⚡ electrical engineering

Mount Selection Strategy: Ensuring Compliance for Noise and Vibration in Shipboard Critical Machinery

This paper argues that ensuring compliance with shipboard noise and vibration limits requires a collaborative, customized approach to machinery mount selection between designers and OEMs, detailing critical design parameters, the efficacy of double-stage raft isolation, and the influence of mount stiffness on structure-borne noise to meet performance targets and support underwater radiated noise management.

Original authors: Akula Chaturvedi, Sharad S Dhavalikar

Published 2026-07-14

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CC BY 4.0

Original authors: Akula Chaturvedi, Sharad S Dhavalikar

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a massive ship chugging across the ocean. Inside, there's a giant, grumpy engine that loves to shake, rattle, and roll. If that engine is bolted directly to the ship's floor, every thump travels straight into the metal hull, turning the whole vessel into a giant drum. This "Structure-Borne Noise" (SBN) is bad news: it makes the crew's lives miserable and, for military ships, gives away their secret location to anyone listening underwater.

For a long time, people thought the engine maker (the OEM) should just pick the right rubber feet for their machine and call it a day. But this paper argues that's a bit like asking a shoe designer to fix a broken leg without talking to the doctor. The authors, Akula Chaturvedi and Sharad S. Dhavalikar from the Indian Register of Shipping, say that picking the right "shoes" for the engine is a team sport. It requires the ship designer and the engine maker to work together, because the engine, the rubber mounts, and the ship's floor all dance together in a complex rhythm.

The One-Step vs. Two-Step Dance

To figure out the best way to stop the shaking, the researchers built a digital playground. They didn't build a real ship in a lab; instead, they used computer simulations (mathematical models) to test two different ways of mounting an engine.

The Single-Stage Mount: This is the standard approach. The engine sits on a set of rubber mounts, which sit directly on the ship's floor. It's like wearing a single pair of thick sneakers. It helps, but the thumps still get through.
The Double-Stage (Raft) Mount: This is the fancy upgrade. Imagine the engine sitting on a heavy, floating platform (called a "raft"). The engine is attached to this raft with one set of rubber mounts, and the raft is attached to the ship's floor with a second, stiffer set of mounts. It's like wearing a pair of high-tech sneakers, but then standing on a bouncy trampoline that is itself sitting on the floor.

What the Simulations Showed

The team ran the numbers with some specific weights: a 20,000 kg engine and a 10,000 kg raft. They found that the "raft" system is a total game-changer for noise control.

In their computer models, the single-stage system had one big "bouncy" moment (resonance) where it shook the most. The raft system had two bouncy moments, but here's the magic: between those bounces, it was incredibly good at blocking noise.

When they looked at the frequency range from about 10 Hz up to 1,000 Hz (the range that matters most for underwater noise and crew comfort), the raft system blocked more than 20 dB more noise than the single-stage system. To put that in perspective, if the noise at the engine was a loud 150 dB, the raft system would drop the noise hitting the ship's floor significantly lower than the single-stage system could.

However, the paper warns that nothing is perfect. Around 100 Hz, the raft system gets a little less effective. This happens because the stiffness of the rubber mounts and the "heaviness" of the ship's floor start to match up in a way that lets vibrations slip through. It's like trying to walk on a trampoline when the springs are just the right tension to let you bounce right through to the ground.

The Secret Sauce: Stiffness and Timing

The researchers also discovered that the "stiffness" of the rubber mounts is the most critical ingredient. But it's not just about how hard the rubber is; it's about how that hardness changes as the engine spins faster or slower.

They found that if you treat the rubber as having a fixed stiffness, you get one result. But in the real world, rubber gets stiffer or softer depending on the speed of the vibration (frequency). When they simulated this "dynamic stiffness," the resonance peaks (the bouncy moments) shifted to lower frequencies, and the overall noise transmission dropped even further.

The Bottom Line

This paper doesn't claim to have solved the noise problem for every ship in the world. Instead, it offers a clear roadmap based on these simulations. It suggests that if you want to keep a ship quiet—whether to protect marine mammals from underwater noise or to keep the crew from losing their minds—you can't just let the engine manufacturer pick the mounts.

You need a custom plan. You need to decide if a simple mount is enough or if you need the heavy-duty "raft" system. And if you go with the raft, you have to be super careful about how stiff the rubber is and how that stiffness behaves at different speeds. By getting the math right early in the design phase, shipbuilders can create vessels that are not only powerful but also whisper-quiet.

