Exhausting noise

Noise onboard is very tiring. Noise often creates sea-sickness. For the commercial operator it takes the pleasure out of work. For the private users it turns pleasure into hard work. objective is to achieve an exhaust noise reduction of around 40% with its basic silencers, 70% with its special dual chamber silencers, or even 80% if the exhaust system is fitted with a silencer and a water separation unit or a combi-separator silencer.

What does an effective and advanced marine exhaust system achieve?

First of all, an effective marine engine exhaust system must eject the cooling water as well as silencing the engine’s combustion noise and removing the exhaust gases (cooling of a marine engine may be direct, if it utilizes the raw sea water from the circulating pump which sucks up the sea water from a seacock under the keel, or indirect through an heat exchanger fitted between the sea water and the liquid cooler which runs in a separate circuit and laps the inside engine walls. This solution avoids the growing of chalk and salt deposits on the inside engine water-casing, while the cold sea water tekes away the heat through the exchanger and then cools the exhaust gases by mixing water). This mixing muffles the exhaust noise and drastically reduces in the same time the temperature of gases. The system must cause minimal restriction to the flow of exhaust gases, known as "back pressure", or the performances and fuel compsumption may suffer great consequences and the engine itself may be damaged.

An effective and advanced exhaust system must achieve and ensure four main objectives :

1 - To reduce noise level by between 40% and 90% through the insertion of silencers end/or silencer-separators;

2 - To cool the exhaust, and eject the engine cooling water through the exhaust without excessive back pressure;

3 - To site the outlets at the stern of the boat to minimise exhaust fumes on board;

4 - To minimize, by the insertion of an adeguate capacity silencer, the risk of water running back up the exhaust and into the engine cylinders, causing serious engine damages.

Regarding this last aspect, will never end to advise anybody who's cold starting a marine engine, not to insist in turning on the starter motor more than around twenty seconds, if the engine doesn't start (the time however depends on the length of the hoses and on the silencer volume). In fact, even if the engine doesn't start, nevertheless the cooling water pump runs with the turning of the starting motor. The water, once injected into the water jacket manifold enter the muffler and fills it. Insisting in trying to start the engine, the water fills also the inlet hose between the engine manifold and the muffler. In fact, the outlet hose from the muffler to the stern is gooseneck shaped not to allow the outside sea water running back up the exhaust into the muffler. The water into the muffler doesn't flow out down the outlet hose because of exhaust pressure missing (the engine doesn't operate). If someone persists in turning on the starter motor, the water will pass through the inlet hose into the engine cylinders and the resulting damage would be catastrophic.

How we make our silencers

The whoole production process of components is managed under Total Quality Assurance system, according to UNI EN ISO 9001 procedures.

manufactures AISI 316L stainless steel (EN 1.4432) silencers and pipes, mainly intended to "dry" exhaust systems. For "wet" exhausts, produces both classic rubber-stainless steel or GRP, glass reinforced plastics, silencers and separators. These products are R.I.Na. (Registro Navale Italiano - Italian Naval Registry) type approved.

produces GRP silencers using specialist resins, allowing our silencers both to resist to the corrosion and to survive remarkably high temperatures of the gases if the cooling water supply blocks for short periods.
This failure of cooling water flow may be caused by an obstruction of the water seacock under the keel owing to plastic bags, seaweeds or other floating materials. Silencers are usually made from filament wound GRP tube, an aircraft borrowed technology. Path and strain of the resin saturated glass filament on a winding spindle are driven by a computer aided control system (“Filament winding technology”). Each single silencer produced is pressure tested individually to guard against leaks and then is fire-resistant gel-coated after completion to give the distinctive "Blu Oltremare" colour to whoole GRP products.

The specialist resins are used as the base material to avoid corrosion and acoustic problems, and also to avoid the low melting points of plastic silencers. In fact, although normal marine exhaust systems "work" at not more than 60°C, SILENCERMARINE GRP silencers are efficient even at temperatures exceeding 150°C, while a plastic unit, maybe heaper, but certainly less reliable, will melt at around 100°C. More costly resins allow to offer silencers for even higher temperatures for specific applications.
Following what explained, GRP silencers will be replaced free of charge if they prove faulty within five years from the dispatch to the Customer. The Registro Navale Italiano and many other Classification Societies have readily accepted GRP silencers as those safest and reliablest among this type of products. Most major engine manufacturers recommend GRP silencers and several list them as options.

SILENCERS AND COMPONENTS FOR WET EXHAUST SYSTEMS

GRP lift silencers - "H Y D R O L I F T" series

When the onboard marine engine is positioned nearby or below the waterline, a lift silencer became necessary. Our GRP lift silencers are available for all those Customers for which excellent acoustic specifications and good temperature and corrosion resistance are paramount. All silencers are fitted with a drain plug to evaquate the remaining water when the boat is garaged at the end of the season.

GRP side inlet top outlet lift silencers - "HYDROLIFT" SI/TO (SIDE IN/TOP OUT) series

These GRP silencers are actually the best selling in the market. These silencers are made from enormously strong filament wound tube, which is hand-laminated with flat sheet bases, giving the maximum possible protection if the exhaust back-fires. Every unit is pressure tested before being fire-resistant gel-coated "Blu Oltremare" coloured.

GRP side angled inlet top outlet lift silencers - "HYDROLIFT" AI/TO (ANGLED IN/TOP OUT) series

Frequently, a 45° angled inlet pipe may solve some narrow space problems in the engine comparment and makes easier the junction from the engine to the muffler placed near below the manifold. All their acoustic and corrosion resistance specifications remain the same as SI/TO type, included the strict quality control tests.

GRP top inlet top outlet lift silencers - "HYDROLIFT" TI/TO (TOP IN/TOP OUT) series

Often, a top position of the inlet pipe may solve some narrow space problems in the engine comparment and makes easier the junction from the engine to the muffler. All the acoustic and corrosion resistance specifications remain the same as SI/TO type, included the strict quality control tests.

GRP dual chamber lift silencers - "HYDROLIFT - DUAL" AI/AO (ANGLED IN/ANGLED OUT) series

Dual chamber silencers are the most effective wet exhaust silencers on the market today. Exhaust noise reductions of up to 80% can be achieved and the back pressure created by the unit is minimal. Most units are hand-built with 45° angled inlet and outlet positioned to suit the greater part of the exhaust layout of the power boats and workcrafts. Dual chamber silencers with inlet pipe or pipes specifically angled may be produced to suit the particular Customers' specifications and the outlet points straight to the position provided on the hull.
dual chamber lift silencers are used by some of major Boatbuilders specialized in powerboats today, as well as by commercial operators looking for comfort on board.

GRP rectangular top inlet top outlet lift silencers - "H Y D R O L I F T - C O M P A C T" series

Rectangular lift silencers "HYDROLIFT - COMPACT" series, represents the most recent answer to the narrow space problems in the engine comparment of the sailing yachts, where the muffler is placed nearby the auxiliary engine alongside the propeller shaft.

GRP rectangular top inlet top outlet lift silencer - "HYDROLIFT - COMPACT" TI/TO (TOP IN/TOP OUT) series

This type of silencers is very useful in sailing yacht auxiliary engine comparments. Their compact and oblong dimensions and the inlet and outlet pipes position easily meets the limited space availability into the narrow engine comparments. All the acoustic and corrosion resistance specifications remain the same as cylindrical silencer types, included the strict quality control tests.

GRP in-line silencers - "H Y D R O L I N E" series

"HYDROLINE" in-line silencers are used where the engine is well above the waterline, or where a high-riser has been fitted to achieve about 10 degree of steepness from the engine to the transom outlet. They are simple to specify and fit, and ideally need to be 0,5 to 1 meter behind the engine. The internal design requires the exhaust gasses and water to bubble through a layer of water on the bottom of the silencer to reduce noise. They offer excellent noise reduction and are widely used on both pleasure and commercial engines.

