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Online Acoustical Consultant for Yachts

 General Acoustic Engineering for Yachts

This section outlines those items of concern in the design of a quiet vessel.

  • Cavitation and Propeller induced Vibration:
    The propeller tip clearance should be a minimum of 20% of the propeller diameter; for very quiet vessels – a wake survey should be taken and wake adapted propeller should be designed; tip velocity should be held to less than 105 fps; preferred that propeller induced pressure pulses not exceed 1.1 psi at blade freq. & 0.5 psi at twice blade freq.

  • Foundations:
    Foundations are to be designed with due regard for noise and vibration; dynamic stiffness should by ten times the stiffness of isolation mounted equipment; cantilever foundations should be avoided; foundations should not be placed on bulkheads adjacent to noise sensitive compartments.

  • Shaft Struts:
    Struts are to be V form and airfoil shaped or faired to minimize resistance and cavitation.

  • Acoustic Insulation:
    (See Separate design guidance below)

  • HVAC:
    Particular attention should be directed toward designing and installing an HVAC system with minimal airborne and structureborne sound transmission (see separate design guidance). High velocity systems (above 1500 fpm) should be avoided.

  • Vibration and Noise Control Features:
    All diesel generator sets, air compressors, fans and blowers should be installed on resilient mounts; select rotating equipment in lieu of reciprocating equipment; utilize equipment that vendors certify as having low noise and vibration source levels.

  • Combustion Air Systems:
    Each diesel engine is to be fitted with an air filter/silencer. Ducts to weather are to have either silencers or sound traps. Louvers in hull sides are not to be adjacent to noise sensitive compartments.

  • Combustion Exhaust Systems:
    Each diesel engine is to be fitted with a high attenuation (high Dynamic Insertion Loss – DIL) silencer. Resilient mounting of the exhaust system may be necessary to meet the noise requirements.
    Particular attention shall be given to the arrangement of the ship service generator resilient mounting system when selecting expansion joints.

 Design Guidance: Acoustic Insulation & Privacy

Acoustic Insulation:  The following treatments shall be provided as a minimum:

  • Accommodations:
    Overhead absorptive insulation which shall consist of a minimum of at least 2-inches thick absorption material, which may be covered by a perforated sheathing or vapor barrier or soft finish surface. Minimum material Noise Reduction Coefficient (NRC) equal to 0.75.

  • Engine Rooms:
    Dry boundaries adjacent to noise sensitive compartments and any opening in these boundaries shall be fitted with acoustic treatments incorporating high transmission loss (HTL) materials. Required fire treatments shall be in addition to these treatments. The forward bulkhead and deck surfaces exposed to engine room shall be insulated on the engine room sides with a limp mass layer (usually sheet lead of at least 1 lb/ft2) sandwiched midway between two absorptive materials, minimum two outer surface. Composite may be covered with perforated galvanized sheet metal, minimum 30% open.

  • Pump/Fan Room:
    All boundaries common to noise sensitive spaces shall be treated with at least a two-inch thick, 3 pound density absorption material.

Key Points:

  • To be considered ‘acoustic insulation,’ the material must be the finish surface. For instance, fiberglass insulation placed between a structural bulkhead and a joiner bulkhead is not considered acoustic insulation (though it will help increase the airborne transmission loss, not the absorption). All acoustic material should have a Noise Reduction Coefficient (NRC) of 0.75 or better. Material vendors can provide this information.

  • If the ‘acoustic insulation’ is to be covered by a sheathing, the sheathing must be perforated. Perforated lining usually have 0.125 inch holes on staggered center and must be painted prior to installation. The builder should cover and protect perforated linings during construction to assure that the perforations are not plugged by paint and debris. To obtain the chosen color, the perforated sheathing should be anodized not painted.

  • Thermal or fire insulations, if left as the finish surface, can also qualify as acoustic insulation treatments.

  • Ceilings panels, it used, shall have sufficient suspension attachments to preclude any sagging, vibration or buckling. All continuous ceiling systems shall have modules with perforations of 1.8mm holes on 3.5 mm centers. All units shall have an inlay of 25mm or greater fiberglass or mineral wool.


