4.10 Currents: Set, Drift & Current Tables

Current Is Horizontal Motion

If tide is the water going up and down, current is the water going sideways. On a Master 100 vessel, current decides whether you make good the course you steered, how hard the docking will be, and whether an inlet bar is a passenger-friendly crossing or a white-knuckle event. Q162 tests definitions and tables; Q163 Chart Plot will force you to apply set and drift numerically. Learn the language once and use it on both modules.

Set and Drift — Exact Definitions

Memorize these two words as if they were COLREGS:

Critical trap: set is not "where the current came from." If the table says set 270°, the water is flowing toward the west and will push your vessel westward. Drift of 2.0 kn means two nautical miles of offset per hour if you did nothing but drift with it.

On a plot, current is often drawn as a vector: set is the direction of the arrow; drift is the length scaled to speed × time. Heading and speed through water produce a water track; current produces a ground track (COG/SOG).

Flood, Ebb, and Slack

In tidal waters, the horizontal flow is the tidal current:

• Flood current: generally the incoming flow associated with the rising tide (water moving into a bay, river, or harbor)
• Ebb current: generally the outgoing flow associated with the falling tide
• Slack water: the period of little or no horizontal flow near the turn between flood and ebb

Slack is not the same as high or low water. High/low water are vertical extremes (stand of the tide). Slack is a horizontal minimum. In many channels the current continues for a time after high water, so slack may occur after high or low. Current tables list slack times and maximum flood/ebb times and strengths — use those products, not an assumption that "high tide means no current."

Strength and timing

Maximum flood and maximum ebb often occur roughly midway between slacks (varies by location). Spring ranges tend to produce stronger currents; neap ranges produce weaker ones. Narrow inlets, bridge openings, and river mouths can amplify drift to several knots even on small coasts — enough to stop a underpowered vessel dead in the water or set you onto a jetty.

Rotary vs Reversing Currents

Exam questions may describe a current that "sets in successive directions around the compass" → rotary. A channel that "floods north and ebbs south" → reversing.

Current Tables, Tidal Current Charts, and Atlases

Use the right product:

• Tidal Current Tables (or NOAA current predictions): times of slack, times and strengths of max flood/max ebb, and set directions for reference stations; corrections for secondary stations
• Tidal current charts / atlases: graphic snapshots of set and drift across a bay or sound at successive hours relative to a reference station — excellent for planning a passenger route through complex waterways
• Coast Pilot / local knowledge: notes on dangerous rips, overfalls, and recommended transit windows

Read set carefully (toward which direction?), drift in knots, and whether the prediction is for surface current — wind can layer a surface set different from deeper flow, but exam problems stick to table values unless stated otherwise.

Effect on COG, SOG, and Course to Steer

• Speed through water (STW): how fast the hull moves relative to the water (log/speed through water)
• Speed over ground (SOG): how fast you move over the bottom (GPS)
• Course over ground (COG): the track made good over the bottom

A following current (set nearly the same as your course) increases SOG. A head current decreases SOG and can make you late or unable to stem an inlet. A beam current sets you off track: COG diverges from heading even if you "steer the charted course." To make good a desired track you must steer a course to steer (CTS) that aims up-current by the amount of the set/drift triangle — the heart of many Q163 problems and of real inlet approaches.

Practical Master 100 examples

1. Harbor transit: 3 kn ebb on the nose; vessel makes 8 kn STW → about 5 kn SOG. Time-to-dock doubles versus slack.
2. Crossing a channel: flood sets you onto a shoal to starboard if you hold heading without offset.
3. Meeting situation: both vessels are being set; COLREGS still apply, but your aspect and CPA change with current — another reason lookout and radar judgment matter in strong flow.

Docking: Stem the Current

When current is significant, professional docking doctrine is to stem the current — present the bow into the flow so the current becomes a controllable force against the hull rather than a sideways shove off the pier. Benefits:

• Lower SOG for the same STW when stemming, giving more time for lines and engine orders
• Predictable set against the dock or into the slip when you angle correctly
• Less risk of the stern swinging wildly with a strong beam set

If wind and current disagree, decide which force is stronger on your hull and freeboard, then plan the approach for the dominant force. Brief the deck crew: which lines first, where the set will pin you, and when to abort.

Visual Cue: V-Shaped Ripple and Towed-Under Buoys

Strong current can pull a buoy under or hold it awash. A classic visual: a V-shaped ripple or wake pattern on the surface whose point (apex) indicates the upstream direction — water is flowing toward the open end of the V after it hits the obstruction. That pattern may mark a buoy, stake, or wreck being dragged under. Do not cut close "because nothing shows on the surface." Reduce speed, keep clear of the ripple, and assume solid metal is just below.

Other current visuals for the watch: rips and overfalls where opposing flows meet, eddies behind jetties, and a vessel's ground track crabbing across the channel while heading "straight."

Master's current pocket card

1. Set = direction current flows toward; drift = speed in knots.
2. Slack ≠ high/low water automatically — use current tables for slack.
3. Flood in / ebb out as general tidal-current sense; local tables win arguments.
4. Current changes COG/SOG; plan CTS and ETA with set/drift.
5. Stem the current when docking in flow.
6. V-ripple pointing upstream may hide a towed-under buoy — stay clear.

Master currents on paper so Chart Plot and real docks do not master you.