Hull vibrations and shudder

Dear members,
I have a problem with one of our existing ships.
There is a high level of hull vibration. We have done some vibration trials. One problem is that the entire hull expriences a 'shudder' phenomenon. We have done some trials and the max levels are around 30 m/s (velocity). The interesting thing is that:-
1. The mas intensity is observed in an area about 2 mtrs above the bilge keel
2. The frequency of vibration corelates well with the natural frequency of the forward portion of the bilge keel (we've modified it a couple of times with the same observation)
3. The leves are about 1.5 times higher on the Stbd side as compared to the Port side.
4. There is no correlation with the shaft rpm. However, the effect is maximum at quartering and following seas.

I am not sure about the orientation of the bilge keel since the tests results are not held with us. It is however, documented that the ships 'as designed' and 'as built' have a differnec of about 0.8m in trim (and a displacement difference of
350t). The displacement as built is around 3.8m and the displacelemt around 3500t.

Can I request the informed members to forward comments????

Bharat Tyagi


HULL VIBRATION COMMENT

Hi this is Engr. Dennis Sianghio, based on my experience in the shipyard the usual causes of hull vibration comes from underwater machineries such as misalignment of tailshaft or strut, poor condition of transmission or gear box, ripped portion at propellers, clearance of propellers to appendages.