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05 / 07 — Life-support stack

Fins

Turning leg muscle into thrust, efficiently, for an hour.

Fins multiply the surface area of your feet so each kick moves real water. The shape, stiffness and even the splits down the blade decide how much oxygen you burn for every metre of water.

Drag rises with the square of speed — slow down

v²

Cruise speed
~0.4 m/s (0.8 kt)
Blade length
30–40 cm (scuba)
Kick styles
Flutter · frog · helicopter
Drag
∝ v² — double speed, 4× drag

Blade, rail, pocket, strap

Drag or use the arrow keys to orbit. Press plus or minus to zoom, Home to reset. Use the parts manifest to move between callouts.

Fins. This 3D plate needs WebGL; the parts list below carries the same content.

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FIG 5.1 — Paddle, Assembled

Scale: the bar is 10 centimetres on the model.

Try the variations

Blade design

Toggle paddle vs split and watch the blade flex pattern change as it kicks.

01Foot pocket

Where your foot lives. Full-foot pockets slip on bare feet (travel, boats); open-heel pockets pair with booties and an adjustable strap — the only sane choice for shore diving over rocks.

Parts list5 parts

  1. Foot pocket

    Where your foot lives. Full-foot pockets slip on bare feet (travel, boats); open-heel pockets pair with booties and an adjustable strap — the only sane choice for shore diving over rocks.

  2. Heel strap

    Open-heel fins use spring straps or adjustable buckles. Spring straps are nearly unbreakable and self-adjusting with thick gloves — one of the cheapest, best upgrades in diving.

  3. Blade

    The working surface. Longer and stiffer blades move more water per kick but demand stronger legs; softer blades are forgiving and sip air gently. Material sets the snap: plastic, composite, or carbon.

  4. Side rails

    Thickened rubber ribs along the blade edges that stop water spilling sideways off the fin, channelling it backward into thrust instead. They also control how the blade flexes and snaps back.

  5. Vents / split

    Vented fins let water through at the start of each stroke, cutting drag where the blade is least effective. Split fins go further: the two halves flex apart at lower angles of attack and shed water off the gap, so a fast flutter kick costs less drag — efficiency, not extra thrust.

Pushing water backwards, efficiently

  1. More area, more water

    Thrust comes from accelerating water backward. A fin multiplies your foot’s area several times over, so the same leg muscles push a useful mass of water instead of slipping through it.

    The drag equation (F ∝ v²)

    Drag F = ½ ρ C_d A v²: it rises with the square of speed, and the power your legs must deliver — force times speed — with the cube. Doubling your pace quadruples the drag and costs eight times the effort, and your breathing rate follows. Fin efficiency is really about cruising slowly with long, relaxed kicks, not sprinting.

  2. Flex is a spring

    A good blade bends during the power stroke, storing elastic energy, then snaps straight at the end — adding a free flick of thrust. Too stiff wastes energy; too soft stores nothing.

    Thrust is momentum

    Thrust equals the mass of water you push back each second times how fast you push it. A big blade moving a lot of water slowly wastes less energy than a small blade whipping a little water fast — the same reason a large, slow propeller beats a small, fast one.

  3. Flutter vs frog

    The flutter kick (vertical scissor) is fast and intuitive. The frog kick — knees bent, fins sweeping out and snapping together, then a glide — is efficient, silt-safe, and the technical diver’s default. Kick, glide, kick.

    Split fins, honestly

    Split blades shed water off the gap at lower angles of attack, so the drag penalty of a fast flutter kick falls. Measured studies find lower oxygen cost at cruise speed and lower peak thrust — efficiency, not free thrust. They do not “generate thrust on the upstroke”, whatever the box says.

  4. The upstroke problem

    The up-kick of a flutter cycle produces far less thrust than the down-kick — much of it is just dragging the blade back into position. Vents and splits reduce that drag, and the frog kick hides its recovery inside the glide: the heels come back slowly with the blades feathered edge-on, so it costs almost nothing — less oxygen for the same distance.

Kick, and count the cost

Dive profile — one timeline, seven instruments

T+12 minAL80 · SAC 18 L/min · 5 mm suit

Cylinder

136bar

Wing

1.6L

Suit

2.5mm

Mask air

113mL

NDL left

10min

Ceiling

0m

Ascent

0.0m/min

Depth 30.0 m · ambient 4.00 bar. Drag the needle, or use the arrow keys.

Drag or use the arrow keys to orbit. Press plus or minus to zoom and Home to reset the view.

A fin blade flexing through a kick cycle. This view needs WebGL.

Kick cycle — watch the blade load and snap

Thrust vs. oxygen

Match the fin to the kick

Every combination below is a real trade. Split blades make a flutter kick cheaper; they do not make it stronger, and they take your back kick away. Speed costs the most of all — the power your legs deliver rises with the cube of it.

Challenge

Push the range past 150 m per 100 litres. Two of the three controls have to move.

Blade

Kick

Back kick needs a blade

0.40 m/s

Gas at this pace

20.7L/min

Range per 100 L

116m

Thrust vs. paddle flutter

100%

Back-kick precision

70%

Powerful and thirsty. Half of every cycle is the recovery stroke dragging a solid blade back into position.

Takeaway — locked

Get the range past 150 m per 100 L to unlock what this proves.

Reference rate 18 L/min at 0.40 m/s. Blade and kick multipliers are the direction and rough size of published flume spreads, not one manufacturer’s numbers.

Pick the fin for your kick

Match the fin to the kick you actually use. A paddle blade does every kick — frog, back, helicopter — and is what instructors and technical divers wear. A split fin makes a flutter kick cheaper and the other kicks worse. A long freedive blade is a different sport.

Paddle blade

What it is
One solid blade — direct, powerful, predictable. The benchmark every other fin is compared against.
For
  • Strong, immediate thrust
  • Precise for frog, back and helicopter kicks
  • Works in current
Against
  • Recovery stroke is wasted energy on a flutter kick
  • Stiff models cramp weak legs
Best for
Technical divers, photographers backing up, anyone fighting current.

Split fin

What it is
A blade divided down the middle so the halves flex apart — engineered to make a flutter kick cheaper.
For
  • Lower oxygen cost on a flutter kick at cruise speed
  • Easy on knees and ankles
  • Comfortable cruise
Against
  • Weak for frog and back kicks
  • Lower peak thrust — feels mushy in strong current
  • More edges to snag
Best for
Recreational divers who flutter kick and want longer, easier dives.

Long freedive blade

What it is
Blades of 70–90 cm in plastic, fibreglass or carbon — massive surface, minimal kicks, maximum glide.
For
  • Huge thrust per kick cycle
  • Unmatched efficiency for straight-line swimming
  • Beautiful glide
Against
  • Awkward on land and boats
  • Useless for fine positioning
  • Expensive, fragile (carbon)
Best for
Freedivers and spearfishers; overkill (and annoying) on scuba.

What fails, and what you do

Why slow divers breathe less

Air consumption is the real spec. Lab flume studies find real, measurable spreads in oxygen cost between fin designs at the same speed — but the biggest lever is you: slow, long kicks beat fast, short ones with any fin.

Full-foot fins over rocks will end your dive day before it starts — open-heel plus booties for anything but boats.

Split fins genuinely lower the cost of a flutter kick but offer almost nothing for the frog kick — match the fin to your kick style, not the marketing.

Spring straps survive boat decks, salt and years; rubber buckle straps are the part most likely to snap at the worst moment.

Fins too stiff for your legs cause cramps; too soft and you cannot hold position in a current. Try them in water, not on the shop floor.

Where this chapter connects