Figure 6. Applied leg force versus distance of foot travel.
The fin performance factors and the mechanical efficiency were
calculated. The mechanical efficiencies were determined by assuming a
direct relationship between fin performance factor and mechanical effi-
ciency. These computations were based on one of the three commercial
fins evaluated, fin B, shown in Figure 1. This fin had an efficiency of 48 o/o
and a performance factor of 1.86. The tabulations of results for six types
of fins are shown in Table 1.
Table 1. Performance factors and efficiencies for six fin types
Change in Distance/ Performance
tank pressure Time kick cycle factor (m/ Efficiency
Fin type (lb/inches2) (sec) (m/cycle) cycle/ psi-sec) (%)
A 250 26S 1.30 1.96 X w-s so
B 285 270 1.43 1.86 48
c 27S 25S 1.50 2.14 ss
D 27S 28S 1.15 1.47 38
E 280 280 1.50 1.91 49
F 1.15
296 Lorch and Lewis
These tests and calculations indicate that commercial swim fins are
much more efficient mechanically than anyone had suspected (48-55%).
However, if the determination of the power input is incorrect by lOo/o or
20o/o, the efficiency is far greater than the 3 o/o physiological efficiency
reported by Goff, Brubach, and Specht (1957). This would indicate that
there must be some physiological reason why an underwater swimmer
utilizes so much oxygen. Perhaps it has something to do with the body's
utilizing oxygen to maintain body temperature through increased oxida-
tion of fats and protein. Even if a diver wears a wet suit, he will still lose a
great deal of heat through his head. It would be easy to conduct tests to
verify this point.
Also, if swim fms were 100% mechanically efficient and the diver
maintained a comfortable 0.82 kick cycle/ sec, he would be able to attain a
speed of only 1.1 m/sec. This is not very encouraging. In fact, when the
maximum possible speed was calculated using the maximum possible
mean thrust, 178 N, it was determined that a diver could reach the top
speed of 1.28 m/ sec.
The drag data (Figure 5) and the empirical equation for drag as a
function of swim speed can be used for future studies of underwater
swimmers.
Performance of New Fins
The base plate functioned well, and the swimmer experienced small ankle
loads with both fin blades, D and E. Actually, much larger fin forces
could have been handled with this base plate, but it was not possible to
make the fms wider without having them interfere during swimming, and
if the length were increased, ankle loads would be too large. The rigid
blade did not seem to offer any advantage over a flexible rubber one.
The type E fin performed well at low ankle forces. Much larger forces
could have been handled, but interference of the fms with alternate kick-
ing prevented this. Because it seems that the tail of a fish is shaped to
operate with a vortex system generated by the fish's head and because no
such vortex system is generated by the swimmer, the V fin offers no advan-
tage other than to shift the forces closer to the ankle.
The type F leg fins allowed large loads to be applied without ankle
fatigue, but they were so awkward to use that the disadvantages out-
weighed any possible advantages.
The fin performance factor seems to be a very good measure of
overall fin performance; a fin that is large and stiff will enable a diver to
travel farther per kick cycle, but he will consume more oxygen in the pro-
cess. If it is assumed that there is a direct relationship between fin perfor-
mance factor and mechanical efficiency, the mechanical efficiencies of the
fms tested vary from 38 to 55%.
Swim Fin Design 297
CONCLUSIONS
The following conclusions are based on the results of this study.
1. The mechanical efficiency of most commercial swim fins is between
48 o/o and 55 o/o •
2. If underwater swimmers continue to use foot fins in the present man-
ner, sustained underwater speeds will not exceed 1.5 m/sec. If higher
speeds and greater ranges are to be attained, radically different types
of fins will have to be used.
3. The fin with the best performance characteristics is type C. This type
is presently being used by U. S. Navy underwater swimmers.
REFERENCES
Bret, J. R. 1965. The swimming energetics of salmon. Sci. Am. 213(2).
Goff, L. G., Brubach, H. F., and Specht, H. 1957. Measurement of respiratory
responses and work efficiency of underwater swimmers utilizing improved in-
strumentation. J. Appl. Physiol. 10.
Kent, J. C. 1961. Flow visualization and drag about a swimming fish. NOTS Con-
tract 123(60530)20579A. Fisheries Research Institution, College of Fisheries,
University of Washington.
Lang, T. G., and Daybell, D. A. 1960. Results of performance tests conducted on
a porpoise in a salt water tank. Technical Note P 508-13. U.S. Naval Ordnance
Test Station, Pasadena Annex.
Rosen, M. 1959. Water flow about a swimming fish. NOTS TP 2298-ASTIA
238395. U.S. Naval Ordnance Test Station, China Lake, Cal.
Rosen, M., and Cornford, N. 1972. Pacific barracuda. Sea Frontiers 18(4).
