In this article, you will learn the following:

• Why coastal and inland waterways have strict regulations regarding noise and distance to keep anyone in proximity safe.
• How certain foam materials, like polyurethane, help absorb noise effectively for marine vessels, especially the motor, where the majority of the noise comes from.
• Why custom molded foam is a preferred solution for motor insulation because it provides a precise fit and streamlined assembly for manufacturers.

Recreational boating offers freedom and fun on the water, but excessive motor noise can quickly spoil the experience. Loud motors not only disturb nearby residents and wildlife but can also make it hard for passengers to hear signals or communicate safely onboard. Many states now enforce strict noise limits to improve safety, and new innovations have helped keep the noise down.

Marine motor manufacturers are developing quieter designs that balance performance with compliance. These include advanced mufflers and underwater exhaust systems, of course, but the type of insulation used to absorb any noise made from the propulsion components will need to be addressed too.

We’ll explore some specific noise challenges depending on the type of motor, and how custom molded foam can help close any performance gaps in insulation and ensure a quieter experience.

How Noise Makes Others Less Safe on the Water

Many coastal and inland waterways have regulations stating maximum sound levels that are measured at specific distances from the vessel. For example, a common rule is no more than 75 dB of noise at 50 feet from the coastline. These laws aim to reduce the impact of engine and exhaust noise.

Excessive noise can make it difficult for passengers on the boat to talk, fish, or simply unwind. Outside the boat, high noise levels can disturb nearby anglers, paddleboarders, and waterfront residents. Even worse, high noise levels add to fatigue and reduce situational awareness, making it even more difficult to stay safe in or on the water.

What Makes Boat Motors So Noisy?

Recreational boats can have a range of propulsion systems that contribute different levels of noise. The main sources of excessive noise in the engine are mechanical vibration, intake and exhaust flow, and propeller or hull resonance. To understand how these factors come together, it helps to look at the two of the most common marine motor types and how their designs influence acoustic performance.

Outboard Motors

Outboards are mounted externally on the transom. They combine the engine, gearbox, and propeller in one unit, which means noise and vibration are less isolated from the environment. We’ll explore this further when we explore a recent case study on outboard motor acoustics. For now, it’s important to distinguish between two-stroke and four-stroke outboards.

Two-Stroke Outboards

Two-stroke (or two-cycle) outboard motors are traditionally lighter but punch above their weight in horsepower. Still, they are typically louder due to their frequent combustion cycle and less efficient exhaust control.

Four-Stroke Outboards

Four-stroke (or four-cycle) outboard motors typically have better fuel mileage and are much quieter at idle for operators and passengers. However, they can still be somewhat loud while running depending on the manufacturer.

Inboard Engines

Inboard engines are housed inside the hull and connected to a propeller shaft or outdrive. The enclosure helps dampen airborne noise, but these systems still generate deep, low-frequency sound that resonates through the hull and bulkheads. Vibration from the engine mounts or drive system can travel through the structure, creating a steady background noise that requires different mitigation strategies than airborne sound.

While outboard motors are most common for most recreational boats, both configurations present unique acoustic challenges that call for targeted solutions.

Why Die-Cut Foams May Not Cut It for Engine Insulation

Acoustic foams made from open-cell materials like polyurethane and hydrophobic melamine provide excellent sound absorption for many marine vehicle applications. The honeycomb structure of these materials helps trap and dissipate sound without adding unnecessary weight to the vessel. Foam insulation is one of the most effective ways to control noise, but not all foam solutions perform equally for every design.

Traditional die-cut foam sheets usually offer good results but come with several limitations:

Imprecise Fit: Flat or die-cut pieces often leave small gaps or overlaps around curved surfaces, mounts, and access panels. These are weak points where noise often leaks through.

Complex Assembly: Multiple pieces must be aligned and trimmed, adding time to installation and increasing labor costs.

Inconsistent Performance: Gaps and seams reduce acoustic efficiency, especially for low-frequency engine noise that can travel through small openings.

Wear and Delamination: Over time, exposed edges can peel or degrade under heat, oil, and moisture.

That’s why manufacturers are increasingly turning to custom molded foam for marine acoustic solutions. Polymer’s Molded Product (PMP) division designs molded polyurethane foam components engineered to fit the motor components precisely for seamless coverage without any gaps. This improves both noise absorption and vibration damping while also simplifying installation. The result is a cleaner assembly process, consistent quality, and quieter performance.

How Our PMP Division Helped a Marine Manufacturer Add an Acoustic Barrier to Their Design

We have worked with many marine motor makers to reduce excess noise and provide a quieter ride on the open water. While traditional die-cut solutions are sometimes suitable, our PMP division is often called to deliver a bold solution to meet strict sound restrictions for coastlines and waterways. This happened with one of our clients, but we were able to help them think outside the box for a robust solution that met their acoustic needs.

The challenge was the manufacturer’s two-stroke motor was producing sound levels 11 to 12 decibels higher than their competitor’s four-stroke motor. This can often happen with two-stroke motors, as we previously discussed, and the excess noise levels were uncomfortable for passengers and people nearby. We were asked to develop a sound barrier to limit noise comparable to the competitor’s four-stroke motor.

Ultimately, we determined the best way to do this was to replace the 15-piece die-cut solution under the motor housing, as it was unsuccessful in absorbing the noise emitted from the outboard motor, which was just above the waterline. Our team was up to the challenge, though, and we were able to replace the cumbersome insulation setup within the engine compartment with POLYFORM® molded marine foam.

• We developed a two-piece custom molded foam solution for this project. POLYFORM® High-Density Molded Foam with Integral Skin was added to the lower part of the motor below the waterline to reduce noise and prevent water absorption.
• A non-skinned version of POLYFORM® was used to line the upper motor cover to provide a higher level of noise absorption, as the upper motor created the most noise. The two-piece solution helps streamline assembly and reduce the number of parts required.

This solution was a complete success, as it helped reduce noise levels down to those of the four-stroke motor while reducing assembly costs and time. If you are interested in thinking forward about your energy management challenges for the open water, contact us today. We develop innovative solutions for a wide range of marine vessels to reduce the noise and ensure compliance with local regulations.