  • If privacy is a concern each stateroom and cabin should be sound insulated for privacy along all boundary bulkheads. An acceptable design standard for privacy is an airborne sound insulation index (Ia) of 30 dB. Joiner vendors should be made aware of this requirement as soon as possible. Divisional bulkhead should be continuous from deck to deckhead. If no, curtain plates or limp mass layers should be used between the top of the joiner and the deckhead.

  • To provide adequate privacy, the construction needs to present a uniform transmission loss from deck to deckhead and side to side. To ensure privacy in critical areas, penetrations of the bulkhead should be minimized. If penetrations are required, they should be grouped and sealed light tight.

Criteria for Acceptable Noise Levels

Table 1: Passenger Ships-Passenger Accomodations

For yachts intended for overnight crusing, the rules for passenger ships are to apply.

Noise Levels in db(A) cm=comfort rating number
--------50m below--------
Passenger top grade cabin
Passenger cabins, standard
Public Spaces
Open deck recreation

Table 2: High Speed and Light Craft-Length above and below 50m

Noise Levels in db(A) cm=comfort rating number
----50m below----
----Above 50m----
Passenger localities
Nav. bridge
Service areas/shops

Table 3: Levels specified for transit conditions apply to yachts not intended for overnight crusing.

For yachts intended for overnight crusing, the rules for passenger ships are to apply for he transit condition.

Noise Levels in db(A) cm=comfort rating number
----In Harbor----
Sleeping Room
Outdoor recreation
Nav. bridge      

 Design Guidance: HVAC System Design

This section covers guidance for heating ventilation and air conditioning (HVAC) systems. These systems are of concern for compartments with requirements below 60dB(A). Particular attention should be directed toward the design and installation of an HVAC system with minimal airborne and structureborne sound transmission.

Noise from HVAC in machinery spaces is generally not a concern, because the ambient machinery noise levels are higher than those created by HVAC equipment.

These are general design rules that can minimize noise generated by HVAC systems. Noise generation in HVAC system occurs from four sources:

  • Fans and Blowers
  • Flow Noise generated at Branches
  • Flow Noise generated at Turns
  • Flow Noise generated at Diffusers

Noise from these sources can be transmitted to compartments either down stream or upstream of the fan. Noise is also transmitted into or out of a compartment by exposed ducts. This is called break-out and break-in noise.

General recommendations are as follows:

Air flow velocities should be kept as low as possible. A target for quiet systems should be velocities no higher than 1,000 ft/min., and never exceed 3,000 ft/min. Higher velocities require extensive treatment, particularly near each diffuser terminal.
Attached is the recommended maximum airflow velocity for various installations:

Duct Type
Noise Rating in Compartment
Max. Airflow in Duct, fpm
Square Turn
Radius turn or Sq. w/ Short
Square Turn, Long Vanes

