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Engine Room

Look beyond alignment to props to find source of vibration

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When a vessel is plagued with vibration, alignment isn’t always the issue. Propellers can often be a source of serious vibration for many reasons. Here’s a brief look at what can cause propellers to be a cause of vibration, and how they can be avoided or corrected.

Propellers are usually S Class, Class 1, or Class 2. Class 2 is typically on a workboat. Greater care goes into the engineering and build of an S Class or Class 1 propeller, so they are innately going to provide for better performance.

A quick glance at a propeller report will offer a good idea where a propeller stands. There should be minimal deviations between the pitch numbers, and blade thicknesses. When large deviations are present, they are not consistent with ISO-R484, the standards that distinguishes the different classes of propellers.

When the thickness of the blades varies greatly, it throws off the dynamic balance of the propellers. The differences in the pitch prevent the propellers from rotating smoothly in the water (the torque), which causes a wobbling effect. A comparison would be trying to swim with one hand cupped. The result is poor performance and vibration.

When a vessel is fitted with props that have too little blade area, they can easily become overloaded, even if the correct diameter and pitch have been spec’ed. Overloading can cause cavitation, which, in turn, causes vibration.

Propellers are able to move a vessel forward because of the negative and positive pressure generated on their forward and rear-facing surfaces, respectively. When the pressure around the blades falls below the surrounding vapor pressure, cavitation occurs. If these bubbles of vapor hit the blades with enough force, they can erode the surfaces and cause significant noise.

When calculating the necessary DAR (disc area ratio) or BAR (blade area ratio), total brake horsepower and flywheel power for each engine is examined, as well as the maximum speed of the vessel.

Tip noise describes the noise generated when there is not enough clearance between the prop’s blade tips and the hull. When there is not enough clearance, vortices – pockets of energy generated by the props — usually dissipate, and they create a noise problem when they implode on the blades.

To increase blade tip clearance, a viable solution is to reduce the diameter of the propellers, while increasing the amount of prop blades.

Another common complaint is the singing prop. A propeller that makes the high-pitched humming noise is more of an annoyance than anything harmful. The singing is thought to be a symptom of a combination of the propellers’ diameter, the speed of the vessel, and the geometry of the propeller blades’ trailing edges (thickness and shape).

While not completely understood, singing propellers are sometimes corrected by switching the position of the propellers. Another remedy is to alter the trailing edge of the blade, giving it an “anti-singing” edge.

Improper maintenance, namely improper polishing or too infrequent cleaning, can cause excessive wear and decrease the life of a propeller. Polishing to restore a propeller’s smooth surface can be done both underwater and at dry dock. By their very design, propellers are prone to scratches, dents, marine growth and cavitation, all of which can hamper their efficiency. By polishing propellers at least every six months, their performance is maintained and their lifespan extended.

However, improper polishing performed with a disc or grinding wheel can do more harm than good.

To best maintain propellers — and best minimize their noise — begin by knowing the vessel. Become acquainted with its machinery and how it performs. Do some due diligence and ensure the correct propellers are on the vessel.

When servicing propellers, always opt for propeller shops that offer two-plane dynamic balancing. Propeller reports after servicing should show minimal deviation between the blade thicknesses and pitch. Reputable shops will also have proper polishing techniques down to ensure they have the smoothest surface possible.

Like all forms of maintenance, consistency is key to keeping propellers operating at optimal performance.

Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale. Contact him through www.AMEsolutions.com.

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