The gentle hum of an engine can be soothing when under way. In fact, some people sleep soundly to the low vibrations.
But when vibrations begin to shake the chandelier, it’s time to have things checked out.
Vibration expert Rich Merhige talked about the basics of vibrations and the latest tool his company, Advanced Mechanical Enterprises (AME) in Ft. Lauderdale, uses to diagnose problems with misalignment, imbalance and vibration.
What once was raw waveform data (the plot of a signal whose amplitude varies with time) can now be broken down into a series of sines and cosines of particular frequencies and amplitudes. Thanks to microprocessors and their ever increasing power, vibration analyzers have gotten more powerful, too, so that today, engineers and maintenance technicians can diagnose specifically what’s causing a vibration.
Turns out, the most common cause of vibration problems on yachts is related to machinery installation and alignment, and more specifically, the main propulsion machinery isolation mounts, Merhige said.
“Because these take thrust and torque, they wear and deform, requiring readjusting and realignment periodically,” he said. “Often times this is overlooked in the vessel maintenance.”
Merhige gave a presentation on vibration in Ft. Lauderdale in September to regional members of SNAME, the Society of Naval Architects and Marine Engineers. In addition to the basics of what causes vibration, he offered attendees the science to back it up.
After the presentation, we asked Merhige a few more yacht-specific questions.
Q. What should yacht captains and engineers pay attention to if they suspect a vibration issue?
Merhige: It is helpful if the vessel crew is able to note the location of the greatest vibration and at what speed and conditions (water depth, bunkers onboard, sea state, etc.). They can also make visual observations of any shakes and wobbles of the machinery, exhaust system piping, propeller shafts, even vessel appointments (such as chandeliers). I had one vessel where a piano started to hum at a certain rpm. It turned out to be out-of-pitch propellers.
Q. What can captains and engineers do to prevent or avoid some of these problems?
Merhige: Perform an annual vibration survey of the vessel hull and machinery to have an accurate idea of propulsion system performance and condition. Make engine and exhaust system mounts and support inspections and measurements part of the vessel’s planned maintenance and dry-docking inspections, along with vessel main shafts TIR (total indicator run out) measurements. Inspect engine torsional and shaft flexible couplings for deformation or cracks. Keep propeller in good clean condition.
Q. Once they have determined a vibration issue, what can they do to correct or at least diminish problems before things get too bad?
Merhige: Main propulsion machinery, depending on the arrangement, are subjected to high forces of engine torque and propeller thrust. This leads to changes in the alignment, which need to be checked. If excessive vibration is noted, don’t run at high rpms and loads as this could have catastrophic consequences.
Q. During your presentation, you mentioned that many yachts will launch with misalignment in the shaft, giving it vibration problems from Day 1. How can captains and engineers identify this issue?
Merhige: The shaft will have excessive friction in the bearings. This will be particularly noticeable when maneuvering as the shaft may squeal and stop suddenly when shifting. The engine may also have higher load; difference in load between main engines is a sign. Misalignment can also manifest itself as a shaft seal leak.
Also part of the SNAME event was Paul Nailor of Wartsila who talked about his company’s launch of bio-oil seals to support the new EPA regulations and the use of water-lubricated seal and bearings to the commercial market.
New EPA regulations on environmentally acceptable lubricants are coming into effect in December, he said, on commercial vessels larger than 79 feet trading in U.S. waters. In those vessels, environmentally acceptable lubricants (EALs) must be used in all oil-to-water interfaces and must be changed out by the next dry dock period after this date.
On Oct. 1, Wartsila will launch the Bio Seal Ring, a seal compatible to EAL-approved oils and offer the same level of performance as existing seals used with standard oils.
One step further to support these legislations would be to adopt water lubrication for stern tube applications like the ones the military has used for decades. The advantages, he said, are that they offer an environmentally friendly solution for which maintenance of the seals can be performed without going into dry dock.
Earlier this year, Wartsila named AME its authorized distributor of seals and bearings for Florida.
“I have been in this field since its infancy,” Merhige said. “In old days, the guys used fax paper and a machine that looked like something out of World War II with the needle going back and forth on a page. Then the guys would have to figure this stuff out in the field.”
Vibration monitoring, he said, helps identify problems before they become failures, resulting in more efficient maintenance and less down time for vessels.
While the practice is common in the commercial shipping world, not many yachts make this a regular part of vessel maintenance.
“Where it would help 30-100m yachts is in monitoring pumps, gears, A/C systems and engines,” Merhige said. “Identifying problems early would eliminate breakdowns and unpleasant surprises. When things fail, they don’t fail in a straight line. They get bad, and get bad, they fail exponentially. But you could see it coming.”
Lucy Chabot Reed is editor of The Triton. Comments on this article are welcome at firstname.lastname@example.org.