Chapter 3: Fins & Flukes — Thrust vs Maneuvering

Created by Sarah Choi (prompt writer using ChatGPT)

Fins & Flukes — Thrust vs Maneuvering

Limbs, Wings, Fins & Tails for Creature Concept Artists

When you put a creature in water (or any dense fluid, including exotic gases or magic fields), fins and flukes become what wings are to the sky and legs are to the ground. They determine:

  • How fast it can move.
  • How tightly it can turn.
  • How it brakes, hovers, or hides.

As a creature concept artist, you don’t need to simulate hydrodynamics—but you do need to design fins and flukes that clearly communicate thrust vs maneuvering roles, and that can actually be animated and staged in a game.

This article will walk through:

  • Fin and fluke appendage classes.
  • Thrust engines vs steering surfaces.
  • Dorsal, pectoral, pelvic, anal, and tail fin functions.
  • Silhouette reads for speed vs agility.
  • Production‑side considerations for rigs and readability.

We’ll keep it equally useful for concepting‑side and production‑side creature artists.

1. Appendage Classes Underwater: Engines vs Rudders

Think of aquatic appendages as part of a vehicle design.

Broadly, fins and flukes fall into two main functional classes:

  • Thrust generators (“engines”) – push the creature forward, up, or down.
  • Control surfaces (“rudders”/“ailerons”) – steer, stabilize, and fine‑tune motion.

Most fins contribute a bit to both, but you should assign a primary role:

Is this fin/fluke meant to drive the creature forward, or to help it turn and balance?

Clear classification makes your designs more readable and easier to animate.

2. Thrust Fins & Flukes — Power & Propulsion

Thrust appendages are where the big muscles attach. Their design signals strength, cadence, and speed.

2.1 Tail Flukes as Main Engines

In many aquatic vertebrates, the tail fluke (or tail fin) is the primary engine.

Types:

  • Vertical tail fin (fish‑like) – side‑to‑side tail sweeps.
  • Horizontal fluke (whale/seal‑like) – up‑and‑down tail strokes.
  • Multi‑lobed or forked tails – combine thrust with some steering.

Design cues for powerful thrust:

  • Broad surface area near the end of the tail.
  • Thick musculature at the tail base.
  • Smooth, streamlined connection to the body.

Silhouette read:

  • A large, well‑defined tail fin suggests strong propulsion and often long‑distance swimming.

Production notes:

  • Tail flukes are typically driven by a root joint at the tail base with a chain of joints down the tail.
  • Animators will often use sine‑wave motion; clear segmentation along the tail helps.

2.2 Pectoral & Pelvic Fins as Auxiliary Engines

While tail flukes are the main engines for many creatures, pectoral (side) fins and pelvic (ventral) fins can also contribute to thrust:

  • Broad, flexible pectorals can provide strong paddle strokes and rapid braking.
  • Some creatures use synchronized flapping of pectoral fins for hovering or slow cruising.

Design cues:

  • Pectorals that are large and wing‑like suggest they contribute to glide and thrust, not just steering.
  • Pelvic fins that are small and closer to the midline often act more as stabilizers.

Production:

  • Large pectorals require rigs similar to arm‑wings; consider joint counts and fold states.

2.3 Undulatory vs Oscillatory Thrust

There are two common thrust patterns you’ll imply in design:

  • Undulatory – waves traveling along the body or fin (eels, sea snakes, some rays).
    • Long, ribbon‑like fins or bodies.
    • Silhouette suggests flexibility and stealth rather than raw sprint speed.
  • Oscillatory – discrete flapping or tail strokes (tunas, dolphins, sharks).
    • More rigid bodies with big, discrete fins or flukes.
    • Silhouette suggests explosive bursts and cruising efficiency.

Design choice:

  • Long continuous fins (dorsal/anal ribbons) → undulatory.
  • Distinct, robust fins → oscillatory.

3. Maneuvering Fins — Steering, Braking & Hovering

Maneuvering fins keep a creature upright, oriented, and capable of quick turns.

3.1 Dorsal Fins as Stability Vanes

Dorsal fins (on the back) are primarily stabilizers:

  • They resist rolling and help keep the creature upright.
  • They can act as directional rudders during turns.

Design traits:

  • Smaller dorsal fins → subtle stabilization, more roll freedom.
  • Larger, taller dorsal fins → strong directional stability, iconic silhouettes.

