A semisubmersible, 200 feet in length between draft marks with the LCF 10 feet aft of amidships, records the following drafts: Port Forward 64'-9"; Port Aft 68'-9"; Starboard Forward 59'-9"; and Starboard Aft 63'-9". What is the true mean draft?
• Mean draft vs. true mean draft and how they differ when the vessel is not even keel • How to convert feet-inches to decimal feet for calculation accuracy • Effect of LCF (Longitudinal Center of Flotation) location relative to amidships on correcting mean draft for trim
• First, how do you find the apparent mean draft from the four draft readings on port and starboard, forward and aft? • Once you have forward and aft mean drafts, what is the vessel’s trim, and is it trimming by the bow or by the stern? • How does the LCF being 10 feet aft of amidships change the correction you apply to the apparent mean draft to get the true mean draft?
• Be sure all four drafts are converted to the same units (decimal feet) before averaging • Confirm that you correctly computed the mean forward draft and mean aft draft before finding trim • Double-check the sign and magnitude of the trim correction based on whether the LCF is forward or aft of the vessel’s midpoint and whether the vessel trims by the bow or stern
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