# Swim Hydrodynamics and Equipment · Triathlon Level 8: Expert

Triathlon · Level 8: Expert · Insporta Book (https://book.insporta.app/triathlon/level-8/swim-hydrodynamics-and-equipment/)

Swim Hydrodynamics and Equipment in Triathlon (Level 8: Expert) on Insporta: 100 sourced facts behind 10 tests. Covers: drag in water; and more.

The physics of moving through water: drag in water, buoyancy and body position, wetsuit science, swimskins and suit rules, propulsion, measuring drag and force, drafting, currents and navigation, streamlining and the energetics of swimming.

## Test 1: Drag in Water

- Skin friction, pressure (form) drag and wave-making drag together resist a surface swimmer. (Source: Toussaint & Beek, Sports Med 1992)
- 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)
- 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)
- Wave-making resistance rises sharply with speed as the swimmer pushes up a larger bow wave. (Source: Vennell, Pease & Wilson, J Biomech 2006)
- 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)
- Travelling below the surface avoids wave-making resistance, so a streamlined glide keeps speed longer. (Source: Vennell, Pease & Wilson, J Biomech 2006)
- Passive drag is measured on a towed, still body; active drag includes the body movements of real swimming. (Source: Toussaint & Beek, Sports Med 1992)
- A body sloping downward presents a bigger cross-section, leaving a larger turbulent wake. (Source: Toussaint & Beek, Sports Med 1992)
- 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)
- 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)

## Test 2: Buoyancy and Body Position Physics

- The buoyant force equals the weight of the displaced water. (Source: Toussaint & Beek, Sports Med 1992)
- 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)
- 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)
- Sea water at about 1.025 g/ml gives more buoyant force than fresh water. (Source: Toussaint & Beek, Sports Med 1992)
- Less air in the lungs means less displaced volume and lower buoyancy. (Source: Toussaint & Beek, Sports Med 1992)
- 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)
- 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)
- The buoy adds buoyancy low on the body, countering the legs-down torque. (Source: Toussaint & Beek, Sports Med 1992)
- 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)
- Buoyant force equals weight, so the body stays at its depth without effort. (Source: Toussaint & Beek, Sports Med 1992)

## Test 3: Wetsuit Science

- Closed-cell foam traps nitrogen bubbles, making the material lighter than water and insulating. (Source: Toussaint et al., Med Sci Sports Exerc 1989)
- Both rule sets cap wetsuit thickness at 5 mm to limit buoyancy. (Source: World Triathlon Competition Rules (2025))
- At about 1.25 m/s, a wetsuit reduced active drag by roughly 12-14%. (Source: Toussaint et al., Med Sci Sports Exerc 1989)
- Buoyancy is most useful at the hips and legs, while the shoulders need flexibility for the stroke. (Source: Chatard & Millet, Sports Med 1996)
- Slick coatings reduce skin friction and shed water. (Source: Chatard & Millet, Sports Med 1996)
- A tight suit resists chest expansion, adding to the load of breathing in water. (Source: Chatard & Millet, Sports Med 1996)
- Loose seals at the neck or wrists let water circulate, cooling the swimmer and adding drag. (Source: Chatard & Millet, Sports Med 1996)
- Insulation that helps in cold water traps heat in warm water, risking overheating during hard efforts. (Source: Chatard & Millet, Sports Med 1996)
- 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)
- Removing sleeves reduces shoulder restriction; the legs, where buoyancy matters most, stay covered. (Source: Chatard & Millet, Sports Med 1996)

## Test 4: Swimskins and Textile Suit Rules

- 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))
- Low-permeability panels added buoyancy and squeezed the body into a more streamlined shape. (Source: FINA Requirements for Swimwear Approval (2010))
- 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))
- 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))
- Swimskins must be textile with no neoprene, so they cannot give flotation. (Source: World Triathlon Competition Rules (2025))
- Holding soft tissue firmly gives a smoother outline, reducing wake and vibration. (Source: FINA Requirements for Swimwear Approval (2010))
- Hydrophobic fabric sheds water, lowering friction and avoiding a heavy, waterlogged suit. (Source: FINA Requirements for Swimwear Approval (2010))
- Approval tests measure thickness, buoyancy and air permeability among other properties. (Source: FINA Requirements for Swimwear Approval (2010))
- The swimskin gives lower drag in the water and is taken off in transition. (Source: World Triathlon Competition Rules (2025))
- Any buoyant material can make the garment a wetsuit, which is banned when wetsuits are not allowed. (Source: World Triathlon Competition Rules (2025))

