Tidal Theory: Gravitational Forces
Tides are caused by the differential gravitational forces exerted by the moon and, to a lesser degree, the sun acting on the earth and its oceans. The moon is far closer than the sun and despite being much smaller, it exerts roughly 2.2 times the tidal force of the sun. The side of the earth facing the moon experiences a stronger gravitational pull, drawing water toward it. On the opposite side, water bulges outward because centrifugal forces from the earth-moon orbital system exceed the weaker lunar gravity there. The result is two tidal bulges: one facing the moon, one on the opposite side.
Moon's Role
- • Primary tidal force (2.2x the sun)
- • Lunar day = 24 h 50 min
- • Completes orbit in 27.3 days
- • Declination varies north/south
- • Drives semidiurnal tidal pattern
Sun's Role
- • 45% of moon's tidal effect
- • Modifies lunar tide up or down
- • Aligns with moon at new/full moon
- • 90° offset at quarter moons
- • Creates spring and neap cycles
Earth's Role
- • Rotation creates daily tidal cycle
- • Land masses restrict water flow
- • Basin shape amplifies or dampens
- • Resonance creates extreme ranges
- • Bay of Fundy: up to 53 ft range
Spring Tides & Neap Tides
Spring Tides
Occur at new moon and full moon when the sun, earth, and moon align (called syzygy). The gravitational forces of sun and moon reinforce each other, producing the greatest tidal ranges of the month.
Higher high tides than average
Lower low tides than average
Greatest tidal range of the month
Strongest tidal currents
Occurs twice per lunar month (~every 14 days)
Greatest danger to draft-sensitive vessels
Neap Tides
Occur at first and third quarter moons when the sun and moon are approximately 90° apart as seen from earth. Their gravitational forces partially cancel, producing the smallest tidal ranges of the month.
Lower high tides than average
Higher low tides than average
Smallest tidal range of the month
Weakest tidal currents
Occurs twice per lunar month
Best conditions for shoal-draft passages
Diurnal, Semidiurnal & Mixed Tidal Patterns
The number and equality of daily tidal cycles vary by geographic location. The USCG exam tests all three types. You must recognize each type and know where it occurs.
| Tide Type | Highs & Lows Per Day | Typical Locations | Key Characteristics |
|---|---|---|---|
| Semidiurnal | 2 highs, 2 lows (roughly equal) | U.S. East Coast, Atlantic coasts | ~6 h 12 min between HW and LW; both highs nearly equal in height |
| Diurnal | 1 high, 1 low per day | Gulf of Mexico, parts of Southeast Asia | One 24 h 50 min cycle; large diurnal inequality |
| Mixed semidiurnal | 2 highs, 2 lows (unequal heights) | U.S. West Coast, Hawaii, Pacific Islands | Higher High Water (HHW) and Lower Low Water (LLW) differ markedly |
Exam Note: Diurnal Inequality
On the West Coast (mixed semidiurnal pattern), the two daily high waters are not equal in height. The higher high water (HHW) and lower high water (LHW) differ, as do the higher low water (HLW) and lower low water (LLW). NOAA tide tables list all four. The datum MLLW is referenced to the average of the lower low waters.
Tide Tables & Tidal Datum
NOAA publishes tide tables for thousands of stations worldwide. The primary reference is the nearest reference station. Secondary (subordinate) stations use correction factors applied to the reference station data.
Tidal Datums — Definitions
| Datum | Full Name | Definition & Use |
|---|---|---|
| MLLW | Mean Lower Low Water | U.S. chart datum. Average of the lower of the two daily low waters over 19 years. Charted depths are measured from MLLW. Most important for navigation. |
| MLW | Mean Low Water | Average of all low waters. Used in some older charts and for certain tidal predictions. |
| MHW | Mean High Water | Average of all high tides. Legal boundary between private and public land (coastal property law). Determines shoreline location. |
| MHHW | Mean Higher High Water | Average of the higher of the two daily high tides. Used for bridge vertical clearances on NOAA charts. |
| MSL | Mean Sea Level | Average sea level over 19 years. Reference for topographic and land elevation data (USGS). NOT the same as MLLW. |
Critical Rule: Actual Depth Calculation
Actual Depth = Charted Depth + Tidal Height above MLLW
When the tide is above MLLW, add to the charted depth — you have more water than charted. When the tide falls below MLLW (negative tide), subtract — you have less water than charted. Negative tides are common on the West Coast and can expose normally submerged hazards.
