You land your inflatable boat on a sandbar at high tide in Southeast Alaska, which has a large semi-diurnal tide. You intend to be away from the boat for 12 hours, but come back early after only 8 hours. Your boat is, unfortunately, 400m from shore. How long will you have to wait to be able to use your boat (assuming you can't carry it)?

The time we need to wait to be able to use the boat is approximately 4 hours

Explanation:

A semi-diurnal tide is a tide with two daily approximately equal high and low tides. The period of each semi-diurnal tide is 12 hours and 25 minutes and the wavelength of the tide is over a half of the circumference of the Earth.

Therefore, approximately every 12 hours, there is an high or low tide, which means that in 12 hours from the inflatable boat was landed, it will return to its original position.

The time we need to wait is given by

12 - 8 = 4 hours

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