tides, currents, and leeway 119
“Your Call, Skipper”
You’re the skipper or most knowledgeable crewmember on board.
What actions would you take in the following situations?- You are planning to arrive in Marblehead, Massachusetts, late in the afternoon
of May 25. Turning to the tide tables, you read the following information: high
water will be 9.5 feet at 1335 hours; low water is −0.7 foot at 1940 hours. What
fi nal step must you take before making your tidal graph? - Slack water in The Race, at the eastern end of Long Island Sound, is at
1400 hours DST. Three hours later the ebb reaches a maximum of 3 knots. You
plan on entering the The Race from the eastward around 1500. You expect the
current to be from dead ahead. In a sailing vessel traveling at 5.5 knots, how
would this affect your SOG? - You expect a Gulf Stream current of 000 degrees true at 3.5 knots between
Miami and Bahama Cay. In a power vessel traveling at 12 knots, you lay a TR of
93 degrees magnetic for Bahama Cay. The variation is 3 degrees west and the
distance is 46 nautical miles. You begin the crossing at 0900. What is the initial
course to steer and the estimated time of arrival? - Your latest position places you a quarter mile to the right of your TR.
You’re not too concerned but want to make sure this course will be safe.
What is the safest method to use before making a decision? - You are close-reaching on port tack, bound for Martha’s Vineyard in
a stiff southwesterly. Your estimated leeway is 10 degrees. How do you
apply this to your present course?
Answers - Add 1 hour to each tide time for Daylight Savings Time. In the United
States, DST lasts from March into November. Then make your tide graph
using the listed information. (Note: remember, if using tide data from the
NOAA online tide tables, it isn’t necessary to add an hour; those times are
corrected for Daylight Savings Time when applicable.) - Using the 50-90-100 Rule, you estimate that the current will reach
50% of its maximum during the first hour after slack. At 1500 hours there
would be 1.5 knots ebb. This would slow your SOG to 4 knots (5.5 knots
of boatspeed − 1.5 knots ebb current). - The true course to steer is 107 degrees. Convert this to a magnetic course
to steer: 107 degrees true + 3 degrees west variation = 110 degrees magnetic.
Your ETA would be 1300, since the SOG would slow to 11.5 knots (46 miles ÷
11.5 = 4 hours). (Measure the second side [vector] of the set and drift triangle.
This vector shows 11.5 nautical miles, which represents speed over ground.) - Draw a line from your starting point to your latest position. This is your
COG. Extend it for one or more hours ahead, and study the resultant track
on all sides for danger. - Change course 10 degrees toward the wind, or to port.