Radius w/ Vanes



1. Fan inlet air flow should be straight for at least three duct diameters. Turns closer than three diameters cause air flow turbulence that increase noise.
2. The fan discharge should be straight for a distance of at least three major duct widths to avoid turbulence. Noise will significantly increase due to turbulence. Noise will significantly increase due to turbulence from elbows too close to the discharge (or inlet).
3. Five to ten diameters of straight duct is required for turbulence to die out and flow to equalize. If the air flow does not become smooth before the next fitting or terminal device, the flow noise generated in the next element will increased. For high velocity systems, it is critical that offsets or transitions near the fan can be avoided.
4. Design the HVAC system so that fans operate near peak efficiency. Fans that do not operate near peak efficiency will have higher noise levels than fans operating at peak efficiency.
5. All ventilation fans should be resiliently mounted and all connections to the fan, including ducting and electrical cables, should be flexible and at least as effective as the vibration isolators.
6. It possible, fans should not be attached to bulkheads, decks and overheads that form the boundaries of noise sensitive spaces.
7. Square turns, mitered elbows and zero radius elbows should generally be avoided. Transforming sections should be symmetrical. Avoid using abrupt changes in cross-section or duct turns. Noise increase when expansion in a transition section exceeds a 7° angle from the straight ahead.
8. Where possible, use circular metallic ducts instead of rectangular ducts. Circular ducts are more effective in preventing "breakout noise" from the duct walls than are rectangular ducts. In addition, circular ducts attenuate noise moving down the duct better than rectangular ducts. Finally, low frequency rumble caused by turbulence, is less likely to occur in circular ducts than in square ducts because circular ducts are usually more rigid.
9. To avoid cross-talk between rooms served by the same duct system, provide duct layout offsets and 90° lined turns.
10.The use of vane turns is advised when the turn is within five duct diameters of a fan, regular elbows with ample radii should be used as they cause less pressure drop in the flow.
11. The area of the duct that engages a fan should be at least equal to, if not greater than, the active area of the fan. Also, fan intakes should be kept symmetrical with respect to the fan.
12. Acoustically treated plena should be used to reduce noise created by the flow, especially when outlet ports are near fans or other noise sources. The plena should be designed so that the inlet and outlet ports do not lineup. Also, the plenum should be made as large as possible.
13. The inside of the ducts should be as smooth as possible to avoid turbulent flow. Seams should be faired and protruding objects avoided. Improperly fitted gaskets and flax couplings should be avoided as well. Leading edges of dampers, splitters and deflectors should be rounded or folded back. When using vaned or un-vaned elbows use the following table values for straight length of duct preceding and following the fitting.

Turn Size (Degrees)
Diameters of Straight Duct

14. Splitters should be located as far from the terminal as possible. The flow rate over the splitter should be as low as possible. The noise generated by the splitter varies as the sixth power of the flow rate. Thus, a doubling of speed corresponds to a 18 dB increase in noise. Orifices are preferred for balancing of systems.
15. External lagging used as an acoustic treatment is not in general satisfactory. Internally lined duct, with one-inch or two-inch thick fiberglass, should be used extensively for noise control.
16. Rat guards on flexible couplings should not bridge the flexible coupling. The guard should be rigidly attached to only one end of the ductwork and folded back at the other end providing a 1/4 inch gap.
17. Diffuser terminals should have a manufacturer’s noise rating that is at least 10 dB lower than the compartment’s noise criterion. Acoustical treatment of all air mixing boxes should be considered.
18 . Noise from HVAC systems should be designed to be at least 5 dB below the compartment noise specification so that when combined with other noise sources the noise specification is not exceeded.

A Practical guide to Noise and Vibration Control for HVAC Systems, ASHRAE Inc., 1991.

 Design Guidance: Resilient Isolation Mounts and Foundations

The general rules below should be followed when mounting equipment on isolation mounts. Keeping these guidelines in mind will result in a well-designed system with fewer changes.

Design Parameters:

1. Resilient mounts must be sized so that the resonant vertical natural frequency of the mounted system is less than one third (1/3) of the machine disturbing frequency. The machine disturbing frequency is generally equal to the machine’s rotational rate. The rotation rate, expressed in rpm, is divided by 60 to determine frequency in Hertz (Hz or cycles per second). The calculation of the resonant frequency is shown on Figure 1.
2. In sizing resilient mounts the total weight on the mounts should include the machinery, subbases (above the mounts), associated fluids, and weight of piping carried by the mounts.
3. The resilient mounts should be located equidistant from the isolated equipment’s center-of-gravity (CG) in both horizontal planes. In many cases, mounts are located equidistant from the machinery’s center-of-geometry. This is only acceptable if the CG coincides with the center-of-geometry.
4. It is preferable to locate resilient mounts in the CG vertical plane. This is generally not possible, and vertical distance from the mounts to the CG should be minimized, as shown in Figure 2.
5. Keep the isolation mounts to a reasonable number. It is preferable to have fewer rather than more mounts. Four is usually the minimum number of mounts.
6. A minimum one-inch clearance envelope should be maintained around the mounted equipment to prevent the unit from striking adjacent ship structure, adjacent machinery or other objects during maximum deflection.
7. Resiliently mounted equipment should have flexible hose, exhaust, and/or cable connections. The preferred design for fluid systems is to incorporate two flexible hoses in 90° “dogleg” or “V” configuration. Double arch piece flexible hose, with motions that match the maximum possible excursions of the mounted equipment without over stressing the attached piping or components can also be utilized. Tie rods on flexible hose should only be used as limit stops and should be supplied with rubber grommets to prevent metal-to-metal contact. The first three pipe hangars should have resilient elements. Electrical connections should be made with generous service loops.

