Minimizing Underwater Noise: Quieting Boat Engines

Reducing underwater noise in boat engines is crucial to mitigate the harmful effects on marine life, including whales, dolphins, and even invertebrates like octopuses. Excessive noise can disrupt their feeding, communication, and breeding behaviours, leading to hearing loss, physical injury, and chronic stress. To address this issue, a combination of ship design, maintenance, and policy solutions can be implemented.

One effective method is to focus on propeller design, specifically the selection of the appropriate propeller diameter, blade number, and blade pitch, which can reduce cavitation and the resulting noise. Additionally, advancements in quieter propellers, such as those with composite materials or rubberized hubs, can be retrofitted to existing ships. Maintaining polished and clean hulls and propellers can also reduce noise, as buildup of aquatic organisms can contribute to higher noise levels.

Engine noise can be mitigated by soundproofing the engine room, using dense foam or caulk to block or absorb noise transmission. For inboard motors, adding thick rubber to the top and bottom faces of the mounts can help dissipate vibration levels and reduce overall noise.

Soundproof the engine room

Soundproofing the engine room of a boat can significantly reduce the amount of noise emitted. This is especially important for larger crafts, as a noisy engine room can disturb guests and employees in other areas of the boat.

To soundproof the engine room, you can use dense foam or caulk to seal doors, hatches, and bulkhead openings. It may also be beneficial to soundproof neighbouring rooms and hallways to further reduce noise levels. While some noise will still escape due to the requirement of vents, soundproofing can make a significant difference in reducing overall noise levels.

When soundproofing the engine room, it is crucial to use only materials that are certified for marine use. Fire and water resistance are critical factors to consider. It is also important to ensure that the materials used are not too thick, as this can create clearance issues.

In addition to soundproofing the engine room, addressing the noise generated by the propeller is also essential. This can be achieved by selecting a propeller with an appropriate diameter, blade number, and blade pitch to reduce cavitation and, consequently, the noise produced.

Use a high-efficiency prop

Using a high-efficiency prop is a highly effective method to reduce underwater noise in boat engines. A high-efficiency prop is a well-made prop that is designed to minimise vibration and noise. It is constructed from robust materials that enable it to smoothly slice through the water, reducing the amount of noise produced.

The International Maritime Organization (IMO) recommends that propeller design should focus on selecting the appropriate propeller diameter, blade number, and blade pitch to reduce cavitation and, consequently, the noise produced. A high-efficiency prop typically has a higher number of blades, which distributes the workload more evenly, resulting in quieter operation.

The benefits of a high-efficiency prop are not limited to noise reduction. The additional blades also provide improved stability and thrust, making them ideal for rough weather conditions and heavy loads. Furthermore, the advanced geometry of the blades enhances fuel efficiency by creating a more direct flow of water, which minimises cavitation and noise.

When selecting a high-efficiency prop, it is crucial to ensure it is compatible with your boat's engine and follows the manufacturer's recommendations. Consulting a marine specialist is advisable to ensure the propeller is appropriately sized and pitched for your vessel.

In addition to the number of blades and pitch, the diameter of the propeller also plays a role in noise reduction. A larger diameter propeller is generally quieter as it reduces blade loading. However, it is important to consider the intended use of the vessel. For boats operating at low speeds, a larger diameter and lower pitch are recommended, while high-speed boats require a smaller diameter and higher pitch to prevent overworking the engine.

By investing in a high-efficiency prop, you can significantly reduce underwater noise, improve performance, and enhance your overall boating experience.

Add flex to the motor

Adding flex to the motor is a great way to dampen the sound of your boat engine. This is especially effective for outboard motors, where vibrations are often transferred through the boat, creating a rattling noise. A simple solution is to install a strip of flex hose under the motor, which will absorb some of the vibrations and reduce the noise. This is an inexpensive and easy fix, which can make a big difference in the overall noise levels of your boat.

