Swim Hydrodynamics and Equipment

Skin friction, pressure (form) drag and wave-making drag together resist a surface swimmer.

Source: Toussaint & Beek, Sports Med 1992 · recall

Water is roughly 800-830 times denser than air, which is why speeds in water are so much lower.

Source: Toussaint & Beek, Sports Med 1992 · recall

As in air, drag in water scales roughly with velocity squared, so the power needed rises about with the cube.

Source: Toussaint & Beek, Sports Med 1992 · recall

Wave-making resistance rises sharply with speed as the swimmer pushes up a larger bow wave.

Source: Vennell, Pease & Wilson, J Biomech 2006 · recall

Wave drag fell to little at depths of roughly 0.5-0.7 m, which is why glides and push-offs are done underwater.

Source: Vennell, Pease & Wilson, J Biomech 2006 · recall

Travelling below the surface avoids wave-making resistance, so a streamlined glide keeps speed longer.

Source: Vennell, Pease & Wilson, J Biomech 2006 · applied

Passive drag is measured on a towed, still body; active drag includes the body movements of real swimming.

Source: Toussaint & Beek, Sports Med 1992 · recall

A body sloping downward presents a bigger cross-section, leaving a larger turbulent wake.

Source: Toussaint & Beek, Sports Med 1992 · recall

Gross efficiency of front crawl is low; much energy is lost to heat and to moving water rather than the body.

Source: Toussaint & Beek, Sports Med 1992 · recall

With drag rising roughly with v squared, power rises about with v cubed: 1.1 cubed is about 1.33.

Source: Toussaint & Beek, Sports Med 1992 · applied

The buoyant force equals the weight of the displaced water.

Source: Toussaint & Beek, Sports Med 1992 · recall

When floating still, the air-filled lungs put the centre of buoyancy closer to the head than the centre of mass, which sits nearer the hips; the two forces form a torque that drops the legs. Mid-stroke the picture changes, as Yanai showed.

Source: Toussaint & Beek, Sports Med 1992 · recall

Fat (about 0.9 g/ml) floats while muscle (about 1.06 g/ml) sinks, so body composition changes buoyancy.

Source: Pendergast et al., J Appl Physiol 1977 · recall

Sea water at about 1.025 g/ml gives more buoyant force than fresh water.

Source: Toussaint & Beek, Sports Med 1992 · recall

Less air in the lungs means less displaced volume and lower buoyancy.

Source: Toussaint & Beek, Sports Med 1992 · recall

Greater fat mass and a smaller legs-down torque gave women a more horizontal position and lower drag.

Source: Pendergast et al., J Appl Physiol 1977 · recall

Lifting the legs gives a flatter body line and a smaller cross-section to the water.

Source: Toussaint et al., Med Sci Sports Exerc 1989 · recall

The buoy adds buoyancy low on the body, countering the legs-down torque.

Source: Toussaint & Beek, Sports Med 1992 · applied

Dense, lean bodies sink more, especially at the legs, raising drag and the effort needed to kick.

Source: Pendergast et al., J Appl Physiol 1977 · applied

Buoyant force equals weight, so the body stays at its depth without effort.

Source: Toussaint & Beek, Sports Med 1992 · recall

Closed-cell foam traps nitrogen bubbles, making the material lighter than water and insulating.

Source: Toussaint et al., Med Sci Sports Exerc 1989 · recall

Both rule sets cap wetsuit thickness at 5 mm to limit buoyancy.

Source: World Triathlon Competition Rules (2025) · recall

At about 1.25 m/s, a wetsuit reduced active drag by roughly 12-14%.

Source: Toussaint et al., Med Sci Sports Exerc 1989 · recall

Buoyancy is most useful at the hips and legs, while the shoulders need flexibility for the stroke.

Source: Chatard & Millet, Sports Med 1996 · recall

Slick coatings reduce skin friction and shed water.

Source: Chatard & Millet, Sports Med 1996 · recall

A tight suit resists chest expansion, adding to the load of breathing in water.

Source: Chatard & Millet, Sports Med 1996 · recall

Loose seals at the neck or wrists let water circulate, cooling the swimmer and adding drag.

Source: Chatard & Millet, Sports Med 1996 · recall

Insulation that helps in cold water traps heat in warm water, risking overheating during hard efforts.

Source: Chatard & Millet, Sports Med 1996 · recall

The force saved rises with speed, though weaker swimmers gain more time because of their body position.

Source: Toussaint et al., Med Sci Sports Exerc 1989 · recall

Removing sleeves reduces shoulder restriction; the legs, where buoyancy matters most, stay covered.

Source: Chatard & Millet, Sports Med 1996 · applied

Dozens of world records fell in 2008-09 in polyurethane suits, leading FINA to restrict suits to textiles.

Source: FINA Requirements for Swimwear Approval (2010) · recall

Low-permeability panels added buoyancy and squeezed the body into a more streamlined shape.

Source: FINA Requirements for Swimwear Approval (2010) · recall

Approved suits must be textile; men's suits reach from waist to knee, women's from shoulder to knee.

