Effect of Swim Suit Design on Passive Drag

Abstract

The drag (D) of seven (7) male swimmers wearing five (5) swimsuits was investigated.
The drag was measured during passive surface tows at speeds from 0.2 up to 2.2 m x s and during starts and push-offs. The swimsuits varied in body coverage from shoulder-to-ankle (SA), shoulder-to-knee (SK), waist-to-ankle (WA) and waist-to-knee (WK) and briefs (CS).
Differences in total drag among the suits were small, but significant. In terms of least drag at 2.2 m x s, the swimsuits ranked: SK, SA, WA, WK and CS. The drag was decomposed into its pressure drag (DP), skin friction drag (DSF) and wave drag (DW) components using nonlinear regression and classical formulations for each drag component. The transition-to-turbulence Reynolds number and decreasing frontal area with speed were taken into account. The transition-to-turbulence Reynolds number location was found to be very close to the swimmers' "leading edge," i.e. the head. Flow was neither completely laminar, nor completely turbulent; but rather, it was transitional over most of the body. The DP contributed the most to drag at low speeds (<1.0 m x s) and DW the least at all speeds. DSF contributed the most at higher speeds for SA and SK suits, whereas DP and DW were reduced compared with the other suits.
The decomposition of swimmer drag into DSF, DP and DW suggests that increasing DSF on the upper-body of a swimmer reduces DP and DW by tripping the boundary layer and attaching the flow to the body from the shoulder to the knees. It is possible that body suits that cover the torso and legs may reduce drag and improve performance of swimmers.

Full text

Effect of Swim Suit Design on Passive Drag
JOSEPH C. MOLLENDORF
1–3
, ALBERT C. TERMIN II
4
, ERIC OPPENHEIM
2
, and DAVID R. PENDERGAST
1,3
1
Center for Research and Education in Special Environments;
2
Department of Mechanical & Aerospace Engineering,
School of Engineering and Applied Sciences;
3
Department of Physiology and Biophysics, School of Medicine and
Biomedical Sciences; and
4
Division of Athletics, University at Buffalo, Buffalo, NY
ABSTRACT
MOLLENDORF, J. C., A. C. TERMIN II, E. OPPENHEIM, and D. R. PENDERGAST. Effect of Swim Suit Design on Passive Drag.
Med. Sci. Sports Exerc., Vol. 36, No. 6, pp. 1029–1035, 2004. Introduction: The drag (D) of seven (7) male swimmers wearing five
(5) swimsuits was investigated. Methods: The drag was measured during passive surface tows at speeds from 0.2 up to 2.2 m·s
⫺1
and
during starts and push-offs. The swimsuits varied in body coverage from shoulder-to-ankle (SA), shoulder-to-knee (SK), waist-to-ankle
(WA) and waist-to-knee (WK) and briefs (CS). Results: Differences in total drag among the suits were small, but significant. In terms
of least drag at 2.2 m·s
⫺1
, the swimsuits ranked: SK, SA, WA, WK and CS. The drag was decomposed into its pressure drag (D
P
),
skin friction drag (D
SF
) and wave drag (D
W
) components using nonlinear regression and classical formulations for each drag
component. The transition-to-turbulence Reynolds number and decreasing frontal area with speed were taken into account. The
transition-to-turbulence Reynolds number location was found to be very close to the swimmers’ “leading edge,” i.e. the head. Flow was
neither completely laminar, nor completely turbulent; but rather, it was transitional over most of the body. The D
P
contributed the most
to drag at low speeds (⬍1.0 m·s
⫺1
) and D
W
the least at all speeds. D
SF
contributed the most at higher speeds for SA and SK suits,
whereas D
P
and D
W
were reduced compared with the other suits. Conclusion: The decomposition of swimmer drag into D
SF
,D
P
and
D
W
suggests that increasing D
SF
on the upper-body of a swimmer reduces D
P
and D
W
by tripping the boundary layer and attaching
the flow to the body from the shoulder to the knees. It is possible that body suits that cover the torso and legs may reduce drag and
improve performance of swimmers. Key Words: FRICTION DRAG, PASSIVE DRAG, WAVE DRAG, TURBULENT FLOW,
LAMINAR FLOW
Swimming performance is measured to the nearest
0.01 s, with swimmers in the top 16 of meets sepa-
rated by only 0.10 s. Because drag is a major factor in
the energetics of swimming, small decreases in the swim-
mer’s drag can affect performance. With the availability of
a new generation of suits that cover larger parts of the body
and are made of different materials than the traditional suits,
there is a potential for drag reduction. These new suits are
reported, in a nonrefereed swim magazine, to reduce skin
friction of the material itself (without a swimmer) by 16%
and by 10% when worn by a swimmer (17). A Lycra
specially designed suit covering the torso of male swimmers
reduced the energy demand of swimming, compared with a
standard racing suit, presumably due to the drag-reducing
characteristic of the suit (14). The opinion that these new
suits improve performance is, however, not universally ac-
cepted. It was concluded (based on active drag measure-
ments and a statistical analysis of performance times), in
another nonrefereed swim magazine (11) that these types of
suits do not reduce drag or improve performance.
