Chapter 2: Propulsors & Wake Patterns

Created by Sarah Choi (prompt writer using ChatGPT)

Propulsors (Props, Waterjets) & Wake Patterns for Sea‑Surface & Subsurface Vehicles

Propulsion defines the silhouette behind the hull. For boats, ships, and submarines, the choice and placement of propulsors—open propellers, ducted props, azimuth pods, waterjets, pump‑jets—sets efficiency, maneuvering, noise, and the wake signature your audience sees. This article gives concept‑side artists the physics cues to sketch convincing drives and production‑side artists the details to make them buildable and animatable.

1) Thrust, Slip, and the “Momentum Tube” Picture

A propulsor accelerates a tube of water aft. The net thrust equals mass flow times velocity change plus pressure terms. Designers trade jet speed against efficiency: moving a larger mass of water a little is more efficient than a small mass a lot. This is why big, slow props suit heavy ships and small, fast jets suit light craft. In your sketches, think in streamtubes: clean, straight inflow into the propulsor, accelerated outflow aligned with the craft’s axis, and minimal obstructions.

Slip is the difference between theoretical advance per revolution and actual advance. Some slip is necessary; too much slip implies cavitation or poor loading. Visualize slip through wake: long, coherent streaks with modest froth for efficient props; short, frothy plumes and micro‑bubble clouds for overloaded ones.

2) Open Propellers: Geometry that Reads

Key parameters—diameter, pitch, blade area ratio, skew, rake, and blade count—have visible tells. Larger diameter, fewer blades, and low rpm imply high efficiency and low noise. High blade area and more blades resist cavitation at high load but add drag. Skewed blades (trailing edge swept aft) soften cavitation inception and reduce tonal noise; raked blades (leaning aft) favor tip clearance and hub flows. Controllable‑pitch propellers (CPP) have hubs with visible pitch‑actuation links; model the fat hub and blade roots with trunnion caps.

For readability, keep tip clearance to the hull and free surface plausible; props too near the surface ventilate (suck air) and lose bite. Shaft angles should be shallow; steep shafts waste thrust by pushing water downward.

3) Ducted Props and Nozzles (Kort, High‑Thrust)

A duct (nozzle) around a prop can increase bollard pull at low speeds by recovering swirl and preventing tip loss. Nozzles look like short airfoils with a foil‑shaped cross‑section. They shine on tugs, trawlers, and slow workboats; at higher speeds, the nozzle adds drag. Draw supports (struts) and fairings that smoothly tie the nozzle into the hull; leave maintenance space for rope cutters and debris knives at the hub. High‑lift nozzles create a denser, narrow jet and a tighter, more coherent wake ribbon.

4) Azimuthing Pods and Thrusters

Pods put the motor in a streamlined torpedo that rotates 360°, with a pulling or pushing prop. They leave a clean hull and create superb maneuverability for ferries and cruise ships. Visual cues: a vertical steering column, a torpedo pod with an ovoid section, and a prop in front (puller) or behind (pusher). Tunnel thrusters mount transversely in the bow or stern for low‑speed side force; show grilles or doors to reduce drag underway.

5) Surface‑Piercing & High‑Speed Drives

Surface‑piercing propellers run with only the lower blade arcs submerged. They draw splashy, highly aerated wakes and rooster tails; blade tips scythe air and water, producing dramatic sheeted spray. Drive legs rake aft with trim adjusters; transoms are notched or stepped. These suit racing hulls and aggressive interceptors. Stepped planing hulls re‑energize boundary layers; wake shows discrete white patches aft of steps and a high‑energy fan off the transom.

6) Waterjets: Intakes, Pumps, and Buckets

Waterjets ingest water through a hull intake, accelerate it with an impeller and stator, and eject through a steerable nozzle. Reverse buckets deflect the jet forward for braking and back‑and‑fill maneuvers. In plan views, show flush intakes with grates, S‑ducts leading to pump housings, and compact jet nozzles at the transom. Jets make clean wakes at speed—a narrow, glassy lane with a faint central boil—because there are no props ventilating near the surface. At low speed with heavy throttle, jets blow a distinct turbulent plume and can kick air down the intake if the hull squats.

Jets excel in shallow water and at high speed; they lose at low‑speed bollard pull compared to props. Sell this by making tugboats and trawlers prop‑driven while patrol craft and fast ferries often run jets.

7) Submarines: Quiet Propulsors and Pump‑Jets

Submarines hide their wakes. Large‑diameter, slow, highly skewed seven‑blade props reduce cavitation onset and spread loading over time. Shrouded pump‑jets (ducted mixed‑flow fans) mask blade‑rate tones and let designers tailor pressure gradients; the wake becomes a tight, low‑contrast jet with minimal tip‑vortex signatures. Show an annular nozzle, stator/rotor hints, and fairwater cones at the hub. Around the hull, depict anechoic tile patterns and clean seams to suggest acoustic discipline.

