Tuneable Hull Plate Reduces Propeller-Induced Noise/Vibration

Propeller-induced noise and vibration can be a serious problem particularly on smaller, higher-speed vessels. If intense enough, these foreign forces reduce power and fuel economy, add to the discomfort of passengers and crew and can negatively influence the operation of shipboard electronics and other equipment.

The problem is traceable to the ratio of propeller power to ship weight; as power increases on lighter weight ships, the problem is amplified.

One recent solution to the problem is a CIRR-PSA system (patent pending), developed by Pal Francis Hansen, that is designed to reduce the transmission of the propeller shocks into the hull structure of the vessel from 60 to 90 percent. The design is predicated on the vibration and shock absorbing characteristics of helical wire rope isolators supplied by Aeroflex International Corporation.

The CIRR-PSA system entails a box built into the hull immediately above the propeller tip, Figure 1. The hull plate of the PSA box has the same shape and contour as the surrounding bottom plating, and is flexibly mounted on helical isolators located inside the box, Figure 2. The flexible hull plate mass, combined with specific shock and vibration ca- pabilities of the isolators, provides a specific damping level. To tune the system for any particular installation, it is only necessary to alter the hull plate mass and select the correct isolators.

Propeller-induced turbulance establishes force levels acting upon the hull plate. The hull plate moves against the counter force produced by the isolators, reducing the amount of vibration that migrates into the ship's hull.

Acceleration measurements in the aft body have shown up to 90 percent reduction of vibration amplitudes after installation of the flexible-seated PSA hull plate in the nearby structure at specific propeller-blade frequencies.

The helical isolators are sealed inside the box by a special watertight double rubber seal. The elastomer material is flexible enough to permit smooth movement of the hull plate with low drag at the point of isolation force crossover.

Helical wire-rope isolators are stable mounting assemblies of stranded-steel wire rope formed between metal retainers, Figure 3. Their large dynamic displacement attenuates heavy shocks, while their inherent damping enables them to absorb and dissipate large amounts of low- and high-frequency vibration.

Of particular importance for motion control at resonance is their flexure hysteresis.

Damping characteristics are related to the strain applied to the isolator. Large motions are highly damped, whereas small amplitude motion is moderately damped with the isolator appearing to act as a nonlineal element.

The cable is wound in a helical fashion between two metal bars to assure shock and vibration control, regardless of the direction of applied force.

The helical cable isolators resist destructive environments because they are made entirely of stainless steel or stainless-steel and corrosion- resistant aluminum alloy; or even more exotic metals such as Inconel. The isolators require little or no maintenance and usually will outlast the equipment they isolate. They also can be painted to match equipment.

A prototype of the system is in operation onboard a new high-speed rescue vessel, R/S Olav V, operated by the Norwegian Sea Rescue Service. The vessel is equipped with twin 1,500-hp engines; propellers have a 1.20 metei* diameter The CIRR-PSA system will be delivered with isolators, seal and an engineering package with all technical data necessary to build a PSA system for a specific project.

For more information on the system, Write 40 on Reader Service Card

Maritime Reporter Magazine, page 93, Apr 1984

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