SOFT or RIGID Engine mounts? Achieving Stability & Isolation on a Yanmar 2YM15

Vessel: 1977, Islander 32 based in East Greenwich Rhode Island, USA.

2 ELLEBOGEN 75 (Ref. 128270-08341) and 2 ELLEBOGEN 100 (Ref. 128377-08351)


The Sailboat was designed by the naval architect Robert Perry. The overall length of the boat is 31,96 ft /9,74m with a beam of 11.08 ft / 3.38 m. The displacement of the boat is 10,500 lb / 4,763 kg with a draft of 5.33 ft / 1.62 m. Constructed by Islander Yachts (USA).

Below, blueprint of the boat:

Fig 1: Overall view of the Sailboat
Fig 1: Overall view of the Sailboat

The Sailboat was originally powered by a Volvo MD7A. It was given a new engine in 2017 which was a Yanmar 2YM15 purchased in a used condition. At that time the engine had approximately 100 hours of use. The engine is at least 10 years old. The owner did the initial engine upgrade using the original mounts from the previous engine as superficially they “looked” fine. From the outset the owner was never satisfied with the vibration levels while the engine was idling, he said, “I always believed that the issues were related to some fault in the engine, not the mounts.”

The owner mans the boat alone and lives aboard the cruiser in the months of July and August. Sailing the coast of Maine, New Brunswick and the northern side of Nova Scotia. He sails approximately 1200 nautical miles a year on average and puts about 140 hours a year on his engine.  He is a 77 years old, experienced sailor with an extensive mechanical background and an enviable workshop.


The owner of the sailboat decided that he could easily lift this relatively light engine by rigging a cable winch to the lifting points on the engine and connecting it to the sailboats boom. He took most of the weight off the boom by supporting the it with a halyard and only needed to raise it a few inches to remove and install the new mounts.


Once he had disconnected the shaft from the gear flange, he was able to raise the engine without disconnecting any linkage or exhaust/cooling connections.

Then he removed the oil filter, air cleaner cover, the raw water pump, bulkhead mounted fuel filter and the alternator.

Before raising the engine, he removed all the 3/8 lag bolts that secured the mounts to the beds. A variety of flex sockets and wobble extensions helped access some of the more hidden bolts. A small battery impact driver was found to be useful with this process. He also made good use of a 24mm combination box/open end wrench that he cut in half, giving him 2 stubby wrenches that were useful in the final alignment as his engine sits very tightly between the bulkheads.

He got through the whole conversion in about two half days work, working entirely alone with the boat in its mooring. He claims “a younger man could have done the whole thing in a day.  A helper to hand me tools while in the bowls of the cockpit lockers would have been very helpful as well…

The alignment process was more or less as he had expected it to be having done it numerous times in the past. He commented “It’s a rather tedious process but I managed to achieve a very satisfactory result.”



A suspended element will have a resonance frequency (also called fundamental frequency or natural frequency) following a below formula.

Fig 4: Natural frequency formula.
Fig 4: Natural frequency formula.

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.

On the graph below an engine disturbing frequency 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.

Fig 5: Transmissibility Graph/Curve.
Fig 5: Transmissibility Graph/Curve.

The yellow line sweeps from left to right depending on the engine running speed, from low idle to high idle.

Fig 6: Transmissibility Graph/Curve.
Fig 6: Transmissibility Graph/Curve.

Transmissibility 0 = means no transmission of vibrations, in other words. 100% of isolation.

Transmissibility = means transmission of vibrations at 100%, in other words. 0% of isolation.

In the example above the complete engine running speed range would be far below the resonance so the system would work well.

A negative scenario can be seen in the example above, the engine at low idle matches the resonance frequency. Any proximity to the resonance peak would be felt as excessive engine shaking, vibration transmitted to the vessel.

Fig 7: Transmissibility Graph/Curve.
Fig 7: Transmissibility Graph/Curve.

At this point it is evident that the further the blue peak from the yellow zone the better. The yellow zone location cannot change as it has a fixed value for each engine.

What can be tuned is the location of the resonance, softer mounts would move the peak to the left and stiffer ones to the right.

Fig 8: Natural frequency formula.
Fig 8: Natural frequency formula.

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.

Fig 9: Transmissibility vs Operating Speed Range Graph.
Fig 9: Transmissibility vs Operating Speed Range Graph.

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).

Fig 10: 6 Degrees Of Freedom of the engine.
Fig 10: 6 Degrees Of Freedom of the engine.

The main goal is to select engine mounts that keep the engine resonance frequencies at a minimum of 2-3 times below the engine’s operating speed range.


No matter the type of rubber used, hardness, color, all elastomers are composed of polymeric chains. Vibrations create stress and strain on the material. This creates tension on the polymeric chains.

Over the years the polymeric chains are subjected to many strain stress cycles, they will break down proportionally to the number of cycles they are subjected to. In the below image this is represented in a load vs deflection chart showing two marine engine mounts, one brand new and the other previously used.


Load vs Deflection curve on a new mount and a used mount.

Load vs Deflection curve on a new mount and a used mount.

As indicated above, the strain and stress caused by the dynamic loads and vibrations on the elastomer, cause polymeric chains to break. Therefore, the marine engine mounts, over time, have a lower amount of polymeric chains to withstand the same load. This affects the deflection of the mount. As can be seen in the graph, with time, the marine engine mount deflection goes from S1 to S2. This is because the remaining polymeric chains have resisted as much as possible but obviously become damaged over a long period of time and use.

From the viewpoint of isolation we need to understand that stiffness in flexible engine mounts plays a key role. But what do we mean by the stiffness? The stiffness is the proportion between force and displacement. This is to say, the amount of force that is needed to provide a given displacement or deflection.

The stiffness is represented by a dotted brown line, showing the proportion or slope of the curve at a given force (F1). The stiffness 0 is the stiffness of the new mount and the stiffness 1 is the stiffness of the used mount. The stiffness of the used mount is higher than the new one. Stiffness plays a major role in the isolation of the engine. It determines the resonant frequency of the system. The higher the stiffness of the suspension the higher the natural frequency, so the isolation will be lower. So, if the engine moves more and the system is showing more elasticity you might think that the engine is better isolated against vibrations, but the case is in fact the opposite. The vibrations are felt higher than ever, since the system is more elastic, the misalignments of the shaft are more pronounced.


When the mounts reach a certain level of use, they become stiffer, the stiffer they are, the higher the natural frequency of the system. This will decrease the vibration isolation. In a parallel effect, the more mobility the marine engine has, the more deflection will be noted in the mounts. This will cause higher stress on the rubber which will lead to the rupture of the remaining polymeric chains. Creating more and more degradation in the material.

This is explained in more details in the videos below.

Sailing on the coast of Maine, New Brunswick and Northern side of Nova Scotia

Mr Leo Constantino is an American sailor based in East Greenwich Rhode Island, USA.  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.

Leo sails the coast of Maine, New Brunswick and sometimes the northern side of Nova Scotia. For those interested in finding out more about sailing these areas, the following webpages may be of interest: