[Updated] The Option of a Two-Stage Rudder


By Harry Valentine 2017-04-09 21:20:22

Recent German research has applied a well-proven aeronautical wing concept to improve maritime rudder performance. There are other proven aspects of aeronautical wing technology that could further improve maritime rudder performance.

The development of maritime rudders preceded the development of aeronautical wings by several millennia. Very recently, a German company applied aeronautical concepts to maritime rudder design and greatly improved vessel steering control. Aeronautical wings develop a smooth airflow over the top surface to produce a vacuum-effect known as lift and the same result can with water flowing on the shadow surface of maritime rudder design. There are many other well-proven aeronautical wing-related concepts that could have future application in maritime rudders. Travelers who occupy a window seat near a wing can often observe such concepts during take-off and landing. 

Commercial aircraft wings are built in multiple sections that are connected by hinged joints. During take-off and landing, spaces can clearly be seen between these sections and allow air to flow from the underside of the leading section to the upper surface of the trailing section to enhance the vacuum effect. The modification to the concept maritime rudder is based on aeronautical precedent and has improved rudder performance and other well-proven aeronautical wing features provide further scope for additional future improvements in rudder performance.

Improved Ship Steering Control

The development of the steerable propeller on small pleasure craft powered by outboard motors mounted on steerable hinges greatly improved directional control of small boats. The need to improve steering control on tug boats led to the development of the twin counter-rotating vertical-axis propulsion units developed by Voith that provide extraordinary directional control as well as high propulsive efficiency at reduced sailing speeds. Well-proven diesel-electric and steam-turbine-electric ship propulsion led to the development of ship azipod steering mechanism that includes electric motor and propeller and has set a new standard in large-vessel directional control.

While some cargo ships utilize azipod propulsion, the mainstream of the cargo maritime sector prefers direct connection between diesel engine and propeller, leaving the classical design of rudder to provide directional control. It is possible that in the future, the modified rudder design could appear on large cargo vessels and provide improved vessel directional control while sailing through narrow navigation channels and perhaps reducing the need for tug assistance. Commercial aircraft wings are a hinged assembly of multiple wings and provide precedent from which to borrow to further develop rudder design for large vessels.

Multi-Rudder Concept

Commercial airliners have a main fixed flight wing with a leading section that protrudes forward during take-off and land and trailing flaps that reset to different angles in relation to the main wing. Adapting such a concept to a maritime rudder could result in a multi-section rudder that rotates on a main kingpin, with leading and trailing sections that turn on main-rudder mounted hinges. While such a concept could increase smooth water flow over the shadow side of such a rudder, the added initial cost and higher maintenance cost could restrict its application to smaller vessels.

A second option could involve two rows of rudders all mounted on their own hull-mounted kingpins. The first row would involve a single leading rudder placed to the rear of and closest to the propeller(s). The second row would involve a pair of parallel trailing rudders placed to the rear of the leading rudder. Using the revised concept, the leading rudder steered to 30 degrees would generate a smooth flow over its shadow surface with the opposite surface simultaneously redirecting water flow to the shadow side of either the leaf or right side trailing rudders steered to 60 degrees.

In Service

The revised rudder technology applied to a large ship would improve directional control as a result of water flowing over the shadow surface during relatively small steering angles, when leading and trailing rudders would operate in parallel. At more severe steering angles, the leading and trailing rudders would operate in series as the leading rudder redirects water flow toward the shadow side of the trailing rudder that is steered to a greater angle. The result could be a large ship capable of sailing through a narrow and possibly winding channel without the assistance of a tug. 

Kinetic Ferries

The idea of leading and trailing rudders designed to improve water-flow over the shadow surface could enhance the performance of water current-driven kinetic ferries. Such presently operate across rivers in several nations and that could further be developed to sail across ocean channels through which powerful unidirectional currents flow. Depending on channel depth, there could be scope for a large-ship size rudder assembly to be installed under a kinetic ferry that is a fraction of the size of the large ship. Used for propulsion, such a rudder assembly could generate greater power and yield higher sailing speed.

A kinetic ferry using series rudders for propulsion could combine a pair of upstream rudders with a trio of trailing or downstream rudders. While water would flow smoothly over both surfaces of the upstream rudders, they would redirect much of that water flow over the shadow surfaces of a pair of trailing rudders. The combination of higher sailing speed and greater propulsive power could open a possible application for kinetic ferries to sail between nearby islands in locations such as Caribbean, Philippines and Indonesia where unidirectional currents flow.


German research has proven that a well-proven aeronautical concept can successfully be applied to maritime rudder design and improve vessel steering control.

There is much else in proven aeronautical design that has not yet been applied to maritime rudder design that could be applied to future propeller design to both improve directional control of large vessels that use direct drive between engine and propeller.

Evolving and future rudder design could be applied to future kinetic ferry design to both increase propulsive power and sailing speed, perhaps making the technology for ferry service across narrow ocean channels through with unidirectional currents flow.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.