VIBRATION AND NOISE CONTROL IN SMALLBOATS

Lawrence Zeitlin

VIBRATION CONTROL

I like my boat engines to be completely as completely unobtrusive as myhouse cat. To paraphrase the old Rolls Royce commercial, the loudest mechanicalnoise aboard should be the ticking of the chronometer.

But - making your propulsion engine purr like a pussycat is a lot morecomplicated 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 farworse than it would have been had you just bolted the engine to the bearers. I'mnot suggesting that satisfied boat owners rip out their engines and install a newmounting system, however those of you planning a new boat or rebuilding orrepowering an old one might consider the following suggestions.

Think of an engine as a block of iron suspended on a Slinky like spring. Ifyou pull the engine down slightly and let it rebound, it will bounce up and downslowly. The rate that it moves up and down is called the NATURALFREQUENCY of the system and is dependent on the engine weight and thestrength or restoring force of the spring. The heavier the engine and the weaker thespring, the slower the engine moves and the lower the natural frequency. If theengine 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 beconsidered 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 themounts a good deal and the natural frequency of the system will be low. With stiffmounts and/or a light engine, the frequency will be higher. Bolting the enginedirectly to the bearers is the equivalent of using very stiff mounts and the naturalfrequency will be high.

The natural frequency of an engine mounted on flex mounts can be estimatedby measuring the deflection of the mounts and working through the followingequation:

Fn = 3.13 x sq. rt. (1/ Ds)

where:Fn = natural frequency in cycles/secondDs = static deflection of engine mounts in inches

Lets put numbers to this. Assume that we have a Perkins 4-108 enginemounted on four flex mounts. The deflection of the mounts is .05 inch. Thenatural frequency (Fn) of the system would be 14 Hz or roughly equivalent to840 oscillations per minute. If the mounts were softer and deflected more, the Fnwould be lower. Less deflection and Fn would be higher.

Now engines don’t sit quietly in a boat. They are forced into vibration bythe explosions within the cylinders which produce the power. Pistons move upand down, the crank rotates, the valves open and close. It is the imbalance of themoving masses within the engine which produces this forced vibration. Thenumber of oscillations per second is called the FORCING FREQUENCY. Thisforcing frequency (Fd) is largely a function of engine design. A one cylinder enginehas all its masses unbalanced and exhibits a strong vibration at every revolution. Atwo 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 aredisplaced slightly from each other, secondary rocking forces appear.

A four cylinder engine such as the Perkins 4-108 can balance these forces toa great extent but not completely. The cylinder firing sequence also adds its bit tothe vibration. It takes at least a six cylinder engine to fully balance all the internalforces. There is no simple way of calculating the forcing frequency withoutknowing a good bit about the engine design. A four cylinder engine like the Perkinsusually has most of its vibration at twice the crankshaft rpm and directed in avertical direction. A two cylinder engine has a vertical vibration component attwice the rpm and a rocking component equal to the rpm. A one cylinder enginevibrates vertically and horizontally at the crankshaft rpm.

The moving masses of six and eight cylinder engines can be completelybalanced and their blocks tend to be far heavier than the lighter four cylinderPerkins, Yanmar, and Lehman Fords. Big engines with six cylinders or more can befirmly 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 onits mounts then the vibration will not be fully transferred to the boat’s structureand vibration isolation will occur. That, of course, is what we want. Assume thatour Perkins is running at 1000 rpm. Most, but not all, of the forcing vibration willbe at 2000 oscillations per minute. This is more than twice as high as thecalculated natural frequency of 840 oscillations per minute and vibration isolationoccurs. In this case the isolation is about 80% and only 20% of the enginevibration is transmitted to the structure as compared with an engine bolted firmlyin place.

