Posted on: January 26th, 2017 by Alex Wills

Mounts for Propulsion

Propulsion engines are typically the largest source of noise and vibration aboard any vessel. An investment in a highly effective mounting system will reduce the weight, space, and cost of insulation treatments required to meet a given noise target.

Choosing the most effective system will depend largely on the drive line configuration. Isolating the propeller thrust from the engine by using a remote mounted gear or thrust bearing allows for a higher deflection and thus a quieter installation. In cases where the engine does carry thrust, Soundown has unique solutions to achieve the highest possible performance.

Some of the most important criteria for the acoustic performance of your mounting system are deflection and rubber stiffness. Higher deflections result in lower natural frequencies and improve the reduction of low frequency (bass) tones.  Softer rubber provides lower transmissibility, which means less acoustic energy can pass through the mount and into the vessel. When selecting mounts, Soundown will aim for soft rubber and high deflection while ensuring that the functionality of the system is not compromised. For example, in the first system below vertical deflection must be limited to avoid misalignment of the shaft as the engine rotates under torque load.

Most engine mounts offered by Soundown are type approved by ABS, DNV, GL, Lloyd’s of London, or RINA.

The illustrations below show three of the more common drive line layouts. While there are many possible variants, these illustrate the types of equipment available and ranks their performance in descending order, with the highest performance at the bottom. All systems are shown with Rubber Design Flexible Shaft Couplings which are effective for reducing bothersome gear tones in the aft cabin and salon of many vessels.

Close coupled engine and gear box with thrust carrying mounts and a flexible shaft coupling (good)

  • Mounts capable of handling this thrust include the AMC marine mounts and Rubber Design TT mounts. The unique design of the TT orients the rubber elements so that the vertical and lateral loads are taken in shear and the axial load in compression. This design results in very good isolation and minimal axial displacement.



A high deflection mounting and remote gear with semi-elastic gear mounts and a flexible shaft coupling. (better)

  • When the gear box is attached to the engine by a high displacement coupling, higher deflection mounts such as Rubber Design conical mounts are a very quiet solution.  The semi-elastic gear box mounting effectively isolates higher frequency gear noise and improves shaft stability.




A close coupled engine and gear box with a remote thrust bearing on high deflection mounts, also with a remote thrust bearing and double coupling.  (best)

  • The use of a Rubber Design remote thrust bearing and double coupling assembly provides a fixed attachment point for the shaft and a flexible connection to the engine.  This allows all engine and gear components to be isolated on high deflection Rubber Design Conical mounts.

Finally, this illustration shows a driveline that includes an oil bath shaft system. Oil bath shaft systems ares similar to the “best” option above and add the improved efficiency and smoothness of system with oil lubricated bearings.