Water is 800 times denser than air. Swim smarter, not harder.

Three kinds of drag act on a swimmer at the surface. Form (pressure) drag comes from the shape you push through the water: a head lifted high or hips dropped low present a bigger, blunter shape. Skin-friction drag is the water dragging along your skin and costume, small at swimming speeds but not zero. Wave drag is the energy you spend making waves; it rises steeply as you swim faster. Together they follow roughly D = ½ × ρ × CdA × v², where ρ is water density and CdA lumps your shape and size into one "drag area".

Why streamlining beats strength

Because drag scales with CdA, a small change in shape is worth as much as a big change in effort. A long, flat, narrow body (head in line with the spine, hips high, legs kicking inside the body's "shadow") lowers CdA. A 10% lower drag area gives 10% less drag at any speed. Getting the same gain from fitness alone would take 10% more power.

Waves, and why racers go underwater

At the surface, part of your effort becomes the bow wave in front of your head and the trough behind. A streamlined body some way below the surface makes far smaller waves, so research on towed swimmers finds wave drag drops away once the body is roughly half a metre or more down. That's why racers glide and dolphin-kick after starts and turns. World Aquatics rules allow you to be fully submerged for no more than 15 metres after the start and each turn in freestyle, backstroke and butterfly; by then your head must have broken the surface.

Archimedes: the water pushes up on you with a force equal to the weight of the water you displace. Fat is less dense than water and lean tissue and bone are denser, so people differ, but most adults end up very close to the density of water. The big variable is the air in your lungs: a full breath adds several litres of volume for a few grams of mass.

Why the legs sink

Upthrust acts at the centre of buoyancy, around your air-filled chest. Your weight acts at the centre of mass, nearer your hips. These two points are a few centimetres apart, so even when the forces balance, they twist you: chest up, legs down. With 687 N of weight and an offset of, say, 2 cm, that's a turning effect of about 14 N·m, enough to swing still legs towards the bottom.

Swimmers fight this with body position, not strength: head neutral (eyes down, not forward), a slight press of the chest into the water, a long reach in front (moving mass towards the head), and a small, steady kick. Lifting the head to look forward drops the hips and raises form drag (Ch 1).

Start with what's certain: Newton's third law. To move forward, you have to accelerate water backwards. The thrust you get equals the mass of water you move each second multiplied by how much you speed it up: thrust = ṁ × Δv.

The two theories

Drag-based propulsion treats the hand and forearm as a paddle pushing water straight back. Lift-based propulsion treats the hand as a small hydrofoil: angled and moved partly sideways (sculling), it generates a force at right angles to its motion, like a wing. In the 1970s, the lift idea became very popular and coaches taught a pronounced S-shaped pull to "use" it.

Later research, with better measurement and flow visualisation, found the picture more mixed. In much of the front-crawl pull, pushing water backwards (drag) does a large share of the work, while lift contributes to varying degrees depending on hand angle, stroke phase and swimmer. Both simple models also assume steady flow, but a real stroke is unsteady: the hand accelerates, sheds vortices and drags water with it. The honest summary: both mechanisms contribute, the exact split varies, and it's still an active research topic. Coaching has moved towards a straighter, deeper pull with the forearm held near vertical early in the stroke, so a big surface pushes water backwards.

Arms, legs and the rest of you

In front crawl over distance, the arms provide most of the propulsion. For many distance swimmers the legs mainly keep the body level and balanced (Ch 2), adding less thrust while costing a lot of energy, since leg muscles are big. Sprinters kick hard; open-water swimmers usually kick lightly.

Breathing in front crawl

You breathe by rotating your head to the side, into the trough your head's bow wave makes beside your face, so your mouth clears the water without lifting your head. Breathe out steadily underwater so that the brief moment of turning is only for breathing in. Breathing every three strokes alternates sides and keeps the stroke symmetrical; every two strokes gives more air at higher effort. In open water, breathing to both sides lets you breathe away from waves and see more.

Why breath-holding games are dangerous

Your urge to breathe is driven mostly by rising carbon dioxide, not falling oxygen. Over-breathing (hyperventilating) before a long underwater swim washes out CO₂ but adds very little oxygen. The warning then comes too late: oxygen can fall low enough to cause a blackout underwater, without any feeling of distress first. Never hyperventilate, never do breath-hold challenges, and never swim underwater alone.

The energy cost of a kilometre

The human body turns only a small fraction of food energy into useful forward work in swimming; studies put the gross efficiency of front crawl at only a few per cent to around ten per cent. Physiology studies of front crawl put a good swimmer's energy cost at very roughly 0.7–1 kJ per metre. For a 70 kg runner it's about 0.3 kJ per metre, so swimming costs around 2.5–3.5 times as much per kilometre, and beginners far more.

Currents are just vector addition

Your speed over the ground is your swimming speed plus the water's speed, with directions. With the current, they add; against it, they subtract. Across it, you have to aim upstream to hold a straight line, at an angle where sin(angle) = current ÷ your speed.

Rip currents

A rip is a narrow channel of water flowing out to sea from the beach, often faster than a swimmer. Fighting one head-on exhausts you. Stay calm, float to save energy, raise an arm to signal for help, and swim parallel to the shore to get out of the channel before heading in. Best of all, swim only in the zone the lifeguards mark with flags.

Sighting: swimming straight without a line

Every few strokes, lift your eyes just above the water to spot the next buoy or a big landmark behind it, then turn your head to breathe as normal. Small errors add up fast: drifting 10° off line over 1 km puts you 1,000 × sin 10° ≈ 174 m off course. Goa's swimathon places sighting buoys every 150–200 m and asks swimmers to keep them on their left. Breathing to both sides helps you correct a stroke that pulls you one way.

Mass starts and drafting

Open-water races start in a crowd. Organisers ask swimmers to "self-seed": slower swimmers start towards the back, so nobody is swum over. Swimming just behind or beside another swimmer (drafting) means less effort for the same speed, because they've already set the water moving. It's allowed in most open-water races, but stay clear of other swimmers' feet.

Why water chills you so fast

Water's thermal conductivity (about 0.6 W/m·K) is roughly 25 times that of air, and a litre of water soaks up far more heat than a litre of air. Air at 20 °C feels pleasant; water at 20 °C can chill a slim swimmer within an hour. Sudden immersion in cold water also triggers cold shock: an involuntary gasp and fast breathing in the first minute, which is dangerous if your face is underwater. Enter cold water slowly and float until your breathing settles.

Warm seas: the Indian problem is heat

Swimming hard, your muscles produce several hundred watts of heat (most of the food energy in Ch 4 ends up as heat). In cool water that heat drains away easily. In warm water there's little temperature difference to carry it off. Goa's swimathon expects water at around 26–28 °C and bans wetsuits, warning that they would make swimmers "dehydrate and overheat rapidly". Hydrate before the start, use the feed stations, and respect the sun on long swims.

Jellyfish, kit and good habits

• Jellyfish: Goa's organisers warn they're always possible, and suggest full-length suits or jammers and a barrier of coconut oil or petroleum jelly on exposed skin.
• Be seen: a bright cap (events issue one, and it's mandatory). Tow floats help visibility, but Goa allows them only in its 5 km and 10 km races, because lines tangle in crowded short races.
• Medical: Mumbai's Sunk Rock race asks every swimmer for a doctor-attested medical certificate; Goa asks swimmers over 60 for one for its longer races.
• Never alone: swim with lifeguards, a club or an organised event. Don't swim within 12 hours of drinking alcohol (a Goa race rule, and good advice anywhere).