Reading NOAA Tide Tables: Reference & Subordinate Stations
Reference Station
- • NOAA publishes full daily tide tables (HW and LW times and heights) for each reference station
- • Tables include: date, time (local standard or daylight), height in feet above MLLW
- • There are about 70 reference stations around the U.S. coastline
- • The nearest reference station may be tens of miles from your actual location
Subordinate (Secondary) Station
- • Thousands of subordinate stations are listed with correction values
- • Time correction (minutes): add or subtract from reference HW and LW times
- • Height correction: either a ratio (multiply) or a fixed offset (add)
- • HW and LW corrections may differ from each other — apply each separately
Worked Example: Subordinate Station Correction
Reference station (Boston): HW 0845, height 9.5 ft. Subordinate station time correction for HW: +23 min. Height ratio: 0.95. Find HW time and height at the subordinate station.
Reference HW time
Boston HW = 0845
Apply time correction
0845 + 0023 = 0908
Apply height ratio
9.5 ft × 0.95 = 9.0 ft
Subordinate Station HW
0908 / 9.0 ft
Always apply time correction first, then height correction. Use the corrected time for any subsequent Rule of Twelfths calculation.
Tidal Window Planning
A tidal window is the period during which sufficient water depth exists to allow safe transit of a shoal or bar. Planning a tidal window requires knowing the charted depth, your vessel's draft, and a safety margin (typically 1 ft minimum under keel).
Tidal Window Formula
Required tidal height = Draft + Safety margin - Charted depth
If required height < 0, you can transit at any tide
If required height > 0, find times when tide exceeds that value
Worked Example: Tidal Window
Bar charted depth: 4.5 ft MLLW. Vessel draft: 5.5 ft. Safety margin: 1.0 ft. HW height: 7.2 ft at 1100. Rule of Twelfths range: 7.2 ft (LW = 0 ft). When can you cross?
Required tidal height = 5.5 + 1.0 - 4.5 = 2.0 ft above MLLW
Tidal range = 7.2 - 0.0 = 7.2 ft. 2.0 ft above MLLW = 2.0/7.2 = 27.8% of range
By Rule of Twelfths, tide passes 1/12 (8.3%) after 1st hour, 3/12 (25%) after 2nd hour, 6/12 (50%) after 3rd hour.
The 27.8% threshold is crossed during the 3rd hour before HW (i.e., about 3 h before HW and 3 h after HW).
Tidal window: approximately 0800 to 1400 (centered on 1100 HW)
Rule of Twelfths
The Rule of Twelfths is a quick, non-tabular method for estimating tidal height at any time during the 6-hour rise or fall cycle. It assumes a sinusoidal tide and a 6-hour half-cycle — close enough for most practical and exam purposes.
| Hour of Cycle | Change This Hour | Fraction of Range | Cumulative Change | Current Speed |
|---|---|---|---|---|
| 1st hour | 1/12 of range | 8.3% | 1/12 (8.3%) | Weakest — slack water |
| 2nd hour | 2/12 of range | 16.7% | 3/12 (25%) | Increasing |
| 3rd hour | 3/12 of range | 25% | 6/12 (50%) | Strongest — max current |
| 4th hour | 3/12 of range | 25% | 9/12 (75%) | Strongest — max current |
| 5th hour | 2/12 of range | 16.7% | 11/12 (91.7%) | Decreasing |
| 6th hour | 1/12 of range | 8.3% | 12/12 (100%) | Weakest — slack water |
The Memory Pattern: 1-2-3-3-2-1
The sequence 1-2-3-3-2-1 represents the twelfths of range that change each hour. The 3rd and 4th hours together account for exactly half the total tidal range. This is also when tidal currents run fastest — the middle two hours of any tidal cycle are the most energetic and most hazardous in restricted channels.
1-2-3
3-2-1
Twelfths per hour
Worked Example 1: Height at a Given Time
LW = 1.5 ft at 0600. HW = 10.5 ft at 1200. What is tidal height at 0900?
Tidal range = 10.5 - 1.5 = 9.0 ft
0900 is 3 hours after LW (end of 3rd hour)
Cumulative rise = (1 + 2 + 3) / 12 = 6/12 = 1/2 of range = 4.5 ft
Tidal height at 0900 = 1.5 + 4.5 = 6.0 ft above MLLW
Worked Example 2: Actual Depth Calculation
Charted depth in a channel: 8 ft (MLLW). Tidal height at 0900 is 6.0 ft (from Example 1 above). Your vessel draws 12 ft. Can you transit safely with a 1 ft safety margin?
