CASE STUDY YANMAR 3YM30. TECHNICAL EXPLATION OF AN IMPROVEMENT ON NOISE AND VIBRATION ISOLATION
Sailboat: Beneteau 343 Clipper, moored in Port Solent, Portsmouth
2 ELLEBOGEN 75 (Ref. 128270-08341) and 2 ELLEBOGEN 100 (Ref. 128377-08351)
The aim of this report is to explain the case of a user of a Yanmar 3YM30 that had high levels of vibration on a Beneteau 343 Clipper and the steps that were followed to fix the vibration problem.
We will start first describing the details of the Sailboat. The engine is a 3-cylinder Yanmar reference 3YM30. It is a sailboat with a length of 10.82 metres (35.50ft) and a beam of 3.48m (11.42ft) giving a total displacement of 5380Kg.
Designed by Berret-Racoupeau and built by Beneteau Shipyard in Saint Hilaire de Riez, FRANCE.
This Sailboat well maintained and properly stowed can accommodate up to 5 people in comfort.
INITIAL SITUATION OF THE ENGINE.
The owner of the Sailboat observes that the engine moves too much. After making a video he gets an advice of changing the mounts.
On the below graph we can see the alteration of elasticity of a new mount and used engine mount. The Force expresses the load exerced on the mount in vertical direction and on the X axis, the compression of the mounts is shown (this is also called deflection).
As it can be seen, the mount exhibits an alteration on the elasticity. At the same give force, the used engine mount has more deformation under the same given load. This is the reason why the engine shakes more. The engine has more movement because the engine mounts allow more deflection.
The Sailboat owner decides to change the mounts by another design that has a higher stiffness.
The outcome is not as the sailboat owner expected and asks for advice on a mechanical forum for Assistance.
To explain why this is happening, we need to understand the relationship between the stiffness of the mounts and vibration transmissibility.
An elastically suspended engine will have a resonance frequency (also called fundamental frequency or natural frequency) following the formula below.
Where “k” is the stiffness of the marine engine mount, and the “m” is the mass of the marine engine.
The main objective here is to try to avoid any coincidence or proximity between resonance frequency and an engine disturbing frequency. (Rpms at idle or tick over)
On the graph below an Iddle speed would be represented by the yellow line and the dark blue line represents the potential impact of this (also called a Transmissibility curve). The worst scenario would be if the yellow line matched the dark blue peak, at which point the system would have reached a resonance which could damage engine components, mounts, transmissions etc… so clearly something to be avoided.
The yellow line sweeps from left to right depending on the engine running speed, from low idle to high idle.
Transmissibility 0 = means no transmission of vibrations, in other words. 100% of isolation.
Transmissibility 1 = means transmission of vibrations at 100%, in other words. 0% of isolation.
At Resonance point, the vibrations are not isolated, they are amplified.
If we use an engine mount with higher stiffness we increase the Resonant frequency (Fundamental frequency):
This makes that the resonant peak moves to the right. Making getting closer to the idle speed. This effect makes a decrease of vibration isolation.
In order to avoid the resonant system the sailboat owner decides to increase the idle rpm to 1000rpm.Since 1000rpm idle is not good for the gearbox in long term, there is a reason to be concerned.
CHANGING THE STIFFNESS TO THE CORRECT VALUE.
The stiffness of the mount plays a key role on the resonant frequency of the engine. The engine mounts change their stiffness during the time, showing a higher stiffness with the time. The below graph shows the stiffness of the mount when it is new (Stiffness 0) and when the mounts has been used (Stiffness 1).
The Stiffness 0 is lower than the Stiffness 1. Therefore if we continue with the natural frequency formula, we can understand that the lower the stiffness is the lower the resonant frequency of the engine.
This moves back the resonant peak to the left side. Allowing an important increase on the isolation or diminishing the idle speed to lower rpms.
For further information on the stiffness alteration process, it is explained in depth on this article. https://www.ellebogen.com/en/how-do-the-marine-engine-mounts-loose-their-elastic-properties/
On the below images we can see the new Ellebogen engine mounts on the Yanmar 3YM30 marine engine at 825 rpm\850rpm\900rpm\950rpm\1000rpm.
The position of the Ellebogen marine engine mounts follow the diagram as shown below.
Sailing on the Solent Strait
Frank Thompson, a British sailor from Grimsby up in the east coast in the mouth River Humber, which can have it’s challenges. He learned to sail on the north coast of Wales at the Conwy. His boat is based down south Port Solent, Portsmouth. He is an active helper (contributor) on Yanmar forums, always keen to help fellow Seamen by troubleshooting problems and finding solutions. An example of the brotherhood between Sailors.
Frank sails in the waters of the Solent and South of UK. The Solent is a major shipping lane for passenger, freight and military vessels. It is an important recreational area for water sports, particularly yachting, hosting the Cowes Week sailing event annually. It is sheltered by the Isle of Wight and has a complex tidal pattern, which has benefited Southampton’s success as a port, providing a “double high tide” that extends the tidal window during which deep-draught ships can be handled. Portsmouth lies on its shores. Spithead, an area off Gilkicker Point near Gosport, is known as the place where the Royal Navy is traditionally reviewed by the monarch of the day.
The area is of great ecological and landscape importance, particularly because of the coastal and estuarine habitats along its edge. Much of its coastline is designated as a Special Area of Conservation. It is bordered by and forms a part of the character of a number of nationally important protected landscapes including the New Forest National Park, and the Isle of Wight AONB.
The following links may be of interest for those who are interested in finding out more about sailing on these waters: