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ShipStability_Basic Terms

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[MARITIME TRAINING SERVICES INC.] [In case of any conflict between the requirements shown in the movie and the company's safety management system (SMS), please follow the company's SMS requirements] [SHIP STABILITY TERMS & DEFINITIONS] In January of 2006, the tugboat Valor was pulling a barge off the coast of North Carolina when the weather turned bad. 30 knot winds and 10 foot waves buffeted the Valor and while she should have been up to the challenge, she instead developed a bad list to port and capsized. The Valor, along with three of her crew was lost. A ship does more than simply sit on top of the water, it balances in it. Understanding this balance is critical for ships officers. They need to know how to keep their vessel upright in the water. A ship's ability to keep itself upright is called stability. [What keeps a SHIP afloat is the] What keeps a ship afloat [FORCE of BUOYANCY acting upward] is the force of buoyancy acting upward [against the hull] against the hull, [a force equal to the weight of water of VOLUME of water it displaces] a force equal to the weight of the volume of water it displaces. [But there is a twist] But there is a twist. [The upward force of BUOYANCY is not in the same place] The upward force of buoyancy is not in the same place [as the downward force of GRAVITY] as the downward force of gravity. [The relative position of the two] The relative position of the two [is the ESSENTIAL balance that all vessels must find] is the essential balance that all vessels must find [and is the root of their STABILITY] and is the root of their stability. [This video will EXPLORE this balance and] This video will explore this balance [EXPLAIN how a vessel rests in the water] and explain how a vessel rests in the water. [By taking time to UNDERSTAND the principles] By taking the time to understand these principles, [officers and sailors alike will better understand] officers and sailors alike will better understand [the STABILITY of their vessel and how to best keep it safely] the stability of their vessel and how to best keep it safely [UPRIGHT] upright. [Working up from the basics this video will BUILD] Working up from the basics, this video will build [understanding of where a ship's STABILITY and TRIM come from] an understanding of where a ship's stability and trim come from. [STABILITY] The following are the standard terms used to describe the hull of a ship even before it is put into the water. These first few terms are fairly basic. [DEPTH] Depth is the height of a hull from the highest point of its main deck to it's lowest point. [BEAM] In the other direction a ship's beam or breadth is its width at it's widest point. [CENTERLINE] The centerline is a vertical plane that runs the length of the ship at the midpoint of it's beam [BASELINE] and the baseline is a horizontal plane perpendicular to the centerline located at the lowest point of the hull. [KEEL] The keel is the principal structural member of a ship running lengthwise along the centerline from bow to stern to which the ships frames are attached. The lowest point of the keel, or K, [K] is the point from which vertical distances are measured on a ship. K is located at the intersection of the centerline and the baseline. [WATERLINE] The waterline is the intersection of the surface of the water a ship is floating in with the sides of the ships hull. When a ship is designed, the Naval Architect determines [DESIGN WATERLINE, DWL] the design waterline or DWL that represents the waterline of a ship under a full load or maximum draft conditions on an even keel. [FORWARD PERPENDICULAR, FP] The forward perpendicular or FP, is a vertical line drawn at the intersection of the design waterline and the foreside of the stem of the hull. [AFTER PERPENDICULAR, AP] The after perpendicular or AP, is a vertical line drawn at the intersection of the design waterline and the aft most point of a ship's hull. For most commercial vessels, this is generally where the rudder post is located. [MIDSHIPS] Midships is the horizontal point halfway between the forward and aft perpendiculars. [LENGTH BETWEEN PERPENDICULARS, LBP] And the length between perpendiculars or LBP, is the total horizontal distance between the forward and aft perpendiculars. [LENGTH OVER ALL, LOA] Length over all or LOA, is the total length of a ship at it's longest point. Note that this may be a little longer than the LBP because a ship can extend slightly past the perpendiculars. Distances on board ships are measured in one of three directions. Longitudinally, transversely, and vertically. [LONGITUDINAL] Longitudinal is the horizontal direction along the length of a ship. Longitudinal distances are measured from one of three places, the forward perpendicular, the aft perpendicular or midships where longitudinal measurements are taken from will vary from ship to ship. [TRANSVERSE] Transverse is the horizontal direction across the beam of a ship. Transverse distances are measured port or starboard from the centerline with one written as a positive distance and the other as negative. It is not standard which is which however and this vary's from ship to ship as well. [VERTICAL] Vertical distances on a ship are measured upward from the baseline or lowest point of the keel. With these terms in mind, we can now look at how a ship interacts with the water. These concepts are the foundation of stability and trim. What holds the shop above the water is the force of buoyancy, [BUOYANCY] a force that equals the weight of the water the ship displaces. Displacement is the amount of water pushed aside or displaced [DISPLACEMENT] by a ship when it is floating. A ship's displacement is always equal to the total weight and is measured in tons. Depending on your vessel, ship stability can be calculated in either the metric or imperial system so you must be familiar with both. Displacement was first understood by the Greek thinker Archimedes over 2000 years ago. The King of Syracuse had asked Archimedes to determine if a crown he had commissioned was pure gold or if the jeweler had cheated him by mixing in some lesser metal. While he was still thinking about it, Archimedes noticed the water in his bathtub rise as he stepped into it. "Eureka!" he shouted and ran, naked, into the streets in his excitement. By using displacement, he understood that he could measure the exact volume of the crown. This allowed him to find its density telling him if the metal was pure or not. It wasn't. [DRAFT] Draft is the vertical distance between the waterline and the lowest point of the hull. A ships draft can be found or taken, by reading the draft marks that are welded onto a ships hull, forward and aft. [TONS PER in OF IMMERSION] Tons per inch of immersion or TPI, [TPI] is the number of tons necessary to change the draft of a vessel by one inch. [TONS PER cm OF IMMERSION, TPC] In the metric system this is tons per centimeter of immersion or TPC. Load lines are marks welded on the side of a vessels hull at midships showing the draft under maximum safe loading conditions. This mark is called the plimsoll mark. If a vessel is overloaded, and it's draft is deeper than it's load line, it is unsafe and is violation of the International Load Line Convention. It will be subject to fines or other legal actions and it's insurance is void. [TRIM] Trim is the difference in draft, forward and aft. The trim of a vessel can be found by reading the draft marks on it's hull. These draft marks are placed as close to the perpendiculars as the shape of the hull allows. [FREEBOARD] A vessel's freeboard is the vertical distance between the waterline and the highest water tight deck of the hull, usually the main deck. Freeboard is important because it is part of what determines the volume of the space above the waterline. [WATERPLANE AREA, AWP] The waterplane area or AWP, of a vessel is the horizontal intersection of the waterplane and the vessel's sides. The bigger a vessels waterplane area is, the greater the surface of the hull is that buoyancy will be acting against. For this reason, in the case of two vessels with the same weight added, the one with the greater waterplane area will have a smaller change in draft. As a vessel rolls in the water, it's waterplane area increases as long as there is freeboard available to add to it. The larger a vessels waterplane area becomes when it rolls, the more buoyant force it develops. This is why freeboard is indicative of a ships reserve buoyancy or additional buoyancy at larger angles of roll. [LONGITUDINAL CENTER OF FLOTATION, LCF] Longitudinal center of flotation or LCF, is the geometric center of the waterplane area. The LCF is the point about which the vessel trims. Note that the LCF is not necessarily located at midships. It's location is determined by the shape of the waterplane area and the trim of the vessel. Stability is the tendency of a vessel to right itself. When a vessel is tilted by an outside force, such as wind or waves, and it returns to its original position, it has positive stability. If it does not, it had neutral or negative stability.

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Duration: 11 minutes
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Language: English
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Posted by: maritimetraining on Feb 16, 2018

ShipStability_Basic Terms

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