Visual messaging:

  • A tall, sharp dorsal fin signals predator, cutting through water.
  • Multiple dorsal ridges or sails can read as ancient, armored, or exotic.

Production:

  • Dorsals often have limited motion; small sway and flex is enough.

3.2 Pectoral Fins as Rudders & Brakes

Pectoral fins are your primary steering and braking tools:

  • By angling them, the creature pitches up/down and yaws left/right.
  • Spreading them wide can act as a water brake.

Design cues for maneuvering‑heavy pectorals:

  • Moderate size but high articulation at the base.
  • Shapes with variable attack angles (triangular, rounded wings).

In silhouette:

  • When extended downward or upward, pectorals signal turning or braking.

Production:

  • Pectorals need enough rig controls to pitch, roll, and fold; they’re like short wings.

3.3 Small Fins & Finlets

Small fins (anal fins, finlets along the tail) fine‑tune movement:

  • They help prevent tail oscillations from becoming chaotic.
  • They can add subtle control for rapid maneuvering.

Design:

  • Use them to break up big shapes and add visual rhythm.
  • Don’t overdo the count; choose a pattern and stick to it for faction consistency.

4. Silhouette: Speed vs Agility Reads

Fins and flukes sculpt the body plan’s language of motion.

4.1 Fast Sprinters

Visual traits:

  • Streamlined body.
  • Big, crescent tail fluke with narrow tips.
  • Relatively small pectorals (just enough for steering).
  • Modest dorsal fin.

Read:

  • Built for long, fast runs or powerful chases.

4.2 Agile Maneuverers

Visual traits:

  • Larger pectorals and pelvic fins.
  • More surface area distributed around the body.
  • Tail fluke may be smaller or multi‑lobed, trading raw speed for control.

Read:

  • Built for tight turns, reef environments, complex terrain.

4.3 Hoverers & Drifters

Visual traits:

  • Broad pectoral fins, sometimes wing‑like.
  • Extended dorsal/anal fins that act like stabilizing skirts.
  • Tail may be small or primarily used for minor adjustments.

Read:

  • Built for slow positional control: ambush predators, camouflaged watchers, or magical floaters.

4.4 Heavy Tanks & Juggernauts

Visual traits:

  • Thick bodies, minimal streamlining.
  • Fins that are short and stout, more like barges than race cars.
  • Tail fluke broad but not sharply tapered.

Read:

  • Built for sheer mass and short, brutal surges, not speed.

5. Fins & Flukes Across Body Plans

Different body plans use fins and flukes differently. As a creature artist, you can think in families.

5.1 Vertebrate‑Like Swimmers

Core fins:

  • Tail fluke – main engine.
  • Pectorals – steering, some thrust.
  • Dorsal/anal – stability.

Align fin shapes with:

  • Predator vs grazer roles.
  • Open ocean vs reef vs deep‑sea biomes.

5.2 Arthropod & Insectoid Swimmers

Instead of typical fins, they may have:

  • Leg paddles.
  • Plate fins along segments.
  • Tail fans (shrimp‑like).

Design logic:

  • Use repeated segments for undulatory thrust.
  • Reserve larger tail fans for escape bursts and direction changes.

5.3 Cephalopod & Soft‑Body Swimmers

Cephalopods often use:

  • Mantle jets for thrust.
  • Fins for stabilization and fine motion.
  • Arm/tentacle webs as gliding surfaces.

Design cues:

  • Ear‑like side fins for hovering and slow cruising.
  • Tail‑like webs for sudden directional flips.

5.4 Hybrid & Fantasy Swimmers

Your creature might combine:

  • Dragon‑like bodies with finned tails.
  • Insect wings adapted to water as stiff paddles.
  • Magic‑driven propulsion with decorative fins as only steering.

Just maintain:

  • One clear propulsion system (jets, tail, pectoral flapping) and
  • A set of recognizable steering surfaces.

6. Appendage Classes by Function Underwater

You can classify fins and flukes in a simple table in your mind:

  • Engines: tail flukes, large pectorals, whole‑body undulation.
  • Rudders: dorsal fins, smaller pectorals, finlets, tail tips.
  • Brakes: enlarged pectorals, flaring tail fans.
  • Displays: exaggerated dorsal sails, bioluminescent fins, ornate flukes.

As a concept artist, pick one primary class per fin/fluke when you design it, then add secondary functions as accents.

