Knowing the load per mount is key for the selection a marine engine mount. For this purpose, we need to know the position of the mounts and the position of Centre of Gravity (CoG). The center of gravity is the point where all load is concentrated and the engine+transmission would find its equilibrium point. This point is referenced with a symbol of â
â. This information is very important and unfortunately not all engine manufacturers reveal it.
In this article we will follow a real case using the data sheet of a Universal 25 engine. The engine is discontinued, and the owner of a Catalina 28 sailboat wants to replace the marine engine mounts as they are already worn. The metal to metal contact of the engine mounts make a concerning unwanted noise inside the hull and a high level of vibration that is causing wear on nearby elements of the sailboat. The below image shows the status of the mounts on this engine.
The Sailboat was designed by the naval architect Mr Gerry Douglas.  The overall length of the boat is 28.50 ft / 8.69 m with a beam of  10.17 ft / 3.10 m. The displacement of the boat is 8,300 lb / 3,765 kg with a draft of 5.25 ft / 1.60 m.    The name of the shipyard is Catalina Yachts (USA).
Below is an overall view of the Sailboat.
The engine is a Universal Motors M25 model. The engine has 3 cylinders developing a continuous horsepower of 21 at 3200rpm
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The manufacturer of this engine was based in Wisconsin (Oshkosh) who started in WW2 producing very reliable diesel engines for many applications, one of them life boats. The distribution and marinization was done by the well-known Massachusetts company, Westerbeke. More information:Â https://www.westerbeke.com/Category/M-25/5399ADA2DAF73A26740BF7A5#techdocs
The below video shows the marine engine at idle and increase of speed having the gear engaged.
The marine engine mounts were not definitively redrilled as the sailboat owner wanted to have a preliminary idea of how the isolation was.
Our sincere thanks to Mr Chris Sandes , owner of the Catalina 28 Sailboat and Mr Geoff Mealing who is a friend of Chris and has helped Chris on the replacement. Both of them were kind enough to send us these emails.
The datasheet of the engine does not reveal the position of Cog. In the axis Y, we could assume that the center of gravity is centered. However, it is very unlikely that the cog in the axis X is in the geometric center.
Therefore, we will have to estimate a position of Cog. For this, we have considered the dimensions shown in the drawing of the engine, and we have followed the next calculation process to define the position of the center of gravity.
2nd step: we have calculated the position of the center of gravity of each square. In a square, we know the center of gravity is in the middle, this way we obtain the following.
3rd step: we position both centers of gravity on the same coordinate axis, obtaining a distance X for each one.
4th step: with the previously calculated values we can follow the following formula to obtain the position of the center of gravity of both squares.
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Once we obtain the value for Xcg, we have all the information to position the center of gravity and perform the calculation.
For the positioning of the supports, the center of gravity has been considered as the origin. In addition, as the weight indicated in the technical data (134kg) is empty weight, a higher weight of 150kg has been considered to avoid overloading the supports.
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The load that we have calculated has to be understood as static load. Waves, movement of the sailboat originated by strong winds or navigation style will input dynamic loads on the mount. The severity of the movement will have a proportional impact on the dynamic loads.
Dynamic loads can input additional deformations on the rubber of the marine engine mounts and this will affect the durability.
Due to this fact, it is not advisable that the mounts are selected to their maximum static load capacity. It is advisable not to overpass the 90% of their static load capacity. Being a safe approach to dimension them around the 70% of their maximum load capacity.
Ellebogen marine mountâs data sheets indicate the maximum static load capacity on a graph indicating the deflection of the mount also.
GTS-45:Â https://www.ellebogen.com/wp-content/downloads/datasheet/technical-data-sheet-ellebogen-gt-s45.pdf
GTS-55Â :Â https://www.ellebogen.com/wp-content/downloads/datasheet/technical-data-sheet-ellebogen-gt-s55.pdf
| NÂș | Description | X (mm) | Y (mm) | F (Kg) | s (mm) | % (MAX) |
| 1 | ELLEBOGEN GT-S55 (Ref. 120450-06801) | -141,5 | -146 | 46,2 | 2,28 | 66 |
| 2 | ELLEBOGEN GT-S55 (Ref. 120450-06801) | -141,5 | 146 | 46,2 | 2,28 | 66 |
| 3 | ELLEBOGEN GT-S45 (Ref. 120450-06400) | 227 | 203 | 28,8 | 2,58 | 64 |
| 4 | ELLEBOGEN GT-S45 (Ref. 120450-06400) | 227 | -203 | 28,8 | 2,58 | 64 |
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Following the example of the Catalina 28, Universal M25 XT engine we have done the following table.
This table indicates the position of the mounts in X and Y, the load per mount (F), the deflection (S) and the loading vs max load capacity of the mount. (%).
The table shows that the front mount has less load than the transmission mounts. In this case, we can not use the same hardness of rubber. As this would make us very unevenly deflected mounts. This is the reason why 55 Shore mounts were used on the transmission side and 45 Shore on the front side.
The engine resonant frequency is the rpm in which the engine has a resonance or a rattle. Normally we are able to see excessive movement of the engine, having too much movement of the propeller shaft too. This resonant rpm is called âResonant frequencyâ. This resonant frequency should not be close to the idle rpms recommended by the engine manufacturer. The engine mounts should provide us a natural frequency well below, so we do not have too much vibration at idle.
Steps to know the engine resonant frequency:
1st Step: we plot the deflection of the mount from the technical data sheet on the Y axis.
2nd Step: we trace a horizontal line until we reach the vertical line, natural frequency.
3rd step: In this case, we have 2 mounts that are at 580rpm and 2 mounts at 595rpm. This is an approx. indication of the engine resonant frequency.
The engine vibration isolation is expressed in percentage, this would be the reduction of the vibration speed amplitude. There is a simplified way of know what vibration isolation can be expected. Here are some steps.
1st Step: we plot the deflection of the mount from the technical data sheet on the Y axis.
2nd Step: we trace a horizontal line until we reach the vertical line, natural frequency.
3rd step: If we want to know the isolation at 2000rpm, we plot a vertical line till we get between the black lines of deflection.
4th Step: we follow the diagonal line until we get the the percentage of isolation. In this case 90%
For those interested on deeper information on Vibration isolation this free course of MIT may be interesting. https://ocw.mit.edu/courses/2-003sc-engineering-dynamics-fall-2011/resources/lecture-21-vibration-isolation/
It is possible to sail from Ottawa to Montreal. The whole route is very navigable and well-buoyed where needed. There are about 360 km of well-populated and serviced Ontario shores between the two cities. The Canada Shipping Act dictates all boats travelling within 30 metres of marinas must keep to a maximum speed of 5 knots. The below link: provides a list of all the marinas on the way:
https://marinas.com/search?category=marina&country=CA®ion=ON&city=Ottawa
Nepean Sailing Club:Â https://nsc.ca/web2/
The below link provides a nice explanation of the trajectory.