It is well known that surface characteristics have an effect
on fluid mechanical drag. In general, the effect of uniformly
distributed surface roughness is to decrease the transition-
to-turbulence Reynolds number. This will increase skin-
friction drag because turbulent skin-friction drag is higher
than laminar skin-friction drag. The same is true for single
protuberances with height a fraction of the boundary layer
thickness. Surface roughness and protuberances may alter
transition-to-turbulence on the swimmer, and may attenuate
overall drag.
The purpose of this study was to combine theoretical
models with passive drag measurements to determine
whether these new generation suits are drag reducing, and if
so, what the physical mechanisms behind the reduction are.
Passive drag was used to concentrate on the effects of the
suit alone, and not the swimmer technique or style.
METHODS
Seven male University Division I swimmers participated
in this study. They average 20.2 ⫾0.5 yr of age, 171 ⫾12
cm in height, 71.80 ⫾1.66 kg in weight, 51.3 ⫾0.9 cm in
chest width, 25.4 ⫾0.4 cm in chest depth, and 8 ⫾1% body
fat. The study was approved by the University’s Institutional
Review Board; subjects completed a medical history, were
given a physical examination, and completed an informed
consent form.
Address for correspondence: Joseph C. Mollendorf, Ph.D., Center for
Research and Education in Special Environments, 124 Sherman Hall,
University at Buffalo, Buffalo, NY 14214; E-mail: molendrf@buffalo.edu.
Submitted for publication February 2003.
Accepted for publication February 2004.
0195-9131/04/3606-1029
MEDICINE & SCIENCE IN SPORTS & EXERCISE
®
Copyright © 2004 by the American College of Sports Medicine
DOI: 10.1249/01.MSS.0000128179.02306.57
1029

Four swimsuits that covered various body surface areas
(Fastskin by Speedo) and a traditional competition suit
(Speedo) were purchased commercially for each swimmer,
and their drags were determined and compared. These suits
also represented the suits most commonly purchased by
competitive swimmers (personal communication from
Adolph Kiefer and associates). The body surface areas cov-
ered were shoulder-to-ankle (SA), shoulder-to-knee (SK),
waist-to-ankle (WA), and waist-to-knee (WK). The suits
were made of microfiber polyester and Lycra material.
All testing was done with the suits initially dry, and the
order of the suits in testing was randomly assigned for
each swimmer. All of the swimmers wore a conventional
swimming cap.
The densities of the swimsuits were determined using
standard densitometric techniques. The suits, when dry,
were weighed in air and then placed into a press device to
eliminate any air, and then were weighed in water while
still in the press. This technique was repeated with the
suits after they were immersed in water (while wet) by
the same techniques.
Measured Towing Drag
Two sets of drag measurements were taken, passive tow-
ing at the surface in an annular pool 58.6 m in circumfer-
ence, over the path of the swimmer, and 2.5-m wide and
2.5-m deep and during starts and turns in a standard Olym-
pic pool. Both starts and turns were considered separately,
because the initial speeds of each are different and the starts
involve motion in air and water. The pool temperatures were
maintained at 28°C⫾0.2°C with the air temperature at
22°C⫾2.0°C.