Resilient Mount Features

8. Spring type should have limit stops. Limit stops should be inherent in the isolator design. If not, external, adjustable-limit stops will need to be designed into the system. Spring mounts typically provide the low frequency required of the mounts. Other mount types, i.e., rubber or rubber & spring may also prove suitable, since they offer greater isolation at the mid- to high- frequencies. There are many suppliers of vibration isolation mounts. As noted above, the mounts should be designed to have captive elements (i.e., equipment does not come loose if mount fails). Choose from vendors who make mounts designed for marine applications. US Navy type rubber mounts should be considered along with commercial manufactures of spring and spring/rubber marine mounts. Barry, Lord, Shock-Tech, Metalastik, and Christie Grey are several recommended manufacturers.
9. For applications in oily environments such as the Engine Room, the resilient element should be made of neoprene or nitrile rubber. Natural rubber may be used if in a protective housing is provided. Mounts used on hot equipment, such as exhausts systems, shall be protected from the direct radiation of heat, unless the mount is rated to take the expected heat. Thermal break material shall be installed between the hot side and the mount to limit the mount temperature to the manufacturer’s tolerance. Mounts used in wet areas, where there is a possibility for more than occasional immersion of the mount in sea or bilge water, should be installed using non-corrosive hardware.
10. The drawings should note the load direction and orientation for the mounts.
11. Fasteners should follow directions of the mount manufacturer.
12. For dynamic analysis, each mount vendor should provide both the static and dynamic stiffness of the isolation mount. Furthermore, the following equipment information will need to be provided: total dry and wet weight (including subbase and attached equipment), location of center-of-gravity, translational (NOT rotational) mass moments of inertia of the equipment, and location of mounting feet.

Resilient Pipe Hangers

13. Resilient pipe hangars are necessary with the use of flexible connections.
14. Resilient pipe hangars must be attached to heavy, rigid parts of the ship structure for the isolation treatment to be most effective. Pipe hangars must not be attached to plating sections between stiffeners.
15. Resilient pipe hangers should be used for first three hangars locations from a resiliently mounted component.
16. Resilient pipe hangars should have a natural frequency less than or equal tot hat of the mounts supporting the machinery to which the associated piping is connected.
17. Do NOT attach pipe hangars to a machinery subbase or piping directly to rigid ship structure without a rubber grommet isolator or pipe hangar. Downstrea, of the pipe hangars, the piping should be held in rubber lined supports attached to structural frames.

Foundation/ Subbase Design

18. The foundations used for vibration isolated equipment have a different criterion from hard mounted equipment. As mentioned in item 3, the foundation must be able to accommodate mounts located at the center-of-gravity, not at the center-of-geometry.
19. Local stiffeners should be added to points where mounts attach to the foundation. This should generally consist of gussets on either side of the isolation mount. The goal is to have local stiffness at least one hundred (100) times the stiffness of the resilient mount. A minimum acceptable ratio of local to mount stiffness is ten (10). The local foundation stiffness can be computed by empirical or finite element analysis (FEA).
20. Built-up foundations should be anchored to hull framing. Noisy equipment should be attached to structures having high impedance such as stiff bulkheads or hull frame-stringer intersections. Foundations should he designed to stress structural members in tension or compression, bending and torsional stress being less desirable.
21. The foundation should accommodate shock excursion of various appendages of the mounted equipment, (such as oil pan, cantilevered motors, etc.).
22. If a resiliently mounted machinery item consists of the two or more components connected by shafting, use of a common subbase under both units should be considered as a means to provide proper alignment. A subbase may also be used when it is necessary to secure resilient mounts at some position other than that of the mounting feet provided with a machine. Subbases should be rigid.
23. Cantilevered foundations should be avoided. Foundations should not be attached to surfaces that are common with noise sensitive spaces, e.g., accommodations, offices, and passageways in those areas.

