Vibration And Noise Control In Small Boats

VIBRATION AND NOISE CONTROL IN SMALL BOATS Lawrence Zeitlin VIBRATION CONTROL I like my boat engines to be completely as completely unobtrusive as my house cat. To paraphrase the old Rolls Royce commercial, the loudest mechanical noise aboard should be the ticking of the chronometer. But - making your propulsion engine purr like a pussycat is a lot more complicated than just slapping in a few flexible mounts and a flex shaft coupling. In fact, by selecting the wrong mounts, you can actually make the vibration far worse than it would have been had you just bolted the engine to the bearers. I'm not suggesting that satisfied boat owners rip out their engines and install a new mounting system, however those of you planning a new boat or rebuilding or repowering an old one might consider the following suggestions. Think of an engine as a block of iron suspended on a Slinky like spring. If you pull the engine down slightly and let it rebound, it will bounce up and down slowly. The rate that it moves up and down is called the NATURAL FREQUENCY of the system and is dependent on the engine weight and the strength or restoring force of the spring. The heavier the engine and the weaker the spring, the slower the engine moves and the lower the natural frequency. If the engine is light and the spring is stiff, the natural frequency is high. Now, mount the engine in a boat on flexible mounts. These mounts can be considered as small springs that deflect or compress slightly by the engine weight. If the engine is heavy and/or the mounts are soft, the engine will compress the mounts a good deal and the natural frequency of the system will be low. With stiff mounts and/or a light engine, the frequency will be higher. Bolting the engine directly to the bearers is the equivalent of using very stiff mounts and the natural frequency will be high. The natural frequency of an engine mounted on flex mounts can be estimated by measuring the deflection of the mounts and working through the following equation:

Fn = 3.13 x sq. rt. (1/ Ds) where: Fn = natural frequency in cycles/second Ds = static deflection of engine mounts in inches Lets put numbers to this. Assume that we have a Perkins 4-108 engine mounted on four flex mounts. The deflection of the mounts is .05 inch. The natural frequency (Fn) of the system would be 14 Hz or roughly equivalent to 840 oscillations per minute. If the mounts were softer and deflected more, the Fn would be lower. Less deflection and Fn would be higher. Now engines don’t sit quietly in a boat. They are forced into vibration by the explosions within the cylinders which produce the power. Pistons move up and down, the crank rotates, the valves open and close. It is the imbalance of the moving masses within the engine which produces this forced vibration. The number of oscillations per second is called the FORCING FREQUENCY. This forcing frequency (Fd) is largely a function of engine design. A one cylinder engine has all its masses unbalanced and exhibits a strong vibration at every revolution. A two cylinder engine can be configured to balance most large motions of the pistons (i.e., one moves down while the other moves up), but since the pistons are displaced slightly from each other, secondary rocking forces appear. A four cylinder engine such as the Perkins 4-108 can balance these forces to a great extent but not completely. The cylinder firing sequence also adds its bit to the vibration. It takes at least a six cylinder engine to fully balance all the internal forces. There is no simple way of calculating the forcing frequency without knowing a good bit about the engine design. A four cylinder engine like the Perkins usually has most of its vibration at twice the crankshaft rpm and directed in a vertical direction. A two cylinder engine has a vertical vibration component at twice the rpm and a rocking component equal to the rpm. A one cylinder engine vibrates vertically and horizontally at the crankshaft rpm. The moving masses of six and eight cylinder engines can be completely balanced and their blocks tend to be far heavier than the lighter four cylinder Perkins, Yanmar, and Lehman Fords. Big engines with six cylinders or more can be firmly fastened to the boat structure without transmitting too much vibration. Fairly noise tolerant owners of boats with these engines need read no further.

