Rudder and steering technology tends to evolve gradually, with few big bangs, but commercial pressures are now spurring the pace of development
Historically, most of the developments around rudders and steering systems have occurred through incremental gains, rather than headline-grabbing breakthroughs. It is an interesting fact that improvements in rudders alone can be measured at a rate of just 1% a year, according to Rolls-Royce.
But lately the pressure on operators to run their ships as efficiently as possible has sparked more interest in rudder design and consequent technological innovation.
One outcome of this is rudders for all occasions. For instance, Rolls-Royce’s Type CB is designed to improve manoeuvrability at low speeds. Featuring a bulbous shaped cross section and large end plates, it shares the same profile as the company’s FB flap rudder that is popular with offshore supply vessels, cargo, fishing, seismic and other vessels that are not usually in a hurry.
In another slow-speed innovation, Becker Marine has developed the Type SA flap rudder, most suited for small and low-speed vessels such as river boats, supply vessels and tugs among others.
In the same vein, MM-Offshore’s Empire rudder matches a flap with a full spade rudder, a package suitable for low- to medium-speed ships with a lot of port operations, such as feeder vessels or those operating in difficult passage ways, module carriers, ropax and ferries. MM-Offshore might have achieved a record with its flap, which can operate at an angle of up to 100°.
In a flap
In terms of manoeuvrability, most recent progress has been in flap rudders, such as Becker’s FKSR design. Decades in development and refinement, the latest flap design can be pushed to an angle of 65° for the main rudder, plus an extra 45° for the flap.
Netherlands-based Van der Velden Marine Systems - a specialist in manoeuvring and propulsion - has developed a high-lift, slim-profile, low-drag flap rudder called Timon, designed for manoeuvrability among higher-speed vessels. Addressing a constant concern of operators, the company has developed a quick-service device on the flap linkage that minimises maintenance time.
Battle of the bulb
Apart from the flap, one of the biggest incremental improvements in rudder design has been in the bulb. Japanese manufacturer Nakashima Propellers claims energy-saving effects of about 6% for its Ultimate Rudder, which includes a bulb.
Basically a rounded cap, the bulb is placed on the leading edge of the rudder in line with the propeller. Among other gains, rudder bulbs smooth out the wake and recover the so-called 'hub vortex' between the prop and rudder. According to Nakashima, the latter effect “brings an increase in negative pressure at the rudder’s leading edge, leading to reduced rudder resistance, [which] increases hull efficiency.”
Rudders operate in one of the most hostile environments, enduring extreme differences in temperature, dangerous proximity to solid objects such as rocks, reefs and floating objects, and huge loads during sharp manoeuvring. But because they are under water, it is difficult and expensive to monitor their condition.
To make things easier, Rostock-based Becker Marine has devised a system that measures remotely the wear and tear on the vulnerable rudder neck bearing. Four electrical sensors are mounted on the neck-bearing bush and the readings transmitted to a processing unit in the steering-gear room where the crew can spot any abnormalities. A small touch panel allows the crew to calibrate the system and display the monitored values. The readings can also be hooked up to an alarm system.
Similarly, Becker has devised an intelligent monitoring system that directly measures the forces imposed on the rudder so that the crew can avoid putting too much pressure on the steering system and, in extreme circumstances, triggering a failure.
IMO's regulations recognise the stresses and strains that are routinely imposed on the main steering gear, and the body sets specific tests to ensure they are up to the job. For instance, the system including the rudder stock should be capable of steering 35° on one side to 35° on the other with the ship at its deepest seagoing draught. And the rudder must be able to survive the vessel being driven at maximum astern speed. The regulations also impose stringent standards of redundancy. Every ship must, for instance, have an auxiliary steering system that starts up within 45 seconds of the main system’s failure.
Look, no rudder!
Not every ship needs a rudder, at least not all the time. For nearly 18 months two sister ferries, the Copenhagen and Berlin, have been working the Gedser-Rostock route between Denmark and Germany. Designed specifically for the shallow waters along the 26-mile route, both vessels feature Rolls-Royce’s integrated CP Promas propeller and rudder, configured with two wing Azipull thrusters that help turn them in Gedser harbour at the Danish end of the route.
The props used in Azipull thrusters not only deliver vectored power for quick turns, they are housed in streamlined underwater bodies that provide a generous rudder-like effect under straight-ahead sailing. Result? No separate rudders are required. On bigger vessels though it may be necessary to install a conventional rudder, but, as Rolls-Royce pointed out, that would depend on factors such as the properties of the hull and the steering mechanisms employed.
Demonstrating the importance of integrating the steering function into hull design according to the operator’s specific needs, the ferries have a designed draught of just 5.5 m, so the underwater equipment can be located at a safe distance from the harbour bottom.
A lot of computational fluid dynamics went into these ferries. The Promas system, which combines the propeller, a hubcap, rudder bulb and the rudder into a single unit, has a five-bladed propeller that reduces cavitation, which in turn makes the rudder more efficient. And the spade rudder features a Costa bulb and a twisted leading edge so that the energy of swirling water in the propeller’s slipstream is recovered and converted into extra forward thrust.
Rolls-Royce claims an increase in propulsive efficiency of up to 8% from the Promas configuration.
It 's blowing hard and the wind is turning the side of the container ship into a sail as it approaches the berth. The rudder is struggling to counteract the force of the wind. That is when a bow-mounted rudder system comes into its own. Developed specifically for vessels with large wind surface areas, it helps greatly in specific situations such as on winding rivers, particularly those passing through flat and exposed country, and in buffeting cross winds.
Van der Velden Marine Systems has developed the RMS 2000 rotor bow rudder system for just such occasions. Quick-acting, the system allows the crew to steer from port to starboard in a few seconds. There are other bow rudder systems, but in shallow waters the RMS 2000 has the particular advantage that the rotor automatically retracts if the vessel runs aground.
Technically speaking, the rotor bow rudder system is based on the Magnus effect, whereby a pressure differential is generated by placing a rotating cylinder in oncoming water currents. The result is a thrust force that is larger than a conventional rudder, enabling much-improved manoeuvrability.
It may not grab all the headlines, but constant innovation like this is keeping rudder design at the forefront of propulsion technology.