2.10 Currents: Set, Drift & Current Tables

Currents: Set, Drift and Current Tables

Where a tide is the vertical rise and fall of water, a tidal current is the horizontal movement of that water past a point. The two are driven by the same forces but they are separate problems, and a competent captain reads them from separate tables.

The vocabulary of current

• Set - the direction toward which the current flows, stated in true degrees. A current with a set of 090 flows toward the east.
• Drift - the speed of the current, in knots.
• Flood - the incoming current, generally flowing upstream or toward land as the tide rises.
• Ebb - the outgoing current, flowing downstream or seaward as the tide falls.
• Slack water - the brief moment of no horizontal flow at the turn between flood and ebb.

A frequent trap: set is named for where the current is going, whereas wind is named for where it comes from. A "westerly wind" blows from the west; a current that "sets west" (set 270) flows toward the west. Keep the two conventions straight and half the current questions answer themselves.

Current tables and their timing

The NOAA Tidal Current Tables predict the clock times of maximum flood, maximum ebb, and slack at reference stations, with time-and-speed corrections for thousands of subordinate stations - exactly like the tide tables. The essential lesson: slack water and maximum current do not line up with high and low tide. In many channels the current keeps running for an hour or more after the tide has turned, so you must read current from the current tables, never assume it from the tide.

As a rough field estimate of current strength between slack and maximum, mariners use the 50-90-100 rule of thumb: about 50% of maximum drift one hour after slack, ~90% at two hours, and full drift at three hours (the peak), then back down - a current analog of the Rule of Twelfths.

Effect on the vessel: the current triangle

A current pushes your boat off the course you steer. To find your real path over the bottom you build a current (velocity) triangle:

1. Draw the vessel vector - the course you steer and your speed through the water.
2. From the tip of that vector, draw the current vector - the set and drift.
3. The line from the start of the first vector to the tip of the second is your course made good (CMG) and speed made good (SMG) - your actual track and speed over the ground. This is what a GPS reports as COG and SOG.

Worked set and drift example

You steer 090 True at 10 knots through the water. A current sets 180 True (due south) at 3 knots.

Resolve into east/north components:

• Vessel 090 at 10 kt -> 10 kt east, 0 kt north.
• Current 180 at 3 kt -> 0 kt east, 3 kt south (i.e. -3 north).
• Sum -> 10 kt east, 3 kt south.

The resultant magnitude is sqrt(10^2 + 3^2) = sqrt(109) = 10.4 knots - the speed made good. The direction is just south of due east, about 107 True - the course made good. So a 3-knot beam current has pushed your track 17 degrees to the right of your heading, while barely changing ground speed.

Compare the pure fore-and-aft cases: a current setting 090 at 2 kt dead astern of a vessel steering 090 simply adds - SMG = 12 kt, CMG = 090. Setting 270 at 2 kt dead ahead subtracts - SMG = 8 kt, same course. A head-on or following current changes speed only; a beam current bends the track.

Practical seamanship

• Fair current (with you) shortens the passage; foul current (against you) lengthens it and burns fuel. Timing a tidal gate - leaving to ride a fair current through a narrow pass - can save hours.
• Current runs strongest in the deepest part of a channel and around points and constrictions; it is weaker in the shallows and in back eddies.
• Where a strong current meets an opposing wind or an uneven bottom, it kicks up a steep, dangerous tide rip.
• Offshore currents can be rotary - continuously changing direction through the tidal cycle - rather than simply reversing like a channel current.

Finding set and drift from your track

Current is not the only thing that pushes a boat off course - leeway, the sideways slip caused by wind on the hull and topsides, does too, and in practice the two are lumped together. The practical way to measure the combined effect is to compare where you think you are with where you actually are: run a dead-reckoning position from your steered course and speed, then take a fix. The direction and distance from the DR to the fix is the set and drift the water and wind gave you over that interval. Divide the distance by the time elapsed and you have the drift in knots - a number you can then use to work out a course to steer that cancels the current on the next leg (covered with the plotting problems in Chapter 5).