Thruster noise spotlighted by smaller cruise ships
Thrusters can be a source of noise and vibration onboard ships. Rapid expansion of expedition cruise fleets in the past few years has brought this matter to the spotlight as these vessels are much smaller than most mainstream ships, yet they cater largely for the upper end of the market. There are ways to tackle the problem, but some of them affect the wider design of the vessel.
“A problem with thruster noise is that it is usually very quiet until the thruster comes on and suddenly, the cabins in the forepart of the ship can experience a lot of noise,” says Lars Myklebust, executive sales manager at Brunvoll, the Norwegian thruster manufacturer that is a leading supplier of this equipment to the cruise industry and mega yachts.
Owners sometimes think that the thrusters are not used for a long period at each time, so they can be tempted to opt for less expensive but noisier technology. “Noise levels of 75-88 dB(A) are typical in cabins next to and above conventional tunnel thrusters. Brunvoll’s advanced technology can cut this by up to 15dB(A). Combined with other measures, noise reductions of up to 20-25dB(A) have been verified,” Myklebust tells The Naval Architect.
Thrusters are, however, an investment that often lasts the lifetime of a ship and quieter sailing means better sleep, improving the alertness and efficiency of the crew, while passengers enjoy a new degree of comfort if the owner opts for more advanced technology.
In good weather conditions, only half of the power of a thruster installation is often required to manoeuvre a ship into or out of port. However, under strong winds, 80-100% of the output is needed, with a consequent sharp increase in noise levels.
Thrusters have little water around them, which can result in cavitation that causes noise and vibration. Therefore optimising the size of the tunnel and the propeller and how the tunnel is mounted to the hull are important aspects in reducing noise and vibration.
Resiliently mounted units that offer stepless speed control and variable pitch propellers produce the best results. Brunvoll’s double tunnel systems feature an inner tunnel with an electric motor that is fitted with a frequency conversion drive and a controllable pitch propeller. The inner tunnel is resiliently mounted to the outer one.
“In this way the propeller and the electric drive motor is decoupled from the hull structure by elastic mounts. The idea behind this concept is to reduce the noise from the noise source itself (the thruster), which will make the noise reduction job easier on the decks above the thruster compartment to get an overall comfort for both the passengers and the crew,” Myklebust points out.
Noise insulation and design
“It is very important to pay attention to the welding in of the thruster to the hull structure, to the arrangement of the tunnel entrance, design of grids, blade design, choice of electric drive motors, etc. to avoid resonant vibrations and that the noise and vibration is transmitted at a high level up into the different decks above the thruster compartment,” he continues.
“Thrusters must be able to keep the ship under control while manoeuvring even in strong wind conditions. A large cruise ship can have four thrusters of 3MW each, an expedition vessel two of 1.5MW each,” says Peter Albrecht, senior advisor at Elomatic, the Finnish engineering consultant company.
Several alternatives come under consideration here. The necessary thrust can be obtained by using the power to provide a rapid flow of water through the thruster tunnel. This allows reducing the diameter of the tunnel, but increases the noise level. A larger tunnel results in a slower water flow and lower noise levels, but this option obviously means that the installation will take up more space onboard the ship.
A grid is often installed at each end of the tunnel to allow adjusting the direction of the water flow – the resistance is lower when the ship is moving forward and a radial grid can increase the thrust of the installation by up to 10% compared to a traditional grid.
Installation of the electric motor that drives the thruster in the propeller hub in the tunnel produces less noise and vibration than an installation where the motor is placed above the thruster tunnel and power is transmitted via shafts and gears. However, the first named option is the more expensive of the two.
For the full article please see the April 2021 edition of The Naval Architect.