Theoretically isolation begins when the forcing frequency is 1.4 times thenatural frequency (Fd = 1.4 x Fn). For a system with a natural frequency (Fn) of840 oscillations per minute, isolation would start at a forcing frequency (Fd) of1187 oscillations per minute. If the forcing frequency is closer to the naturalfrequency, vibration AMPLIFICATION occurs and there will be more vibrationtransmitted to the boat than with a solidly bolted engine. When the forcingfrequency and the natural frequency coincide (Fd = Fn), resonance occurs and theengine will literally shake itself out of the boat.

If boat engines ran at only one speed, there would be little problem. Youwould simply run the engine at a speed where the forcing frequency was at twiceas 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, theforcing frequency may well drive the system into the amplification mode andexcess vibration will be transmitted to the hull. There are a couple of ways tominimize the problem.

First, choose mounts soft enough so that the natural vibration frequency isbelow the normal operating range, even when idling. This means that you shouldchoose mounts rated for the size and weight of the engine. Stronger and firmer isNOT better. For the Perkins 4-108, a comparatively light engine, the mountsshould be chosen to support only the actual engine weight at each attachmentpoint. This is usually given in the engine specification drawings. If you don’t havethis information, the simplest way to determine mount ratings, with the engine outof the boat, is to put a bathroom scale under each attachment point and note theweights. Mounts are chosen which have an appropriate static deflection for theweight at that point. You may need different strength mounts for the front andrear of the engine since weights are not necessarily symmetrical. Catalogs usuallygive 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 anarbitrary 80% isolation at idle.

Second, select mounts made of high damping material. These tend to absorbsome of the vibration energy when passing though critical engine speeds. Mostengine mounts are already made of this type of rubber so you don’t have to worrytoo much about it. If your engine rocks at certain rpms you can fit a small shockabsorber type vibration snubber, available at most auto stores for transverselymounted car engines.

Third, move through the critical rpm range quickly and/or avoid it ifpossible. If your engine vibrates at idle, set the idle speed slightly higher. Peaceand 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 shockloads, it is necessary to limit excessive motion of the engine. High damping isrequired to control engine motion during startup and shutdown. It is mandatorythat in the event the vibration mount fails, the engine must be kept in place. Thisusually means that if the rubber breaks, a metal bolt and retaining washer stop theengine from falling out of the mount.

It is not necessary to restrict yourself to marine engine mounts. A number ofother mounts would be suitable for specific applications but you would have todo the engineering work yourself. There are several manufacturers of industrialvibration control mounts who provide catalogs with full engineering information.Among these are Lord Industrial Products, 1952 West Grandview Blvd., P.O. Box10040, Erie, PA, phone (814) 868-5424; and Barry Controls, 700 Pleasant St.Watertown, MA 02172, phone 617-923-1500. A do-it-yourself custom madeapproach is far cheaper than a factory made system but only if you can do thecalculations and fabrications on your own. My favorite factory made mounts forsmall engines are the liquid damped Van Den Ouden mounts, although they cost abundle.

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. Thisis generally a bolt and nut arrangement extending upward from the mount. Lackingthis 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 equippedwith 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 toresist propeller shaft thrust although propeller shaft thrust bearings can eliminatethis requirement. Finally, marine mounts are up to ten times more expensive thancomparable industrial motor mounts (surprised?).

OK, why not fit the softest mounts you can and keep the natural frequencyas low as possible. Well, there is more to vibration isolation than keeping it fromthe structure. All flex mounts let the engine move. That is their intent. This meansthat all connections to the engine must be able to tolerate motion without breaking.It means flexible fuel lines, flexible controls, flexible exhaust systems, and mostimportant, a flexible connection between the engine and propeller shaft. The lessthe motion, the less the problem. Back in the days of wooden boats and iron menthe problem was handled by simply bolting the engine to wooden bearers. Engineswere far heavier in those days, hence the natural frequency would be lower, andthe 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 ofabsorbing as much lateral deflection as the engine is expected to move at itsmaximum vibration point. For a motion less than 0.1 inches, a Drivesaver, PYIFlexmount or equivalent rubber doughnut will do. This is all that a correctlymounted four or six cylinder diesel needs. On one of my boats, I have a softly flexmounted two cylinder Volvo MB10. The engine moves a quarter of an inch in eachdirection at its low idle resonance point. Max rpm on this engine is 2000 and itidles at 650 so I need a high deflection to keep the natural frequency low enoughto get appreciable isolation. In this case I needed a couple of very soft cylindricalmounts at the front, mounted at 30 degree angle, so that the vertical and rockingmotion would be absorbed partly in compression and partly in shear, and a coupleof soft mounts at the rear which were stiff enough radially to counter the propellerthrust. The half inch total horizontal and vertical motion of the coupling was toomuch to use a rubber flex coupling. Instead I used a double chain coupling betweenthe engine and propeller shaft. The Boston Gear manual will tell you how to makeone. This is a poor man’s version of a double universal coupling, but better since itpermits sufficient axial motion to avoid a spline joint. A properly sized chaincoupling 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 acousticinsulation 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 themoving engine to the shaft.