Installation Details

24. Welding or flame cutting near the mounts shall be finished prior to installation of the mounts. A note to this effect should be put on appropriate drawings.
25. The actual height of each isolator should be measured before and after the mounts are completely loaded. This determines the mount deflection. Equal deflection indicates equal loading. Deviations of greater than 15% from equal loading should be investigated. The mount should not be cooked by more than 5 degrees.
26. The mount height should be measured again after ten days to determine the amount of creep.
27. The resilient elements should never be painted. A note to this effect should be put on appropriate drawings.
28. Located resilient mounts or pipe hangars such that they can be inspected regularly without having to remove structural work.
29. Provision should be made for periodic removal and replacement of resilient mounts or pipe hangars. The requirement should be reflected in headroom space and lateral clearance.

Calculation of Vertical Resonant (Natural) Frequency and Requirements for Effective Isolation

Rack guards, if required, should not impede the free motion of the flexible connections. Care should be taken in their design and installation to ensure there is no metal-to-metal contact during maximum excursion of the equipment under ship slamming conditions.

Where break-out noise (the escape of noise from within the duct into the noise-sensitive compartment) could adversely affect a compartment’s airborne noise level, special attention to the design of the flex connection is required.

Flexible duct connections shall not be used to correct for misalignment. Duct work and resiliently mounted equipment shall be aligned within 3 mm per 25 mm of flexible duct length as a minimum.

Exhaust Piping

Resilient mounted exhaust systems shall be connected to their prime movers and between various parts of the system with flexible metallic bellows connections, or if water cooled, with a suitable rubber hose connection. Metallic bellows shall be of small corrugations, extremely flexible type such as Lo-Corr by Flexonics, or equal. They shall be of sufficient length to allow free motion of all resiliently-mounted elements under all ship conditions.

Particular care shall be exercised in the location of the bellows and the resilient supports of the system and system design to prevent the imposing of undue loads and torques on the bellows pieces due to ship motion and thermal loads. The first flexible joint should be located at the exhaust flange. These flex bellows shall, as a minimum, have a life expectancy of 10 years.

Insulation in the way of bellows pieces shall be prevented from filing or jamming the corrugations.

Exhaust systems should be mounted resiliently to rigid ship structure, not to plates between stiffeners. Care should be taken to ensure that the exhaust system does not resonate and that the proper loads are placed on the mounts.

Flexible Wiring Connections:

As with piping and ductwork, wiring to resiliently-mounted equipment must be arranged in such a manner as to not restrain the equipment under any ship’s motion. Wiring shall run in a 90 degree or Z configuration with a minimum free length of 30 cm for outside diameters of up to 20 mm, 60 mm for diameters from 20 mm to 40 mm and 1 m for diameters greater than 40 ,,. At least 75 mm of slack should be provided to allow for motion of the equipment under ship slamming.

Flexible Shaft Couplings:

Selection of proper coupling is determined by load carried by the shaft, by the angular and parallel misalignment that must be anticipated, and by the variation in the axial separation between the two coupled shaft ends.
Reasonable standards for shaft balance, alignment, and runout should be invoked. The engine/gearbox manufacturer should supply guidance on these limits.

 Design Guidance: Flexible Connections

Flex Hose:

  • Flexible hose shall be installed on all piping that crosses a resilient mount interface. Synthetic reinforced flexible hose should be used wherever system requirements and regulatory bodies permit. The intent is to use as compliant a hose as possible based on system constraints. To obtain freedom of motion in two planes, flexible hose shall be installed wither in a dog leg or right angle configuration. The free length (that length of hose unconstrained by clamps, fittings, nipples, spigots, etc.) shall be at least equal to 18.0 cm plus 4 hose diameters for each leg.

