![]() The assessment for seakeeping and design considerations in achieving the same is centred around three main objectives or cornerstones for design: So, this frame of reference is said to have the seakeeping perspective in the most accurate sense, as shown. The best analogy can be a person or point moving alongside the vessel in a hypothetically undisturbed (no values of motion in Y or Z co-ordinates whatsoever) platform which is traversing in the same heading as the vessel, with the same forward speed and without any deviation, i.e., the observer is in a relative linearly fixed condition concerning the traversing vessel. When we talk of seakeeping motions specifically, the best frame of reference for analyzing the dynamics in the vertical plane (roll, pitch and heave) is the ship-fixed-earth-relative frame or the seakeeping frame. When we talk of describing Seakeeping and Manoeuvring, collectively the motions and dynamics of the vessel at sea, it is also necessary to understand the frame of visualization. All of these are explained in the figure below. The bow moving towards the starboard direction of the ship, i.e., the vessel turning in a clockwise sense in the +Y direction, the yaw is considered positive and vice versa. ![]() Yaw is the rotational motion of the vessel about the vertical axis of rotation (preferably passing through the longitudinal centroid of floatation and perpendicular to the longitudinal plane). Sway alludes to the sideways motion of the vessel (along the Y-axis), the directions in the way of starboard taken positively as per convention. Surge deals with the to and fro (forward and backward motions) of the vessel, i.e., along the X-axis of reference, the positive surge being the direction towards the bow. The three principal motions in this regard are (i) Surge, (ii) Sway and (iii) Yaw. The motions under consideration for the concurrent Horizontal plane (parallel to the sea surface) are under the purview of another domain of study, namely Manoeuvring, which is of equal importance as seakeeping. ![]() The following figure gives the proper explanation as described. When the bow moves up, i.e., upwards with respect to the sea level or in the -Z direction (and consequently the stern moves down), the pitch is said to be positive as per convention. Here the bow and stern move up and down in opposite directions (bow moving up, stern moving down and vice-versa). Pitch is the longitudinal oscillatory motion of the vessel about the transverse axis of the boat. When the direction of this motion is downwards (+Z), i.e., in sinkage, the ship has a positive heave. Heave is pure up and down vertical motion of the vessel concerning the sea surface. When the starboard side is down, and the port side is up, i.e., the vessel is turning in a clockwise sense when viewed from in a longitudinal direction looking forward towards the bow from aft (+X direction), the roll angle is said to be positive. Recapitulating, Roll is the oscillatory motion of the vessel athwartships about the ship’s longitudinal axis (fore to aft). The three feasible degrees of motion along this plane occurring discretely or in interplay are: Seakeeping deals with motions encountered by the vessel in the vertical plane only (perpendicular to the sea surface). Rarely does a seagoing vessel remains perfectly upright and equanimous. What are the three basic motions of a ship? Design Philosophy And Criteria For Seakeeping And Optimization.Assessment Of Sea And Environmental Conditions.It does not consider the structural considerations, resistance and propulsion conditions, and statical stability, all of which are separate entities and comprise exclusive realms of study. ![]() When we talk of ‘efficacy’, it pinpoints precisely the motions and dynamics of the ship. The Seakeeping ability, also known as seaworthiness, is thus a yardstick to estimate the vessel’s behavioural efficacy during sea conditions underway its voyage. While Stability is a widely discussed facet of a given vessel’s response to loading and surrounding conditions, there is more to it. The term ‘Seakeeping’, evident from the nomenclature itself, alludes to the performance and response of the vessel in various ensuing seagoing conditions. So when it comes to ships, after main parameters like structural strength, and propulsive efficiency, designing to attain maximum feasible adeptness in response to these seagoing conditions is a big challenge in the Design Spiral. Waves of various degrees, choppiness, currents, ripples, vortices, tidal variations, inclement climatic conditions, and the oceanic surface are rife with disturbances in eclectic forms throughout. ![]() As they say, ‘A smooth sea never made a skilled sailor’! So, seldom are the conditions in the sea placid. ![]()
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