GRP in-line silencers - "HYDROLINE" series

These low lying units can be very useful in sailing yachts, where the silencer needs to be placed along the propeller shaft. in-line silencers offer approximately 40% noise reduction. These silencers, like those vertical lift silencers, are made from enormously strong filament wound tube, giving the maximum possible protection if the exhaust back-fires. Every unit is pressure tested before being gel-coated. To fit them, we recommend to follow the enclosed mounting instructions to each delivered unit. All the acoustic and corrosion resistance specifications remain the same as the vertical silencer types, included the strict quality control tests.

In-line silencers in AISI 316L (EN 1.4432) stainless steel/reinforced rubber - "H Y D R O L I N E - S S R" series

AISI 316L (EN 1.4432) stainless steel/reinforced rubber in-line silencers - "H Y D R O L I N E - S S R" series

"HYDROLINE-SSR" stainless steel/rubber in-line mufflers, looking like some great engine manufacturer's silencers such as VOLVO PENTA, offer resistance and durability characteristics that compare with constructions in GRP, suggest at the same time such a retro style in their way of fabrication, which is pleasant to find on classic sailing yachts, increasingly retrieved from oblivion and refurbished. The “HYDROLINE – SSR” series is also characterized by some special details, which make them “state-of-the-art” constructions. Amongst these, there is the internal check valve placed into the inlet pipe to avoid water running back up the exhaust; a threaded thimble placed on the inlet pipe, eventually to fit an exhaust temperature sensor; the AISI 316L stainless steel bottom end caps, equipped with a drain plug in order to evaquate the water remaining inside, when garaging is necessary at the end of the season; the silencer body made of reinforced rubber (R.I.Na. approved), resistant to high temperatures and corrosion; AISI 316L stainless steel adjustable feet and other items.

GRP WATER SEPARATORS AND SILENCER-SEPARATORS

has developed the art of water separation, for generators and propulsion engines, to end the noise created by water sploshing from the exhaust outlet. Fitting a water separator into the exhaust to replace the gooseneck removes water once it has done its cooling and silencing job, draining this quietly away below waterline. has developed the concept of water separation and actually designs and produces systems, often in coupling with silencers, for engines ranging from small diesel generators through to main engines fitted on superyachts and commercial craft. Recently built combined silencer separators particularly dedicated to large generators. Today designs and produces silencer separators which are so efficient that the dry gases can even exit up the funnel, achieving noise levels which are effectively inaudible against normal backgrounds. Back pressure is drastically reduced. The separator removes the principal cause of back pressure and stops the sploshing noise which accompanies every marine engine ticking over in a harbour, or those generators sploshing away in a quiet haven somewhere, to the fury of nearby boats.

GRP water separators - "HYDROSEP" series

develops, produces and offers through its online catalog a wide range of GRP water separators. They are manufactured using the same materials and proven technology as GRP silencers. The offered "HYDROSEP" series covers a huge variety of body dimensions and inlet/outlet pipe bores for right fitting to a wide range of applications, whether generators or main or auxiliary propulsion as well. Through their fitting, four basic beneficts may be achieved:

- The cooling water is separated and discharged overboard and the water-free exhaust gases can be piped in a variety of newly available paths

- The irritating noise of water "sploshing" out from the transom outlet is removed completely

- The exhaust noise from generators running in quiet anchorages is reduced by up to 90%.

- Back pressure is minimized.

The right choice of its dimension must be carried out as a consequence of the cooling water flow rate. Individual applications must be checked for correct sizing.