Actual depth = Charted depth + Tidal height = 8 + 6.0 = 14.0 ft
Required depth = Draft + Safety margin = 12 + 1 = 13 ft
Available margin = 14.0 - 13 = 1.0 ft
Transit is possible — exactly at the safety margin. A prudent mariner would wait for the tide to rise further.
Worked Example 3: Working Backward — What Time?
LW = 0.5 ft at 0700. HW = 6.5 ft at 1300. When does the tide first reach 5.5 ft?
Range = 6.5 - 0.5 = 6.0 ft
Required rise = 5.5 - 0.5 = 5.0 ft = 5/6 of range = 10/12 of range
After 5 hours: cumulative = (1+2+3+3+2)/12 = 11/12. Too far.
After 4 hours: cumulative = (1+2+3+3)/12 = 9/12 = 75%. Rise = 6.0 × 0.75 = 4.5 ft. Height = 0.5 + 4.5 = 5.0 ft. Still short.
The 10/12 threshold is exceeded during the 5th hour (between 4 h and 5 h). Interpolating: (10 - 9) / 2 = 0.5 h into the 5th hour.
5.5 ft reached at approximately 0700 + 4.5 h = 1130.
Current Tables: Slack Water, Flood & Ebb
NOAA Tidal Current Tables (distinct from Tide Tables) list the times and speeds of maximum flood, maximum ebb, and slack water for reference current stations. Subordinate current stations provide correction factors. Understanding these tables is critical for passage planning through tidal gates, inlets, and channels.
Flood Current
Current flowing landward — into harbors, up estuaries, and rivers. Generally occurs while the tide is rising. Expressed as a direction toward which water flows (e.g., flood direction 045 means water is moving northeast).
- • Aids vessels heading into port
- • Creates greater boat speed over ground when following
- • Can make docking maneuvers challenging
Ebb Current
Current flowing seaward — out of harbors, down rivers, and away from shore. Generally occurs while the tide is falling. Ebb currents often carry debris and reduce visibility in harbor approaches.
- • Opposes vessels departing harbor
- • Increases speed over ground when following
- • Can carry logs, debris, ice floes
Slack Water
The brief period of near-zero current between flood and ebb. Best time for transiting narrow, hazardous channels where strong currents can set a vessel onto rocks or shoals. Plan passages through tidal gates at slack.
- • Best time to transit tidal gates
- • May last only minutes in fast channels
- • Can be offset from HW/LW by hours
Applying Subordinate Current Station Corrections
Time Corrections
- • Listed separately for Max Flood, Slack before Flood, Max Ebb, and Slack before Ebb
- • Expressed in minutes (add or subtract from reference station time)
- • Example: reference Max Flood at 0930, correction +15 min = subordinate Max Flood at 0945
- • Note: corrections for slack and max are typically different values
Speed (Drift) Corrections
- • A speed ratio factor is given for Max Flood and Max Ebb separately
- • Multiply reference station max speed by the ratio to get subordinate station speed
- • Example: reference Max Flood = 2.4 kts, ratio = 0.8, subordinate = 1.9 kts
- • Flood and ebb ratios are often different — always check both
Worked Example: Current Table Lookup
Reference station: Max Flood 1045 at 3.2 kts, set 280. Subordinate station corrections: time +20 min, speed ratio 0.75. Find subordinate Max Flood.
Time
1045 + 0020 = 1105
Speed
3.2 × 0.75 = 2.4 kts
Set
280°T (same as reference)
The set direction is usually the same at the subordinate station. Always confirm with the current table notes — some stations list separate flood and ebb directions.
Current vs Tide: Key Distinctions
One of the most commonly tested distinctions on the USCG exam is the difference between tide (vertical) and tidal current (horizontal). They are related but do not occur simultaneously. A vessel running aground at low tide is a tidal problem; a vessel being set onto a shoal by a flooding current is a tidal current problem.
| Attribute | Tide | Tidal Current |
|---|---|---|
| Direction of motion | Vertical (up and down) | Horizontal (flood or ebb) |
| Maximum intensity | At HW and LW (extremes of range) | At mid-tide (between HW and LW) |
| Minimum intensity | At mid-tide | At HW and LW (slack water) |
| Reference tables | NOAA Tide Tables | NOAA Tidal Current Tables |
| Primary hazard | Grounding when depth decreases | Being set off course or onto hazards |
| Units measured | Feet (or meters) above datum | Knots (speed) and degrees (set) |
| Named directions | Rising tide / Falling tide | Flood (landward) / Ebb (seaward) |
The Timing Offset — Critical Exam Point
In open coastal waters, tidal current slack roughly coincides with HW and LW. But in rivers, estuaries, and channels, slack water can lag high or low tide by one to three hours. The lag depends on channel geometry, river flow, and distance from the open sea. Always use the Tidal Current Tables, not the Tide Tables, to find slack water in a channel. The USCG exam may include questions that test whether you know to use the correct table.
Set, Drift & Piloting in Currents
Every navigator must be able to account for current when planning and executing a course. The USCG exam tests this with both calculation problems and chart plotting exercises. The three key tools are: the current vector triangle, the ferry angle (crab angle), and dead reckoning with current correction.
Core Definitions
Set
The direction toward which the current is flowing, expressed in degrees true. A current with set 090 flows eastward. NOTE: Set is named for where the current is going, unlike wind which is named for where it comes from.
Drift
The speed of the current in knots. A current with drift 2.0 kts moves the water (and any vessel in it) at 2.0 knots in the direction of set.
Course Made Good (CMG)
The actual path of a vessel over the ground after accounting for current. This is what a GPS track line shows — not your heading.
Speed Made Good (SMG)
Your actual speed over the ground after current effects. May be faster or slower than vessel speed through water.
Course to Steer (CTS)
The compass heading you must maintain to achieve a desired CMG despite a cross-current. The CTS is always upstream of the desired track.
Current Vector Triangle — Method
From your departure point (A), draw your vessel's water track vector: length proportional to boat speed, direction = your course through water.
From the end of that vector, draw the current vector: length proportional to drift, direction = set.
The line from A to the tip of the current vector is your CMG (direction) and SMG (length, using same scale).
To find CTS for a desired CMG: from A, scribe an arc of radius equal to boat speed. Where it intersects the current vector line is the tip of your water track — the direction from A to that point is CTS.
Quick Rules
- • A following current (same direction as course) increases SMG by drift
- • A head current (opposing course) decreases SMG by drift
- • A beam current produces the largest crab angle requirement
- • When SMG is zero, the current equals your boat speed — you cannot make progress
Ferry Angle (Crab Angle)
The ferry angle is the angular difference between the course to steer (CTS) and the desired course made good (CMG). It is always applied into the current — if the current sets you to port, steer to starboard. The term "crab angle" describes the sideways appearance of a vessel ferrying across a current.
Worked Example: Ferry Angle with Vector Triangle
Desired CMG: 000°T (due north). Boat speed: 8 kts. Current set: 090°T (eastward), drift: 2 kts. Find CTS and SMG.
The current pushes east (to starboard if heading north), so steer left (west) to compensate.
Ferry angle = arcsin(drift / boat speed) = arcsin(2 / 8) = arcsin(0.25) ≈ 14.5°
CTS = CMG - ferry angle = 000° - 14.5° = 345.5°T (steer about 346°T)
SMG = √(boat speed² - drift²) = √(64 - 4) = √60 ≈ 7.7 kts
Note: This formula (arcsin) gives the exact ferry angle when current is exactly perpendicular. For non-perpendicular currents, use the full vector triangle.
// Vector summary
CMG desired: 000°T
Current set: 090°T at 2.0 kts
Boat speed through water: 8.0 kts
CTS: 346°T (steer 14° into current)
SMG: 7.7 kts
Ferry angle: 14.5° W of track
Worked Example: Finding CMG with Opposing Current
Course steered: 090°T at 10 kts. Current set: 270°T (westward, directly opposing), drift: 3 kts. Find CMG and SMG.
Current is exactly opposing the course. Both vectors are collinear (opposite directions).
SMG = boat speed - drift = 10 - 3 = 7 kts
CMG = 090°T (no lateral deviation; direction unchanged but speed reduced)
A directly opposing current reduces SMG by the drift value. A directly following current increases SMG by the drift value. These are the two simplest cases for exam questions.
Tidal Current Prediction
Between the published slack and maximum values in the current tables, the speed of the current varies approximately sinusoidally. NOAA provides a Tidal Current Prediction Graphic (similar to a tide curve) for many stations, but for the exam the key method is interpolation between table values.
Speed at Intermediate Times
The current speed between slack and maximum follows the same sinusoidal curve as the Rule of Twelfths for tide height — the 1-2-3-3-2-1 pattern applies. The 3rd and 4th hours (middle of the flood or ebb cycle) carry the strongest currents.
Speed at time T (approximate):
V ≈ V_max × sin(πt / T_cycle)
where t = time from slack, T_cycle = hours to max
Using NOAA Current Prediction Tables
- • Table 1: Max flood and ebb speeds for reference station
- • Table 2: Subordinate station corrections (time and ratio)
- • Table 3: Speed ratios for intermediate times (fraction of max)
- • Enter Table 3 with hours before/after slack or max to get speed factor
- • Multiply speed factor by max current speed for the prediction
Worked Example: Current Speed at Intermediate Time
Slack before flood: 0800. Max flood: 1100 (3 kts). What is current speed at 0930?
Time from slack to max = 1100 - 0800 = 3 hours
Time from slack to 0930 = 0930 - 0800 = 1.5 hours
Fraction of cycle elapsed = 1.5 / 3 = 0.5 (halfway to max)
Using sine curve: speed factor at 50% of cycle ≈ sin(90° × 0.5) = sin(45°) ≈ 0.707
Speed at 0930 = 3.0 × 0.707 ≈ 2.1 kts
The NOAA Table 3 interpolation method gives essentially the same answer without needing to calculate sine directly.
Tidal Current Hazards: Eddies, Rips & Overfalls
Tidal currents create a variety of hazardous sea conditions, particularly in constricted channels, at headlands, and over shoals. The USCG exam tests your ability to recognize and avoid these dangers.
Eddies
Eddies are circular or swirling current patterns that form on the downstream (lee) side of obstructions such as points of land, jetty ends, bridge pilings, and rocky outcroppings. The main current flows past the obstruction; on the sheltered side, a counter-rotating eddy forms.
How Eddies Affect Navigation
- • Can spin a vessel unexpectedly
- • May reverse apparent current in a small area
- • Kayaks and small vessels can be trapped or capsized
- • Debris, logs, and ice accumulate in eddy zones
Where to Expect Eddies
- • Downstream of channel markers and buoys
- • Behind rock outcroppings and headlands
- • Inside bends of tidal rivers
- • Downstream of harbor entrance jetties
Tide Rips
Tide rips are areas of rough, choppy water caused by strong tidal currents flowing over shallow banks, through channel constrictions, or where opposing currents meet. The turbulence disturbs the water surface creating steep, short-period waves that can swamp small craft and make steering difficult.
Characteristics
- • Appear as a line or zone of rough water
- • Can occur in otherwise calm conditions
- • Worst during spring tides and strong winds
- • Wind against current greatly amplifies severity
Common Locations
- • Race channels and tidal gates
- • Entrance bars and harbor mouths
- • Meeting zones of flood and ebb currents
- • Current boundaries in open water (current rips)
Overfalls — Most Dangerous
Overfalls are particularly dangerous standing waves or breaking seas that form when strong tidal currents run over a shoal or submerged ridge, especially when the current flows against the wind. The shoal forces the current upward; the opposing wind steepens the waves until they break. Overfalls can capsize even large vessels and appear with little warning.
Conditions that Create Overfalls
- • Strong current over a shoal (3+ kts)
- • Wind opposing the current direction
- • Spring tide conditions
- • Shallow water forcing current upward
- • Swell from offshore entering shoal water
- • Channel entrance bars at max ebb
Overfalls are charted on NOAA charts with the symbol "overfalls" or tide rip annotations. Always check the chart and Coast Pilot for locations in your area.
| Hazard | Cause | Primary Danger | Avoidance |
|---|---|---|---|
| Eddy | Current shear behind obstruction | Unexpected heading change, debris | Give obstructions wide berth downstream |
| Tide Rip | Current turbulence over shoal or meeting currents | Short steep seas, swamping small craft | Avoid during max current; transit at slack |
| Overfall | Strong current over shoal, wind against current | Breaking seas, capsize, structural damage | Avoid entirely; wait for slack, calm conditions |
COLREGS & Tidal Current Interaction
The Rules of the Road (COLREGS) do not suspend the give-way obligation based on current. A vessel being pushed by a following current cannot claim it was unable to maneuver — it must account for the current in its CPA calculations. However, current does affect several COLREGS-related considerations tested on the USCG exam.