For outboard motors, the connection point between the electric motor and the boat often creates a rigid link that allows vibrations to travel through the boat. By adding a bit of flex to this connection, you can reduce the amount of vibration that is transferred to the boat, resulting in a quieter ride. A strip of flex hose is a cheap and easy solution that can be fastened to the craft to dampen the sound.

For inboard motors, the mounts are the key to adding flex and reducing noise. By placing thick rubber on the top and bottom faces of the mounts, you can dissipate some of the vibration levels, which will reduce the overall noise output. This won't change the sound of the propeller, but it will help to minimise the noise coming from the motor itself.

The addition of flex to the motor is a simple and cost-effective way to reduce the noise of your boat engine, particularly for outboard motors. By minimising the vibrations that travel through the boat, you can enjoy a quieter and more pleasant boating experience.

Slow down

Slowing down is one of the most effective ways to reduce underwater noise from boat engines. This is because the noise emitted by a ship is directly related to its speed. As a general rule, for every 1-knot reduction in speed, there is a corresponding 1-decibel reduction in noise. This means that even a small decrease in speed can significantly reduce the amount of noise produced. For example, an analysis by Russell Leaper found that slowing the global shipping fleet by just 10% could reduce total sound energy by around 40%.

In practice, this has proven successful. For instance, the Vancouver-Fraser Port Authority in British Columbia implemented a policy asking ships to slow down to either 11 or 14.5 knots as they approached the port, in order to reduce noise disturbance to the critically endangered Southern Resident orca population. With over 80% of large vessels participating in the slowdown in 2022, underwater sound intensity in the area decreased by up to 55%.

In addition to reducing noise, slowing down has the added benefit of decreasing fuel consumption and lowering harmful emissions. This not only reduces the environmental impact of boating but also saves money.

While slowing down is a simple and effective way to reduce underwater noise, it may not always be a desirable or feasible solution. For example, in certain situations, a minimum speed may be required to maintain safety or efficiency. Additionally, slowing down can increase travel time, which may be undesirable for time-sensitive journeys. Furthermore, slowing down may not address other sources of underwater noise, such as propeller cavitation or engine vibrations. In such cases, complementary measures, such as engine insulation or propeller design changes, may be necessary to achieve a significant reduction in underwater noise.

Use sound-reducing gear

Using sound-reducing gear for your boat motor is a great way to minimise underwater noise. There are several methods and materials you can use to achieve this.

Firstly, you can use acoustic enclosures or hoods to reduce the sound produced by your engine. These enclosures are made of sound-absorbing materials that capture sound waves and prevent their transmission. They are rated based on their efficiency in blocking sound, so you can choose one that suits your needs.

Another option is to install a marine-grade muffler, which can reduce engine noise by 15 to 20 dB. This will help to quieten the sound produced by your boat's exhaust system. Additionally, you can use vibration-damping mounts to control the displacement of vibration caused by the engine. These mounts should be chosen based on the installation load your engine operates under.

You can also redesign your boat's exhaust system to direct engine emissions underwater, creating a quieter structure. This method's efficiency will depend on your boat's design and operation, so careful consideration is necessary.

To further reduce underwater noise, you can use vibration-damping materials such as rubber, polyurethane, and polyvinyl chloride (PVC) sheets. These materials prevent the transmission of vibrations from the engine to the hull, minimising the noise experienced by those on board.

Another effective method is to use acoustic barriers, such as mass-loaded vinyl (MLV) acoustic barriers. These dense, sound-absorbing panels reduce airborne sounds transmitted through floorboards, enclosures, and other spaces on your boat.

Additionally, applying acoustic absorbers made of polyester fibres can prevent sound reflection off hard surfaces like engine enclosures. These absorbers offer low-frequency absorption, thermal resistance, and compliance with UL94 HF1 standards.

By using a combination of these sound-reducing techniques and materials, you can significantly reduce underwater noise in boat engines, creating a more peaceful and environmentally friendly boating experience.

Frequently asked questions