GRP silencers combined with water separators - "HYDROCOMBISEP" series

GRP silencers combined with gas/water separators take part of a new generation of products. These separators utilize gravity and centrifugal force to separate exhaust gas from cooling water. The water can be discharged directly through the hull below the waterline. It is most advantageous on those propulsion engine installations where there is a problem with routing the exhaust piping through interior structures with a constant downward grade for draining. An "HYDROCOMBISEP" separator was recently designed and built specifically for a super-yacht. These units may be part of a system which extracts all water from the exhaust, and allows the gas from a 150kVA generator to be piped up the main engine funnel. The fumes from the generator will then disperse silently, and well above the heads of those on deck below while the cooling water will discharge silently below the waterline without sploshing effect.

PARTS TO MAKE AN EFFECTIVE AND ADVANCED MARINE EXHAUST SYSTEM

From reinforced stainless steel hose clamps to sophisticated special rubber bellows and bends to absorb vibration, carries the stock to supply every part of the exhaust from the engine to the outlet: rubber hose (R.I.Na. - Italian Naval Registry approved), GRP silencers and exhaust pipes, siphon breakers, clamps, stainless steel exhaust thru-hulls with flapper that are strong structures featuring refined aesthetic style with high quality finishing, transom outlets, bellows, rubber and stainless steel mountings, heat protection jackets, rubber and GRP bends and elbows and every other kind of component strikes the most skilled and....fanciful designer's mind.

WATER INJECTION ELBOW (RISER) IN AISI 316L STAINLESS STEEL (EN 1.4432) EQUIPPED WITH “TMS” (TWIRL MIXER SYSTEM) WATER INJECTION COOLING SYSTEM

When the exhaust manifold of a marine engine is positioned nearby the waterline level, the exhaust hose path from the engine to the stern is nearly flat. In this case the water, whether for exhaust cooling or seawater coming from the stern wave due to pitching by the hull in a heavy sea or to sudden reversing as well, could run back into the engine, causing serious damage. In this case an high riser needed on the engine. This riser allows the exhaust section of the manifold to be raised well above the waterline of the boat, in order to re-stablish an inclination of at least ten degrees from the cooling water injection point and the exhaust exit point to the stern.
develops and produces risers of excellent workmanship, exclusively made in AISI 316L stainless steel, to Customer specifications, any type of riser required, equipping them with a patented mixing system, called “TMS” (Twirl Mixer System), developed by the company itself.

Water injection elbow (riser) in AISI 316L (EN 1.4432) stainless steel - "T M S" (TWIRL MIXER SYSTEM) series

Many engine manufacturers make a standard spray head for cooling the exhaust's gases. When the engine is only just above the waterline, the exhaust hose path from the engine to the transom is nearly flat. Water could run back into the engine if the vessel pitches on its mooring, or a wave could do the same. An "high riser" is needed on the engine and a "TMS" rise can help. can also produce water injection elbow with particular spray patterns for installations where cooling is critical. high risers have a carefully calibrated sequence of water spray slots sized to make water jet into the exhaust gases at full power (and not only) to maximize the cooling effect.

Reinforced, high temperatures, corrosion and hydrocarbons-resistant rubber exhaust hoses, R.I.Na. (Italian Naval Registry) approved - “H Y D R O H O S E” series.

Reinforced rubber exhaust hoses – “HYDROHOSE” series

offers its Customers a wide range of reinforced rubber exhaust hoses in the “HYDROHOSE” series, all R.I.Na. approved and resistant to corrosive agents, high temperatures and hydrocarbons. These hoses are made of rubber and are reinforced with high tensile textile cords and with embedded steel helix wires reinforcements. On the inside they are lined with a PVC layer that is resistant to high temperatures, abrasion, corrosion by acids and hydrocarbons with up to 50% aromatic content. Their internal bore sizes cover a range that is compatible with the exhaust and the cooling water flow in engines ranging from 10 to 300 KW and over. In order to prevent exhaust leakages from the junctions,

it is advisable to carry out connections with other components (rises, silencers, gas/water separators and exhaust thru hulls) by inserting the hoses for a length at least equal to one diameter and then fastening them with two hose clamps of the “HYDROTIGHT” series to each end.

