Propulsion & Wake Engineering
Maritime Coordination RGB
Every boat writes on the water. Its propeller pushes, swirls, roars and, too often, cuts. For most of maritime history that signature has been treated as an unavoidable cost of moving.
This page is about rewriting it. We explore propulsion that moves vessels efficiently, shapes wakes and gentle mixing on purpose, and stays safe for anything swimming nearby. It brings together old ideas that never went mainstream, a new idea borrowed from the engine bay, and one firm rule: nothing that spins should be able to hurt what lives in the water.
- Florida manatees killed by boats, 2025
- 98, a quarter of all necropsied deaths
- Living manatees with propeller scars
- Nearly all of them
- Where ship noise sits
- Mostly below 300 Hz, the band marine life uses to communicate
- A main source of propeller noise
- Cavitation at the blade tips
Beyond the screw
Rotating-blade families worth reviving
The familiar screw propeller is efficient at speed, but it is also fast, exposed and loud. Other families of rotating propulsion move water differently, often more slowly and more like a living fin.
Cycloidal propellers
Picture a turntable set flush in the hull with upright blades standing around its edge. As the disc turns, each blade pivots so it always meets the water at a useful angle, like a wing. The most famous version, the Voith Schneider propeller, steers by moving a single control point off-centre: shift that offset and thrust swings to any direction, no rudder required. Tugs and ferries use it for precise manoeuvring.
Trochoidal propellers
Push the offset far enough (an eccentricity greater than one) and the blades stop circling smoothly and begin to sweep back and forth, much like a fish or a whale's tail. Laid on its side and spanning the stern, this layout has been studied as the Whale Tail Wheel: a cross between a living creature's tail stroke and the wheel. It suits higher speeds while keeping blade motion slow and foil-like.
Offset as a control
In both families, the offset of the blade-pitch control point is the master setting. Small offsets give gentle, steerable thrust for harbours and reefs; larger offsets give speed. We treat the offset as a stewardship control too: it lets a vessel turn down its intensity in sensitive water without changing hardware.
Wankel-inspired rotors
The Wankel engine's triangular rotor traces an epitrochoid inside its housing, the same family of curves cycloidal blades follow. We are exploring a rotor of that shape, enclosed in a matching housing within the hull, that moves water in sweeping chambers rather than slicing through it. It is a hypothesis, not a product: sealing, efficiency and fouling are open questions we intend to test before making any claims.
Two axes, two jobs
Parallel and perpendicular motors
Most boats push only one way: backward, to go forward. Adding a second axis of thrust opens up a different kind of control over what a vessel leaves behind it.
Propulsion
Thrust along the line of motion moves the vessel. This is where efficiency lives, and where quiet, enclosed, slow-turning designs pay off most.
Steering and shaping
Cross-axis or vertical-axis thrust holds position, turns without a rudder, and can move water up or down through the column rather than only backward.
Wakes by design
Blending the two lets a vessel shape its wake: flattening it near fragile shorelines and seagrass, where wake wash erodes banks, or directing it into gentle mixing where stagnant water would benefit.
Ocean welling
Moving water up and down, carefully
Natural upwelling, where deep, nutrient-rich water rises to the sunlit surface, feeds some of the most productive fisheries on Earth. Researchers have spent decades testing artificial versions using wave-powered pumps and air-lift systems, including the European Ocean artUp project led by GEOMAR in Germany.
The same research is clear about the dangers. Pumping deep water can lower oxygen, release methane, reshuffle which species thrive, and add to acidification, with effects that can cross borders to fishers and coastal communities. Climate models also show a sharp rebound if large-scale pumping ever stops.
So our use of perpendicular thrust for welling stays deliberately small: local, monitored, reversible mixing, such as oxygenating a stagnant harbour or a crowded aquaculture site. Planet-scale ocean engineering is not something we will attempt on a hunch.
- Scale we work at
- A harbour, a lagoon, a farm site
- Required before running
- Oxygen, temperature and nutrient baselines
- Required while running
- Continuous monitoring
- Off switch
- Must leave the water no worse than before
Safer by design
A hierarchy for safer propellers
Workplace safety ranks protections from strongest to weakest: remove the hazard before you guard it, and guard it before you rely on warnings. We apply the same order to propulsion.
- Enclose anything that spins fastRotary propellers live inside ducts, channels or the hull itself. Rim-driven thrusters go further: the motor sits in a ring around the blades, so there are no exposed blade tips at all. Kongsberg Maritime reports that removing those tips eliminates much of the tip-vortex cavitation behind propeller noise, and hubless designs leave little for lines or animals to tangle on.
- Open the rest into slow hydrofoil vanesWhere blades must meet open water, make them large, slow and foil-shaped, like the vanes of a cycloidal or trochoidal propeller: rounded leading edges, low tip speeds, and motion closer to a fin than a blade.
- Screen and slow every intakeEnclosure creates its own hazard: suction. Intakes are screened with grates sized to keep animals out, and sized large enough that the water enters slowly.
- Sense and yieldSonar, thermal and acoustic sensing can spot animals ahead and throttle the drive back automatically, the same way cars now brake for pedestrians.
- Slow down where it mattersThe oldest protection still works: slower boats in marked wildlife zones. Good engineering makes that choice easier, never unnecessary.
Our working concept
A flow channel under the hull
Our current direction brings these ideas together. Instead of hanging a propeller beneath the stern, we run a shaped channel along the underside of the hull. Water enters through a screened intake, is driven by an enclosed rotor inside the channel, and leaves as a smooth, shaped jet at the stern. A cross-axis rotor in a side branch adds perpendicular thrust for steering, station-keeping and gentle mixing.
Concept sketch, not to scale.
Benefits
- Nothing spinning is exposed to animals, swimmers or lines
- The hull itself shields the drive
- The channel conditions inflow, which hull and propeller designers note is key to reducing noise
- Room for both parallel and perpendicular thrust
- Suits cycloidal, rim-driven or Wankel-style rotors
Trade-offs
- Added drag and weight from the channel
- Fouling and debris inside an enclosed space
- Safe intake velocity and screen sizing
- Access for cleaning and repair
- Proving efficiency against a good conventional propeller
The idea has respectable ancestors: bow thrusters already work inside transverse tunnels, waterjets already draw water through the hull, and tunnel hulls already channel flow beneath the boat. What we want to show is that these pieces can combine into a drive that is quieter, safer and just as useful.
Next steps
How we plan to test it
Simulate
Computational fluid dynamics models comparing screw, cycloidal, rim-driven and Wankel-style rotors in the same channel.
Scale models
Printed models in a flume tank to measure thrust, wake shape and intake velocity.
Listen
Hydrophone recordings to compare noise and cavitation across designs.
Publish honestly
Share what works and what doesn't, including designs that lose.
Keep exploring
Related pages
Further reading
- Trochoidal propulsion and the Whale Tail Wheel (Twenty-First Symposium on Naval Hydrodynamics, National Academies Press)
- Quest for quiet: rim-drive thrusters and underwater noise (Kongsberg)
- Artificial upwelling factsheet (CDRmare / GEOMAR)
- Artificial upwelling: risks and trials (Geoengineering Monitor)
- 2025 manatee mortality (Save the Manatee Club)