For the towing experiments, the swimmers were towed
passively at the water surface. The swimmer held onto a
handle that was attached by a wire through pulleys to a
vertically mounted dynamometer (Model TDC 4A, Schae-
vitiz Engineering, Pennsauken, NJ) that was fixed to a
monitoring platform that towed the swimmer. The platform
speed was set by a calibrated impeller flow meter (Model
HP301A2M, Mead Instruments, Riverdale, NJ). The veloc-
ities started at 0.2 m·s
⫺1
and were increased in 0.2 m·s
⫺1
increments up to 2.2 m·s
⫺1
(⫾0.03 m·s
⫺1
, over the entire
speed range).
Swimmers breathed through a swimmer’s snorkel (Finis
Inc., Tracy, CA) and force (total drag; D) was measured
using the vertically mounted dynamometer, the output of
which was conditioned using a Linear Variable Differential
Transformer (Model 300D, Daytronics, Dayton, OH). The
output of the LVDT was processed using an A/D Converter
(Model PPIO-AI08, ComputerBoards, Mansfield, MA) via
software developed “in house”to average and store velocity
and force every min using a personal computer (Model 770,
IBM Thinkpad, Armonk, NY). The data were then plotted as
a function of velocity. All devices were calibrated before
and after each experiment.
During the passive towing experiments, the swimmers
(and a calibration frame) were videotaped through an un-
derwater window using a video camera (DCR TRV 840,
Sony, Oradell, NJ). A simple vertical and horizontal linear
scale was used as a camera calibration frame. After each
swim, the video was replayed and the frontal area, A
f
(
␪
), of
the swimmer was calculated from the whole body angle,
␪
,
from the horizontal (averaged over the body length) using
the following equation:
Af共
␪
兲⫽共Af0兲cos
␪
⫹共AS/2兲sin
␪
[1]
where A
f0
is the frontal area when the swimmer is horizontal
(
␪
⫽0) and A
s
is the body surface area. The measured angle
represents a time average and was assumed to not vary with
suit design.
Measured Glide Drag
For the starts and push-offs experiments, the coefficient
of drag was determined in a competitive swimming pool
after a push-off from the wall or after a dive from the
starting blocks using a device called a “swim-meter”devel-
oped by Craig and Pendergast (3). The swim-meter mea-
sured the decrease in instantaneous velocity as the subject
decelerated passively from the maximal initial velocity. The
swim-meter consisted of a wire secured to the swimmer that
passed through a system of pulleys and turned a DC gen-
erator (3) as the wire was pulled from a fishing reel. The
output of the DC generator was recorded using a computer
and the drag coefficient (C
D
) was calculated using a method
based on a linear mass-damper system. As such, the drag
force is assumed to be the sum of a linear and quadratic
function of the velocity. The constants of proportionality are
determined from the results of a regression of the measured
transient velocity data. Additional details are given in the
thesis, Oppenheim, E. Model parameter and drag coefficient
estimation from swimmer velocity measurements (M.S.
Thesis, Department of Mechanical Engineering, University
at Buffalo, 1997).
Theoretical Calculations
Data analysis. For surface swimming, the most rele-
vant kinds of drag are skin-friction drag, pressure drag and
wave and spray drag. A passively towed swimmer’s surface
can be approximated by an arrangement of a sphere and
circular cylinders in axial flow. Because the boundary layers
are typically thin compared with the radii, the flows can be
modeled to a good approximation as flows over flat plates
(2,6,8).
The Reynolds number, Re
L
, based on a characteristic length,
L, is defined as, Re
L
⫽VL/
␯
, where V is the swimming speed
and
␯
is the fluid kinematic viscosity. For high Reynolds
numbers, as observed in competitive swimming, the flow ex-
periences transition-to-turbulence, separates from the surface,
and the pressure drag is caused by viscosity.