AISI 316L (EN 1.4432) stainless steel exhaust trhu hulls provided with flapper check valve () – “H Y D R O T R A N S O M” series

exhaust trhu hulls belong to the prestigious “HYDROTRANSOM” series, made entirely of first-class AISI 316L stainless steel (EN 1.4432). They are tough structures featuring refined aesthetic style with high quality finishing, which will not fail to satisfy Shipyards and the most demanding Customers. Each “HYDROTRANSOM” transom outlet is equipped with a special corrosion-resistant rubber flapper, which prevents seawater from flowing back towards the engine and causing serious consequences.

AISI 316L stainless steel (EN 1.4432) exhaust outlets equipped with flapper – “HYDROTRANSOM” series

These stainless steel thru hulls in the prestigious “HYDROTRANSOM” series are entirely made of prime AISI 316L stainless steel. They are tough structures characterised by a refined aesthetic style with high quality finishing. All thru hulls are equipped with a flapper check valve in corrosion resistant rubber. They come in a wide range of inlet diameter sizes for connection to exhaust rubber hoses and shapes as well, whether with straight flange or inclined flange, and finished with protruding tail pipes with inclined or curled rim - to avoid dirtying the hull – in order to adapt to the wide variety of both aesthetic and installation requirements.

AISI 304 stainless steel clamps for rubber exhaust hoses fastening – “H Y D R O T I G H T” series

For perfect fastening of reinforced rubber exhaust hoses to other components of the exhaust system, proposes its own reinforced hose clamps, made of AISI 304 stainless steel in the “HYDROTIGHT” series.

AISI 304 stainless steel hose clamps - “HYDROTIGHT” series

These hose clamps are available in a range of diameters that is compatible with the range of external diameters of the rubber exhaust hoses in the “HYDROHOSE” series. In order to prevent exhaust leakages from the junctions, it is advisable to carry out the fastening of the rubber exhaust hose ends to other components, by always using at least two hose clamps for each end, thus guaranteeing an hermetical seal to the whoole exhaust system.

Pipes for connections, either straight or with a 45° or 90° bend, either in GRP “H Y D R O P I P E – S T R A I G H T”, “H Y D R O P I P E – B E N D 4 5”, “H Y D R O P I P E – B E N D 9 0” series or in AISI 316L stainless steel (EN 1.4432), “H Y D R O P I P E – S S – S T R A I G H T”, “H Y D R O P I P E - S S – B E N D 4 5” and “H Y D R O P I P E – S S – B E N D 9 0” series

Many different types of connecting pipes are available, either straight or with a 45° or 90° bend, made of GRP for the “HYDROPIPE” series and of AISI 316L stainless steel (EN 1.4432) for the “HYDROPIPE – SS” series. This is to make installation and the passage of rubber hoses easier where the shape of the hull calls for geometries with a radius of curvature that are not permitted for reinforced rubber hoses. All the GRP and the AISI 316L stainless steel components are available in a range of diameters that are compatible with the range of internal diameters of the reinforced rubber exhaust hoses of the “HYDROHOSE” series.

Special rubber or silicone flexible exhaust bellows "H Y D R O F L E X" series

Special rubber or silicone flexible exhaust bellows - "H Y D R O F L E X" series

These special rubber or silicone flexible bellows, designed and built using particular tensile, compression and share stress-resistant rubber, as well as corrosion and chemical agents activity resistant, are important to isolate engine vibration from the exhaust system, particularly where a silencer or rigid section is close to the engine.