## Test 5: How Swimmers Create Propulsion

- Pushing water backwards creates an equal and opposite force on the swimmer. (Source: Toussaint & Beek, Sports Med 1992)
- Counsilman applied Bernoulli's principle, arguing hands work like foils to create lift. (Source: Counsilman, Biomechanics in Swimming symposium (1971))
- Research suggests propulsion comes mainly from drag forces on the hand and forearm, with some lift contribution. (Source: Toussaint & Beek, Sports Med 1992)
- Some work is wasted pushing water around; propelling efficiency is the useful share. (Source: Toussaint & Beek, Sports Med 1992)
- 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)
- An early vertical forearm presents more area facing backwards, increasing propulsive force. (Source: Toussaint & Beek, Sports Med 1992)
- A bigger surface pushes against more water, increasing force through the shoulder. (Source: Toussaint & Beek, Sports Med 1992)
- When the hand accelerates water rather than holding it, less of the effort moves the body. (Source: Toussaint & Beek, Sports Med 1992)
- Force peaks during the pull and falls during recovery, so speed rises and falls each cycle. (Source: Toussaint & Beek, Sports Med 1992)
- Because drag rises steeply with speed and acceleration costs work, a steadier speed is cheaper. (Source: Toussaint & Beek, Sports Med 1992)

## Test 6: Measuring Drag and Force in the Water

- A winch or tow line pulls a streamlined swimmer, and the force needed is recorded. (Source: Toussaint & Beek, Sports Med 1992)
- 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)
- Swimming against a fixed tether with a force sensor gives propulsive force without forward motion. (Source: Toussaint & Beek, Sports Med 1992)
- Stationary swimming changes stroke mechanics and removes drag, so it only partly predicts free swimming. (Source: Toussaint & Beek, Sports Med 1992)
- Swimmers sprint with and without a small hydrodynamic body attached; the speed change gives active drag. (Source: Kolmogorov & Duplishcheva, J Biomech 1992)
- Accelerometers and gyroscopes on the body record stroke phases, rotation and speed changes. (Source: Toussaint & Beek, Sports Med 1992)
- Arms and legs create both propulsion and drag, so separating them needs indirect methods. (Source: Toussaint & Beek, Sports Med 1992)
- Underwater views reveal what cannot be seen from the deck, such as hand path and hip height. (Source: Toussaint & Beek, Sports Med 1992)
- Doubling tow speed roughly quadruples the drag force. (Source: Toussaint & Beek, Sports Med 1992)
- Holding effort and stroke rate constant, a faster time or longer distance per stroke suggests less drag. (Source: Toussaint & Beek, Sports Med 1992)

## Test 7: Drafting in the Water

- Swimming 0-50 cm behind the feet reduced passive drag by around 21%. (Source: Chatard & Wilson, Med Sci Sports Exerc 2003)
- Lateral drafting at about hip level gave a smaller drag reduction than directly behind. (Source: Chatard & Wilson, Med Sci Sports Exerc 2003)
- Drafting lowered oxygen uptake, blood lactate and rated effort at the same speed. (Source: Chatard & Wilson, Med Sci Sports Exerc 2003)
- At low speeds and with turbulent wakes, the shelter behind a swimmer is less complete. (Source: Chatard & Wilson, Med Sci Sports Exerc 2003)
- The benefit is greatest very close and falls off rapidly with distance. (Source: Chatard & Wilson, Med Sci Sports Exerc 2003)
- Swimming at the same speed for less effort reduces fatigue carried into the next legs. (Source: Delextrat et al., Med Sci Sports Exerc 2003)
- Triathletes who drafted in the swim showed better cycling efficiency afterwards than after swimming alone. (Source: Delextrat et al., Med Sci Sports Exerc 2003)
- A good leader sets the line, reducing how often the follower lifts the head to sight. (Source: Chatard & Wilson, Med Sci Sports Exerc 2003)
- Sitting behind a slower swimmer saves energy but can cost time overall. (Source: Chatard & Wilson, Med Sci Sports Exerc 2003)
- Non-drafting rules apply to the bike leg; swimmers may draft in the water. (Source: World Triathlon Competition Rules (2025))

