CAUSES OF BOAT VIBRATION WHILE UNDERWAY

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CAUSES OF BOAT VIBRATION WHILE UNDERWAY

Many boat owners may experience unwanted vibration in their boat whilst in operation on the water.
A number of aspects can cause this and we at
PRECISION MARINE will try to help you.

First thing we would do is inspect the propeller for dings that would throw it out of balance.
The prop should be balanced to prevent vibration but often when you strike a hard object it may bend one blade towards the other and this will cause vibration.
We can check props for balance and if bent we will send it to a specialist for repair.

Next to check is the prop shaft for any run-out and if over a 1mm or more out we recommend replacement as often they get bent between the skeg and the prop making it difficult to straighten so often less expensive to replace.

The skeg should be inspected by eye and note if the prop shaft is central through the entry of the bull.
If it is not then something has gone amiss since new.
If some wear in the skeg bush is noted it doesn’t make a huge difference to alignment however you may notice some vibration when turning.
Probably best to replace it if the shaft is out.
Rudder with worn bearings sometimes vibrate.

Remove the hose from the log base and note if the prop shaft is in the middle of the hole and not rubbing on the alloy log base.
We have seen bad alignment completely wear through the log base and allow water to leak in.

Next is the prop shaft coupling onto the engine drive connection.
The Prop shaft coupling must be tight on the prop shaft for motor alignment.
When you remove the coupling bolts to the drive and you find it is loose on the prop shaft then most likely it is time to replace as all types of couplings should be tight when fitted to the shaft prior to tightening the bolts or cap screws.
Remove the coupling the correct way and don’t use a big hammer to remove it as it may damage it.

After coupling removal inspect the key and keyway and for any unusual wear to the prop shaft end.
On occasions the key may sheer off and damage the shaft and coupling,
Normally use a brass key in the prop and steel key in the coupling
If worn or damaged the prop shaft requires replacement.

The next important part after all is good it is time to align the motor to the prop shaft which is most important.
The log gland rubber can be loose at this time as to not put pressure on the prop shaft.

The correct method l believe is bring the prop shaft coupling up to the motor coupling, lift it about 10mm.
Then it should slide together if the side alignment is inline. Then check for malalignment
Using about a 3 to 4th. feeler gauge test if you can slide it in-between the top, sides and bottom of the flanges without freeplay.
You may use a much thicker spacer to check the clearance between the flanges keeping in mind that the gap feels the same all-round the coupling.

Most likely you will need to loosen the engine holding bolts and shift it either up, down or sideways ,
using any mount adjustment if present or use washers and or thin metal shims to obtain alignment.
On occasions even tightening down the engine bolts may alter the alignment.
This final operation can take from minutes to hours to get correct however very important operation for smooth vibration free running.

If any vibration is still present the only other possibility is the engine

All off the above is compiled from knowledge gained by PRECISION MARINE over the years.
We hope this write-up may be of help to someone.