YANMAR 2GM20, SOLUTION OF A TRICKY INSTALLATION USING ELLEBOGEN GTS 40 MARINE ENGINE MOUNTS
4 marine engine mounts ELLEBOGEN GTS40 (Ref. 120450-06201)
Brief description of the boat and the engine.
The Sailboat was constructed by Gaffers & Luggers, Mylor Bridge, Cornwall. The overall length of the boat is 23,6 ft /7,19m with a beam of 8.0 ft / 2.43 m. The displacement of the boat is 11,023 lbs / 5000kg with a draft of 4.6 ft / 1.37 m. The model of the boat is called Heard 23.
Celt of Greystones [Heard 23]
|Builder||Gaffers & Luggers, Mylor Bridge, Cornwall|
|LOA||32′ 0″ inc. bowsprit and rudder, 23′ 3″ on deck||Sail area||567 sq ft, main, jib, staysail, and topsail|
|LWL||23′ 6″||Rig||Gaff Cutter|
|Beam||8′ 0″||Draught||4’ 6”|
|Displacement||5000 kg||Engine||Yanmar 2GM20|
|Keel Type||Long keel|
The Sailboat is powered with a Yanmar 2GM20. One of the most popular marine engines for this length range of sailboat.
Vessel: Heard 23, sailing on the Irish coast near Wicklow
Setup and behaviour of the engine.
The sailboat owner could not install the OEM Yanmar marine engine mounts due to the lack of space inside the engine room. The height of the Yanmar OEM marine engine mount was too tall and could not allow the installation. Therefore, he decided to look what the market of marine engine mounts and found a good supplier who supplied a premium brand of marine engine mounts.
Nevertheless, even if the supplier and producer of the engine mounts where of good quality, he was not happy on the outcome. Due to this reason he decided to make a post and ask for advice to a marine engine maintenance forum, since he was not too sure on the selection of the marine Engine mounts. Ellebogen is a proud member of this group.
As it can be seen on the above image of the post, the sailboat owner is asking for advice to check if the mounts were ok, and if the stiffness should be softer or stiffer.
Ellebogen decided to help. The sailboat owner was asked to sent information on the height of the mounts, details of the current marine engine mounts, information of the engine setup and detailed pictures and videos of the “before” installation at different rpms can be seen below.
Calculation of the baseline (1DOF)
Based on the information of the Yanmar 2GM20 engine and the stiffness of the currently installed marine engine mounts a one degree of freedom calculation was done (engines vibrate with 6 degrees of freedom). The stiffness of the currently installed mounts were: Front side: 270N/mm, Transmission side: 200N/mm. The natural frequency of the system was 18,29Hz. This means that if there is something vibrating on the engine at 1097rpm, we would see an amplification of the vibrations.(vertical vibrations)
A Yanmar 2GM20 marine engine is a 2 cyl engine. Based on precedent vibration measurements we see that this engine excites primarily the orders 1,5 (21Hz) and order 2 (28Hz) when the engine is at idle (850rpms) Below picture shows the results of a 2GM20 @867rpm.
Therefore, if the engine is at 1000 rpm the main disturbing frequencies should be at 1,5 or 2 times this frequency. This is to say, 1500(25Hz) or 2000rpm (33Hz). The resonant frequency of the engine (18,29Hz) is distant from these 25 or 33Hz. We would have a theoretically good solution. Unfortunately, the video below shows a different reality.
If the selection of the mount was theoretically correct, then why the sailboat owner is not happy? The reply to this can get complex, but we will try to simplify it to the maximum.
First of all we have to consider that what a 1 degree of freedom calculation does. This means that a suspended element (engine) will have ONE resonant frequency (also called fundamental frequency or natural frequency) following a below formula.
The yellow line sweeps from left to right depending on the engine running speed, from low idle to high idle.
But the reality is more complex, unfortunately. Engines move in 6 possible directions (also called degrees of freedom)
Even if the most important resonance is typically the vertical one, the reality is that a suspended engine will have a total of 6 resonance frequencies.
Why 6? Because the engine can move in 6 different directions 3 translational direction (X, Y and Z) and 3 rotational direction (Pitch, Roll and Yaw).
Advanced Calculation of the baseline (6DOF)
Based on the information from Yanmar 2GM20 engine and the stiffness of the currently installed marine engine mounts a six degree of freedom calculation was done. This calculation allowed us to learn why the sailboat owner was not satisfied.
Results of solution with Current mounts:
The below table shows the natural frequencies of the engine. Based on the “1” we can see the vibration mode. Example, the number 3 natural frequency has an “1” on the Z. This means that the natural frequency has a vibration mode in Z. This is what our 1 degree of freedom calculated. When we see “1” on the Phi means that they are rotational. We can observe that the last 3 natural frequencies of the system would be on the area of the excitation of the engine.
|This table shows the resonant frequencies where the engine will resonate (rattle). When the engine is at 1000rpm (16.66Hz) the order 1,5 will excite the 5th natural frequency (25.52) and also the 6th natural frequency 33,56Hz. This explains the video below.|
|When the engine is at 2100rpm the crankshaft will be vibrating at 35Hz (2100rpm/60). This would explain why we are having this level of vibration.|
The below graph shows is a graphical expression of the below table. We can see where we have the amplification of vibrations when they overpass the 1 on the transmissibility.
To understand better the vibration mode of vibration the below video shows graphically how the engine would move.
Results of calculations with ELLEBOGEN GTS 40
Using the stiffness and damping coefficient of ELLEBOGEN GTS 40 we obtain the following natural frequencies.
We can see that the natural frequencies have been greatly reduced due to the lower stiffness of the mounts (100N/mm) this is about 2 to 3 times lower than the current mounts ( Front side: 270N/mm, Transmission side: 200N/mm), and the amplification to the resonance also. This is obtained thanks to the viscous properties of the mix.
The deflection of the mounts is below 4mm at all the points. This assures us that we are not overloading the mounts. Below is the data sheet of the ELLEBOGEN GT40.
Also achievable on this link: https://www.ellebogen.com/en/ellebogen-gt-s40/
Importance of the viscosity of the mix.
The Ellebogen GTS 40 mounts use a low stiffness and a damped compound that provides an energetic dissipation. This allows that the engine is more stable at with lower resonant frequencies.
The graph below represents a transmissibility curve with two different rubber compounds. In blue, low damping and in black high damping.
|The video shows two rubber balls with these 2 rubber blends with the same hardness..|
The sailboat owner, decided to install the ELLEBOGEN GTS40 mounts and the results speak from themselves.
Other videos at different speeds.
|1000 RPM||1200 RPM|
|1600 RPM||1800 RPM|
|1800 RPM||2400 RPM|
The sailboat owner made this comment on the forum.
We thank Mr Joe Walsh for his trust on Ellebogen and the well documented reports that he sent us.
We are very proud of having him on the Ellebogen community.
Sailing on the coastline of Wicklow, Ireland.
Mr Joe Walsh is an Irish sailor based in Wicklow, Ireland. An active member of Yanmar forums, giving advice to other others by troubleshooting and finding solutions to common and not so common problems onboard sailboats. A good example of the brotherhood that exists between Sailors.
Joe sails the coast of Wicklow. For those interested in finding out more about sailing these areas, the following webpages may be of interest: