At 1727, on 24 August, you pass under the Helena Highway Bridge (mile 661.7 AHP). What has been the average speed of the current since departing Memphis Harbor, McKellar Lake, if you have been making turns for 9 mph?
• Speed made good vs. speed through the water (your turns give speed through the water; the chart gives distance over ground) • Using the basic formula: Speed = Distance / Time and how current affects actual speed over ground • Converting clock times to hours and decimal hours for current calculations
• First, figure out how many statute miles you actually traveled between Memphis Harbor (McKellar Lake) and Helena Highway Bridge using the river mile marks. • Next, convert your departure and arrival times into total hours underway and compute your actual speed over ground from those miles and hours. • Compare your actual speed over ground with the 9 mph you are turning for. Is the current helping or opposing you, and by how much?
• Verify the correct river miles for both Memphis Harbor (McKellar Lake) and Helena Highway Bridge (mile 661.7 AHP) from the exam chart or reference. • Carefully convert minutes to decimal hours (for example, 27 minutes is not 0.27 hours). • When you subtract speeds, be clear which is speed through the water (9 mph) and which is speed over ground so you get the current’s average speed and direction.
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