There are enough thrills in boating without having the additional stress of a vibrating clunker below decks. Obviously some of the older style single cylinder engines can't avoid thumping and shaking around but there are one or two things
you can do to reduce the effect.
In the 'olden days', one would bolt the engine directly to the
engine bearers and direct couple the propshaft to the gearbox output flange.
Hence every time the engine ran all the loud thumps and bangs would shake the
boat and aggravate your best friends migraine , causing her / him never to go
boating with you again. Before the odd one of you rush off to remove the
flexible engine mounts from under the engine, remember that any vibration can
cause the engine, and parts of the boat, to deteriorate at a speed that may out-distance your pocketbook.
If you don't have flexible engine mounts installed already
they can be a little expensive to fit as the engine beds will need to be changed
to suit, however, it is a very worthwhile modification. Never modify the engine mount feet, always modify the engine beds. Beware the installer who tells you otherwise.
To check if you really have a problem or it is in the imagination, 4/5ths fill a beer glass with water and hold it on the top of the rocker cover. If the water slops out, the vibration is excessive.
The common causes of vibration are listed below:
- The engine idle rpm needs setting correctly.
- Engine / propeller shaft misalignment.
- Faulty engine mount. The elastomer deteriorates due to spilled diesel, excessive weight and/or old age.
- Improperly secured engine mounts.
- Incorrectly selected engine mounts. The elastomer has a 'hardness factor' and stiffer mounts must be fitted to compensate for heavier weights, e.g., if a fridge compressor has been added. ( Use the genuine Yanmar mounts, they work well. )
- Mechanical trouble, e.g. an injector not set at the proper
pressure, poor compression on one or more cylinders, valve
clearances incorrect, incorrect fuel pump timing or a sliping
gearbox/transmission clutch cone (slipping cone normally
@ low rpm).
- Modified engine mount brackets. Don't change the engine, always change the engine beds!
- Propeller cavitation. The propeller is too small or the appendages in front of it are causing cavitation.
- Damaged propeller
- Damaged/bent propshaft
- Problem with shaft strut or p-bracket i.e.lose out of
alignment with other stern gear
- Worn or damaged cutlass bearing
- Incorrectly sized propeller shaft tube. The old fixed type 'Admiralty' gland on the prop shaft tube meant the shaft to tube clearance could be about 1/16th inch and the two parts would never touch as the shaft was held in the central position by the gland. With the advent of the modern 'dripless' glands the shaft can move about with the flexibly mounted engine. This can occasionally cause the shaft to bounce on the tube, especially if heeled over while motor sailing. A larger diameter propeller shaft tube (about 2.5mm or 1/8th inch clearance) is one way of resolving this problem. Alternatively, install a thrust bearing or an 'Aquadrive' CV joint which will hold the shaft in the central position.
- Other things in the boat, like the pots and pans in the galley thrashing about.
- Engine imbalance from the manufacturer has happened twice that I know of and both of them were over 10 years old when they were complained about!