Good - or not-so-good - vibrations
Posted on 29 November 2010
Written by Eric Sorensen
Tracking down the source of on-board noise takes some detective work, but the reward is worth the effort
Next to a hard-riding hull, excessive noise and vibration are among the worst powerboat problems. Other than fans of go-fast speedsters, most skippers I know don't think more engine noise is good, and no one likes a boat that rattles and vibrates.
One of the great things about a sailboat is the absolute lack of mechanical noise and vibration when under sail without auxiliary power. All you hear and feel are the waves slapping against the hull and the wind in the rigging.
A quiet powerboat is highly desirable in my book, and I recently had the pleasure of checking one out on Lake Champlain. It was an immaculately finished 20-foot, 2-hp electric-drive, strip-plank composite launch built by Ken Bassett of Onion River Boatworks in North Hero, Vt. What a delight it was to motor around at 5 knots in near silence - no vibrations (other than my BlackBerry on occasion). The loudest sounds came from the seagulls circling nearby.
Noise is nothing more than an excitation of air molecules caused by a vibrating object. Eliminate or reduce the vibration and you greatly reduce noise. Obviously, the loudest sources of noise are the engine and the exhaust system. Therefore, the simplest way to quiet things down is to better insulate the engine compartment and install a better muffler.
But even boats with good acoustic insulation and the best exhaust systems can have noise problems that result from vibrations traveling through the structure - specifically the decks and bulkheads. And, of course, the vibrations can be felt throughout the vessel, adding to the annoyance.
Noise can also be an indication that something - usually a drivetrain component - is out of alignment or inadequately supported. Misaligned drivetrain components wear out faster than they should, and they also can parasitically absorb engine power, decreasing propulsion efficiency.
My $60 Radio Shack noise meter can record overall sound levels aboard a powerboat, but it won't tell me where the noise or vibrations are coming from. So the meter cannot be used as a diagnostic tool. All it can do is confirm whether noise levels are within an acceptable range. There are specialists, however, who can measure noise levels and identify the frequency and source of the offending vibrations. Once you know where the noise (vibration) is coming from, you can do something about it.
Bill Johnson, owner of Engineering Concepts, makes a living tracking down the source of vibrations on boats and offering solutions. Johnson is a Connecticut- and Florida-based marine systems vibration specialist who advised on corrective measures for bearing installations at Pratt & Whitney and worked as a design engineer at Olin Metals Research. He presented seminars at the International BoatBuilders' Exhibition trade show in 2004 and 2007 and at this year's Fort Lauderdale International Boat Show. He uses the IRD Mechanalysis 838 instrument for his full-spectrum testing and analysis, and the SKF bearing analysis instrument for determining the condition of bearings.
Johnson's equipment measures vibration frequencies with an accelerometer and presents the data on a paper graph or electronic chart. He says it's best to think of what he does as giving your boat an EKG, the medical test used to check the health of your heart. The frequency recorded during the initial analysis identifies the offending component, leading the detective to both the problem and the solution. I have not used Johnson's services, but what he has to say about the subject is interesting and rings true, so to speak.
Vibration problems are accentuated when the source is excited at the same natural, or critical, frequency as a part of the boat's structure - the hull, deck or superstructure. The end result is even more excitation.
Think of it as the naturally resonating soundboard in a piano. When the pianist strikes the A above middle C, the string hit by the hammer vibrates naturally at a frequency of 440 hertz because of its tension and length. Similarly, a bass drum is tensioned to produce a sound at a given frequency, creating the desired note. Both the piano strings and the drum, then, have natural frequencies resulting from their tension and physical properties, including thickness, length and density. It's the same thing with the different components of a boat's structure, which will produce a given frequency based on area, thickness and composition.
Johnson conducts natural frequency testing to determine the critical frequencies of the engine mounts, hull stringers, bulkheads and skin, prop strut and rudder foundations, and so on. He uses this testing process to determine if, for example, the propeller is properly seated on the shaft taper, or if it is key-bound with the key preventing less than 80 percent of the recommended surface-to-surface mating.
So a boat's hull and deck have natural frequencies and if they are the same as the source of the noise, whether it's an engine mount or a bent prop shaft, the result will be higher vibration. The higher vibration levels can increase noise. An engine operating at, say, 750 rpm may create vibrations at the same critical frequency as the engine beds because of their length and other physical characteristics. But at 800 rpm and above, the vibrations may greatly diminish.