Boat engines must be coupled to the propeller shaft. If the engine is solidlybolted to the engine bearers AND if the boat maintains a constant shape under allloading and environmental conditions THEN the engine and shaft could be solidlybolted together with rigid, fixed flange bearings. The engine and shaft would bealigned at installation and remain forever married. This was pretty much the casewith very heavily constructed wooden and steel commercial boats.

For pleasure boats, where weight is important, the scantlings are slimmerand the boats work a bit. Some materials like fiberglass are naturally flexible.Engines are mounted on rubber vibration absorbing mounts and prop shafts aresupported by rubber cutlass bearings. All these conspire to induce relative motionbetween the engine and shaft. Conventional alignment techniques try to reducethis motion to an acceptable level for normal boat operation. If the boat works orswells or the engine shifts slightly, alignment must be repeated. Clearly the answeris a flexible coupling between the engine and prop shaft which is capable oftransmitting the rotary and thrust forces but can tolerate the relative motionbetween engine and shaft axes.

Actually all boats have some tolerance for misalignment. The shaft itself is aflexible member and bends slightly under load. Further, with rubber mountedstuffing boxes and stern bearings, the shaft can move a degree or so. For someboats, this is all that is needed. Interestingly, for some boats with slightengine/shaft misalignment, removing a shaft intermediate bearing may allow theshaft to bend a bit more and result in smoother running.

However, no rigid coupling can accommodate the motion of an engineproperly mounted to minimize vibration. The smaller and fewer the cylinders, thegreater the motion. Depending on cylinder configuration, the engine may movevertically, horizontally, yaw, pitch or move in all directions simultaneously. Theworst motion is at slow speeds when we like the engine to be quite andunobtrusive. The engine mounts have to absorb this motion yet at the same timeresist the thrust of the propeller. The preferred engineering answer is to decouplethe 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 somedevice which lets the twisting force get through but does not transmit horizontalor angular motion of the engine. It should also retain the shaft in the boat andallow the propeller thrust to be transmitted to the structure. For moderate degreesof engine motion, a rubber or plastic biscuit fixed between coupling flanges worksOK. This is probably the oldest method. Most automobile steering wheels areconnected to the mechanism which turns the car wheels by a flexible fabricreinforced disc. If you trust you life at 100 mph to this system, why not yourboat? The Globe "Driversaver", the PYI "Flexmount", the Vetus "Bullflex" andabout half a dozen other couplings are of this type. They do not accommodatemore 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, noadditional shaft support or shaft thrust bearing is needed.