If the forcing frequency is higher than the natural frequency of the engine on its mounts then the vibration will not be fully transferred to the boat’s structure and vibration isolation will occur. That, of course, is what we want. Assume that our Perkins is running at 1000 rpm. Most, but not all, of the forcing vibration will be at 2000 oscillations per minute. This is more than twice as high as the calculated natural frequency of 840 oscillations per minute and vibration isolation occurs. In this case the isolation is about 80% and only 20% of the engine vibration is transmitted to the structure as compared with an engine bolted firmly in place. Theoretically isolation begins when the forcing frequency is 1.4 times the natural frequency (Fd = 1.4 x Fn). For a system with a natural frequency (Fn) of 840 oscillations per minute, isolation would start at a forcing frequency (Fd) of 1187 oscillations per minute. If the forcing frequency is closer to the natural frequency, vibration AMPLIFICATION occurs and there will be more vibration transmitted to the boat than with a solidly bolted engine. When the forcing frequency and the natural frequency coincide (Fd = Fn), resonance occurs and the engine will literally shake itself out of the boat. If boat engines ran at only one speed, there would be little problem. You would simply run the engine at a speed where the forcing frequency was at twice as high as the natural frequency and have at least 90% isolation at all times. Unfortunately, engines start from rest, idle, and go slowly. At those times, the forcing frequency may well drive the system into the amplification mode and excess vibration will be transmitted to the hull. There are a couple of ways to minimize the problem. First, choose mounts soft enough so that the natural vibration frequency is below the normal operating range, even when idling. This means that you should choose mounts rated for the size and weight of the engine. Stronger and firmer is NOT better. For the Perkins 4-108, a comparatively light engine, the mounts should be chosen to support only the actual engine weight at each attachment point. This is usually given in the engine specification drawings. If you don’t have this information, the simplest way to determine mount ratings, with the engine out of the boat, is to put a bathroom scale under each attachment point and note the weights. Mounts are chosen which have an appropriate static deflection for the weight at that point. You may need different strength mounts for the front and rear of the engine since weights are not necessarily symmetrical. Catalogs usually give the amount of deflection of a given mount by various weights within the

mounts range. You may have to work through the simple equation above for an arbitrary 80% isolation at idle. Second, select mounts made of high damping material. These tend to absorb some of the vibration energy when passing though critical engine speeds. Most engine mounts are already made of this type of rubber so you don’t have to worry too much about it. If your engine rocks at certain rpms you can fit a small shock absorber type vibration snubber, available at most auto stores for transversely mounted car engines. Third, move through the critical rpm range quickly and/or avoid it if possible. If your engine vibrates at idle, set the idle speed slightly higher. Peace and quiet is usually worth the few extra ounces of fuel.

Most marine engines have four attachment points, one at each corner. Because this is a marine application where the engine is subjected to severe shock loads, it is necessary to limit excessive motion of the engine. High damping is required to control engine motion during startup and shutdown. It is mandatory that in the event the vibration mount fails, the engine must be kept in place. This usually means that if the rubber breaks, a metal bolt and retaining washer stop the engine from falling out of the mount. It is not necessary to restrict yourself to marine engine mounts. A number of other mounts would be suitable for specific applications but you would have to do the engineering work yourself. There are several manufacturers of industrial vibration control mounts who provide catalogs with full engineering information. Among these are Lord Industrial Products, 1952 West Grandview Blvd., P.O. Box 10040, Erie, PA, phone (814) 868-5424; and Barry Controls, 700 Pleasant St. Watertown, MA 02172, phone 617-923-1500. A do-it-yourself custom made approach is far cheaper than a factory made system but only if you can do the calculations and fabrications on your own. My favorite factory made mounts for small engines are the liquid damped Van Den Ouden mounts, although they cost a bundle. The calculations are identical for both industrial and marine motor mounts. Marine motor mounts differ from their industrial counterparts in four respects. Marine mounts usually have some form of adjustment for leveling the engine. This is generally a bolt and nut arrangement extending upward from the mount. Lacking this adjustment screw, the motor would have to be leveled either by shims or by