SILENCERS AND AISI 316L STAINLESS STEEL (EN 1.4432) COMPONENTS FOR “DRY” EXHAUST SYSTEMS

For some working boats, and also for other commercial or some little dated sports craft, therefore equipped with engines with a traditional exhaust system (therefore dry, without a water injection cooling system), reduction of exhaust noise is obtained by the installation of classical silencer components. For economical reasons, “dry” systems are often made of low carbon steel. This material, when placed in an highly aggressive environment due to the presence of eddy currents, high temperatures, corrosive agents in the exhaust gases as well as the continuous presence of salt in sea air humidity and water in the bilge of the engine compartment, has rather limited durability. Systems made with this material, require continuous and expensive maintenance, even today, including frequent replacement of corroded parts. For these dry systems proposes a permanent solution by replacing the old low carbon steel components with a prestigious and incorruptible exhaust system entirely made of AISI 316l stainless steel, including silencers with maximum noise reduction and with minimal back pressure effect, a technical sector in which its own engineers and technicians are highly skilled experts.

Dry silencers in AISI 316L stainless steel (EN 1.4432) – “DRYLINE” series

The high efficiency silencers in the “DRYLINE” series, entirely made of first-class AISI 316L stainless steel (EN 1.4432), can be installed on all marine exhaust systems where the engine exhaust's gases are not cooled by mixing with the cooling water. Noise reduction produced by these three internal chambers dry silencers derives from absorption and reflection principles. Acoustic waves are reflected and interfere with other waves, while the acoustic energy is absorbed using advanced sound absorbtion materials made of continuous filament glass fibre (E-glass). To connect these silencers to the rest of the system, they are proposed both with flanges or simple pipe couplings connectors to fasten with clamps. Inlet and outlet pipes are available either in line with the silencer axis or with the inlet positioned at the side, to comply better with the numerous installation conditions that may be present inside the engine compartments.
Other components that make up the entire system can also be made on request, such as flanged pipes, flexible pipes and contraction/expansion joints with steel bellows, all according to drawings (CAD) and strictly in AISI 316L stainless steel.

Contraction/expansion joints in AISI 321 stainless steel for dry exhaust systems– “D R Y F L E X” series

Contraction/expansion joints in AISI 321 stainless steel – “DRYFLEX” series

The stainless steel contraction/expansion joints in the “DRYFLEX” series are nearly always the compulsory solution to vibrations transmitted from the engine, every time you have to insulate the remaining parts of the exhaust system for dry installations, in which high temperatures are reached along ducts. These special stainless steel bellows are made of multiple layers, also available with an internal steel mesh reinforcement and they are mainly produced according to Customer requirements.

ACOUSTIC TESTS AND RESULTS

For giving better understanding of what explained in the previous sections, where our exhaust components were presented, some grafic test results, managed by a well-known Classification Society and showing the noise reduction due to the installation of some tipical mufflers inside the exhaust systems of some typical boats, are shown in the following pages.

"HYDROLINE" in-line silencer

Noise emission from wet exhaust system of a small 30 Kw non turbo engine with no silencer and then equipped with a "HYDROLINE" in-line silencer was compared. On the left hand, the grafic shows the noise level one metre from the exhaust outlet in connection with the engine rotation (on vertical axis is placed noise level in Decibel dB(a); on orizontal axis the engine rotation in RPM, number of revolutions per minute). Noise reduction obtained with an "HYDROLINE" silencer is around 17 dB. Noise doubles every three decibels, so a 3dB(a) reduction halves the noise.

"HYDROLIFT-DUAL" dual chamber lift silencer

Noise emission from wet exhaust of a great power boat fitted with twin 300 kw turbo diesel engines with no silencer and then equipped with a dual chamber lift silencer "HYDROLIFT-DUAL" was compared. On the left hand, the grafic shows the noise level one metre central from the two exhaust outlets in connection with the engine rotation (on vertical axis is placed noise level in dB(a); on orizontal axis the engine rotation in RPM, revolutions per minute). Remembering that noise doubles every three decibels, the 7dB(a) reduction due to the insertion of an "HYDROLIFT-DUAL" dual chamber lift silencer means the exhaust noise had reduced by 75% in comparison with the same system without silencer.

