Rudder refits promise quick paybacks and improved efficiency

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.