BİLGE AKUSTİK
Noise and Vibration Control Engineering
“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
Hull Vibration
A certain level of boat vibration can be detected by routine measurements during sea trials. There will likely be different levels of vibration measurements and they can be compared with criteria set out in the standards to determine acceptability. Crew discomfort or equipment damage can also indicate excessive vibration levels. If the discomfort is evident, the overall magnitude of the vibration is "disturbing", "very disturbing", etc. can be estimated from standard charts, depending on whether
In many cases, the cost and effort of fixing a ship vibration issue makes it worthwhile to consult a vibration engineer, even if only to confirm the crew's diagnosis.
1. Propulsion Forces
Propeller Cavitation
It is also described as an undesirable situation that occurs when a particular propeller rotates too fast, as the water particles on the blades cannot follow the rotational movement of the propeller blades and their pressure drops below the normal evaporation pressure of the water. The propeller works inefficiently in this state. Holes form on the propeller material. The impeller may cavitate due to poor design or insufficient flow to the propeller. In both cases, the forces are mainly pronounced on the fuselage rather than the shaft, accompanied by large forces at various blade harmonics and blade frequency. Also, there will be a loud noise inside the hull, near the propeller, like hammering on the hull. The techniques mentioned above can be applied to improve flow or increase openings. If the propeller is faulty, a new design or modification may be necessary. Temporarily, a small reduction in velocity usually results in a large reduction in cavitation.
Propeller Imbalance
The mass imbalance will cause lateral vibration of the shaft and thus of the body at shaft frequency.
If the propeller is damaged, the mass unbalance will likely be accompanied by a pitch* unbalance, which will double the axial, torsional and lateral forces on the shaft and possibly the shaft frequencies. To fix this, the propeller, including the balance, must be replaced or repaired.
* Here, pitch is a term that comes from the English 'pitch' expression and the screw analogy. The propeller's propagation through water is similar to the propagation of a screw as it enters a soft wood. Like a screw, the propeller moves a constant amount with each turn. The distance the propeller travels during one full rotation is called the pitch of the propeller.
Water Force
The interaction of the ship with the sea causes vibration through various mechanisms. Striking the bow as it leaves the water and reenters activates the vertical hull modes. This is a temporary phenomenon and usually occurs only occasionally. Normally, the body is straight forward, causing very high impact impact loads. It helps a lot if the body form can be changed to round or v-shaped. Otherwise, a change of course and/or a reduction in speed is required.
Propeller hub fins consist of the same number of smaller blades as the propeller blades. The hub fin and the propeller move together, thus preventing eddy currents in the propeller hub. Therefore, an improvement of 3-5% is achieved in the energy loss caused by eddy currents.
- Using curved propeller blades
- Increasing the axial or radial clearances of the propeller
Each of these techniques should be carefully analyzed by a vibration engineer before being attempted.
2. Location of Vibration
The extent of high vibration levels is another factor in assessing vibration problems. If the ship vibration only appears in the stern in case of blade frequency, this is an indication of excessive propeller forces rather than normal forces amplified by hull resonances. If the wing frequency coincides with a hull resonance, high levels can be seen along the length of the boat even if the excitation forces are not excessive. Similarly deck, deck cabin, masts etc. resonances can be excited by normal propeller forces. If a resonance is known to exist, the stiffness of the structure can be changed, or the excitation frequency can be changed by changing the number of blades or the shaft speed. If there are no practical methods of changing the structural frequency or the excitation frequency, one of the methods of reducing the propeller forces can be considered.
3. Ship Hull Vibration Problem Evaluation and Improvement Chart
The table summarizes the evaluation and troubleshooting of hull vibration issues .