## Test 8: Currents, Lines and Navigation Physics

- Aiming into the current (a ferry angle) cancels drift so the actual path runs straight to the target. (Source: Bowditch, American Practical Navigator (2019))
- Path length scales with 1/cos(10 degrees), about 1.015, so roughly 1.5% extra. (Source: Bowditch, American Practical Navigator (2019))
- Deviations add length and corrections break rhythm, so small errors compound over a long swim. (Source: Bowditch, American Practical Navigator (2019))
- The watch loses signal every time the arm is submerged, so tracks are estimated between fixes. (Source: Bowditch, American Practical Navigator (2019))
- Mounted under the cap it is exposed to the sky more consistently than a wrist unit. (Source: Bowditch, American Practical Navigator (2019))
- Against a current, ground speed is reduced by the current's speed. (Source: Bowditch, American Practical Navigator (2019))
- 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))
- Tidal flows can strengthen or reverse during a race window, changing the best line. (Source: Bowditch, American Practical Navigator (2019))
- Movement of floats, weed or swimmers in the warm-up shows current direction and strength. (Source: Bowditch, American Practical Navigator (2019))
- In conditions that push swimmers off line, more frequent sighting catches drift before it adds distance. (Source: Bowditch, American Practical Navigator (2019))

## Test 9: Body Shape and Streamlining

- 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)
- At hull speed the bow wave length matches the body length and resistance climbs sharply. (Source: Vennell, Pease & Wilson, J Biomech 2006)
- Stacking hands and squeezing the head between the arms minimises frontal area and pressure drag. (Source: Toussaint & Beek, Sports Med 1992)
- Raising the head tilts the body, sinking the legs and increasing drag. (Source: Toussaint & Beek, Sports Med 1992)
- After shaving, swimmers had lower blood lactate and better distance per stroke at the same speed. (Source: Sharp & Costill, Med Sci Sports Exerc 1989)
- Rolling onto the side reduces the cross-section facing the water during parts of the stroke. (Source: Toussaint & Beek, Sports Med 1992)
- Legs splaying beyond the body line increase frontal area and turbulence. (Source: Toussaint & Beek, Sports Med 1992)
- Longer levers and larger surfaces help anchor water and cover more distance per stroke. (Source: Toussaint & Beek, Sports Med 1992)
- Reflected waves add turbulence; wave-absorbing lane ropes reduce this in fast pools. (Source: Vennell, Pease & Wilson, J Biomech 2006)
- Rotating to breathe with one goggle in the water keeps the body line and hips up. (Source: Toussaint & Beek, Sports Med 1992)

## Test 10: The Energetics of Swimming

- 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)
- Energy cost is metabolic energy divided by distance, reflecting drag and efficiency. (Source: di Prampero et al., J Appl Physiol 1974)
- 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)
- Economy in swimming depends heavily on technique, far more than in running. (Source: Toussaint & Beek, Sports Med 1992)
- Differences in drag and propelling efficiency dominate swim speed at a given aerobic capacity. (Source: Toussaint & Beek, Sports Med 1992)
- Arms-dominant work in a horizontal, immersed position reaches a lower peak oxygen uptake. (Source: Toussaint & Beek, Sports Med 1992)
- Swimmers who cover more distance per stroke at a given speed tend to be more economical. (Source: Toussaint & Beek, Sports Med 1992)
- A wetsuit reduces drag, so less metabolic energy is needed per metre. (Source: Chatard & Millet, Sports Med 1996)
- Technique gains lower drag and raise propelling efficiency, improving speed without more aerobic power. (Source: Toussaint & Beek, Sports Med 1992)
- 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)

Sources: Peer-reviewed sports engineering and physiology literature, with published test data and standards for settled facts

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