Rolls Royce: In search of the perfect propeller

By Nancy Bazilchuk
January 29th 2007

The ocean can be a tough place for a propeller. Just ask Sverre Steen, the head of the Rolls-Royce University Technology Center based at NTNU and Marintek in Trondheim. He knows.

 

Illustrasjonsbilde/FOTO

Steen has seen how rough seas can pluck a ship’s stern out of the water so that the propeller is forced to spin against a frothy air-and-water mix. Complex waves may toss a ship from side to side, placing weird torque on propeller shafts. And even the water itself can be the enemy: when a propeller spins at speed, it builds up tiny bubbles of low pressure around certain parts of the blade, and then the bubbles literally implode. The force can be so great it can chew pits in the propeller blades.

At their worst, these problems can mean disaster, with unacceptably damaged or inoperative propellers. More commonly, however, propeller problems translate into engine inefficiencies. So when it awarded its 25th University Technology Center in May 2005 to NTNU and Marintek, Rolls-Royce Marine was betting that Norway’s maritime researchers could figure out how to design ships and propulsion systems to better handle heavy seas. “You are the best in the world in marine technology,” Dr. Mike Howse, Rolls-Royce Director of Engineering and Technology said to the opening crowd. “There are really good people here to carry out the research.”

Illustrasjonsbilde/FOTOSafe and efficient
As humankind expands its reach into inhospitable corners of the maritime world, more and more ships find themselves in situations that put incredible stresses and strains on propellers and propulsion systems, Steen says. Doubters need look no further than the North Sea, where a whole fleet of ships has evolved to provide the services demanded by the petroleum industry. The demand continues today, with the opening of the Barents Sea to natural gas production.

“A lot of offshore vessels are operating at low speeds in rough seas – seismic boats, anchor boats – and that’s a tough environment,” Steen says. Historically, ship designers have been hampered by their inability to understand the complex forces at work on a boat in real ocean conditions. Marine engineers tend to build extra safety margins into equipment to make up for this difficulty. But those same extra safety margins may make the boat sluggish, less manoeuvrable, or simply inefficient. The trick in successful ship and propulsion design is to find the correct safety margin that also maximizes efficiency, Steen says.

From ocean basin to kindergartens

Engineers and researchers at the NTNU/Marintek UTC have a powerful ally in their quest to improve ship and propulsion design: the Ship and Ocean Laboratory facilities at Tyholt, comprised of a 280-metre long towing tank, a 50- by 80- metre ocean basin with a depth adjustable to 10 metres, a cavitation tunnel and a sloshing tank. These tools allow scientists to build scale models to test their ideas, to go beyond the predictions and simulations that computer models can offer.

“It helps to be able to test things,” says Kourosh Koushan, a principal research engineer with the Ship and Ocean Laboratory. Koushan is also working on a post doc at the UTC, studying forces on propellers.

The walls of the Ship and Ocean Laboratory bear silent witness to the benefits of testing. Three walls are covered with row after row of model-sized propellers, dating from the 1950s to the present day. The walkways around the testing tanks are similarly crowded with mustard yellow scale models of everything from oil tankers (with one model fully 9 metres long) to a baker’s dozen of free-fall lifeboats in different torpedo-shaped configurations.

“We make about 50 models a year,” Steen estimated. Most are destroyed after testing is complete, to preserve any trade secrets that may be encapsulated in their designs. Other, more generic models have found their way to area kindergartens, where children, future maritime engineers perhaps, play among scale model cruise ships and anchor handling vessels.

Go with the flow
The NTNU/Marintek UTC, which is properly known as the “Performance in a Seaway” UTC, is also investing heavily in expanding its ability to use computer tools to predict the flow of water around propellers, hulls and propeller shafts.

The key to understanding is the developing field of computational fluid dynamics, in which powerful computers are set to solving intricate mathematical formulas that describe and predict water flows. The NTNU/Marintek UTC has one PhD student working on using computer models to conduct these analyses. The work has the dual goals of avoiding mechanical damage to the propellers, while at the same time increasing propulsion.

“It’s an interesting challenge, to understand all these dynamic forces,” Steen said. The combination of being able to use computational flow dynamics along with information from the different testing tanks provides a world of new information, he added. “There’s a lot to discover – and it’s incredibly rewarding,” he said.

FACTS:

Name:
Rolls-Royce University Technology Center "Performance in a Seaway"

What:
A research centre sponsored by Rolls-Royce at the Norwegian University of Science and Technology and Marintek, based at the Ship and Ocean Laboratory in Trondheim. NTNU is doing the most basic research, done by academic staff, students, Ph.D.-students and Post. ddocs.

Budget:
NOK 6 million per year

Director:
Professor Sverre Steen, NTNU

Staff:
Number of employees: 6
Number of graduate students: 3

Contact:
Tel: +47 73 59 58 61
Email: sverre.steen@ntnu.no