To clarify some of the points Bill Johnson raised in my interview with him, I reached out to Chris Murray, director of sales for Soundown, a company recognized in the marine industry for noise and vibration control products. Murray offered practical insights into diagnosing and fixing common boat noise and vibration problems.
Murray notes that many boats can be improved with upgraded components: better engine mounts, upgraded mufflers and enhanced sound‑deadening materials can all reduce noise and vibration. While advanced diagnostics sometimes require specialized tools and expertise, owners don’t need to be experts to know when something is wrong. You can often feel a vibration or hear excessive noise and localize its source—for example, a 200 Hz buzz often points to excessive prop‑tip speed or insufficient prop‑tip clearance.
When Soundown’s technicians investigate, they always start by asking the owner what they hear and feel at cruise speed. That information helps prioritize solutions, which may include one or more of the company’s products or installation changes.
Diagnosing the main sources
Most noise and vibration issues on boats up to 60 feet stem from three areas: the engine, the propeller and the exhaust system. Between about 20 and 25 knots, drivetrain‑related noise usually dominates; at higher speeds, wind and water noise become the primary contributors. The aim is to reduce engine, prop and exhaust noise to the level of ambient wind and water noise so the drivetrain is not the dominant nuisance at cruise speed.
For example, a typical lobster yacht cruising at 20 knots may have wind and water noise around 85–87 dBA. If drivetrain noise is below that range at cruise rpm, the installation can be considered successful. On larger boats with loud diesel engines you generally can’t escape hearing the engines from a short distance, so the practical target is keeping drivetrain noise below ambient levels at cruise speed.
Airborne vs. structure‑borne noise
Noise control has two broad categories: airborne noise and structure‑borne noise. Airborne noise leaks through gaps and openings; to control it, use barrier materials such as sound‑absorbing backing pads under carpeting and mass‑loaded sheets to seal passages. Structure‑borne noise is transmitted by vibration through hull stringers, deck supports or rigidly mounted exhaust lines. Mitigation for structure‑borne noise includes better engine mounts, mounting decks and bulkheads on vibration‑absorbing mounts, and installing soft couplers between the transmission and propeller shaft.
Interrupting the transmission path is key. Pleasure boats commonly use soft engine mounts attached to much stiffer, more massive engine beds; this decoupling prevents vibrations from easily traveling into the hull. Navy warships have used air‑filled belts with tiny holes across the hull bottom to block machinery noise—an extreme example of interrupting vibration and sound paths for stealth.
Engine noise leaks and simple fixes
If air can pass through a deck or bulkhead, sound will too. Sealing gaps often yields large benefits for modest cost. Murray points out that inexpensive gasketing can reduce noise by several dBA—every 10 dBA reduction roughly halves perceived noise because decibel measurements are logarithmic.
Another efficient option is a sound‑absorbing carpet backing pad. A quality backing pad under carpeting can reduce noise 3–7 dBA, depending on the vessel and installation, and is often more practical than trying to line the underside of the deck with irregular surfaces and openings. Sound blankets or one‑piece acoustic panels are also effective and easier to fit around complex shapes.
Sound‑deadening materials are rated by weight per square foot; heavier mass blocks sound more effectively. For example, a one‑pound‑per‑square‑foot material might be lead thinly spread or a heavier mass‑loaded vinyl. Mass‑loaded products combine plastic with dense fillers like barium sulfate, increasing mass without becoming bulky. Using combinations of mass, absorptive foam layers and separation distances (decoupling) addresses different frequency ranges: greater separation helps absorb lower frequencies with longer wavelengths, while layered materials handle higher frequencies. Because marine diesel engines produce a mix of high‑ and low‑frequency noise, a blend of materials and decoupler spacing often produces the best result.
Engine beds and mounts
Sometimes the mounts are blamed when the real issue is the engine bed. Engine beds are typically the hull stringers that run fore and aft. In fiberglass boats these stringers can be wood‑cored and encapsulated, hollow high‑hat sections, or foam‑filled cavities. For quiet operation, engine beds should have sufficient mass and stiffness to absorb vibration transmitted through the mounts. Wood‑cored beds may need to be beefed up; hollow fiberglass stringer sections used as engine beds should be solidified—commonly with wood—rather than left hollow or only foam‑filled.
Murray summarizes the relationship succinctly: the engine bed should be significantly stiffer than the mounts to form an effective vibration control system.
Propeller noise
Propeller noise can be addressed by adding mass inside the hull over the propellers—sound‑dampening tiles or panels above the props reduce vibration transmission. Maintaining adequate prop‑tip clearance (Soundown recommends approximately 20 percent clearance) and controlling prop‑tip speed are important: Murray prefers cruise tip speeds closer to 120 feet per second and sets an upper practical target of about 150 feet per second to avoid noise issues.
Changing propeller design can also help. Switching from a three‑blade to a four‑blade propeller raises blade‑pass frequency by roughly 33 percent and can reduce noticeable noise and vibration in many cases. Hydrodynamics and hull construction also matter: cored or foam‑filled hull cavities typically perform quieter than solid laminates because they reduce vibration transmission and hull resonance.
Exhaust systems and mufflers
Mufflers supplied as stock, one‑size‑fits‑all pot‑style units are common but often inadequate for optimal quieting. Proper muffler selection considers the engine’s firing rate and cylinder count; water‑drop vertical or horizontal mufflers sized to the application perform better. Well‑engineered exhaust systems measured at the transom can be reduced from typical levels of 90–95 dBA down toward prop‑wash levels of about 80 dBA. For mid‑sized boats, replacing stock mufflers with properly matched units can produce substantial reductions in perceived noise—often cutting noise levels by 40–50 percent at cruise rpm—and underwater discharge options can deliver even quieter results.
Costs vary by engine power and accessibility for installation; higher‑capacity engines and difficult installations raise the price of replacement mufflers and labor.
Practical advice and safety
Owners should be aware of occupational noise exposure standards: prolonged exposure to sound levels at or above 85 dBA can require hearing protection. Excessive noise causes fatigue and can lead to permanent hearing damage, and high cockpit or helm noise reduces the enjoyment and safety of boating by making communication difficult and increasing operator fatigue.
Murray emphasizes the quality‑of‑life benefits of quieter boats. On well‑treated boats he’s seen cruise‑speed noise levels in the mid‑70s dBA even with powerful engines, making conversation and a relaxed cruise possible. A quieter helm reduces fatigue and the risk of mistakes, and it preserves hearing in the long term.
For practical monitoring, an inexpensive sound‑level meter that uses the A‑weighted scale and a slow response setting will give useful readings approximate to how humans perceive sound. If cabin, pilothouse or cockpit levels exceed 85 dBA at cruise speed, consider taking action.
For further reading, Soundown publishes guidance such as a Handbook of Noise Control Materials and practical suggestions on typical treatments and components for marine installations.
See related story: Good — or not‑so‑good — vibrations
This article originally appeared in the December 2010 issue.