You will be loading in Boston Harbor to a maximum draft of 32'06". The charted depth of an obstruction in the channel near Boston Light is 30 feet and you wish to have 3 feet of keel clearance. The steaming time from the pier to the obstruction is 01h 05m. What is the latest time (ZD +4) you can sail on 17 May 1983 and meet these requirements?
• Under-keel clearance: relationship between ship’s draft, required keel clearance, and charted depth • How to use a tide table to find when the water level is high enough above chart datum at a specific location (Boston Light) • Effect of steaming time from the pier to the obstruction on your latest allowable sailing time
• First, determine the minimum total water depth you need over the obstruction and compare it to the charted depth to find the required tidal height • Use the tide data for 17 May 1983 at Boston Light to identify time periods when the predicted height of tide is at or above this required value • Work backward from the time of sufficient water over the obstruction by subtracting the 1h 05m steaming time to find the latest possible departure time
• Be sure your minimum water depth = draft + required keel clearance (in the same units) before comparing to charted depth • Confirm you are using Boston Light tide predictions for the correct date and time zone (ZD +4) • After you find a candidate sailing time, re-check the tide height at the obstruction time to ensure the under-keel clearance requirement is actually met
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