sliding paired wedges under the mount. Second, marine mounts are often equipped with a shield to prevent diesel or oil drips from harming the mount rubber. Third, marine mounts are typically designed to be stiffer in the fore and aft direction to resist propeller shaft thrust although propeller shaft thrust bearings can eliminate this requirement. Finally, marine mounts are up to ten times more expensive than comparable industrial motor mounts (surprised?). OK, why not fit the softest mounts you can and keep the natural frequency as low as possible. Well, there is more to vibration isolation than keeping it from the structure. All flex mounts let the engine move. That is their intent. This means that all connections to the engine must be able to tolerate motion without breaking. It means flexible fuel lines, flexible controls, flexible exhaust systems, and most important, a flexible connection between the engine and propeller shaft. The less the motion, the less the problem. Back in the days of wooden boats and iron men the problem was handled by simply bolting the engine to wooden bearers. Engines were far heavier in those days, hence the natural frequency would be lower, and the compressibility of the wood in the bearers served as natural shock mounts. Watersoaked structures have a natural damping ability, unlike fiberglass or metal, and engine vibration was absorbed before it could rattle out the fastenings. Still, the engines had to be aligned yearly and the mounting bolts snugged up. Coupling a flex mounted engine to the propeller shaft requires a way of absorbing as much lateral deflection as the engine is expected to move at its maximum vibration point. For a motion less than 0.1 inches, a Drivesaver, PYI Flexmount or equivalent rubber doughnut will do. This is all that a correctly mounted four or six cylinder diesel needs. On one of my boats, I have a softly flex mounted two cylinder Volvo MB10. The engine moves a quarter of an inch in each direction at its low idle resonance point. Max rpm on this engine is 2000 and it idles at 650 so I need a high deflection to keep the natural frequency low enough to get appreciable isolation. In this case I needed a couple of very soft cylindrical mounts at the front, mounted at 30 degree angle, so that the vertical and rocking motion would be absorbed partly in compression and partly in shear, and a couple of soft mounts at the rear which were stiff enough radially to counter the propeller thrust. The half inch total horizontal and vertical motion of the coupling was too much to use a rubber flex coupling. Instead I used a double chain coupling between the engine and propeller shaft. The Boston Gear manual will tell you how to make one. This is a poor man’s version of a double universal coupling, but better since it permits sufficient axial motion to avoid a spline joint. A properly sized chain coupling is stronger than the shaft. All the maintenance it requires is a few drops

of oil every season. This approach works so well that even without any acoustic insulation my guests usually can’t tell when the engine is on. OK - let's go back to square one and deal with the problem of attaching the moving engine to the shaft. Boat engines must be coupled to the propeller shaft. If the engine is solidly bolted to the engine bearers AND if the boat maintains a constant shape under all loading and environmental conditions THEN the engine and shaft could be solidly bolted together with rigid, fixed flange bearings. The engine and shaft would be aligned at installation and remain forever married. This was pretty much the case with very heavily constructed wooden and steel commercial boats. For pleasure boats, where weight is important, the scantlings are slimmer and the boats work a bit. Some materials like fiberglass are naturally flexible. Engines are mounted on rubber vibration absorbing mounts and prop shafts are supported by rubber cutlass bearings. All these conspire to induce relative motion between the engine and shaft. Conventional alignment techniques try to reduce this motion to an acceptable level for normal boat operation. If the boat works or swells or the engine shifts slightly, alignment must be repeated. Clearly the answer is a flexible coupling between the engine and prop shaft which is capable of transmitting the rotary and thrust forces but can tolerate the relative motion between engine and shaft axes. Actually all boats have some tolerance for misalignment. The shaft itself is a flexible member and bends slightly under load. Further, with rubber mounted stuffing boxes and stern bearings, the shaft can move a degree or so. For some boats, this is all that is needed. Interestingly, for some boats with slight engine/shaft misalignment, removing a shaft intermediate bearing may allow the shaft to bend a bit more and result in smoother running. However, no rigid coupling can accommodate the motion of an engine properly mounted to minimize vibration. The smaller and fewer the cylinders, the greater the motion. Depending on cylinder configuration, the engine may move vertically, horizontally, yaw, pitch or move in all directions simultaneously. The worst motion is at slow speeds when we like the engine to be quite and unobtrusive. The engine mounts have to absorb this motion yet at the same time resist the thrust of the propeller. The preferred engineering answer is to decouple the transverse motion of the engine from the shaft, only letting the engine twist

