Seakeeping performance of vessel is one of the determinative factors of hydrodynamic characteristic of vessels. Seakeeping performance in fact the response of vessel in different dynamic condition as a result of wave and wind encounter. As an effective factors on vessels performance in different weather condition following factors can be mentioned:
- Vessel speed
- Wave encounter angle
- Significant wave height
- Wave spectrum
- Wave encounter frequency
- Hull form and natural frequency of hull
Seakeeping analysis of vessel calculate the impact force of vessel in stormy sea condition, so most optimum vessel structure can be design and also with calculation of acceleration and frequency condition of body movement, the possibility of vessel crew inhabitancy will be investigate. In high speed vessel, planning vessel (crew boat, search and rescue boat, pilot, patrol, etc.) and passenger boat (ferry boat, water bus, yacht, etc.) seakeeping performance have an important role in design steps, for example vessel acceleration value and acceleration duration on specific point will be calculation and check with standard criteria in ISO 2631. In fact, crew motion sickness incidence on vessel investigate in accordance with above standard.
RAO & Resonance
Each vessel according to the hull form in different wave frequencies and angles have different value of RAO number. RAO number express the amplitude of vessel movement due to specific encounter wave amplitude and frequency, so according to RAO number the value of acceleration and movement amplitude of roll, heave and pitch can be determined. By using RAO curve of vessel, captain decided which direction and speed of vessel in any weather condition cause the ship have lowest response value to encounter wave. In the following some use of seakeeping concept will be express:
- Because of free surface effects and sea waves, submarines move in depth more than half of wave
- Because of vessel buoyancy far from sea free surface in SWATH, vessel movement in wave will be decrease in accordance with lower change in buoyancy.
Resonance phenomena occur when the hull natural frequency and wave encounter frequency become equal, in this situation RAO number will increase and ship response amplitude will be increase. Below curve shown Monohull and 2 type of catamaran hull form in response to encounter wave. Resonance phenomena occur in range of wave length to vessel length ratio between 1.7 and 2 for catamarans, in fact in this range the RAO number (heave (pitch) amplitude to wave amplitude (slope) ratio) will be approximately 2 for 2 type of catamaran hull forms. Notwithstanding better stability performance of twin hull vessel than monohulls, below curve shown the twin hull vessels in seakeeping performance weaker than monohull vessels.
Seakeeping Criteria between Monohull, Catamaran & Trimaran
In this section 3 types of hull form comparison together base on heave, pitch and roll motion in different wave heading. For each type Froude number is 0.3 and weather condition considered sea state 5.
As a result heave motion when ship at head sea become more for trimaran, catamaran and monohull respectively and when ship at beam sea become more for monohull, trimaran and catamaran respectively. Pitch motion when ship at head sea become more for trimaran, monohull and catamaran respectively. Roll motion when ship at beam sea become more for monohull, catamaran and trimaran respectively.
In general catamarans versus monohulls have lesser roll angle, better stability and lesser natural rolling period. According to low differential between natural period of rolling and pitching motion of catamarans versus monohulls, the motion sickness incidence is more in catamarans than monohulls. In order to decrease roll, pitch, slamming and bending moment of ship hull, some appendage install on catamarans main hull. (T-Foil and Flaps are some of this appendages)
Research and development department of Marinnor Shipbuilding Company, analysis the vessel seakeeping performance in design process for the better seakeeping criteria, which effects on passengers’ comfort, better operational condition, speed increase and increase the life of structure.
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