For flow over a flat plate at zero angle-of-attack, transi-
tion-to-turbulence begins at about Re
L
⫽5⫻10
5
and ends
at about Re
L
⫽10
7
(7). For a swimmer 170 cm tall, the
Reynolds numbers based on swimmer length (height) cor-
responding to V ⫽0.3 m·s
⫺1
and 2.2 m·s
⫺1
are 5.10 ⫻10
5
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Official Journal of the American College of Sports Medicine http://www.acsm-msse.org

and 3.74 ⫻10
6
, respectively. Consequently, the flow over
the length of the swimmer’s body is neither completely
laminar nor completely turbulent, it is transitional and ill-
defined. For example, in a completely “still”ambient, free-
stream turbulence will be absent and transitional flows can
persist as laminar beyond about, Re
cr
⫽5⫻10
5
.Onthe
other hand, in a “noisy”ambient, both the beginning and end
of transition-to-turbulence will occur at lower Reynolds
numbers. That is to say, the flow is “trippable”(to
turbulence).
As a result, the forward portion of the swimmer, modeled
as a flat plate, will be in laminar flow, and most of the aft
portion of the swimmer will be in transitional flow. It is
important to note that in the present study the length of the
swimmer is defined as the swimmer height. Thus, for a
general scenario, laminar, transition and turbulent flow re-
gime limits can be described in terms of swimmer speed and
swimmer height. For example, it can be seen from Figure 1
that for a speed of 2 m·s
⫺1
, the laminar flow ends at about
25 cm aft of the swimmer’s head and that most of the body
of the swimmer is in the near-laminar/transition-to-turbu-
lence region. For the towing procedure used in the present
study, the swimmer’s arms are outstretched as indicated on
Figure 2. It seems reasonable (as was assumed here) that the
outstretched arms are quite streamlined and they do not
significantly disturb the flow or contribute to drag. The
head, followed by the shoulders, represents the “leading
edge”of the main part of the body. In this sense, the
outstretched arms can be thought of as an extension of the
towing cable. The actual extent of the laminar flow region
depends upon both swimming speed and the transition-to-
turbulence Reynolds number. The latter is determined as
one of the fit parameters in our regression of the data
reported herein.
The skin-friction drag of the swimmer, D
SF
, was calcu-
lated as for a flat plate in the transition-to-turbulence region
(7) as:
DSF ⫽qA
S关0.074/ReL1/5 ⫺1740/ReL兴[2]
where q is the dynamic pressure, q ⫽1/2
␳
V
2
,
␳
is the fluid
density, and A
S
is the surface area. The corresponding
pressure drag (D
P
) was calculated using the results of
Schmitt (12). As a starting point in the data regression, the
skin friction drag was calculated using one half (1/2) of the
body surface area, because it is assumed that the swimmer
would be partially out of the water. The actual body surface
area is not needed, however, because it is “buried”in the
regression coefficient. This can be seen in the equation for
D
SF
on Figure 2. The D
P
was formulated to be proportional
to the second power of the velocity and directly proportional
to the frontal surface area, which is a measured function of
the body angle variation with speed. The D
W
was formu-
lated to be proportional to the fourth power of the velocity.
Thus, the experimental data were used as the basis for the
theoretical calculations. Drag decomposition consisted of
summing the drag components and then determining the
proportionality constants as well as the transition Reynolds
number using a standard multiple, nonlinear regression
package (NonlinearFit, Mathematica, Wolfram Research).
Statistical Analysis
Mean and SD were calculated for all data to describe and
present the individual values in the figures and tables. The
data for the different suits were compared using an ANOVA
for repeated measures for suits and speeds. Data were also
fit, using regression analysis, with the best statistical fit
being reported. The 0.05 level of significance was accepted
for all comparisons.
RESULTS
Complete drag and drag coefficient data were obtained on
the seven swimmers in the five suits. The suit weights in air
ranged from 0.32 to 0.34 kg, and the densities ranged from
1.0536 to 1.0883 both wet and dry. All of the suits tested had
a density greater than water, and there were no significant
differences among the suits tested.
Measured towing drag. The test-retest reliability of
the passive drag measurement (Fig. 2) was 0.94 (N⫽77).
