Tips For Noise-Critical Tunnel Thruster Installations

Tunnel thrusters have a bad reputation for being noisy.  Since some thrusters are used only for short periods of time during docking, a noisy thruster may occasionally be acceptable for certain applications.

However, there are many applications where noise is critical, and a noisy thruster is not acceptable, even if it is used only for short durations.  There are also many vessels that use their tunnel thrusters for longer durations, such as vessels with dynamic positioning systems or fishing trawlers that use their thrusters while pulling the nets.

There are a number of noise sources in a hydraulic tunnel thruster system.  For all of those noise-critical applications, the following guidelines apply:

First, there is noise generated by water rushing through the thruster tunnel.  This noise is like a very loud rumble and travels through the whole vessel as a structure-borne noise.

The amount of thrust produced by a tunnel thruster is directly proportional to the product of water flow through the tunnel and water velocity through the tunnel.  Larger tunnel diameters allow greater flow rates at lower velocities.  Friction losses and noise increase proportionally by the square of the water velocity through the tunnel.  With this in mind, it is clear that a larger tunnel diameter with larger water flow at lower velocity is more efficient and less noisy than a smaller tunnel diameter producing the same amount of thrust.

There are advantages in using thrusters with small tunnel diameters.  They are easier to accommodate in the hull design.  A smaller tunnel has less drag and less displacement loss.  However, a smaller tunnel diameter makes for a noisier and less efficient thruster.

Larger tunnels also require less acceleration of the water, resulting in less cavitation at the propeller.  This has a favorable effect on noise.  For noise-critical applications, always use the larger tunnel diameter.

Fairing the tunnel openings (see Tips on Thruster Installation) improves flow patterns and reduces inlet losses and cavitation at the tunnel openings, further reducing noise while improving thruster efficiency.

The second noise source is the thruster itself.  Mechanical noise from gears can be substantial. Fortunately, Thrustmaster hydraulic tunnel thrusters use a direct podded drive and do not use any gears.  The hydraulic propulsion motor is of low-noise design, so mechanical thruster noise is not a big factor when using these thrusters.

Hydraulic piping noise, however, can be quite a nuisance.  It is a high-pitched whine that, when transmitted through the vessel structure, may cause resonant noise in different parts of the vessel.  The simple way to avoid piping noise is to place the hydraulic power system close to the thruster and use only flexible hoses and no piping at all.  Of course, this method is not always possible or practical. 
When using hydraulic piping, use straight runs where possible, provide proper piping support, and size the piping adequately to keep fluid velocities low.  To connect piping to the thruster, the pump, and the control valve, always use flexible hydraulic hoses.  This allows relative motion between piping and major components and prevents component noise and vibration from being transmitted into the piping system.

Do not use elbows in the main pressure and return lines.  If you cannot use straight runs, use long radius sweeping bends.  Use the hoses at the piping ends to make the turns to the connecting ports. 
Size the main pressure lines for fluid velocities of not more than 20 feet per second.  Pay special attention to fittings, hose ends, and adapters to make sure they have adequate inside diameters.  An undersized fitting can act as a flow-restricting orifice causing local flow acceleration and cavitation, not to mention a lot of noise.

Use good pipe supports, like Stauff Heavy Series or another of equal quality.  Pipe supports should be spaced not more than five feet apart, and supports should be placed at pipe ends, close to bends and watertight bulkhead penetrations.  Make sure each support is solidly attached to a rigid structural member of the vessel structure, such as a transverse frame.

Do not use watertight bulkheads as pipe supports.  Use stuffing tubes with resilient pipe seals, like the CSD Rise or Riswat system.  This prevents resonance of the bulkhead plating.

Hydraulic directional control valves can create a fair amount of noise as well.  For noise-critical applications, we recommend using a closed loop hydrostatic system.  This system does not use a directional control valve at all and provides for a less noisy and more efficient hydraulic system (see Hydraulic Power Systems and Controls).

If other considerations dictate the use of an open-loop system, you can use either a “bang-bang” directional control valve or a proportional valve.  Size the valve and subplate adequately with low-pressure drop at full flow.  This reduces noise and increases efficiency.  A bang-bang valve needs to be slow shift type, with adjustable pilot throttles to prevent shocks during shifting.  When using a proportional valve, load sensing is preferred to reduce throttling at partial load conditions, again reducing noise and improving efficiency.

The final noise source to consider is the hydraulic pump and its driver.  Gear pumps are generally noisy and should be avoided.  When a fixed displacement pump is used, use a vane pump.  When using a pressure-compensated pump or a hydrostatic transmission pump, select one that operates well below its maximum RPM.  When doing so, you effectively oversize the pump for the application, resulting in reduced fluid velocities inside the pump. This has a favorable effect on pump noise.  The pump should be flange-mounted to the engine flywheel housing or electric motor flange, using a flanged bell-housing adapter.  This ensures accurate shaft alignment and allows for the use of spring-type mounts under engine or motor foot, preventing the transmission of the motor noise and vibration into the vessel structure.  Engine room insulation reduces the radiation of airborne noise from pump and driver.

When following the guidelines outlined here, the thruster system should be relatively quiet.  However, most of these guidelines result in a higher cost.  It is therefore important that if you want Thrustmaster to provide a quote for a thruster system, you express the noise requirements for your particular application.