the shaft. This is the job that a flexible shaft coupling tries to do. A flexible shaft coupling simply joins engine and prop shaft with some device which lets the twisting force get through but does not transmit horizontal or angular motion of the engine. It should also retain the shaft in the boat and allow the propeller thrust to be transmitted to the structure. For moderate degrees of engine motion, a rubber or plastic biscuit fixed between coupling flanges works OK. This is probably the oldest method. Most automobile steering wheels are connected to the mechanism which turns the car wheels by a flexible fabric reinforced disc. If you trust you life at 100 mph to this system, why not your boat? The Globe "Driversaver", the PYI "Flexmount", the Vetus "Bullflex" and about half a dozen other couplings are of this type. They do not accommodate more than a small amount of misalignment but they are comparatively cheap. Because they do not let the engine and prop shaft get much out of line, no additional shaft support or shaft thrust bearing is needed. Mechanical flexible couplings allow more misalignment. These usually consist of engine and propeller shaft flanges connected by a mechanical device which is torsionally rigid yet allows horizontal and angular play between the flanges. The chain coupling is probably the best example. The length of double roller chain wrapped around the hardened sprockets that substitute as coupling halves has sufficient play that the couplings can move slightly with respect to each other yet still transmit the torque. Contrary to what you might expect, the chain does very little flexing, and because the radius of the coupling is so small, centrifugal forces are very low. In fact the chain could rust solid and the coupling would still work. The extreme case is the flexible gear coupling in which the chain is replaced by a loosely fitting external gear connecting both halves. A single mechanical coupling can accommodate perhaps 2 to 3 degrees of misalignment and up to 1/8 inch of horizontal play. If that is not enough for a very flexibly mounted engine, two such couplings mounted back to back with a 6" to 8" stub shaft between them can handle up to 1/2 inch motion and 6 to 8 degrees of misalignment. The downside of any mechanical linkage is rust and noise. If these couplings are kept reasonably dry and squirted with oil occasionally, they will last almost forever. For bad conditions or to minimize noise, they can be wrapped in a grease filled neoprene boot, just like the wheel universals on your front drive car. Really finicky boat owners can get chain couplings made of stainless steel, originally intended for food processing machinery. Boat use of properly specified mechanical flex couplings is considered light loading for industrial quality components. Most are intended for 24 hour continuous heavy duty work.

For extreme engine motion or bad, bad misalignment problems, nothing beats a double universal joint system. Thousands of them are in use in boating already. How else do you think the power gets to the outdrive in an inboard/outdrive installation? A double universal joint system will allow up to 30 degrees of misalignment between engine and prop shaft. Actually, add a few more joints and you can point the engine athwartship while driving a stern mounted prop. several models, including Aquadrive, Scania, and the Evolution system incorporate constant velocity joints to further decrease the vibration. The downside is expense, expense, expense and thrust intolerance. As the angle between the engine and shaft increases, less and less of the thrust reaches the thrust bearing in the reduction gear and more is directed sideways tending to rotate the engine. That eliminates the athwartship mounted engine. The fix is to incorporate a thrust bearing on the prop shaft to transfer the thrust to the boat structure before it reaches the engine. To sum up: 1. Flexible couplings let the engine axis move with respect to the shaft simplifying vibration mounting of the engine and alignment problems. 2. Couplings arranged order of increasing flexibility: Rigid, rubber biscuit, rubber in shear, single mechanical (chain, gear, flange), dual mechanical couplings, universal joints, dual universals. 3. Very flexible couplings (say > 10 degrees) will require a thrust bearing on the shaft. 4. Properly designed couplings do not explode, shatter, or otherwise cause catastrophe if they are kept out of the bilge and oiled once a generation. 5. The real weak spot in the power transmission system is the propeller shaft setscrew and key arrangement. For security, shafts should be kept in place by USCG approved clamp couplings or by through pins of reasonable diameter.

MINIMIZING NOISE IN TRAWLERS Like many others on the T&T list, I started my boating career in sailboats. For me, one of the great appeals of sailing was its comparative quiet. It is very difficult to escape from noise in our modern environment but while on the sailboat the loudest noise I had to contend with was the rattling of the halyards. My transition to trawlers provided a rude awakening. Trawlers are noisy. I was told by other trawler owners to live with it. It goes with the territory.