Mechanical flexible couplings allow more misalignment. These usuallyconsist of engine and propeller shaft flanges connected by a mechanical devicewhich is torsionally rigid yet allows horizontal and angular play between theflanges. The chain coupling is probably the best example. The length of doubleroller chain wrapped around the hardened sprockets that substitute as couplinghalves has sufficient play that the couplings can move slightly with respect toeach other yet still transmit the torque. Contrary to what you might expect, thechain 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 couplingwould still work. The extreme case is the flexible gear coupling in which the chainis replaced by a loosely fitting external gear connecting both halves. A singlemechanical coupling can accommodate perhaps 2 to 3 degrees of misalignment andup to 1/8 inch of horizontal play. If that is not enough for a very flexibly mountedengine, two such couplings mounted back to back with a 6" to 8" stub shaftbetween them can handle up to 1/2 inch motion and 6 to 8 degrees ofmisalignment. The downside of any mechanical linkage is rust and noise. If thesecouplings are kept reasonably dry and squirted with oil occasionally, they will lastalmost forever. For bad conditions or to minimize noise, they can be wrapped in agrease 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 specifiedmechanical flex couplings is considered light loading for industrial qualitycomponents. Most are intended for 24 hour continuous heavy duty work.

For extreme engine motion or bad, bad misalignment problems, nothing beatsa 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/outdriveinstallation? A double universal joint system will allow up to 30 degrees ofmisalignment between engine and prop shaft. Actually, add a few more joints andyou can point the engine athwartship while driving a stern mounted prop. severalmodels, including Aquadrive, Scania, and the Evolution system incorporateconstant velocity joints to further decrease the vibration. The downside isexpense, expense, expense and thrust intolerance. As the angle between the engineand shaft increases, less and less of the thrust reaches the thrust bearing in thereduction gear and more is directed sideways tending to rotate the engine. Thateliminates the athwartship mounted engine. The fix is to incorporate a thrustbearing on the prop shaft to transfer the thrust to the boat structure before itreaches 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 onthe shaft.

4. Properly designed couplings do not explode, shatter, or otherwise causecatastrophe 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 propellershaft setscrew and key arrangement. For security, shafts should be kept in placeby 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 verydifficult to escape from noise in our modern environment but while on the sailboatthe loudest noise I had to contend with was the rattling of the halyards. Mytransition to trawlers provided a rude awakening. Trawlers are noisy. I was toldby 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 classhydroplane or a Harley with a gutted muffler. Of course you may need a hearingaid in a few years. An alternative approach is to use ear plugs or external "MickeyMouse" sound absorbing ear protectors. These are cheap. Single use ear plugssuch as those sold by Flents or E.A.R. sell for about 25 cents a pair. External earprotectors 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 mustbe carried out by signing.

If you are technically inclined you can try a noise cancelling system. Thisapproach has been suggested for aircraft cockpits and luxury cars. A microphonepicks up the ambient sound. An amplifier circuit reverses the phase by 180degrees and drives a loudspeaker. The two out of phase sound waves cancel eachother and comparative quiet reigns. The zone of silencing is limited in area. If youmove very far from the loudspeaker, the sounds may no longer be out of phaseand may even be amplified, creating a noisier environment than before. Normallyautomobile drivers and airplane pilots don't get out of their seats and wanderaround the cockpit. A more portable solution is a pair of sound cancellingearphones such as those sold by Bose. These cost about $300 each. A considerateskipper will provide a pair for each crew member and guest.

Naturally, if you throw enough money at the problem, it can be solved in amore direct manner. In its new line of fiberglass high performance powerboats, theDS 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 theengine room and the boat's wheelhouse and living quarters. Of course these areluxury boats with a base price of over $1,000,000. Patrick Gerety, former managerof Willard Marine's recreational line once told me that the last W30 series oftrawlers carried $10,000 worth of acoustical insulation because upscale buyersdemanded silence. I tend to believe him.

But I suspect that Willard, and many other trawler manufacturers are goingabout sound and vibration minimization the wrong way. You can't just muffle anoise producing device cheaply. It takes sound absorption materials and mass,correctly applied in layers of sound insulation. Automobile manufacturers facedthis problem years ago and came up with other solutions. Look under the hood ofyour car. You will find very little insulation to contain the sound and vibration ofa 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 autobody. Only a small amount of acoustic insulation is necessary to keep the airbornenoise out of the passenger compartment.