Johnson often cites a boat that actually had several critical frequencies generated by the engine mounts as a good example of this concept. The culprit in this yacht was a spiral staircase railing and bow railings vibrating vigorously at 1,100 rpm for the port engine and 1,300 rpm for the starboard engine. By avoiding these rpm settings, the problem was eliminated, but the fix in this case was to increase the thickness of the engine-mount bracket from a 1/2 inch to 3/4 inch, or adding two 1/8-inch washers at each rear stud mount.
While the marine industry does not have standards for maximum noise or vibration levels, there are well-known industrial vibration standards that can be used as a guideline. Johnson considers anything below 0.15 inch per second for diesel-driven boats to be acceptable, but his goal is always 0.06 inch/second, since the difference is noticeable. Boats that he is called to evaluate frequently have vibration levels of 0.30 inch/second and some are as high as 0.60. That's 10 times the amount of Johnson's ideal vibration levels.
Whatever the levels, the problem is finding the source and coming up with a cure. Some vibrations occur at a frequency that is too low to hear - you just feel them. A bent propeller shaft or an unbalanced propeller usually causes low-frequency vibrations, with increased vibration levels causing more substantial damage to the equipment. Bad bearings and worn gears are more often high-frequency noise and vibration sources.
Let's look at some of the common sources and causes of vibrations aboard boats. Johnson says engine-mount loading is the cause of 60 percent of the vibration problems he investigates. Fortunately, he says, this problem is often fairly easy to fix and it might not require a sea trial.
Evening out the loading from one engine mount to the next often solves the problem and it can also reduce fuel consumption if misalignment is causing the shaft to bind. Johnson looks for any deviation in concentric loading. He uses a feeler gauge around the perimeter of the mating surface and he aims to keep tolerances within 0.002 inch, with more than 0.01 inch resulting in uneven engine loading. He checks this with the engine shut down and when it is idling.
The rest of the drivetrain can also be problematic. For example, the fit between the prop shaft taper and the prop hub must have the previously mentioned 80 percent contact along the mating surface or vibrations are likely to result. This calls for precise hand-fitting of the shaft and the prop hub to make sure they mate well. In the case of a shaft-to-prop taper, it's a little like fitting one ice cream cone on top of another. The taper absorbs the thrust of the propeller against the shaft, while the nut is there to hold the propeller when the engine is in reverse. A key in the taper prevents the prop from rotating in relation to the shaft and this key can also cause vibration if it prevents the tapered surfaces from mating properly.
Engine transmissions are another common cause of vibrations, with gear wear or damage, journal or roller bearing wear and inadequate lubrication all checked as potential sources. Johnson recommends staying away from 2-to-1 gears because of the amplified natural vibrations producing what he calls "looseness" frequencies between two mating parts. He says gear ratios of 1.9-to-1 or higher and 2.1-to-1 or lower are OK. As with the engine-mount checks, Johnson conducts transmission vibration tests mostly at low speeds.
By virtue of their size and composition, mufflers are subject to critical frequency amplifications and loose muffler supports can also be an issue. On the other hand, if the exhaust lines are visibly moving, it may be an indication that the engine mounts are the root cause of the muffler vibrations.
The hull skin area between longitudinal and transverse framing may resonate if it is inadequately supported and stiffened, and if it happens to resonate at the same critical frequencies as the drivetrain components. Of course, this panel resonation can also occur if the tabbing that holds the hull skin to the supporting structure has come loose. If it has, it's a structural issue that negatively impacts the boat's seaworthiness, not just vibration and noise levels.
The cockpits in sportfishing boats are another area in which these natural vibrations commonly develop. If the deck is the right size and made of the right materials, it can turn into a giant guitar soundboard, resonating at just the right (or wrong) frequencies to amplify the naturally occurring drivetrain vibrations.
To prevent the boat's structure from vibrating, the fix could be as simple as spraying or gluing on vibration-dampening materials to increase mass, which better absorbs and attenuates vibrations. Or it could be that the scantlings of the engine beds and supporting stringer structure need to be increased. If the boat has run hard aground or has been dropped from its lifting slings, a shaft or rudder strut could have been whacked out of alignment, creating a source of vibrations.