There are several approaches to dealing with a high trawler noise problem. You can simply do nothing, pretending you are driving a slow unlimited class hydroplane or a Harley with a gutted muffler. Of course you may need a hearing aid in a few years. An alternative approach is to use ear plugs or external "Mickey Mouse" sound absorbing ear protectors. These are cheap. Single use ear plugs such as those sold by Flents or E.A.R. sell for about 25 cents a pair. External ear protectors cost about $10. Both will provide about 30 dB of sound attenuation. Either can be bought at an industrial supply or hardware store or in a gun shop. The downside is that they attenuate all sounds and shipboard conversations must be carried out by signing. If you are technically inclined you can try a noise cancelling system. This approach has been suggested for aircraft cockpits and luxury cars. A microphone picks up the ambient sound. An amplifier circuit reverses the phase by 180 degrees and drives a loudspeaker. The two out of phase sound waves cancel each other and comparative quiet reigns. The zone of silencing is limited in area. If you move very far from the loudspeaker, the sounds may no longer be out of phase and may even be amplified, creating a noisier environment than before. Normally automobile drivers and airplane pilots don't get out of their seats and wander around the cockpit. A more portable solution is a pair of sound cancelling earphones such as those sold by Bose. These cost about $300 each. A considerate skipper will provide a pair for each crew member and guest. Naturally, if you throw enough money at the problem, it can be solved in a more direct manner. In its new line of fiberglass high performance powerboats, the DS 45 and DS 53, Linssen has achieved an astounding level of acoustic silencing. Even at 30 kts. Linssen advertises up to 40 dB of sound attenuation between the engine room and the boat's wheelhouse and living quarters. Of course these are luxury boats with a base price of over $1,000,000. Patrick Gerety, former manager of Willard Marine's recreational line once told me that the last W30 series of trawlers carried $10,000 worth of acoustical insulation because upscale buyers demanded silence. I tend to believe him. But I suspect that Willard, and many other trawler manufacturers are going about sound and vibration minimization the wrong way. You can't just muffle a noise producing device cheaply. It takes sound absorption materials and mass, correctly applied in layers of sound insulation. Automobile manufacturers faced this problem years ago and came up with other solutions. Look under the hood of your car. You will find very little insulation to contain the sound and vibration of a lot of moving machinery. What the auto makers did was to develop ways of

decoupling the noisy and vibrating components from the structure of the auto body. Only a small amount of acoustic insulation is necessary to keep the airborne noise out of the passenger compartment.

BACKGROUND Noise is simply defined as unwanted sound. The operative word here is UNWANTED. If you are a rock fan you may be perfectly happy blasting out Def Jam to the limit of your stereo speakers capability while your chamber music loving neighbor in the next slip is dialing the police with shaking fingers. The sound level in his boat may be one tenth of that in your saloon but to him it is still noise. But, in addition to being irritating, loud noise has other effects. It can make it difficult to hear radio messages and acoustic navigational signals. Extended exposure to high level noise can inhibit the ability to concentrate and can cause ear damage, raise blood pressure and trigger other health problems. Before we get too deeply into noise reduction, let's talk for a minute about how sound is measured. Sound pressure is measured on a logarithmic scale using a unit called the BEL. Named, of course, after Alexander Graham Bell. Each BEL increase or decrease is a 10 times increase or decrease in the sound pressure. For convenience the BEL is divided into 10 parts or deciBELs (dB). A change of approximately 3 dB indicates a doubling or halving of the sound pressure level. But those are physical measurements. They only loosely correspond to the perception of loudness of a sound. The human ear responds to changes in sound pressure with its own logarithmic system. The range of human hearing is enormous and covers a 120 dB range from the threshold of hearing to the onset of auditory pain. That's a trillion times difference in sound pressure. A 3 dB change, or halving of the sound pressure is barely noticeable. A 6dB drop in sound pressure will produce a trivial change in loudness. It takes at least a 30 dB reduction in sound pressure for a sound attenuation method to make a real difference. That represents a 1000 time reduction in the sound pressure level.

