Slamming is a high frequency transient vibratory response of a ship’s hull to wave impact. The impact may be large enough to cause physical damage to the ship.
Like heave, slamming co–exists with pitching and is caused by the emergence of the lower bow in head seas. In shallow draft craft this leads to very high bottom pressures on the hull followed by structural vibrations extending the whole length of the vessel. Bulbous bows of the simple cylindrical type fitted close to the waterline are particularly susceptible to slamming. When the forefoot of the vessel emerges, a suboptimal bulb can contribute to slamming.
In severe slamming situations, the most vulnerable area is identified between 10 and 25 per cent of the ship’s length from the bow. Ship master is required to reduce speed, keeping under control the slamming effect and thus prevent structural damage to the hull. An often used criterion for "voluntary speed reduction" is that a typical ship master reduces the speed if slams occur for more than three out of 100 waves that pass the ship.
Lightly loaded cargo ships are particularly liable to slam and speed reduction up to 40 per cent may be required.
The slamming impact lasts for about 1/30sec and does not perceptibly modify the downward movement of the bow. It can be detected as disturbance in the acceleration record and by the ensuing vibration which can last for about 30sec.
Slamming pressures as high as 0.7n/mm2 have been recorded in merchant ships. Pressure in specially designed high speed vessels are generally less because of their finer form forward.
Martechnic Hellas offers the definitive solution against severe bow slamming situations at rough seas. A simple system, installed as a matter of hours, designed to monitor bow slamming and longitudinal structural bending. While allowing on-screen full visibility to ship master, it is triggering alarm upon detecting measurements exceeding acceptable levels, as they are defined either by the classification societies and/or ship structural design specifications.
The system is primarily based on a bow accelerometer. Strain gauges may be optionally installed on appropriate deck locations, as well as a specialized bow pressure transducer.
Signals are transmitted in a compact central processing unit, typically located on bridge, offering visual interpretation of all measured dimensions to the OOW on a dedicated LCD color screen.
All run time data series and alarms history is archived on direct access hard disk, available for retrieval and intelligent search, at any time.
Call Martechnic Hellas, for a customized study and proposal for your vessel.
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