User:Sydneem/sandbox

Background

The term “set and drift” is used to describe external forces that affect a boat and keep it from following an intended course. To understand and calculate set and drift, one needs to first understand currents. Ocean currents are the horizontal movements of water from one location to another. The movement of water is impacted by: meteorological effects, wind, temperature differences and gravity and on occasion earthquakes. Set is referred to as the current’s direction, expressed in true degrees, and the drift is referred to as the current’s speed, which is usually measured in knots. Ignoring set and drift can cause a mariner to get off their desired course, sometimes by hundreds of miles.

How to Calculate

In order to utilize set and drift in navigation, navigators much first set the course using Dead Reckoning.

A Dead Reckoning, DR, is calculated by using a previously determined position on a chart, and advancing that position based on known or estimated speed over a set amount of time. This can be calculated by using the formula Speed= Distance x Time.

Once an advance position has been plotted, then set and drift can be factored in. If there is a known set and drift, then the corrections can be applied to the Dead Reckoning position to then get an Estimated Position on a chart.

The Course Made Good is the direction in which a ship or vessel has traveled with the effects of current, wind, and helmsmanship. If a current is flowing in the same direction as the ships heading, then the Course Made Good remains the same, but the current speed and ship speed are added together. If the speeds are in opposite directions, then the smaller speed is subtracted from the larger speed.

Set and Drift can be calculated by using a vector diagram and can be drawn and measured on a chart, maneuvering board, even a plain piece of paper, or a Radar Plotting Sheet, which is the most common way to calculate set and drift.

Example Calculation

Set and Drift Example

You are underway on course 150 degrees true at 8 knots. Your vessel is making good a course of 166 degrees true and a speed made good of 8.8 knots. What is your set and drift? 

Step 1. Plot out course of 150 degrees true on Radar Plotting Sheet

Step 2. Measure length of course by using speed of 8 knots and converting into nautical miles via the time, speed, and distance scale

Step 3. Plot the length of speed on Radar Plotting sheet

Step 4. Plot out desired course of 166 degrees true on Radar Plotting Sheet

Step 5. Measure length of course by using speed of 8.8 knots and converting into nautical miles via the time, speed, and distance scale

Step 6. Connect the two ends of the vectors from the current course to the course made good. Thus creating your set and drift vector

Step 7. Using the navigational triangle, place on set and drift vector. Then drag to center crosshairs keeping the same angle and find the degrees on the outer circle

Step 8. Measure the length of the set and drift vector with the compass.  Convert from nautical miles into knots using the time, speed, and distance scale.

Step 9. Your set is 230 degrees true at a drift of 2.5 knots

Tools used to calculate

To use Navigation Triangles, a navigator need two of them to be able to navigate correctly. Navigation Triangles can be used to find Lines of Positions, Dead Reckoning, ranges, Estimated Positions, Running Fixes and so on. They are also used to connect all the types of fixes to be able to determine a course.

Dividers can be used for the measurement of lengths of lines on a chart and approximating the lengths of non-linear lines on a chart. Dividers can also be turned into a compass by replacing the metal leg of the divider with a piece of lead. A compass can measure and draw arc and circles on charts and maneuvering boards.