Manufacturers who sell sound reduction equipment rarely indicate the reduction in perceived loudness, merely the reduction in physical sound pressure as measured in dB. They hope that the typical user will confuse the two measures and believe that a 50% reduction in sound pressure corresponds to a 50% reduction in loudness. It does not. It is a barely noticeable change in loudness. Powerboats are perceived as noisy for several reasons.

First, internal combustion engines are remarkably efficient noise generators. They operate by a series of explosions in the cylinders and release pulses of energy in the exhaust. Valves open and shut interrupting the flow of air in the intake manifold, each interruption creating a wave of sound. And the gears, pumps, turbos and injectors add to the cacophony. Second, human ears are very sensitive. The threshold of hearing at 1000 Hz is .0002 dynes/sq. centimeter. To put it into terms more familiar to the average trawler owner that works out to .00000013 hp. A small amount of energy indeed. Ten watts of acoustic energy, a tiny fraction of a horsepower, in the middle of the hearing range, would drive us screaming from an engine room. Continued exposure would cause permanent hearing loss. No wonder so many band members are partly deaf from playing in front of their 150 watt sound systems. Unfortunately so are many of us from a lifetime of exposure to noise. Finally, the boats themselves are effective at transmitting sound throughout their interior. Most of what we think of as noise in a fiberglass boat is really structure borne vibration which forces the relatively thin boat panels to resonate. Minimize structure borne vibration and you go a long way toward making a boat quiet. And it doesn't require $10,000 of after the fact noise insulation. Just a little attention to the physics of sound and vibration control will do it. NOISE CONTROL ACTIONS If you want to minimize the noise in recreational boats, you should take the following actions, arranged in rough order of importance. First, and probably the most important, is to prevent engine generated vibration from being transmitted to the boat's structure. This was discussed in the previous series of notes. But to recap, the engine should be mounted on soft rubber mounts designed to provide at least 80% vibration isolation. More isolation

is better. Usually that means that the weight of the engine will compress rubber mounts by .05" to .1". But since the engine will move a fair amount with this level of isolation, a flexible connection with the drive shaft is necessary. Elastomeric doughnuts such as the Drivesaver or PYI between the coupling flange halves will provide a minor reduction in vibration. The best approach is to use a double constant velocity universal joint coupling. Examples are the Scania, the Aquadrive, and the Evolution couplings. These all include a thrust bearing that transmits prop thrust directly to the hull and spares the engine mounts the necessity of handling engine thrust as wall as vibration. All engine connections, fuel lines, and controls must be capable of dealing with engine motion. Second, the engine should be provided with adequate mufflers both on the intake and exhaust. Exhaust muffling is handled relatively well in wet exhaust systems using an Aqualift type muffler. The injection of water into the hot exhaust cools the exhaust and extracts some of its energy. The large volume of the muffler smooths out the exhaust pulses. Additionally the long rubber hose to the exhaust outlet used by most boats further attenuates the sound. If the exhaust is not quiet enough after all this processing, an inline exhaust muffler, such as those sold by Vetus, can be added to the exhaust line. Dry stack exhausts must be muffled in the same manner as a car. The muffler should be a large chamber with baffles. Entry and exit pipes should be tuned to minimize sound at the cruising RPM. Just letting the exhaust exit through a tall pipe above the deck doesn't do it. Intake mufflers are often overlooked in marine installations. The intake valves interrupt the air flow into the engine up to ten times a second creating sonic pulses whose higher order harmonics are well within the hearing range. If you look under the hood of your car you will see that the intake muffler/air filter consists of a large cylinder or box with a relatively small intake port. The combination of the air filter and restricted opening attenuates the intake sonic pulses. Run your engine with the air filter open or off and you will hear the difference. On many boat diesels the filter consists of an oil saturated metal gauze which works admirably to keep dust particles from the engine innards but does little to block the sound. An Airsep type filter does a better job of noise control. It is amazing how much sound can get through a small hole. The bulkheads of the engine compartment are penetrated by many ducts and cables, each of which can offer a pathway for engine generated noise to escape. Example: The engine compartment in my Willard is below the pilothouse floor. It has a heavy 1" plywood hatch layered with sound absorbing material. The hatch was penetrated

