EVALUATE shipboard stability by analyzing weight and moment considerations. (JTI 3.2.1, 6.0, 6.1, 6.2)
ENABLING OBJECTIVES:
EXPLAIN why trim and loading affect intact stability.
COMPUTE the impact of longitudinal weight shifts, additions, and removals on the ship's trim and drafts.
Given
a specified loading condition and draft readings, CALCULATE the
required movement of solid or liquid loads to bring the ship to within
acceptable limits of trim.
Given a predetermined amount of flooding damage, COMPUTE final trim and drafts.
DEFINE
drag, trim, trimming moment, trimming arm, longitudinal center of
flotation, parallel rise, parallel sinkage, and plunging.
DESCRIBE the movement of longitudinal stability reference points caused by weight shifts, additions, and removals.
LONGITUDINAL STABILITY AND TRIM
Design Waterline (DWL): | The waterline at which the ship is designed to float in the full load condition. Corresponds to a line in the middle of boot-topping of the ship. |
Forward Perpendicular (FP):  Aft Perpendicular (AP): | A vertical line drawn at the point of intersection of the DWL and the stem of the ship. Important in the study of longitudinal stability as well as in frame numbering. A vertical line drawn at the point of intersection of the DWL and the stern of the ship. |
Length Between Perpendiculars (LBP):  |
Distance from the FP to the AP.
Found in the DC Book Part 1(a), and/or in the booklet of general plans. When not found there, use the Length Between Draft Marks (LBD) usually found on the Draft Diagram and Functions of Form.
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Midships Perpendicular (MP):  |
A vertical line intersecting the ship's centerline, half the distance between the FP and AP.
Symbol
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Longitudinal Center of Flotation (LCF): |
Geometric center of the ship's waterline plane. The ship trims about this point.
May be forward or aft of the MP depending on the ship's hull shape at the waterline.
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Center of Flotation Distance:
 |
Distance from the LCF to the MP. Found using the Draft Diagram and Functions of Form, as a function of displacement. Used to distribute changes of trim between the fwd and aft drafts.
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Center of Buoyancy (LCB):
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The point through which the forces of buoyancy act, longitudinally.
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Drag:
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A design feature where the draft aft is greater than the draft forward.
Primarily done to increase propulsion plant effectiveness.
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Trim:
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The difference between the forward and after drafts, in excess of design drag.
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Parallel Rise/Sinkage (PR/PS):
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When weight is removed/added from/to a ship at LCF, the forward and aft drafts will change by the same amount.
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Change in Trim (CT):
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The sum total of the absolute values of the change in forward and after drafts. EXAMPLE:
DFWD DAFT Trim
Original: 20FT 18FT 2FT b/b
Final: 16FT 21FT 5FT b/s
Change: -4FT +3FT 7FT b/s
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Trimming Arm (TA):
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The distance from the center of gravity of the weight to the LCF. If the weight is shifted, TA is the distance shifted.
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Trimming Moment (TM):
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Moment about the LCF produced by weight additions, removals, or shifts.
 , where w is the amount of weight added, removed, or shifted. |
Moment to Trim One Inch (MT1"):
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The moment necessary to produce a change in trim (CT) of one inch. Found using the Draft Diagram and Functions of Form.
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Tons Per Inch Immersion (TPI):
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The number of Long Tons added or removed necessary to produce a change in mean draft of one inch. (in salt water)
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