The regression analysis allowed the value of the critical
Reynolds number to “float”and be a parameter to be de-
termined by the behavior of the data. As such, it was found
that transition-to-turbulence occurred at a Re of about 3 ⫻
10
3
(lowest curve, lower left corner of Fig. 2, left panel). It
can be seen that the transition from laminar flow occurs very
early (a few centimeters), i.e., near the maximal diameter of
the head, and was not significantly different among the suits
tested (Fig. 2, left panel). Therefore, most of the length of
FIGURE 1—Speed is plotted as a function of the height a hypothetical
average swimmer if transition-to-turbulence began at a Re of 5 ⴛ10
5
and ended at 1 ⴛ10
7
. The region between the laminar flow and
turbulent flow regions is the transition-to-turbulence region (25 cm
and beyond, for a swimming speed of 2.2 m·s
ⴚ1
). For this hypothetical
plot, the laminar region is less than 25 cm from the top of the head for
a swimming speed of 2.2 m·s
ⴚ1
. The more turbulent region is from the
knee to the foot, with the region between the head and knee in the early
transition region. The data reported herein suggest that transition-to-
turbulence begins at a Reynolds number considerably less than 5 ⴛ10
5
.
EFFECT OF SWIM SUIT DESIGN ON PASSIVE DRAG Medicine & Science in Sports & Exercise姞
1031

the body, between the head and feet, is in the transition
phase.
The total drag increased monotonically: 77.5 ⫾3.7, 81.7
⫾3.9, 82.3 ⫾4.8, 83.5 ⫾4.9, and 86.2 ⫾4.3 N at a V of
2.2 m·s
⫺1
, for the SK, SA, WA, WK and CS, respectively
(Fig. 2, right panel). From these data, it can be concluded
that the more surface area of the torso that is covered with
the Fastskin suit material the less the total drag was and
ranged from 3% for WK to 10% for SK.
Measured glide drag. As described in the method
section, the C
D
was calculated from both dives and push-
offs (Table 1). The test-retest reliability of the C
D
was 0.978
(N⫽35).
The peak velocities achieved were not significantly dif-
ferent among the suits for the starts or turns and were 5.829
⫾0.128 m·s
⫺1
for the starts and 2.68 ⫾0.07 m·s
⫺1
for the
push-offs. The C
D
for SA, SK, WA, and WK were not
statistically different from each other, but all were signifi-
cantly different from CS (15, 8, 8, and 10%, respectively).
Theoretical calculations. Decomposing the total drag
yielded expressions for D
SF
,D
P
, and D
W
(Fig. 2, right
panel). D
SF
increased as function of V and was significantly
FIGURE 2—Data for total drag and
drag decomposed into drag due to skin
frictional (D
SF
), drag due to pressure
(D
P
) and drag due to wave (D
W
) are
shown as a function of velocity in the
right panels for the five tested suits in
order (from top to bottom) of decreas-
ing total drag. The left panels show the
suit coverage areas as well as swim-
ming speed plotted as a function of the
swimmer’s height. The shaded area in-
dicates the speeds observed in compet-
itive swimming. All values are the av-
erages of the seven subjects tested. The
three curves in the lower left corner of
the left panel are for transition Reyn-
olds numbers: 3 ⴛ10
3
,3ⴛ10
4
, and 3
ⴛ10
5
. The other two curves are the
same as those in Figure 1; namely, 5 ⴛ
10
5
and 1 ⴛ10
7
. The left panel shows
that the laminar flow region persists
within only the first few centimeters of
the head for the transition Reynolds
numbers shown, whereas the region
between the head and knee has transi-
tional flow. The data for the D
SF
,D
P
,
and D
W
are shown for each suit in the
right panel along with the equations
that describe the data. The D
SF
was
significantly greater for the SA and SK
suits than the WA, WK, and competi-
tion suit, whereas the D
P
and D
W
were
significantly lower, as was the total D.
There was not a statistically significant
difference between the SA and SK
suits.
