Diagnostic Sea Trials

Whether something is shaking, cracking, or just plain annoying, J&A can identify the source and use our years of experience and engineering expertise to recommend practical and effective solutions. We also provide pre- and post-refit and/or sale surveys to protect owners and yards from costly disputes and unmet expectations. Typical diagnostic tasks include:

• Defining the problem by amplitude, frequency, mode of operation

• Identifying the source, and its mechanism of noise/vibration generation

• Outlining a range of solution options (at Source, Path, or Receiver)

• Designing the selected treatment option

Examples of Past Noise and Vibration Problem Diagnosis & Fixes

•  Skeg Induced Vibration – A 20m boat experienced excess vibration after a repowering. J&A found that the vibration’s speed dependence, its frequency, and other vessel characteristics indicated that problem was caused by Karmen vortex shedding. Analysis pointed to the keel’s trailing edge as the excitation source, with peak vibration occurring when the shedding frequency coincided with a hull resonance mode. The fix was a simple modification of skeg’s trailing edge, which, when implemented, resulted greatly reduced vibrations and quieter and more comfortable boat.

• Stabilizer Fin Induced Vibration – A 50m boat was experiencing major excess vibrations, particularly when the vessel was maneuvering at cruising speed. After expensive shaft and propeller checks and changes yielded no improvement, J&A was called on to perform diagnostic tests that identified the source as shedding from stabilizer fin fit-up to the hull.  Inexpensive fairing blocks at the fin root eliminated the problem.

• Carbon Shaft Vibration Issues - A new workboat with twin carbon shaft drive lines was experiencing high vibrations and shaft seal rubbing. J&A conducted non-contact shaft motion measurements while underway and shaft dynamics calculations afterward to identify the problem. J&A then suggested several practical fixes, which were implemented by the shaft designer/supplier. The result: problem solved!

• Shaft/bearing Induced Vibration – Excess hull vibration and airborne noise were found in a 70m yacht in cabins close to shaft seal. J&A tests and calculations showed the cause to be the interaction of a torsional shaft mode and the seal/thrust bearing. As the yacht was already in service, it was desired to avoid “hot work” around the shaft system. J&A engineered a fix using steel and structural adhesives that fixed the problem with no requirement for hot work or realignment.

• Quiet Refit - A 20m research vessel was initially built with little regard for noise control. The result was quite noisy, negatively impacting passengers and crew. J&A sea trial measurements were able to identify a series of improvements, including upgrades to various machinery mounting and insulation. The corrections were made during the next available yard period, and a 10 dB(A) noise reduction were achieved.

• Modification of Newbuild Production Boat - The purchaser of a new 38’ sport fishing yacht wanted improved quieting compared to stock models. J&A trials on a stock boat identified practical improvements for his new construction. Fixes for exhaust, propeller, and insulation resulted in significant noise reduction, and a happy owner.

• Simple Solution – An 80’ sport fisherman was experiencing a major increase in airborne noise levels after a yard period. Gearbox and shaft bearings were suspected. J&A sea trials and examination determined that the renewed propeller coating was the cause of the noise, specifically, “drips” in the new coating on the blade trailing edge . The coating was smoothed at the trailing edges and the problem was solved.

• Identification of a Likely Propulsion Impact History - A workboat was experiencing increased noise and vibration after years of satisfactory service. J&A was able to identify a gearbox on one side that was source of the vibrations, which were at the gear tooth frequency. Further, from the spectrum side bands J&A was able to state that the dominant issue had to be related to the bull gear. The history of “sudden” onset of the vibration and its relation to the bull gear allowed for directing further inspections by the gear specialists, which indicated a propeller strike as the likely initiating cause of the vibration.

• Repitched after Repower - A yacht experienced a major increase in propeller blade rate noise and vibration after being repowered, particularly at the higher speeds enabled by the newly increased power. After questioning, it was discovered that the increased power had been accommodated by a major rep-itching of the original propellers in a manner that over-pitched the blades toward the outer diameter. A new set of propellers matched to the new power and speed characteristics of the vessel is on order.

• Before and After - J&A performs before and after haul out noise and vibration trials that provide protections for both shipyards and vessel owners for evaluation of improvements and surprise “it didn’t used to do that” situations. Our work quality is recognized by A.B.S.