8) Cavitation, Ventilation, and What They Look Like

Cavitation forms when local pressure drops below vapor pressure, creating vapor bubbles that collapse downstream. It erodes blades and screams acoustically. Visually, it’s a milky, glittering cloud off the blade face or tip vortex spirals that flash. Ventilation is air ingestion from the surface, producing bubbly, frothy wakes and a sudden rpm surge. Planing boats near cavitation limits show bright, hissing sheets at tips; efficient displacement props show narrow, faint streaks and a quiet, translucent wake.

Temperature, salinity, and depth shift cavitation onset—subs dive deep to suppress it at speed. If you show a fast submarine near the surface throwing a big milky wake, you’re implying it’s noisy or in emergency power.

9) Wake Patterns by Regime

Displacement hull + prop: classic Kelvin wake—a V‑shaped pattern (about 39° full angle) with diverging and transverse waves; the prop wash adds a central turbulent lane. Semi‑displacement: still shows Kelvin structure but with a warmer (frothier) centerline and some spray. Planing: tall, energetic rooster tail and fan‑shaped spray; aft, the waterline detaches cleanly at the transom creating a bright white patch. Waterjet: cleaner lanes with less blade streaking; at high thrust, a tight, high‑velocity core that subsides quickly. Ducted tug: narrow, dense jet that holds together longer, sometimes visibly ‘boiling’ behind the nozzle. Submerged prop deep: minimal surface trace; on sonar visualization, you’d see long, faint tip‑vortex filaments.

Wind and current shear the wake; cold mornings show steam wisps over warm exhaust water; bioluminescence paints the prop wash at night—great cinematic hooks.

10) Maneuvering Signatures

With twin screws, differential thrust yaws the stern; the wake shows two overlapping fans, one hotter than the other. With pods, the entire jet swings; you’ll see a hooked wake tracing the nozzle direction. Bow thrusters make transverse boils at the bow without forward speed—paired counter‑rotating vortices appear near the tunnel exits. Waterjets pivot on a point with buckets down; the wake reverses cleanly with a sudden forward‑boil as the jet redirects.

11) Integration: Inflow & Outflow Cleanliness

Props hate disturbed water. Keep intakes and struts upstream fair; align shafts with the local flow. Avoid putting a transducer, step, or chine break ahead of the prop disk—these seed cavitation. On jets, keep intake lips smooth, with a slight ramp and fences to block air ingestion in hard turns. For subs, fair the sail and control surfaces to avoid cross‑flows into the prop plane; the quietest boats feed uniform wake to the rotor.

12) Materials, Edges, and Wear

Prop blades read as bronze, nickel‑aluminum‑bronze, or titanium with subtle chordwise polish from cavitation abrasion. Leading edges may carry hardfacing; trailing edges are thin and straight to prevent tonal whistle. Nozzles show ring anodes and wear liners. Jet buckets and nozzles darken with soot and salt streaks; intakes collect scuffs and marine growth patterns where flow is high. These are texture cues for production.

13) Safety & Maintenance Reality Checks

Add rope cutters and line guards at hubs in workboats. Provide shaft seals (stuffing boxes) and access hatches inboard. Jets need grate access and impeller service doors. Pods require lifting pads and removable fairings for motor access. Show sacrificial anodes near underwater metals and bonding straps across rudder/shaft joints. For CPP, route hydraulic lines to the hub with rotary unions.

14) Visual Cues to Propulsor Choice by Mission

  • Tugs/Tractors: Ducted props or azimuth thrusters; dense, narrow jet wakes; big nozzle rings.
  • Fishing/Workboats: Open props with guards, moderate rpm, Kelvin wakes with warm centerlines.
  • Patrol/Rescue Fast Craft: Waterjets; clean lanes, tight plumes, agile reversing signatures.
  • Yachts: Twin screws or pods; elegant, symmetric wakes; low noise cues (skewed props).
  • Subs: Large skewed prop or pump‑jet; almost no surface trace; acoustically disciplined details.

15) Common Pitfalls (and Fixes)

Props too small/fast for ships: enlarge diameter, reduce blade count, add skew; slow the wake. Jets on heavy bollard‑pull missions: switch to nozzled props or accept weak low‑speed control. Wakes ignoring hull regime: give displacement hulls Kelvin V and modest froth; give planing hulls transom detachment and rooster tails. Shafts too steep: shallow angles or pockets/tunnels; otherwise you’re wasting thrust pushing water down. No clearance: leave blade tip clearance to hull and free surface.

16) Production‑Side Deliverables

Provide (1) propulsor orthos with diameters, pitch cues, and clearances; (2) shaft/pod/jet routing with access panels; (3) wake style frames for key speeds (Idle/Displacement/Transition/Planing); (4) animation beats for reversing, crash‑stop, and tight turns; and (5) a texture guide for metals, anodes, and wear. If subs are included, add a noise discipline sheet—skew, rpm, and cavitation regimes—to keep shots consistent.

17) Final Advice

Pick the mission and speed regime first, then let propulsor choice, geometry, and wake follow. Clean inflow, adequate diameter, and believable wakes will make your boats, ships, and subs read as real on the first glance—and give production clear hooks for modeling, rigging, FX, and look‑dev.