The shipping industry has spent years rethinking its engines. Fuels, combustion and emissions control have all received enormous attention and enormous investment. Rightly so, says Gert-Jan van Goch, director, Product Line – Propellers and Transmission, Wärtsilä Marine.
But there is a component at the back of almost every merchant vessel that has been left largely alone. The rudder. It sits behind the propeller, in the slipstream, and steers the ship by redirecting the flow at the cost of some additional drag. It has done this in roughly the same way for over 150 years.
The drag cost nobody is counting
Naval architects have long treated the high-velocity propeller wash as a benefit, using it to generate larger rudder steering forces. The drag cost that comes with that arrangement has received considerably less attention. Every course correction interrupts propulsive flow. That has simply been accepted as the price of steering a ship.
The assumption behind it is worth examining. Propulsion and steering are treated as two separate problems, solved in sequence. You specify the propeller. Then you choose a rudder to suit it. An afterthought in the design process tends to perform like one.
This matters more now than it used to. Sustainable fuels are expected to cost three to five times more than today’s fossil fuels by 2030. Every percentage point of efficiency built into a vessel now is worth considerably more in ten years than it is today. So the question of what the rudder is actually doing to propulsive performance starts to matter quite a lot.
What happens when you ask it differently?
Some newer configurations are beginning to challenge the conventional arrangement. Instead of a single blade sitting directly behind the propeller, twin foil arrangements positioned either side of it interact with propeller flow rather than disrupting it. During transit the foils can contribute to thrust performance. During manoeuvring, forces are redirected to generate lateral contribution quickly. Vessels can hold speed more effectively in turns than with a conventional rudder, because the speed drop associated with helm application is less severe.
Wärtsilä’s Gate Rudder, developed with partners in Japan, applies this principle to single-screw merchant vessels. Some vessels have reported fuel savings of up to 20% compared with conventional arrangements, with improvements in manoeuvrability and reductions in noise and vibration alongside.
The key difference is not just the geometry. It is when in the design process the decision gets made. These configurations deliver their full value when propeller, steering device, and hull form are considered together from the outset. That is a different conversation from the one most design processes have, and it needs to happen earlier.
Beyond fuel: what integrated steering design unlocks
The efficiency gains are not the only outcome worth examining.
A twin foil configuration, by positioning steering surfaces alongside the propeller rather than behind it, also improves manoeuvrability and course keeping, particularly at low speeds and during the frequent course corrections typical of coastal merchant operations. Reduced hydrodynamic losses contribute to more stable handling without adding propulsion complexity. As the concept influences aft-ship layout from the outset, it creates greater flexibility in how machinery space and stern volume are arranged. Simply said, the propeller and engine room can be moved a couple of frames aft, providing more cargo space for the same vessel length.
The shipping industry has spent years rethinking its engines. The rudder has waited long enough for the same attention.