7. Concept vs Production: Making Fins & Flukes Buildable

7.1 Concept‑Side Focus

When concepting:

  • Draw the creature in at least two states: cruising and turning/attacking.
  • Make sure fin/fluke shapes still make sense in both.
  • Keep silhouettes clean: avoid too many tiny fins that blur together at game scale.

Ask:

  • Can a player tell at a glance if this creature is built for speed, agility, or hovering?
  • Can they predict whether it will charge, dart, or drift?

7.2 Production‑Side Focus

Closer to production, refine fins and flukes to be rig‑friendly:

  • Simplify fin topology, especially at the edges.
  • Ensure membranes are not so tattered they’re impossible to skin cleanly.
  • Group fins logically so animation systems can reuse rigs across a faction.

Communicate in callouts:

  • Which fins are driven by animation (main motion).
  • Which fins can be secondary or simulated (small flutters, wobble).

8. Practical Workflow for Designing Fins & Flukes

Step 1 – Define Environment & Role

Write a one‑line brief:

“Open‑ocean ambush predator that sprints in short bursts, then glides.”

From that, decide:

  • Main engine (tail fluke vs pectoral fins vs whole body undulation).
  • Expected maneuvering style (tight turns vs long arcs).

Step 2 – Block in Body & Appendage Positions

Sketch a simple body:

  • Place dorsal fin(s) for stability.
  • Place pectorals for steering.
  • Define tail length and fluke size.

Check for balance and silhouette clarity.

Step 3 – Shape Fins for Thrust vs Maneuvering

For each fin/fluke:

  • Decide if it’s mainly an engine or rudder.
  • Give engines more area and stronger root connections.
  • Give rudders more subtle shapes and articulation.

Step 4 – Draw Two Poses: Straight Run & Hard Turn

Pose 1 – Straight run/cruise:

  • Tail and fins in efficient, streamlined arrangement.

Pose 2 – Hard turn or sudden stop:

  • Pectorals angled, tail bending, rudder fins engaged.

Silhouette‑test both poses.

Step 5 – Add Story Cues

Use surface detail to support your design:

  • Scars and chips on tail fluke edges for combat veterans.
  • Bioluminescent tips on maneuvering fins for deep‑sea communicators.

Step 6 – Annotate for Handoff

On a clean callout sheet:

  • Label primary and secondary fins.
  • Note which ones contribute to thrust vs maneuvering.
  • Include simple arrows showing typical movement and turn directions.

9. Exercises for Creature Concept Artists

Exercise 1 – Three Swimmers, Three Fin Profiles

Design three silhouettes:

  1. Sprinter predator – large tail fluke, minimal fins.
  2. Reef maneuverer – larger pectorals, multiple small fins.
  3. Hovering ambusher – wide, wing‑like pectorals, small tail.

Keep the same body core; only change fins and flukes. Compare how role reads change.

Exercise 2 – Tail Fluke Variants

Create a sheet of tail flukes:

  • Crescent, forked, square, rounded, ragged.

Under each, write how it would change thrust vs maneuvering and what kind of creature might use it.

Exercise 3 – Turn & Brake Sequence

For one aquatic creature design:

  • Draw three small frames: straight swim, start of turn, hard brake.

Show how fins and flukes change angle and curvature in each. This helps you think like an animator.

10. Closing Thoughts

Fins and flukes might seem like small details, but they’re the primary language of motion in aquatic and fluid‑based environments.

As a concept‑side creature artist, thinking in terms of thrust vs maneuvering helps you:

  • Make clear decisions about body plans and fin layouts.
  • Communicate speed, agility, and temperament through silhouette.
  • Build coherent aquatic factions with shared fin logic.

As a production‑side concept artist, designing fins and flukes with rigging and animation in mind helps you:

  • Avoid impossible shapes that can’t fold or flow.
  • Provide clear guidance about which appendages are engines vs rudders.
  • Support combat and navigation design with believable movement cues.

Each time you design an aquatic or fluid‑flying creature, ask:

  • What is pushing it forward?
  • What is helping it turn and stabilize?
  • Do its fins and flukes clearly show that difference from every key angle?

If the answers make sense in your sketches—and your silhouettes still look compelling at thumbnail size—your fins and flukes won’t just be decorative. They’ll be fully integrated appendages that make your creatures feel alive, functional, and game‑ready in their chosen medium.