Source: FINA Requirements for Swimwear Approval (2010) · recall

Swimskins are low-friction textile suits worn over the race suit to cut drag in non-wetsuit swims. World Triathlon allows them for age-group athletes in every event and bars them for elite, U23, junior, youth and para athletes in standard and shorter races.

Source: World Triathlon Competition Rules (2025) · recall

Swimskins must be textile with no neoprene, so they cannot give flotation.

Source: World Triathlon Competition Rules (2025) · recall

Holding soft tissue firmly gives a smoother outline, reducing wake and vibration.

Source: FINA Requirements for Swimwear Approval (2010) · recall

Hydrophobic fabric sheds water, lowering friction and avoiding a heavy, waterlogged suit.

Source: FINA Requirements for Swimwear Approval (2010) · recall

Approval tests measure thickness, buoyancy and air permeability among other properties.

Source: FINA Requirements for Swimwear Approval (2010) · recall

The swimskin gives lower drag in the water and is taken off in transition.

Source: World Triathlon Competition Rules (2025) · applied

Any buoyant material can make the garment a wetsuit, which is banned when wetsuits are not allowed.

Source: World Triathlon Competition Rules (2025) · applied

Pushing water backwards creates an equal and opposite force on the swimmer.

Source: Toussaint & Beek, Sports Med 1992 · recall

Counsilman applied Bernoulli's principle, arguing hands work like foils to create lift.

Source: Counsilman, Biomechanics in Swimming symposium (1971) · recall

Research suggests propulsion comes mainly from drag forces on the hand and forearm, with some lift contribution.

Source: Toussaint & Beek, Sports Med 1992 · recall

Some work is wasted pushing water around; propelling efficiency is the useful share.

Source: Toussaint & Beek, Sports Med 1992 · recall

Studies found the kick adds roughly 10% to speed; its main role is body position and balance.

Source: Deschodt et al., Eur J Appl Physiol 1999 · recall

An early vertical forearm presents more area facing backwards, increasing propulsive force.

Source: Toussaint & Beek, Sports Med 1992 · recall

A bigger surface pushes against more water, increasing force through the shoulder.

Source: Toussaint & Beek, Sports Med 1992 · recall

When the hand accelerates water rather than holding it, less of the effort moves the body.

Source: Toussaint & Beek, Sports Med 1992 · recall

Force peaks during the pull and falls during recovery, so speed rises and falls each cycle.

Source: Toussaint & Beek, Sports Med 1992 · recall

Because drag rises steeply with speed and acceleration costs work, a steadier speed is cheaper.

Source: Toussaint & Beek, Sports Med 1992 · applied

A winch or tow line pulls a streamlined swimmer, and the force needed is recorded.

Source: Toussaint & Beek, Sports Med 1992 · recall

The Measuring Active Drag system used underwater pads with force sensors to estimate drag while swimming.

Source: Hollander et al., Int J Sports Med 1986 · recall

Swimming against a fixed tether with a force sensor gives propulsive force without forward motion.

Source: Toussaint & Beek, Sports Med 1992 · recall

Stationary swimming changes stroke mechanics and removes drag, so it only partly predicts free swimming.

Source: Toussaint & Beek, Sports Med 1992 · recall

Swimmers sprint with and without a small hydrodynamic body attached; the speed change gives active drag.

Source: Kolmogorov & Duplishcheva, J Biomech 1992 · recall

Accelerometers and gyroscopes on the body record stroke phases, rotation and speed changes.

Source: Toussaint & Beek, Sports Med 1992 · recall

Arms and legs create both propulsion and drag, so separating them needs indirect methods.

Source: Toussaint & Beek, Sports Med 1992 · recall

Underwater views reveal what cannot be seen from the deck, such as hand path and hip height.

Source: Toussaint & Beek, Sports Med 1992 · recall

Doubling tow speed roughly quadruples the drag force.

Source: Toussaint & Beek, Sports Med 1992 · recall

Holding effort and stroke rate constant, a faster time or longer distance per stroke suggests less drag.

Source: Toussaint & Beek, Sports Med 1992 · applied

Swimming 0-50 cm behind the feet reduced passive drag by around 21%.

Source: Chatard & Wilson, Med Sci Sports Exerc 2003 · recall

Lateral drafting at about hip level gave a smaller drag reduction than directly behind.

Source: Chatard & Wilson, Med Sci Sports Exerc 2003 · recall

Drafting lowered oxygen uptake, blood lactate and rated effort at the same speed.

Source: Chatard & Wilson, Med Sci Sports Exerc 2003 · recall

At low speeds and with turbulent wakes, the shelter behind a swimmer is less complete.

Source: Chatard & Wilson, Med Sci Sports Exerc 2003 · recall

The benefit is greatest very close and falls off rapidly with distance.

Source: Chatard & Wilson, Med Sci Sports Exerc 2003 · recall

Swimming at the same speed for less effort reduces fatigue carried into the next legs.