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Official Journal of the American College of Sports Medicine http://www.acsm-msse.org

greater in the SA and SK suits (42.8 and 45.6 N, respec-
tively, at V ⫽2.2 m·s
⫺1
) than the WA, WK, and CS (20.0,
23.3, and 18.5 N, respectively, at V ⫽2.2 m·s
⫺1
), which
were not significantly different from each other. D
P
in-
creased as a function of V
2
and was significantly lower in
SA and SK (28.3 and 28.9 N, respectively, at V ⫽2.2
m·s
⫺1
) than in WA, WK, and CS (42.4, 41.9, and 44.9 N,
respectively, at V ⫽2.2 m·s
⫺1
), which were not signifi-
cantly different from each other. D
W
increased as a function
of V
4
, and the SA and SK suits values were significantly
lower (10.6 and 3.0 N, respectively, at V ⫽2.2 m·s
⫺1
), than
the WA, WK, and CS (19.9, 18.3, 22.8 N, respectively, at V
⫽2.2 m·s
⫺1
), which were not significantly different from
each other. In summary the higher D
SF
in the SA and SK
suits resulted in a reduction in both the D
P
and D
W
, which
was not observed for the WA or WK suits. Covering the
torso increased the D
SF
, which presumably reduced the D
P
and D
W
by tripping the boundary layer and attaching flow to
the remainder of the body. The absence of significant dif-
ferences between SA and SK (one comparison), and WA
and WK (another comparison) for D
SF
,D
P
, and D
W
may
reside in the observation that flow below the knee is be-
coming turbulent (Fig. 2, left panel) and thus covering this
area does not alter drag. It is tentatively concluded that the
lower D
W
for SK may be due to the absence of coverage
from the knee to the ankle. This effect is apparently coun-
teracted by the increase in D
P
for WA, WK, and CS due to
the absence of coverage from the shoulder to the waist.
Accordingly, coverage from the shoulder to the waist trips
the boundary layer and promotes attached flow, hence low-
ering D
P
.
DISCUSSION
Swimming performance is determined, in part, by the body
drag of the swimmer, which has components of friction, pres-
sure, and wave. The relative importance of these three sources
of drag is velocity dependent; thus, to understand the contri-
bution of these sources of drag in competitive swimming,
velocities of 1.5–2.2 m·s
⫺1
must be evaluated.
Previous studies have shown that the energy requirement
of swimming at all speeds is determined, in part, by the indi-
vidual swimmer’s body density and torque (1,9,10,18,19).
Although these factors are set by the stature of the swimmer,
decreasing his density could lower drag and/or energy require-
ment (11,18,19). This has been shown for a wet suit (15). The
five suits tested in this study had densities slightly greater than
water, and thus would not significantly alter body density. In
this study, the swimmers squeezed-out any air that might have
been trapped between their body and the suit using their hands.
This was done because trapped air could affect body density
during competition (11).
Theoretically, based upon basic fluid mechanics, the way
to reduce the drag of a swimmer at the surface is to: 1)
reduce the wetted surface area, 2) promote laminar flow
over the surface, 3) promote attached flow, and 4) minimize
the production of waves and spray. One potential method to
accomplish drag reduction is to wear drag-reducing suits.
These suits have been purported to reduce skin friction
(11,17), although it should be cautioned that these refer-
ences are nonrefereed swim magazines. However, the re-
sults of a scientific study (16) also suggest a reduction in
drag, although the reduction was not statistically significant.
It should be noted that laminar flow and attached flow are
distinctly independent concepts arising from basic fluid
mechanics. Further, attached turbulent flow typically has
higher skin friction drag but lower pressure drag than de-
tached (separated) laminar flow. It is this trade-off between
the skin friction, pressure and wave drag that suggested the
need for the presently reported study.
It is commonly held that drag-reducing suits reduce D
SF
(10–15%) (17); however, D
SF
is low in swimming
(11,17,20). Consequently, an alteration in D
SF
was previ-
ously believed to hold little promise for drag reduction.
Furthermore, before the presently reported study, the quan-
titative details of the trade-offs between the various kinds of
drag were unknown. A comparison with previously reported
drag breakdown indicates that the pressure drag, D
P
⫽93.5
N (20), is about 2–3 times greater than that reported here.
The skin friction drag, D
SF
⫽0.05 N (20), is approximately
400–900 times smaller than that reported here. The wave
drag, D
W
⫽5 N (20), is about four times smaller than that
reported here. It appears, however, that (20) used the New-
tonian definition of viscosity and the relationship between
surface shear and time-rate-of-change of strain for a New-
tonian fluid for friction resistance. As such, dV/dZ is meant
to represent the slope of the velocity profile at the surface,
not a finite difference approximation to the average slope.
