The quieter and smoother a boat runs, the more pleasant it is to be on.
By John Clemans
Some boats are just plain noisy. Conversing aboard them amounts to yelling. This may make you hoarse, but it doesn't mean that something is amiss. A boat that vibrates, however, usually does so because something is out of whack. Noise and vibration run the gamut from being merely a nuisance to sounding an alarm that some system needs immediate attention. Noise is caused by vibration - it's a waveform that travels through the air as well as through solid stuff, like a deck. Its origin is usually not very hard to detect. It may be caused by something flapping in the breeze or by a poorly seated hatch or a loose piece of hardware.
I was recently aboard a boat with a loose horn, which caused a bell-like sound as it made contact with the hardtop in a chop. It became surprisingly annoying - not the sound itself but the sound plus the fact that we couldn't find where it was coming from. All noise and vibration should be so inconsequential. Stopping sound is usually a matter of locating its origin and either blocking or absorbing it. Stopping vibration often requires professional help. Here are 10 ways to decrease sound and vibration on your boat.
 1. PROPS
One of the most frequent causes of vibration is a damaged propeller. Who hasn't run aground and then felt more vibration right afterwards? We automatically imagine that we've dinged the prop. (Sometimes dings happen.) And dings can cause vibration, as can warped propellers and propellers that are not seated properly. This vibration can affect other parts of the drive train or engine, so it should not be ignored. Make sure the prop nut is tight and check for damaged blades. Run the engine in gear without the prop at the rpm that caused the worst vibration. If the vibration disappears, the prop is the cause. Or simply try another prop. The prop may be out of balance, which is not visible. A prop shop can balance it for you. If no cause of vibration can be discovered, remember that switching from three-blade props to four-blade props can result in a smoother ride.
 2. SHAFTS
A bent drive shaft can cause serious vibration. A bent shaft can cause a visible wobble, but it may be too subtle to spot. Hold a piece of chalk with a clamp secured to something stable so that the chalk can't move and just touches the shaft. When the shaft turns, it should leave a uniform mark all the way around. If it doesn't, the shaft may be bent and should be removed and checked for straightness and consistency of taper. Shafts can be straightened if they're not too severely bent.
 3. STRUTS
Struts can loosen and can also be damaged so that they are slightly misaligned with the shaft log. This will cause cutlass bearings to wear prematurely, cause transmission misalignment and also cause vibration in the drive train that can loosen stuffing-box and shaft-seal systems and result in water intrusion. Struts should be checked by a pro unless they are obviously loose or damaged, in which case they require repair or replacement.
 4. BEARINGS
From the transmission to the propeller, there are bearings that support the shaft. Any of them can wear out, whether from age, constant use or improper alignment of some component. Transmission bearings, shaft-log bearings and strut bearings (cutlass bearings) can be replaced, but it's usually a job for a yard. Vibration is a sign of bearing wear. Sometimes it can be detected by examining the bearing or wiggling the shaft. There should be no play in it. Vibration from a worn bearing may be barely noticeable at first, but it increases over time. Also, unlike vibration from prop damage, which stays the same, vibration from a bad bearing usually increases as the load on the engine does.
5. ALIGNMENT
Engine and shaft must be in proper alignment, or vibration will result. The key components are the transmission and shaft couplings, which may have a flexible insert between them to dampen engine vibration, and the engine mounts. If the engine's position shifts in any direction, it will affect alignment. Coupling alignment can be checked with a feeler gauge - flanges should be within .003" of parallel - but for an alignment check, they may have to be separated, a job that most boat owners are not equipped to tackle. Correcting inaccurate alignment can be a trying task and is best left to the pros.
 6. ENGINE
Before checking the drive train, make sure that your engine is not out of tune, which can cause an increase in vibration. Any number of engine faults may be responsible, including a cylinder that is not firing. A load of bad fuel can cause rough running that translates into vibration, as can a clogged injector or a malfunctioning fuel pump. Clues to combustion-caused vibration are an increase in vibration as rpm increases and a change in the sound of the exhaust. As for noise, engines should have good mufflers. Underwater exhausts are quieter than conventional exhausts. Four-stroke outboards are quieter than two-strokes.
 7. ENGINE MOUNTS
Your engine's mounts are designed to isolate engine vibration from the hull as well as to offer a means of adjusting an engine's position. If your mounts allow too much vibration, it means they are too stiff or not the right size. Mounts should not be adjusted so the engine is raised so high that the mount is subject to great a load. Use a shim under the mount instead. Mounts wear out and should be replaced if necessary. Try to keep them clean. Those with metal shields over the dampers protect the rubber from destructive elements.
8. GENSET
Generators as well as main engines should be seated on flexible mounts that absorb vibration. Even pumps should have rubber mounts or be mounted on a rubber pad. Gensets should also be shielded to quiet them. Sound shields come as either standard or optional equipment on most gensets. Custom covers made out of fiberglass batting faced with a quilted noise barrier are also available.
 9. INSULATION
Noninsulated, slick, light panels act as loudspeakers in a boat. More porous surfaces better absorb sound. Engine rooms should be as well insulated as possible, as should hatches. The more insulation the better, both in terms of thickness and the area covered. Rubber gaskets around hatches also help. Acoustical insulation with a layer of lead in it deadens sound most effectively. A thickness of 1-1/2" to 2" is ideal.
10. CARPET
Soundproof carpet will help keep engine-room noise from entering cabins above. It should be laid first and then standard carpet padding and carpet should be laid over it. Soft headliners are more sound-absorbent than hard, slick ones. One other factor that can really affect sound levels is the position of air intakes. A boat with air intakes located inside the cockpit will be much noisier than one with intakes on the outside of the hull |
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