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EFFECT OF TRIM ON STABILITY





EFFECT OF TRIM ON STABILITY
The Draft Diagram & Functions of Form and Cross Curves of Stability are prepared for ships based on the design condition: No Trim. For most surface ships, so long as trim does not become excessive (more than 1% of the length) these curves are still applicable.
RULES OF THUMB FOR TRIM
   
 1. Maximum acceptable Trim is 1% LBP 
 2. Follow Liquid Loading Instructions 
 3. Watch for Hogging and Sagging Stresses 
   
PLUNGING
Definition: When the Trimming Moment exceeds the Longitudinal Righting Moment, and the ship sinks by the bow or the stern.
Loss of ships by plunging occurs more often in the merchant or auxiliary type ship than in the combatant type, although some destroyers have been lost in this manner. Merchant ships have much larger compartments, and the flooding of these compartments at the bow or stern trims the ship heavily.
TRIM CALCULATION SHEET
It is often desirable to consider the effects of several weights at once when computing draft changes. The Trim Calculation Sheet is a tabular form used to calculate the net trimming moment created by several weight movements.

DIRECTIONS FOR USE

1. In columns 1 and 2, describe the weight, tank number, flooded compartment, etc. and determine the weight (Long Tons) of each object.
2. Sum total the weights in column 2 to calculate the net weight addition or removal.
3. In column 3, determine the Trimming Arm (TA), the distance from the center of gravity of the weight to the ships LCF.
4. In either column 4 or 5, calculate the Trimming Moment by multiplying each weight by its Trimming Arm. A weight change causing the bow to sink lower in the water is a forward trimming moment, a weight change causing the stern to sink lower in the water is an aft trimming moment.
5. Sum total the forward trimming moments and aft trimming moments in columns 4 and 5. Take the difference between these totals as the NET Trimming Moment (will either be forward or aft based on the greater column total.)
6. Calculate Parallel Rise or Parallel Sinkage (PR/PS). Divide the net weight addition/removal by Tons per Inch Immersion (TPI, found using Draft Diagram and Functions of Form and ORIGINAL DISPLACEMENT.)
7. Calculate the Change in Trim (CT). Divide the net trimming moment by the Moment to Change Trim by 1" (MT1", found using Draft Diagram and Functions of Form and FINAL DISPLACEMENT.)
8. Calculate the change in draft forward due to trim (DdFWD) and change in draft aft due to trim (DdAFT) using the equations. If the net trimming moment was forward, DdFWD is positive and DdAFT is negative. If the net trimming moment is aft, DdAFT is positive and DdFWD is negative.
9. Fill in the box in the lower right corner by applying all changes to the original conditions.

Example Problem: USS STEPHEN W. GROVES (FFG 29)
Original Drafts: Fwd: 15'0" Aft: 15'3" LBP = 408 FT
1. 200 LT is added 20 FT forward of MP
2. 20 LT is removed 80 FT forward of MP
3. 50 LT is removed 30 FT aft of MP
4. 40 LT is shifted forward 20 FT
FIND: New drafts and new trim




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