with a 3/4" hole intended to be used as a finger hole. When operating at over 2000 RPM, engine noise in the pilothouse was excessive. Simply sealing the back of the hole with a 1/4" thick square of plywood dropped the noise level in the pilothouse by 10 dB. So the third step in noise control is to close all openings in the engine room leading to the living spaces aboard the boat. Many of these openings can be sealed or gasketed with little interference with function. If a throttle or shifting Bowdon cable runs through a hole in the engine room bulkhead, seal the gap with a blob of silicone caulking. The same for electrical wiring or conduits or even exhaust hoses. Silicone caulking is fireproof and can be pulled out if replacement or repair of the cable or wiring is necessary. Some openings are required for air intake and ventilation. These should exit outside the hull whenever possible and be covered by louvers directing the sound away from the living spaces. Acoustical texts are replete with techniques of suppressing sound transmission through ducts. Most of these involve making several right angled bends in the duct and lining the interior with sound absorbent material. Few are practical for the engine compartment. What is practical is making a single right angled bend in ducts for air intake and ventilation, and using a sound absorbent lining. Alternatively, the hole in the compartment wall can be covered by a large baffle at least 6" in diameter greater than the hole. The baffle, covered with sound absorbent material on the side adjacent to the entry hole, should be spaced 3" from the hole by several dowels or spacers. This will permit air to enter or exit yet block some of the high frequency sound emanating from the compartment. Fourth, the walls and overhead of the engine compartment and large panels in the living space should be treated so vibration is kept to a minimum. If vibration transmitted through the boat structure excites a thin panel or bulkhead into a sympathetic vibration, it doesn't matter how much effort you have expended in sound treating the engine room. It's just like having a loud speaker in the living space blasting out engine sounds. One way of minimizing panel vibration is to add viscous mass. In automobiles a rubber based undercoating is applied to thin metal panels to change their vibration characteristics. Suitable application of a viscous coating can make a tinny car door sound like it is made of armor plate. If a panel vibrates and the back side is accessible, this is a convenient and cheap solution. We used a modified undercoating in the inside of tanker's helmets to stop them from vibrating like a bell when the 50 caliber machine gun was fired. Auto grade undercoatings are generally not used in boat engine rooms because of the potential

fire hazard. However, in the saloon, undercoating is no more dangerous than a teak panel. For engine rooms I have used Silent Running SR 1000, a USCG approved water based vibration and sound dampening material made by Current Composites of New Haven, CT (www.silentrunning.us). Other manufacturers make similar materials. Another way of minimizing panel vibration is to stiffen the panel by gluing or screwing rigid battens to the backside. Shelves or picture frames fastened to the front serve the same purpose. What you want to do is change the natural frequency of the panel so that it does not vibrate in resonance with the engine. In extreme cases, you can make a bulkhead or panel virtually soundproof by fastening another panel to its back, spaced about 1 inch away by wood strips around the edges, and filling the cavity between panels with dry sand. This technique is often used in large Hi Fi enclosures to completely kill panel vibration. The best way to impede sound transmission between rooms or compartments is to use a brick and mortar wall. About 12" of brick and mortar will reduce the sound transmission by 60 dB. It goes without saying that this is only practical in the largest of boats, perhaps in the QE2 range. Eventually, you will want to apply sound treatment to the interior of the engine compartment and living spaces. The mistake most people make is using sound absorbing material as the first step in a noise reduction program when it should come near the end. The material is expensive and a great deal more will be needed if you don't control sound at the source. One of the best ways to do this is to construct a box around the engine using heavy materials. A heavy gauge steel or sturdy plywood enclosure around an engine would work extremely well as long as the box extends down to the engine bearers. This approach is impractical for most trawler main propulsion engines but is the method of choice for generators and auxiliary power sources. The interior of the box should be lined with heat resistant sound damping materials and the air intakes should be baffled. All common sound absorbing materials work by converting the energy of moving air particles in the sound wave into heat which is then dissipated into the surrounding surfaces. To effectively capture the sound wave, the absorbing material must have a thickness of at least a quarter of a wave length of the sound in air. Since sound travels at approximately 1100 feet per second, a wavelength of a 1000 Hz sound is about 11 inches. This means that if we use a fluffy fiberglass absorbing material, it should be 2 1/2 to 3 inches in thickness. Higher frequency sounds require less material, low frequency sounds, more material. We can get by with a thinner sound absorbent layer if we add mass to the capturing medium.