"HYDROLIFT" silencer and "HYDROSEP" water separator

tested a 45Kw at 2700 RPM diesel engine before equipped with a straight-through exhaust, then adding an "HYDROLIFT" silencer, then an "HYDROSEP" separator as well. Test results quoted in the grafic on left hand show that at 2000 RPM the complete system reduced exhaust noise from 86dB(a) to around 61dB(a) that is about a 99% decrease of the acoustic energy. Figures were measured one metre from the exhaust outlet.

HOW CORECTLY DESIGN AN EFFECTIVE MARINE WET EXHAUST SYSTEM

"Where's the waterline?" This is the first and most important question to give a straight answer in designing a marine wet exhaust system

Marine exhausts are cooled by sea water, so it's essential that the water cannot run back up the exhaust and into the engine cylinders. The resulting damage can be catastrophic. To determinate where is the waterline level in ralation with the engine and its exhaust manifold position onboard, few simple rules need to be followed, and each engine installer should seek expert advice if in any doubt. Remember that an unattended craft may pitch on its mooring, and a yacht may heel hard over in a heavy sea. The following sketches explain the main points.

Engine well above the waterline

In this layout, the engine is well above the waterline, and the exhaust system has a downhill gradient of 1 in 8, as it slopes to the stern. The water will naturally run down the slope and away from the engine. A standard "HYDROLINE" in-line silencer is suggested which will achieve a noise reduction of approximately 40-45%. Also a dual chamber lift silencer may be used. A "HYDROLIFT-DUAL" silencer will achieve up to 80%.

Engine just above waterline

In this case water could easily run back into the engine if the boat pitches and the engine is not running. Two solutions may be taken into consideration. Both of them needs to be checked with great care.

First solution : fit an AISI 316L stailess steel "TMS" high riser with an "HYDROLINE" in-line silencer.

The stainless steel "TMS" (Twirl Mixer System) high riser raises the exhaust above the back of the engine before the cooling water is injected. This forms a steeper slope to the stern, and may achieve the 1 in 8 gradient required.

A standard "HYDROLINE" in-line silencer can then be fitted to achieve a noise reduction of around 40%, or a "HYDROLIFT-DUAL" dual chamber lift silencer as well, achieving up to 80%.

Second solution : fit an "HYDROLIFT" lift silencer (or a "HYDROLIFT-DUAL" dual chamber lift silencer)

The "HYDROLIFT" lift silencer or eventually a "HYDROLIFT-DUAL" dual chamber lift silencer means the "HYDROHOSE" rubber hose slopes steeply down from engine to silencer. Hence from the first chamber below to the second upper chamber, placed above the waterline and than from silencer to stern with a downhill gradient of 1 in 8. The geometry needs careful checking, but it can solve the problem and it offers a noise reduction up to 80%.

Engine near or below waterline

Most displacement craft have their engines low down. The above sketch shows a typical "HYDROLIFT" lift silencer positioned below and behind the engine. The reinforced "HYDROHOSE" rubber hose from this silencer runs straight up to a gooseneck, and then gradually down to the stern. Water in the system drains into the silencer when the engine is stopped. A wave coming up the stern is unlikely to climb over the gooseneck through the flapper valve of the "HYDROTRANSOM" stainless steel thru-hull.

Siphon breakers should always be used in the cooling water supply to installations of this type. Normal "HYDROLIFT" lift silencers reduce noise by 45% or so. The inclusion of a "HYDROSEP" water separator in the outlet of lift silencer, instead of gooseneck, makes possible to increase this noise reduction to around 90%.

Dimensions on lift silencing systems vary widely, but a 12 metres power yacht normally needs a drop of 300 millimetres from the water injection point on the engine exhaust outlet to the top of the "HYDROLIFT" lift silencer. The gooseneck should then be 500 millimetres above the waterline, or even more, if the exhaust path runs aside the hull. The "HYDROLIFT" lift silencer should not fill with water by more than 30% when the engine is stopped, and it must be large enough for this.

Previous advices represent very general guidelines and will change with angles of heel, size of vessel, engine choice etc. Individual installations must be checked with qualified naval architets or marine engineers.