Retrofitting new rudders are among the short-term measures being considered by shipowners to help comply with IMO EEDI and EEXI regulations and cut fuel consumption
Under mounting pressure to reduce greenhouse gas (GHG) emissions and meet new energy efficiency regulations, shipowners are taking a closer at retrofits that provide significant efficiency gains with relatively short paybacks. As a result, modifications and advances in the often-overlooked rudder are gaining new attention among shipowners.
At the Riviera Maritime Media webinar Steering gear and rudders: is it time to retrofit your vessel? presenters highlighted rudder technology advances that can variously save fuel, reduce ship motion in heavy seas, improve manoeuvring, and reduce wear and tear, while helping shipowners comply with IMO’s Energy Efficiency Existing Ship Index (EEXII).
Noting EEXI comes into force by 2023, panelist David Wing, director of ship design and engineering at marine design and cleantech consultancy Houlder, warned: “That is not very far away now.”
Among the most recent developments in rudder design is the gate rudder developed in Japan. Unlike a traditional rudder, the gate rudder actually consists of two uniquely shaped, high-lift rudder foils placed on either side of the propeller. One of the patent holders for the gate rudder, Dr. Noriyuki Sasaki, visiting professor in the department of naval architecture and marine engineering at the University of Strathclyde in Scotland, told delegates this innovation has proved its worth since it was first installed on the container ship Shigenobu three years ago.
“After three years of data [we can see] that container and general cargo vessels have achieved fuel savings of more than 15% when sailing at the same speed and draft,” he said. “The ships’ motion is also reduced, with less yaw. In fact, it has been cut by about half.” In another benefit, the gate rudder can be twisted in a way that makes the ship more manoeuvrable in port, hence saving time.
And the payback on the investment? “Three years with one ship,” said Professor Sasaki.
In 2020, Wärtsilä signed a licence and co-operation agreement with Kuribayashi Steamship Co for the future development, sales and servicing of the gate rudder. Wärtsilä plans to make gate rudders an integral part of its propulsion offerings, with an eye on compliance for IMO’s Energy Efficiency Design Index (EEDI) for newbuilds and EEXI for existing vessels. EEDI requires a minimum energy efficiency level per tonne mile for various ship types and sizes. Set to be incrementally tightened every five years, EEDI is expected to drive innovation and technology development for anything that might influence fuel efficiency for a ship when it is in design.
Testing rudder performance with CFD
Reviewing developments in rudder design in recent years, Mr Wing explained that shipowners could expect a drag reduction of between 2% and 8%, depending on the ship, if they adopted the latest developments compared to the simple spade rudder that has dominated shipping for decades.
The biggest advance has been in methods of predicting a rudder’s performance before it is installed on a ship at considerable cost. Instead of testing models in tanks, rudders are now developed by computational fluid dynamics (CFD). “With the increasing advent of CFD and the ability to investigate countless numbers of minor changes to the geometries, we’ve been able to drive further down that optimisation curve,” said Mr Wing. Similarly, the design of propellers, the performance of which is integral to that of the rudder, has made advances too, as simulation techniques reveal where the big losses are and how they can be targeted.
On the issue of EEXI, ships that are currently non-compliant will have to undergo some technical measures before they are deemed compliant, he pointed out. However, as Mr Wing added forthrightly, the easiest way would be to de-rate the shaft power or engine, but that would only serve to “fiddle the numbers as opposed to solving the problem.”
In fact, shipowners have many retrofit options to comply with EEXI that will also make for more economical sailing. “Some of these retrofits will be easier than others,” he explained. “The first thing to understand is the savings potential that will vary significantly, even on ships of the same design.
“CFD and the ability to investigate countless numbers of minor changes to the geometries has driven down the optimisation curve”
“The same ship design operating on perhaps two different operating profiles, maybe two different routes and two different speed profiles, will have two very different fuel savings calculated,” he said. “The benefit of the rudder and alignment is very much tuned to a [particular] design speed. How far off that design speed you operate will affect how effective that optimisation is.”
And, as he went on, there are two ways of retrofitting a rudder – a hard way and an easy way. “[The easy way] is not simple but it’s fairly straightforward. But if you get into adjusting the shaft length or moving the propeller up or down, that very quickly becomes complicated and very difficult,” he said.
Overall, he said, the rudder should not be considered in isolation, but in relationship with several other factors including the propeller – “the capacity of the steering gear, the whole structure and everything to support that is a key consideration.”
Mr Wing’s concluding message is that, particularly for older ships, there are a lot of areas where practical and economically viable retrofits are possible.
Interceptor steering gear
It’s likely that many shipowners have never heard of the interceptor steering gear. However, the manufacturer, Sweden’s Humphree specialising in high-speed marine propulsion, equips some 1,000 vessels a year with the technology that reduces power losses under steering.
Designed for water jet propulsion, interceptor steering gear can deliver significant benefits for most types of ships up to 120 metres-long, according to sales directors Lennart Dobele and Per Landegren. Mounted vertically on the transom, it deploys the blade in a way that creates a steering force on the hull. Fully electric, interceptors assist steering and trim.
As they told delegates, the technology works best on waterjet-propelled vessels by saving fuel, improving course-keeping and enabling tighter turns. The system comes into its own at around 12 knots or above.
Interceptors also have a function on more conventional water jet-powered vessels. “[They] mainly increase or decrease the radius of turns,” they explained. “So they can be used instead on larger rudders.” Clearly, useful in docking.
Suitable for retrofits as well as newbuilds, Humphree claims the return on investment is somewhere between six months and two years, depending on operating hours and type of vessel.
As these new technologies emerge, the shipping industry looks much less conservative than it is often thought to be.