This mass is usually in the form of heavy particles distributed through the absorbing medium or by incorporating lead membranes between layers of foam or fiberglass. The sound wave is weakened by using its energy to move the mass. Lead membranes in sound absorbing material also reflect some of the sound back toward the source. Typical non-weighted sound absorbing materials are made of foam or fiberglass covered with a mylar or vinyl facing. The acoustical properties of foam or fiberglass are the same but fiberglass is much more fire resistant and is suitable for engine compartments. USCG rated 2" thick fiberglass with an acoustic scrim facing costs about $4 a square foot, foam or non-USCG rated fiberglass between $2 and $3 a square foot. The acoustic material is available in sheets up to 4' x 8'. The material is attached to bulkheads and overheads with adhesive. One or two battens per section may be required to hold it in place for security. Particularly noisy engine rooms will profit from using lead composite insulation consisting of one or two lead membranes sandwiched between layers of foam or fiberglass. The lead composite material weighs 1 to 2 lbs per square foot depending on thickness. It is heavy stuff. A typical engine compartment in a 40' trawler might well use 200 lbs of lead composite insulation. Costs are higher than for plain foam or fiberglass. A square foot of 2" thick, 1 lb/sq. ft. fiberglass lead composite insulation with a white mylar overlay costs $6. The 2 lb/sq. ft. fiberglass lead composite insulation costs about $8.50 per sq. ft. As might be expected, lead fiberglass sandwiches using all USCG approved materials are more expensive, each square foot running $7.50 for the 1 lb. weight and $10 for the 2 lb. weight. These are retail prices. A search of the internet might find better values. If you install fiberglass or lead composite fiberglass materials on the bulkheads and overheads of the engine compartment, be aware that fiberglass has a tendency to separate from itself. The edges of the panels should be wrapped with a mylar tape to hold the material intact. Most insulation suppliers will cut the material to a pattern and wrap the edge for you. Installation is by either adhesive, reinforced by judiciously placed mechanical fastenings; or, by covering the insulation with pegboard or perforated aluminum. For steel or aluminum boats, the insulation is usually cut to fit between the angle stiffeners. The final step in silencing a boat is to use acoustic carpet underlays in the inhabited areas. I assume that your boat has carpets rather than teak and holly flooring. Acoustic carpet underlay material is similar to the sound insulation used for walls but is covered with a fiber carpeting material. It is also available in a lead

composite composition. The material is sold in rolls, usually 4 1/2 feet wide, of unlimited length and is cut to fit on the site by the installer. Adhesive holds it in place. Your Persian rugs are placed over it. The cost for the fabric material is between $10 and $15 per linear foot of the 4 1/2 ft. roll. I would use this underlayment in any carpeted boat because it will suppress sound coming up from the mechanicals and it doesn't cost much more than normal carpet underlayment. The Persian rugs are optional. Then hang up a lot of drapes and soft fabrics to prevent reflections from hard surfaces and cover the overheads and bulkheads with fuzzy woolen carpet. Just kidding! I wanted to see if you were paying attention. (But it wouldn't hurt.) There are a number of suppliers of acoustic treatment materials. The most complete listing is in a professional engineering magazine named "Sound and Vibration" available in most engineering libraries. You can also read it on the internet if you google "Sound and Vibration." But rather than buy from individual suppliers, I prefer to get everything from one source. It saves worry and shipping although it hardly guarantees the lowest prices. Recently I've been buying acoustical supplies from the Soundown Corp, a Marblehead, MA company that specializes in marine acoustics treatment (www.soundown.com). Your preference may vary. Most large cities have a supply house which sells architectural acoustics materials which are very similar to those I have described above. If you follow the sequence steps I have listed you can make your boat as quiet as the $1,000,000 Linssens. You may even be able to hear your cat purr. Larry Z Cortlandt Manor, NY