Your vessel measures 126 feet (38.41 meters) long by 21 feet (6.4 meters) in beam. If the natural rolling period at a draft of 8 feet (2.44 meters) is 6 seconds, what is the GM?
• Use the rolling period formula that relates the natural roll period, beam, and metacentric height GM (for small angles of roll). • Remember that beam (B) must be in the same units as GM, and time (T) is in seconds. • You’ll need to rearrange the formula to solve for GM in terms of T and B.
• What is the standard formula that connects roll period (T), beam (B), and GM for a vessel in still water? Write it down first. • Once you have that formula, how can you algebraically isolate GM on one side so that GM is expressed in terms of T and B? • After you compute GM, which answer choice is numerically closest to your calculated value?
• Make sure you are using the correct coefficient (constant) in the rolling period formula used on USCG exams. • Double-check that you are using the beam (21 ft), not the length or draft, in the formula. • When you square or take square roots, verify your calculator steps and units so GM comes out in feet, then compare with the choices.
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