The shaft coupling at the transmission is another possible vibration source, in which case the fix is as simple as tightening the flange coupling bolts holding the prop shaft to the transmission. This has to be done with a torque wrench, using the proper bolt-tightening sequence. The prop shaft might be bent, the bearings between the engine and the propeller may not be lined up and/or the shaft may not be properly aligned with the transmission. Johnson's standard is less than 0.003 inch total run-out deviation, or straightness, for the shaft.
Vibration measurements also indicate whether the propellers are properly balanced. The International Standards Organization, in this case ISO 484-2, has four classes of propeller tolerances: 1) Class S, best, very high tolerance; 2) Class I, very good, high tolerance; 3) Class II, average tolerance; and 4) Class III, low tolerance. Keep in mind that not all propellers, even when they're brand-new, meet Class S or Class I standards, so it pays to have yours checked if you're unsure.
Johnson recommends tuning propellers to at least ISO Class I standards. Propeller blade tuning equipment measures mean pitch at a given radius (distance from the hub), pitch and angular deviation for each blade, and rake and blade contour. If all of these elements are within close tolerances, the blades will be evenly loaded. That improves fuel efficiency while reducing vibration and prop noise, and it decreases wear on bearings and the rest of the drivetrain. Dynamic balancing of the prop after tuning further reduces vibrations.
Propellers can vibrate excessively if they are too large for their shafts. As a rule, the prop diameter should be no more than 15 times larger than the shaft diameter. There should also be roughly one shaft diameter separation between the prop hub and the strut to allow adequate water flow through the strut's water-lubricated cutless bearing. Also, make sure line cutters do not interfere with adequate water flow.
Propeller-tip clearance is another issue, because too little clearance between the prop tips and the bottom of the boat creates pulsating high-pressure areas. The water is essentially trapped with no easy exit path each time one of the blades rotates through this region.
Props also can be overloaded, with too much pressure resulting from too little blade area for the power absorbed, creating cavitation in a prop that is not designed for it. (Some high-performance props are designed to be super-cavitating.) Cavitation, in turn, results when a very low-pressure area is developed on the propeller's leading edge. The water boils off into vapor bubbles that collapse and erode the blade surface of a conventional subcavitating propeller.
Generally speaking, the more blades, the smoother a propeller will operate. A prop's vibration frequency will be a function of shaft rpm and blade count. A shaft rpm of 800 with a four-blade prop sets up a frequency based on 3,200 vibrations per minute.
With the propeller itself up to specs, further troubleshooting can be completed. By the way, Johnson says a prop installed underwater by a diver should only be considered a temporary fix because the fit cannot be accurately evaluated. The boat should be hauled and the fit checked as soon as it's practical to do so. For some owners, this will be setting the vibration bar higher than is practical, especially for owners who rarely haul their boats.
Johnson also looks at alignment tolerances of prop struts and rudder bearings. Rudder ventilation and stalling when the boat is at speed can also be detected by telltale cavitation frequencies. For example, a builder installs foil rudders on a 12-knot boat, but then the owner puts a bigger engine in and speed increases to 20 knots, causing the foil rudders to stall at the higher speeds. Whatever the source, once drivetrain vibration frequencies are measured and known, you can better track down the root cause of rattling structures, such as ladders, decks and railings.
Armed with this information, you may be in a better position to do some vibe sleuthing of your own, or perhaps giving your boat an EKG may be the best place to start if you're experiencing high vibration and sound levels without obvious causes. To minimize drivetrain-caused vibrations and noise, stiff, high-mass (heavy) engine beds combined with soft engine mounts is the best way to go. But this basic engineering approach assumes that alignment tolerances, from the engine mounts back to the rudders, are being met. Vibration frequencies at the problem area (bow railing or galley stove) can be matched to the source (misaligned shaft), making for an accurate root-cause analysis and an efficient correction strategy.
It's also more cost-effective to isolate the source of vibrations and address the root cause, when that's possible, than it is to just add sound insulation materials to cover up the problem. Otherwise, you're going to be flying blind and are likely to spend more time and money than necessary. You may also find problems you didn't know existed in the first place if they are not detectable absent sympathetic vibrations that you can feel traveling through the vessel's structure. Having longer-lasting drivetrain components - and owning a quieter, smoother-running boat - sure beats the alternative.
Bill Johnson can be contacted at (860) 888-3995 or
See related story:
- Filling the holes, muffling the machinery
This article originally appeared in the December 2010 issue.