How to reduce vibrations on a boat ?

Vibrations on a boat can cause noise disturbances, discomfort for the crew, and long-term structural damage. A thorough analysis of vibration sources helps identify effective solutions to reduce them.

As part of a study on a Neel 51, here are the main causes of vibrations and recommendations to mitigate them.

1. Identification of Vibration Sources

Vibrations on a boat mainly come from three sources:

• Main Engine: Vibrations occur at different frequencies depending on the engine speed. For example, an engine running at 1500 rpm generates vibrations at 25 Hz, 50 Hz, 75 Hz, and 100 Hz according to the harmonic order.

• Propeller Shaft: The harmonics generated by the propeller shaft vary depending on the engine speed, with the first harmonic at 11.4 Hz at 1500 rpm and at 22.8 Hz at 3000 rpm.

• Generator: It generates vibrations at 25 Hz, 50 Hz, and 100 Hz, mainly due to the rotational orders of the alternator.

2. Identified Issues and Recommended Solutions

2.1 Generator Base

Issue: In the case of the Neel 51, the generator base rests on a hollow structural plate, creating a resonance box effect that amplifies low-frequency vibrations.

Solution:

• Replace the plate with a transverse and longitudinal foundation covering the entire width of the boat.

• Position the support crossbars according to the mounting points of the vibration isolators to ensure optimal rigidity.

2.2 Generator Mounting

Issue: The generator, originally mounted on double flexible supports (vibration isolators), entered into resonance. Switching to single supports improved the situation, but vibrations are still noticeable.

Solution:

• Add a second resilient mounting system beneath the entire generator assembly (including the enclosure and frame).

• Place the new vibration isolators as close as possible to the existing supports and use models suited to the manufacturer’s specifications (Cummins).

• Reinforce and add weight to the existing frame to reduce resonance.

• Modify the generator's support structure to avoid the low-frequency resonance effect by creating a lattice of rigid plywood partitions, ensuring better distribution of forces on the hull sides and bottom.

  

  Generator mounting base

2.3 Exhaust Connections

Issue: The rigid exhaust pipes transmit vibrations to the boat's structures. The exhaust silencer is rigidly fixed, amplifying these vibrations.

Solution:

• Install flexible silicone bellows at each rigid junction (frame, bulkhead, waterlock, silencer).

• Mount the waterlock and exhaust silencer on suitable flexible supports.

• Isolate the exhaust flexible pipe from the boat’s structures..

  

  Exhaust pipe mounting

2.4 Sealing of Access Hatches and Other Sources of Noise

Issue:

• The access hatch to the engine room is not fully sealed, allowing noise to pass through.

• The black water tank also contributes to noise pollution.

Solution:

• Add a soft EPDM foam seal to improve the acoustic sealing of the hatch.

• Reinforce the closure with at least two locking points for even compression of the seal.

• Insulate the black water tank with a plywood box lined with soundproofing foam.

Engine room hatch

3. Conclusion

Reducing vibrations on a boat requires a comprehensive approach that considers the rigidity of the structures, vibration absorption at the source, and limiting unwanted transmissions. By applying these solutions, noise and vibration disturbances can be significantly reduced, thereby improving both the comfort and longevity of the boat.