Set, Drift & Clearances

Defining Set and Drift

Set is the direction toward which the current is flowing — expressed as a true compass bearing. A current flowing north has a set of 000°. A current flowing east has a set of 090°. Set describes where the current is pushing you.

Drift is the speed of the current — expressed in knots. A drift of 1.5 knots means the water mass is moving at 1.5 knots in the direction of set. Drift tells you how fast you're being pushed.

Together, set and drift fully describe a current's effect on navigation. They are the two components of the current vector — the arrow you draw on a navigation triangle to account for current in your course calculations. Both values can be looked up in NOAA tidal current tables or derived from the difference between your intended track and your actual position after sailing a known time and course.

Calculating Set and Drift from Two Fixes

The most practical way to determine set and drift is by comparing your intended position (where dead reckoning says you should be) with your actual position (from a fix).

Method: (1) Take a good fix and mark it on the chart. (2) Navigate for a known time at a known course and speed, without adjusting for current. (3) Take another good fix. (4) Plot your DR position (where you calculated you should be based on course and speed). (5) Draw a line from your DR position to your actual fix. The direction of that line is the set. The length of that line divided by the elapsed time (in hours) is the drift in knots.

This method is powerful because it requires no current tables — you determine the actual current affecting your vessel from your own navigation observations. In areas where current tables are unavailable, or where local conditions differ from published predictions, this is the definitive method.

Steering a Corrected Course

Once you know set and drift, you can calculate the corrected heading needed to achieve your intended track (the straight-line course between waypoints). This is done by constructing a navigation triangle:

Step 1: Draw your intended track on the chart from start to destination.

Step 2: From the start, draw the current vector — in the direction of set, for a length representing drift × passage time (e.g., 2 knots × 3 hours = 6 miles).

Step 3: From the tip of the current vector, draw an arc equal to your boat's speed × passage time. Where this arc intersects the intended track gives you the corrected heading.

In practice, many sailors use a simpler approach: if current is pushing you 10° to the south, steer 10° north of your desired track and let the current carry you back. This is sufficient for most coastal navigation in moderate currents. The formal triangle method is more precise and essential when the current significantly affects the passage.

Overhead Clearances: Bridges, Cables & Obstructions

Overhead clearance is the vertical distance between the water surface and the lowest point of a fixed obstruction — a bridge, cable, or power line. On nautical charts, bridge clearances are printed at Mean High Water (MHW) for tidal waters, meaning the stated clearance is the minimum you can expect under normal tidal conditions. At lower tides, you will have more clearance than charted; at unusually high tides (storm surge, spring tides higher than MHW), you may have less.

To calculate your actual clearance at a given time: find the height of tide above or below MHW from tide tables, then add or subtract from the charted clearance. If the tide is 2 feet below MHW, add 2 feet to the charted clearance. If the tide is 1 foot above MHW (possible during spring tides), subtract 1 foot. For sailboats, the critical measurement is mast height above the waterline — know this number precisely, and always allow a generous safety margin.

Cable clearances use the same MHW reference but deserve extra caution. Power lines sag in hot weather (thermal expansion) and may hang lower than charted. The charted clearance is measured at the lowest point of the cable's catenary curve between towers. Always assume cables may be lower than charted in summer heat, and never cut it close — contact with a power line is fatal. Submarine cables are marked on charts but pose no overhead risk; however, anchoring near them is prohibited.

Under-keel clearance follows similar logic but in reverse. Charted depths are referenced to Mean Lower Low Water (MLLW) — the average lowest tide. At higher tides, you have more water than charted. At extreme low tides (negative tides), you may have less. Calculate under-keel clearance by adding the height of tide above MLLW to the charted depth, then subtracting your vessel's draft. Always maintain a safety margin of at least 20% of your draft in soft-bottom areas, and more over rocky bottoms where grounding causes damage.