BACKGROUND

Noise is simply defined as unwanted sound. The operative word here isUNWANTED. If you are a rock fan you may be perfectly happy blasting out DefJam to the limit of your stereo speakers capability while your chamber musicloving neighbor in the next slip is dialing the police with shaking fingers. Thesound level in his boat may be one tenth of that in your saloon but to him it is stillnoise.

But, in addition to being irritating, loud noise has other effects. It can make itdifficult to hear radio messages and acoustic navigational signals. Extendedexposure to high level noise can inhibit the ability to concentrate and can cause eardamage, raise blood pressure and trigger other health problems.

Before we get too deeply into noise reduction, let's talk for a minute abouthow sound is measured.

Sound pressure is measured on a logarithmic scale using a unit called theBEL. Named, of course, after Alexander Graham Bell. Each BEL increase ordecrease is a 10 times increase or decrease in the sound pressure. For conveniencethe BEL is divided into 10 parts or deciBELs (dB). A change of approximately 3dB indicates a doubling or halving of the sound pressure level.

But those are physical measurements. They only loosely correspond to theperception of loudness of a sound. The human ear responds to changes in soundpressure with its own logarithmic system. The range of human hearing isenormous and covers a 120 dB range from the threshold of hearing to the onset ofauditory 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 soundpressure will produce a trivial change in loudness. It takes at least a 30 dBreduction in sound pressure for a sound attenuation method to make a realdifference. That represents a 1000 time reduction in the sound pressure level.

Manufacturers who sell sound reduction equipment rarely indicate thereduction in perceived loudness, merely the reduction in physical sound pressureas measured in dB. They hope that the typical user will confuse the two measuresand 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 ofenergy in the exhaust. Valves open and shut interrupting the flow of air in theintake 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 Hzis .0002 dynes/sq. centimeter. To put it into terms more familiar to the averagetrawler 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 thehearing range, would drive us screaming from an engine room. Continued exposurewould cause permanent hearing loss. No wonder so many band members arepartly deaf from playing in front of their 150 watt sound systems. Unfortunatelyso are many of us from a lifetime of exposure to noise.

Finally, the boats themselves are effective at transmitting sound throughouttheir interior. Most of what we think of as noise in a fiberglass boat is reallystructure borne vibration which forces the relatively thin boat panels to resonate.Minimize structure borne vibration and you go a long way toward making a boatquiet. And it doesn't require $10,000 of after the fact noise insulation. Just a littleattention 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 thefollowing actions, arranged in rough order of importance.

First, and probably the most important, is to prevent engine generatedvibration from being transmitted to the boat's structure. This was discussed in theprevious series of notes. But to recap, the engine should be mounted on softrubber mounts designed to provide at least 80% vibration isolation. More isolation

is better. Usually that means that the weight of the engine will compress rubbermounts by .05" to .1". But since the engine will move a fair amount with this levelof isolation, a flexible connection with the drive shaft is necessary. Elastomericdoughnuts such as the Drivesaver or PYI between the coupling flange halves willprovide a minor reduction in vibration. The best approach is to use a doubleconstant velocity universal joint coupling. Examples are the Scania, the Aquadrive,and the Evolution couplings. These all include a thrust bearing that transmits propthrust directly to the hull and spares the engine mounts the necessity of handlingengine thrust as wall as vibration. All engine connections, fuel lines, and controlsmust be capable of dealing with engine motion.

Second, the engine should be provided with adequate mufflers both on theintake and exhaust. Exhaust muffling is handled relatively well in wet exhaustsystems using an Aqualift type muffler. The injection of water into the hotexhaust cools the exhaust and extracts some of its energy. The large volume of themuffler smooths out the exhaust pulses. Additionally the long rubber hose to theexhaust outlet used by most boats further attenuates the sound. If the exhaust isnot quiet enough after all this processing, an inline exhaust muffler, such as thosesold by Vetus, can be added to the exhaust line. Dry stack exhausts must bemuffled in the same manner as a car. The muffler should be a large chamber withbaffles. Entry and exit pipes should be tuned to minimize sound at the cruisingRPM. 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 intakevalves interrupt the air flow into the engine up to ten times a second creating sonicpulses whose higher order harmonics are well within the hearing range. If you lookunder the hood of your car you will see that the intake muffler/air filter consists ofa large cylinder or box with a relatively small intake port. The combination of theair filter and restricted opening attenuates the intake sonic pulses. Run your enginewith the air filter open or off and you will hear the difference. On many boatdiesels the filter consists of an oil saturated metal gauze which works admirably tokeep dust particles from the engine innards but does little to block the sound. AnAirsep type filter does a better job of noise control.

