Your vessel is underway and approaching an...

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Question 1 of 2066636ee728f7522a1c518fc7

Your vessel is underway and approaching an overhead obstruction on Lake Superior. Given the following information, determine the clearance between your vessel and the obstruction. Highest point on vessel: 74 ft. Lowest point of obstruction: 126 (LWD) Monthly lake level: +2 (LWD) International Great Lakes Datum: 600.0 (182.88 meters)

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Question 1 of 20

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🔍 Key Concepts

• Low Water Datum (LWD) and what it represents on the Great Lakes • How a monthly lake level of +2 (LWD) changes the actual water surface height relative to charted clearances • How to compute vertical clearance: obstruction height above water minus vessel’s highest point

💭 Think About

• First, think about whether the 126 (LWD) value is measured from the datum up or down, and whether it represents the bridge height above the datum or above the water at Low Water Datum. • Ask yourself: if the lake is currently +2 above LWD, does that make the available clearance bigger or smaller? By how many feet? • After you adjust the obstruction height for the current lake level, what simple subtraction do you perform with the vessel’s highest point to get the clearance?

✅ Before You Answer

• Be sure you understand that LWD is a reference plane, not the actual water level unless the lake is exactly at datum. • Confirm whether you have adjusted the 126 (LWD) value to reflect the current lake level of +2 (LWD) before subtracting the vessel’s height. • Double-check units: everything here is in feet, so you should not be converting to meters or using the 600.0 datum value in the final clearance calculation.

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