Your vessel has a draft of 24 feet. On 7 April 1983 you wish to pass over a temporary obstruction near Lovell Island, MA, that has a charted depth of 22 feet. Allowing for a safety margin of 3.1 feet under your keel, what is the earliest time after 0100 EST (ZD +5) that this passage can be made?
• Under-keel clearance: how to combine vessel draft, charted depth, and safety margin • Tide height calculations: using predicted tide heights at specific times to see when there is enough water • Time of high water at Lovell Island area on 7 April 1983 and how fast the tide is rising or falling around that time
• First, figure out the minimum total depth you need to safely pass, based on your draft and required safety margin. How does this compare to the charted depth over the obstruction? • Next, determine how tide height adds to charted depth. How many feet of tide do you need above chart datum so that the total water depth meets your requirement? • Then, look at the tide table for that date and place: at what times after 0100 does the predicted tide height reach (or just exceed) the extra depth you need?
• Calculate your required depth carefully: draft + safety margin (in feet) and compare it with charted depth. • Confirm from the tide table the exact predicted heights and times of the tide on 7 April 1983 near Lovell Island, MA. • Check which of the answer times falls at or just after the first moment the predicted tide height meets your required extra depth.
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