It is amazing how much sound can get through a small hole. The bulkheadsof the engine compartment are penetrated by many ducts and cables, each ofwhich can offer a pathway for engine generated noise to escape. Example: Theengine 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 2000RPM, engine noise in the pilothouse was excessive. Simply sealing the back of thehole with a 1/4" thick square of plywood dropped the noise level in the pilothouseby 10 dB. So the third step in noise control is to close all openings in the engineroom leading to the living spaces aboard the boat.

Many of these openings can be sealed or gasketed with little interferencewith function. If a throttle or shifting Bowdon cable runs through a hole in theengine room bulkhead, seal the gap with a blob of silicone caulking. The same forelectrical wiring or conduits or even exhaust hoses. Silicone caulking is fireproofand 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 exitoutside the hull whenever possible and be covered by louvers directing the soundaway from the living spaces.

Acoustical texts are replete with techniques of suppressing soundtransmission through ducts. Most of these involve making several right angledbends in the duct and lining the interior with sound absorbent material. Few arepractical for the engine compartment. What is practical is making a single rightangled bend in ducts for air intake and ventilation, and using a sound absorbentlining. Alternatively, the hole in the compartment wall can be covered by a largebaffle at least 6" in diameter greater than the hole. The baffle, covered with soundabsorbent material on the side adjacent to the entry hole, should be spaced 3" fromthe hole by several dowels or spacers. This will permit air to enter or exit yetblock some of the high frequency sound emanating from the compartment.

Fourth, the walls and overhead of the engine compartment and large panelsin the living space should be treated so vibration is kept to a minimum. If vibrationtransmitted through the boat structure excites a thin panel or bulkhead into asympathetic vibration, it doesn't matter how much effort you have expended insound treating the engine room. It's just like having a loud speaker in the livingspace blasting out engine sounds. One way of minimizing panel vibration is to addviscous mass. In automobiles a rubber based undercoating is applied to thin metalpanels to change their vibration characteristics. Suitable application of a viscouscoating can make a tinny car door sound like it is made of armor plate. If a panelvibrates 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 themfrom vibrating like a bell when the 50 caliber machine gun was fired. Auto gradeundercoatings 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 teakpanel. For engine rooms I have used Silent Running SR 1000, a USCG approvedwater based vibration and sound dampening material made by Current Compositesof New Haven, CT (www.silentrunning.us). Other manufacturers make similarmaterials.

Another way of minimizing panel vibration is to stiffen the panel by gluingor screwing rigid battens to the backside. Shelves or picture frames fastened to thefront serve the same purpose. What you want to do is change the naturalfrequency of the panel so that it does not vibrate in resonance with the engine. Inextreme cases, you can make a bulkhead or panel virtually soundproof byfastening another panel to its back, spaced about 1 inch away by wood stripsaround the edges, and filling the cavity between panels with dry sand. Thistechnique is often used in large Hi Fi enclosures to completely kill panel vibration.The best way to impede sound transmission between rooms or compartments isto use a brick and mortar wall. About 12" of brick and mortar will reduce thesound transmission by 60 dB. It goes without saying that this is only practical inthe largest of boats, perhaps in the QE2 range.