Technical Summary: Mount Selection Strategy for Shipboard Critical Machinery

Problem Statement
Shipboard machinery mount selection is frequently treated as a component-level decision delegated solely to the Original Equipment Manufacturer (OEM). However, this approach often fails to account for the strong coupling between the machinery, the mounting system, and the supporting ship structure. This oversight can lead to inadequate control of Structure-Borne Noise (SBN), which is a dominant contributor to Underwater Radiated Noise (URN), particularly in the low-frequency range below 1 kHz. While strict SBN limits are critical for naval stealth, excessive vibration also compromises habitability and equipment reliability in commercial vessels. As international bodies like the IMO increasingly focus on URN reduction to protect marine habitats, there is a pressing need for a system-level design approach that integrates vessel requirements with equipment vibration characteristics.

Methodology
The paper proposes a design framework that integrates vessel-level SBN limits and equipment source vibration into the mount selection process. To evaluate the effectiveness of different isolation strategies, the authors employ mathematical modeling using single-degree-of-freedom (SDOF) and two-degree-of-freedom (2DOF) representations.

Modeling Assumptions: The analysis assumes undamped multi-degree-of-freedom systems to maintain analytical clarity, focusing exclusively on vertical vibrations and transmitted forces while neglecting rocking and transverse motions. The ship foundation is modeled as a rigid structure with high mechanical impedance.
System Configurations:
- Single-Stage: Modeled as an SDOF system where machinery is directly connected to the ship structure via resilient mounts.
- Double-Stage (Raft Mounting): Modeled as a 2DOF system where machinery is isolated from the ship structure via an intermediate raft. The system includes the mass of the engine/gearbox ( $M_e$ ), the mass of the raft ( $M_r$ ), upper-stage stiffness ( $K_u$ ), and lower-stage stiffness ( $K_l$ ).
Analysis: The study derives force transmissibility ($TF$) transfer functions for both configurations. A parametric study examines the influence of mount stiffness ( $K_u$ and $K_l$ ) and the frequency-dependent (dynamic) stiffness of the lower-stage mounts on the transmissibility characteristics and resulting SBN levels.

Key Contributions

System-Level Design Framework: The paper argues for a shift from OEM-centric selection to a collaborative approach involving ship designers and OEMs. It outlines a framework for selecting isolation configurations (single vs. double-stage) based on specific vessel noise limits and operational constraints.
Mathematical Analysis of Stiffness Influence: The study provides a detailed analysis of how mount stiffness affects the SBN transfer function. It demonstrates that the dynamic behavior of mounts (frequency-dependent stiffness) significantly alters resonance peaks and overall transmission loss.
Quantitative Comparison of Isolation Strategies: By comparing SDOF and 2DOF models, the paper quantifies the performance differences between standard single-stage mounting and double-stage raft mounting, specifically regarding force transmissibility and SBN attenuation.

Results

Resonance Characteristics: The double-stage system exhibits two distinct resonance frequencies, whereas the single-stage system has only one. Parametric studies show that increasing upper-stage stiffness shifts the first resonance peak to higher frequencies, while increasing lower-stage stiffness shifts the second peak higher.
Dynamic Stiffness Effects: Incorporating frequency-dependent dynamic stiffness for the lower-stage mounts results in a shift of resonance peaks toward lower frequencies and a reduction in the overall magnitude of the transfer function.
Performance Comparison: In the frequency band from approximately 10 Hz to 1 kHz, the double-stage (raft-mounted) system demonstrates significantly lower force transmissibility compared to the single-stage system.
SBN Attenuation: Assuming a fixed SBN level of 150 dB at the equipment mounting location, the double-stage configuration provides more than 20 dB of additional attenuation over a broad frequency range compared to the single-stage system.
Limitations: The analysis notes that isolation performance is least effective around 100 Hz, a transition region where the dynamic stiffness of the mounting system becomes comparable to the effective impedance of the ship foundation, reducing isolation efficiency.

Significance and Claims
The paper asserts that effective SBN control requires a coordinated design approach involving the ship designer, the OEM, and other stakeholders. It claims that the proposed framework provides practical guidance for developing mounting systems that meet vessel-specific SBN targets. By emphasizing the critical role of mount stiffness selection—particularly its frequency-dependent behavior—the study aims to help designers optimize mounting systems to achieve stringent noise and vibration performance targets. Furthermore, the paper suggests that these findings contribute to improved Underwater Radiated Noise (URN) management strategies at the vessel design stage, supporting broader efforts to reduce machinery-induced noise in the marine environment.

Technical Summary: Mount Selection Strategy for Shipboard Critical Machinery

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