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This case involves a 1980 Sjøman 22 with a Yanmar 2QM15 Engine, owned by a young student who imported the boat from Norway to Holland. Despite being full of enthusiasm and determination, the owner faced financial constraints. In an effort to assist, Ellebogen undertook the task of refurbishing the boat to minimize noise and ensure a smoother operation.
To achieve this goal, Ellebogen identified suitable marine engine mounts and selected a marine soundproofing foam for the engine compartment. Initially, Ellebogen performed a preliminary calculation based on limited available data. However, the initial samples proved ineffective, causing excessive engine movement during idle, indicative of a resonant condition. The marine engine mounts were also close to their load limit. Lowering their hardness was not a viable option due to potential durability issues.
An alternative considered was raising the idle speed to 1300rpm, but this posed a risk of damaging the cone clutch during gear engagement. After careful analysis, Ellebogen opted to send a second set of samples, utilizing the same mounts with the same rubber hardness (55 Shore A) but incorporating a higher damping compound (SDD55). This adjustment had a positive impact on the engine, stabilizing it at idle (900rpm).
To assess the effectiveness of the new marine engine mounts with the SDD compound, Ellebogen sent an accelerometer to evaluate the vibration efficiency of the mounts.
To attain noise reduction on the boat, Ellebogen developed a prototype of a soundproofing composite by incorporating a higher density material with fire-retardant properties. The accompanying report will feature videos illustrating the noise levels before and after implementing this solution.
Limited information is accessible regarding this Scandinavian motorboat, with the exception of its 1300Kg displacement and a provided drawing. The handcrafted drawings below reveal that this boat was constructed using good old techniques, materials, productivity, and durability.
While designed for Nordic water cruising, this boat is also well-suited for navigating Dutch canals.
As evident in the images below, the mounts have collapsed, which is expected after over 20 years of use.
While designed for Nordic water cruising, this boat is also well-suited for navigating Dutch canals.
After redrilling the engine bed, Niek fits the Ellebogen GTS 55 mounts in place.
Although the mounts looked correct for the application, the result was not positive as it can be seen on the below video. The mounts had already the correct deflection. Despite this, it is clear that is on a resonant condition.
After redrilling the engine bed, Niek fits the Ellebogen GTS 55 mounts in place.
The resonant condition is produced when the engine resonant frequency is the same as the idle speed (rpm).
Opting for a softer rubber compound was not viable; the mounts already exhibited a deflection of 3.5 mm, approximately 75% of the maximum deflection capacity for these mounts. Excessive deflection would compromise the marine engine mount’s durability. Niek proposed raising the rpm to enhance engine stability, but this approach posed additional mechanical challenges, such as premature wear on the gearbox.
In a resonant condition, vibrations are not isolated; instead, they are amplified. This amplification is directly linked to the energetic dissipation of the rubber compound. Lower energetic dissipation results in higher vibration amplification, while higher energetic dissipation leads to lower vibration amplification. A graphical illustration of this phenomenon is provided below.
The SDD high-damped compound provides approximately a 25% mechanical loss factor (damping), enabling a significant reduction in engine movement during a resonant condition. In contrast, conventional natural rubber compounds offer a damping factor ranging from 4% to 8%, depending on the rubber hardness. The impact of this compound was instant, as evident in the videos below.
Each mount underwent testing with accelerometers placed both above and below at idle speed, considered the worst-case scenario. The table below demonstrates a vibration reduction of approximately 90%. These outcomes were deemed successful, as the mounts exhibited the proper level of deflection, ensuring durability.
The full results of the measurements can be found on this link
Marine engine mounts serve as an efficient method to isolate structural noise. However, for effective airborne noise insulation, the choice of a suitable soundproofing composite is crucial. What properties define a proper soundproofing composite? Here are some technical considerations:
The ELLEBOGEN Marine soundproofing marine composite.
In this instance, the material employed is an agglomerated PU foam with a density of 150 Kg/m3, which is five times greater than conventional grey-colored PU foams (30 Kg/m3). This infusion of additional mass into the system is notable. The acoustic performance of this foam was evaluated in accordance with the UNE-EN ISO 140-3 standard (December 1995) titled “Acoustic Isolation of Construction Materials in Laboratory Measurements”, fully aligning with ISO 140-3(1995). The achieved Rw value is 26 dB. The graph below illustrates the transmission loss across various frequencies.
The foam is encased in a facing composed of aluminum bonded to a fiberglass tissue, which serves the dual purpose of acting as a fire retardant and thermal insulator.
The videos below illustrate the noise insulation of this application. The initial video presents the engine without any soundproofing composite, the second one features a lateral enclosure, and the third one showcases a total enclosure.
We thank Mr. Niek Konstra for all his work and dedication on following the steps and instructions of Ellebogen. Without any doubt we are very proud of having him as an Ellebogener.