Eventually, you will want to apply sound treatment to the interior of theengine compartment and living spaces. The mistake most people make is usingsound absorbing material as the first step in a noise reduction program when itshould come near the end. The material is expensive and a great deal more will beneeded if you don't control sound at the source. One of the best ways to do this isto construct a box around the engine using heavy materials. A heavy gauge steel orsturdy plywood enclosure around an engine would work extremely well as long asthe box extends down to the engine bearers. This approach is impractical for mosttrawler main propulsion engines but is the method of choice for generators andauxiliary power sources. The interior of the box should be lined with heat resistantsound damping materials and the air intakes should be baffled.

All common sound absorbing materials work by converting the energy ofmoving air particles in the sound wave into heat which is then dissipated into thesurrounding surfaces. To effectively capture the sound wave, the absorbingmaterial must have a thickness of at least a quarter of a wave length of the soundin air. Since sound travels at approximately 1100 feet per second, a wavelength ofa 1000 Hz sound is about 11 inches. This means that if we use a fluffy fiberglassabsorbing material, it should be 2 1/2 to 3 inches in thickness. Higher frequencysounds require less material, low frequency sounds, more material. We can get bywith 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 theabsorbing medium or by incorporating lead membranes between layers of foam orfiberglass. 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 backtoward the source.

Typical non-weighted sound absorbing materials are made of foam orfiberglass covered with a mylar or vinyl facing. The acoustical properties of foamor fiberglass are the same but fiberglass is much more fire resistant and is suitablefor engine compartments. USCG rated 2" thick fiberglass with an acoustic scrimfacing 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 twobattens per section may be required to hold it in place for security. Particularlynoisy engine rooms will profit from using lead composite insulation consisting ofone or two lead membranes sandwiched between layers of foam or fiberglass. Thelead 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 use200 lbs of lead composite insulation. Costs are higher than for plain foam orfiberglass. A square foot of 2" thick, 1 lb/sq. ft. fiberglass lead compositeinsulation with a white mylar overlay costs $6. The 2 lb/sq. ft. fiberglass leadcomposite insulation costs about $8.50 per sq. ft. As might be expected, leadfiberglass sandwiches using all USCG approved materials are more expensive, eachsquare foot running $7.50 for the 1 lb. weight and $10 for the 2 lb. weight. Theseare retail prices. A search of the internet might find better values.

If you install fiberglass or lead composite fiberglass materials on thebulkheads and overheads of the engine compartment, be aware that fiberglass has atendency to separate from itself. The edges of the panels should be wrapped witha mylar tape to hold the material intact. Most insulation suppliers will cut thematerial to a pattern and wrap the edge for you. Installation is by either adhesive,reinforced by judiciously placed mechanical fastenings; or, by covering theinsulation with pegboard or perforated aluminum. For steel or aluminum boats, theinsulation is usually cut to fit between the angle stiffeners.

The final step in silencing a boat is to use acoustic carpet underlays in theinhabited areas. I assume that your boat has carpets rather than teak and hollyflooring. Acoustic carpet underlay material is similar to the sound insulation usedfor 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, ofunlimited length and is cut to fit on the site by the installer. Adhesive holds it inplace. Your Persian rugs are placed over it. The cost for the fabric material isbetween $10 and $15 per linear foot of the 4 1/2 ft. roll. I would use thisunderlayment in any carpeted boat because it will suppress sound coming up fromthe 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 toprevent reflections from hard surfaces and cover the overheads and bulkheads withfuzzy 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 mostcomplete listing is in a professional engineering magazine named "Sound andVibration" available in most engineering libraries. You can also read it on theinternet if you google "Sound and Vibration."

But rather than buy from individual suppliers, I prefer to get everythingfrom one source. It saves worry and shipping although it hardly guarantees thelowest prices. Recently I've been buying acoustical supplies from the SoundownCorp, a Marblehead, MA company that specializes in marine acoustics treatment(www.soundown.com). Your preference may vary. Most large cities have asupply house which sells architectural acoustics materials which are very similarto those I have described above.

If you follow the sequence steps I have listed you can make your boat asquiet as the $1,000,000 Linssens. You may even be able to hear your cat purr.

Larry ZCortlandt Manor, NY