  • While dog leg configurations are preferred, single right angle hose configurations are permissible provided they are at least equal in length to the dog leg configuration and the hose manufacturer’s minimum bend ratio is not exceeded. Where space is limited, a single or double arch type flex hose may be substituted.

  • Renewable and fittings shall be used whenever possible. These fittings shall be compatible with and similar to the material requirements of the associated piping system.

  • The preferred installed hose configuration is the dog leg in the vertical plane with the elbow downward. Depending on specific arrangement and hose design, configurations with the elbow up may require resilient support at the elbow. Horizontal configurations shall have support at the elbow.

  • Critical (low noise systems) flex hose installations shall have a heavy rigid pipe hanger support at the equipment end of the configuration attached to the equipment sub-base. For those systems that do not require resilient pipe hangers, a similar support shall be attached to the opposite side of the hose and be firmly attached to a structural frame. Resilient supports are to be attached to frames and never plate structures.

  • The use of a single length of flex hose is permissible for gage lines and shall be in a 90 degree, U or Z configuration.

  • Where required by regulatory bodies, flex hose shall have fire protection sleaving. Loose fitting flexible add-on non-metallic sleaving is the preferred method. When and where solid sleaving is required, it shall be arranged in such a manner as to not impede the free motion of the equipment under ship slamming conditions and shall not provide the opportunity for a shorting path between mounted equipment and ship structure.

  • All hoses should be at least one size larger than the piping to which they are connected in order to match the inside diameters as closely as possible.

Flexible Duct Connections:

  • Resiliently mounted elements within a ducted system shall be connected to non-resilient parts of the system by flexible duct connections.

  • Ducts containing fans, whether resiliently mounted or not, should be flexibly connected to their associated duct work. These flexible connections shall, as a minimum have sufficient flexibility to allow full and free motion of the duct under all ship operating conditions. These connections should be non-metallic where system requirements allow.

Piping Systems:

  • Gate/butterfly valves shall not be used for throttling service. Velocity limits are provided in a separate design document. Flexible piping connections should be used to connect machinery and equipment mounted on resilient isolators. These shall consist of an assembly of flexible elements designed to absorb the maximum excursions of the mounted equipment without over stressing the attached piping or components to which the flex connections are attached.

  • Minimum pipe bend radii are provided in this section. Design of piping support hangers is also addressed. Piping to resiliently mounted equipment shall be supported by means of resiliently mounted pipe hangers.

  • There shall be at least two resilient mounts in a "V" configuration with suspension such that each mount will be loaded along its axis and will provide total pipe support.

  • Flexible connections shall not be used to compensate for misaligned piping or carry the weight of attached piping. Documentation should be required of the piping hanger calculations.

 Design Guidance: Equipment, Misc.

Navigation Equipment:

  • Navigation equipment located within the Pilothouse and Radio Room -particularly transformers, gyro repeaters-should be purchased to a noise criteria that is 10 dB below the compartment criterion. If possible, locate transformers and gyro repeaters outside pilothouse.

  • Cooling fans for racks should be back mounted.


  • Refrigeration, dishwasher, ice makers, and other noisy equipment located within galley/scullery areas should be purchased to a noise criteria that is 10 dB below the compartment criterion. If make and model is specified by the owner, he should provide acoustic data.

  • Small food processing appliances are exempted from noise limits.


  • The windows panes should always be mounted in rubber lined channels. The rubber should be a minimum of ½" thick of 30 to 40 durometer.

  • To meet stringent noise requirements, double pane windows may be required.

High Transmission Loss Constructions:

  • Penetrations of bulkheads between machinery spaces and noise sensitive spaces should be minimized and grouped.

  • All gaps at penetrations should be sealed to be gas tight.


Copyright ® NCE 1997 Licensed to Marine Sound Control

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