Source: Delextrat et al., Med Sci Sports Exerc 2003 · recall

Triathletes who drafted in the swim showed better cycling efficiency afterwards than after swimming alone.

Source: Delextrat et al., Med Sci Sports Exerc 2003 · recall

A good leader sets the line, reducing how often the follower lifts the head to sight.

Source: Chatard & Wilson, Med Sci Sports Exerc 2003 · applied

Sitting behind a slower swimmer saves energy but can cost time overall.

Source: Chatard & Wilson, Med Sci Sports Exerc 2003 · applied

Non-drafting rules apply to the bike leg; swimmers may draft in the water.

Source: World Triathlon Competition Rules (2025) · recall

Aiming into the current (a ferry angle) cancels drift so the actual path runs straight to the target.

Source: Bowditch, American Practical Navigator (2019) · recall

Path length scales with 1/cos(10 degrees), about 1.015, so roughly 1.5% extra.

Source: Bowditch, American Practical Navigator (2019) · applied

Deviations add length and corrections break rhythm, so small errors compound over a long swim.

Source: Bowditch, American Practical Navigator (2019) · recall

The watch loses signal every time the arm is submerged, so tracks are estimated between fixes.

Source: Bowditch, American Practical Navigator (2019) · recall

Mounted under the cap it is exposed to the sky more consistently than a wrist unit.

Source: Bowditch, American Practical Navigator (2019) · recall

Against a current, ground speed is reduced by the current's speed.

Source: Bowditch, American Practical Navigator (2019) · recall

The slow upstream leg lasts longer than the fast downstream leg, so the net effect is a slower total.

Source: Bowditch, American Practical Navigator (2019) · recall

Tidal flows can strengthen or reverse during a race window, changing the best line.

Source: Bowditch, American Practical Navigator (2019) · recall

Movement of floats, weed or swimmers in the warm-up shows current direction and strength.

Source: Bowditch, American Practical Navigator (2019) · recall

In conditions that push swimmers off line, more frequent sighting catches drift before it adds distance.

Source: Bowditch, American Practical Navigator (2019) · applied

Hull speed rises with the square root of length, so taller swimmers meet the steep wave-drag rise at higher speeds.

Source: Vennell, Pease & Wilson, J Biomech 2006 · recall

At hull speed the bow wave length matches the body length and resistance climbs sharply.

Source: Vennell, Pease & Wilson, J Biomech 2006 · recall

Stacking hands and squeezing the head between the arms minimises frontal area and pressure drag.

Source: Toussaint & Beek, Sports Med 1992 · recall

Raising the head tilts the body, sinking the legs and increasing drag.

Source: Toussaint & Beek, Sports Med 1992 · recall

After shaving, swimmers had lower blood lactate and better distance per stroke at the same speed.

Source: Sharp & Costill, Med Sci Sports Exerc 1989 · recall

Rolling onto the side reduces the cross-section facing the water during parts of the stroke.

Source: Toussaint & Beek, Sports Med 1992 · recall

Legs splaying beyond the body line increase frontal area and turbulence.

Source: Toussaint & Beek, Sports Med 1992 · recall

Longer levers and larger surfaces help anchor water and cover more distance per stroke.

Source: Toussaint & Beek, Sports Med 1992 · recall

Reflected waves add turbulence; wave-absorbing lane ropes reduce this in fast pools.

Source: Vennell, Pease & Wilson, J Biomech 2006 · recall

Rotating to breathe with one goggle in the water keeps the body line and hips up.

Source: Toussaint & Beek, Sports Med 1992 · applied

Drag in water is so high that covering a kilometre swimming uses several times more energy than running it.

Source: di Prampero et al., J Appl Physiol 1974 · recall

Energy cost is metabolic energy divided by distance, reflecting drag and efficiency.

Source: di Prampero et al., J Appl Physiol 1974 · recall

Because drag rises with speed squared, the energy spent per metre climbs sharply at higher speed.

Source: di Prampero et al., J Appl Physiol 1974 · recall

Economy in swimming depends heavily on technique, far more than in running.

Source: Toussaint & Beek, Sports Med 1992 · recall

Differences in drag and propelling efficiency dominate swim speed at a given aerobic capacity.

Source: Toussaint & Beek, Sports Med 1992 · applied

Arms-dominant work in a horizontal, immersed position reaches a lower peak oxygen uptake.

Source: Toussaint & Beek, Sports Med 1992 · recall

Swimmers who cover more distance per stroke at a given speed tend to be more economical.

Source: Toussaint & Beek, Sports Med 1992 · recall

A wetsuit reduces drag, so less metabolic energy is needed per metre.

Source: Chatard & Millet, Sports Med 1996 · recall

Technique gains lower drag and raise propelling efficiency, improving speed without more aerobic power.

Source: Toussaint & Beek, Sports Med 1992 · applied

A cheaper swim leaves more glycogen and less fatigue for the longer bike and run legs.

Source: Delextrat et al., Med Sci Sports Exerc 2003 · applied