Using the average slope (20) will result in a very significant
underestimate of friction resistance because the velocity
distribution (and hence its slope) is increasing rapidly near
the surface to accommodate the no-slip boundary condition
at the surface. Further, no basis is given for the ranges of
input parameters and one input parameter value used was
not in the range given. Finally, it appears that there are
errors in units and in the numerically computed values. Our
work does not rely on guessing from a range of parameters
but, rather, uses appropriate theoretical formulations and
lets the data (based on measurements) determine the relative
coefficients based on the best fit in a least squares sense. In
the present study, D
SF
was not reduced by the suits tested;
in fact, D
SF
was higher in the SA and SK suits than the WA,
WK, or CS. The density of the swimmer was not affected by
the suits tested; thus, it is unlikely that the wetted surface
area of the swimmer is reduced. The laminar flow region of
the swimmers among all suits tested were not significantly
different (Fig. 1, left column), and transition-to-turbulence
TABLE 1. The mean and SD values for the coefficient of drag, C
D
⫽(drag force)/(
1
⁄
2
␳
V
2
A
S
), determined from the deceleration during a dive and push-off for the five
suits tested.
Suit SA SK WA WK CS
Mean 0.832* 0.897* 0.896* 0.880* 0.978
SD 0.043 0.079 0.081 0.063 0.074
* Significant difference from the competition suit (CS).
EFFECT OF SWIM SUIT DESIGN ON PASSIVE DRAG Medicine & Science in Sports & Exercise姞
1033

occurred at the head, before the areas of the body covered by
any of the suits.
There was significant drag reduction (10–15%) when the
swimmers wore the SA or SK suits compared with the other
suits (Fig. 2, right panel). There was not a statistically signif-
icant difference between the SA and SK suits. The reduction in
total drag occurred, in spite of the observation that D
SF
in-
creased by twofold, due to the reduction in D
P
(37%) and D
W
(60–80%). These data support the conclusion that the in-
creased D
SF
tripped the boundary layer and that the flow
remained attached to the body past the shoulders, in the tran-
sitional phase, and thus lowered both D
P
and D
W
.
The absence of significant drag reduction with drag-
reducing suits that start their coverage at the waist implies
that these suits did not lower D
SF
or trip the boundary layer,
or if they did, the effects were not significant. This finding
is interesting because the lower body coverage suits are the
most commonly ordered men’s suits. It is noted that the
measured glide data in Table 1 indicates that the “leg-only”
body suits (WK and WA) showed a significant reduction in
drag compared with the conventional suit (CS). Further from
Table 1, the SA and SK suits were not significantly different
from each other. On the other hand, using the measured towing
data the SA and SK suits were significantly different from the
WA, WK, and CS suits. We believe that the accuracy of the
measured towing data is better than the accuracy of the mea-
sured glide data because the towing data were taken over a
much longer time period (more than 5 min per speed) whereas
the measured glide data relied on a data fit over a short time (a
few seconds per glide).
The data from this study also demonstrate that the cov-
erage of the leg below the knee is not essential to the drag
reduction of the suits (SK and WK were as effective as SA
and WA). The calf may be in the region of turbulent flow
and thus the boundary layer can not be tripped or this area
is too small to have an effect. Table 1 shows that the SA,
SK, WA, and WK suits exhibited significant difference
from the CS suit (only), not between the SA, SK, WA, and
WK suits. As stated earlier, a more accurate discriminator is
the measured towing data, for the reasons given above.
It should be noted that the observation that the laminar
flow is disrupted at the head suggests that tripping the
boundary layer at the head gives the greatest reduction in
drag. It should also be considered that tripping the boundary
layer at multiple locations may be required as flow separa-
tion may occur at all parts of the body were there are marked
changes in curvature (head, shoulders, buttocks).
An alternative method to reduce drag would be to place
protuberances (vortex generators) at strategic places on the
swimmer. Some models of these suits with protuberances
have been developed and tested and have been shown to
reduce drag, with 2.5-mm protuberances being recom-
mended (11,17). A prototype suit with 10-mm protuber-
ances at the buttocks was tested on six swimmers and was
not found to reduce drag (C
D
of 0.846 ⫾0.119) when
compared with a similar suit without protuberances (C
D
0.869 ⫾0.203); however, both suits were lower than a
competition suit (0.902 ⫾0.115). Additional details are
given in the thesis, Oppenheim, E. Model parameter and
drag coefficient estimation from swimmer velocity measure-
ments (M.S. Thesis, Department of Mechanical Engineering,
University at Buffalo, 1997). The Fastskin suits used in the
present study have microscopic vortex generators and riblets to
channel water flow and have been purported to reduce total
drag by 7.5% (11); however, this claim has been questioned
(11), and the D
SF
in the present study was either not changed
(WA, WK) or increased (SA, SK). The absence of effective-
ness of the protuberances tested to date may be due to their
placement or size, as the thickness of the boundary layer and its
location are not known.
Based on the data from this study, the SA and SK drag-
reducing suits reduce drag significantly, and the effect is
large enough to have an effect on competitive swimming
performance. The suits did not compromise the maximal
velocity of the dive or turn. The drag while actually swim-
ming (active drag) is significantly greater than passive drag,
due to the changes in body position and density and move-
ment of arms and legs changing the direction of flow over
the vortex generators and riblets (4,5,10,11,15). Previous
studies have suggested that there is no reduction in active
drag when wearing drag reduction suits (16). These authors
report, however, that some swimmers did have a reduction
in active drag (21.66 v
2.23
) compared with a competition suit
(23.32 v
2.29
) which represents an 11% reduction at 1.65
m·s
⫺1
. It is unclear what the differences in the subjects that
had a reduction in drag and those that did not were, if any.
The system used to determine drag in these studies requires
the swimmer to use only arms, with the legs floated which
alters active drag and perhaps the effects of the suits.
It has been shown that shaving body hair reduces the
energy cost of swimming, presumably due to reduced D
SF
,
(13) and in this case the suits may not provide any additional
benefit (11). The present study used unshaven swimmers;
however, the C
D
of shaved swimmers determined by the
methods used in this study reduced the C
D
of the same
shaved swimmers from 0.58 to 0.53 (9%), which is less than
the effects of the suits (15%) and previously shown by the
same group for a torso suit (14). Additional details are given
in the thesis, Oppenheim, E. Model parameter and drag
coefficient estimation from swimmer velocity measure-
ments (M.S. Thesis, Department of Mechanical Engineer-
ing, University at Buffalo, 1997).
It is clear that loose fitting competition or drag-reducing
suits can increase drag when compared with tight-fitting
suits (16; 17). We compared a competition suit with a
training suit, using the system described above, and found a
C
D
of 1.04 and 1.29, respectively. The data clearly suggests
that both types of suits should be tight fitting.
In summary, the total passive drag determined in the
presently reported study was significantly reduced by
Fastskin suits that covered the swimmer from the shoulder
to either the ankle or knee compared with models that
covered only the lower body or a conventional suit. The data
demonstrate that laminar flow undergoes transition-to-tur-
bulence in the first few centimeters of the body (head) and
that the flow is transitional over the rest of the body. Con-
1034
Official Journal of the American College of Sports Medicine http://www.acsm-msse.org

trary to common beliefs, the D
SF
is increased by these suits,
and the data are consistent with the concept that the in-
creased friction on the shoulders caused by the suit, trips the
boundary layer, causing attachment of flow to the body, and
results in reduced pressure, wave and total drag. It seems
that at least in some swimmers, the active drag is also
reduced (16). Based on these data, suits that cover from the
shoulder reduce drag and may improve performance, but
only for velocities above 1.5 m·s
⫺1
. Below 1.5 m·s
⫺1
,no
differences were observed. Changing the characteristic of
flow around the head of a swimmer and effective use of
protuberances in strategic locations on the body needs fur-
ther study as does the implications for these passive drag
studies on active drag and performance. Nevertheless, de-
composing total drag, into D
SF
,D
P
and D
W
is believed to be
helpful in understanding the physical mechanisms that de-
termine drag.
We thank Adolph Kiefer and Associates for their data on swim-
ming suit use and the University at Buffalo Swimming and Diving
Team for their participation. We acknowledge the technical assis-
tance of Dean Marky, Frank Modlich, Eric Stimson, and Michael
Fletcher.
The authors gratefully acknowledge support for this research
from the United States Navy, NAVSEA, Navy Experimental Diving
Unit, contract N6133199C0028.
No funding or support was given by the swimming suit manufac-
turer or supplier.
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