Mystery Vibrations on a New Repower

2010 November 22
by Chris Brown

A new 65′ Catamaran was in South Florida for the installation of a brand new set of Volvo v-driven engines.  Unfortunately, after the installation the owner was not happy with the vibration levels and we got a call to come take a look.  Since the vessel was in the water we could only go through an engine room inspection.   There are very few places in S. Florida to haul a 65′ Cat with 33′ beam and a mast over 100′  so inspecting under the waterline would have to wait for another time.

It did not take long to spot one glaring problem.  The latest version of the 150 hp Volvo engine had a change in the steel bracketing system for the heat exchanger.  The new bracket was bottomed out and physically touching the base of engine mount.  You could see the initial signs of wear in the paint and steel after only a few days of cruising.  The exact same problem appeared on both engines.

Any time there is a hard connection between the engine and the vessel the vibration isolation properties of the engine mount are bypassed and the mount cannot do its job.  Engine and running gear vibrations are transmitted directly into the vessel.

Straight Line Marine Machining

Modifying a mount to avoid direct contact with the engine.

We removed the mount on each engine for machining modification.  Fortunately, we were able to remove the single mount while the engine remained in place.  Our machine shop, Straight Line Marine milled a simple relief in the mount base – this was easier than disassembling the motor to modify the bracket.  Mount re-installation and an engine alignment was finished in a day and a sea-trial (inter-coastal waterway trial) was immediately conducted.  Mystery solved……vibration cured…..customer very happy.

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