Maneuvering in Channels with Current
- • Rule 9 (Narrow Channels): vessels should keep to the starboard side — current does not exempt this requirement
- • An overtaking vessel must give way regardless of current — but a strong following current requires much greater caution
- • Anchoring in a channel is prohibited except in emergency — current drag swings the vessel into the fairway
- • Vessels constrained by draft have limited maneuvering room in current and should broadcast their intentions on VHF Ch 16
Current and Anchoring Seamanship
- • Approach the anchorage heading into the current (not necessarily into the wind) — current usually dominates over wind for anchored vessels
- • Anchor scope must account for current load on the rode — use 7:1 in current, not just depth
- • In reversing current (flood to ebb), the vessel will swing 180° — ensure adequate swinging room
- • A vessel at anchor in strong current may appear stationary but is "lying to" the current — it can still drag into danger
Exam Tip: CPA and Current
The USCG exam includes questions about how current affects the Closest Point of Approach (CPA) between vessels. Remember: radar shows targets at their actual positions over ground, so current is already "built in" to radar tracking. However, when computing a CPA manually using plotting, you must use course and speed made good (CMG/SMG) — not course steered and boat speed through water. Using the wrong values produces an incorrect CPA.
Rules of Thumb & Exam Quick Reference
The USCG exam rewards candidates who have internalized the practical rules and relationships governing tides and currents. These rules of thumb are tested frequently and should be second nature before exam day.
1-2-3-3-2-1 (Rule of Twelfths)
- • The tide changes by 1, 2, 3, 3, 2, 1 twelfths of its range each hour
- • Middle two hours = half the total tidal change
- • Middle two hours = maximum current speed
- • Works for both rising and falling tide
Set Naming Convention
- • Set = direction the current flows TOWARD (not from)
- • Wind = named for direction it comes FROM
- • A set of 270 means current flows west
- • A wind of 270 means wind comes from the west
Slack Water Timing
- • Slack = approximately at HW and LW in open water
- • In rivers and channels, slack lags tide by 1-3 hours
- • Always use Current Tables, not Tide Tables, for slack
- • Plan passage through tidal gates at slack
Spring vs Neap Memory
- • Spring = new + full moon (aligned = added force)
- • Neap = quarter moons (90 degrees = partially cancel)
- • Spring has biggest range; neap has smallest
- • Spring currents strongest; neap currents weakest
Actual Depth Formula
- • Actual depth = charted depth + tidal height above MLLW
- • Negative tide = subtract from charted depth
- • Bridge clearance = MHHW; depth = MLLW
- • Gulf Coast: nearly diurnal; East Coast: semidiurnal
Current Compensation
- • Always steer into the current to maintain desired track
- • Following current: adds to SMG
- • Head current: subtracts from SMG
- • Beam current: steer upstream (ferry angle)
Quick Reference: Tidal Terminology
| Term | Definition | Exam Relevance |
|---|---|---|
| Syzygy | Alignment of sun, earth, and moon (new or full moon) | Causes spring tides — maximum range and current |
| Quadrature | Sun and moon at 90 degrees to each other (quarter moons) | Causes neap tides — minimum range and current |
| Priming | When spring tides arrive before full/new moon | Moon's declination and distance effects on timing |
| Lagging | When spring tides arrive after full/new moon | Due to orbital mechanics; tides can 'lag' the moon |
| Perigean tides | Exceptionally large tides when moon is at perigee (closest approach) | Greatest tidal ranges of the year; flooding risk |
| Apogean tides | Smaller tides when moon is at apogee (farthest point) | Smallest spring tides; reduced current speeds |
| Diurnal inequality | Difference in height between two daily high (or low) waters | Characteristic of mixed semidiurnal (West Coast) tides |
| Stand of the tide | Brief period at HW or LW when no vertical change occurs | Can be mistaken for slack water — not the same thing |
Exam Strategy & Common Mistakes
Identify the table first
Is the question asking about depth (use Tide Tables) or current speed (use Current Tables)? Using the wrong table is the most common error on tidal exam questions.
Set goes TO, wind comes FROM
Memorize the difference. A current with set 090 flows east. A wind of 090 comes from the east. Questions frequently exploit this distinction to trap test-takers.
Apply corrections in order
For subordinate stations: apply time correction first to get the corrected time, then apply the height (or speed) ratio to the reference station value. Do not mix up the order.
Use CMG/SMG for radar CPA
CPA calculations require actual motion over the ground. Use course made good and speed made good, not course steered and boat speed through water.
Negative tides subtract depth
On the West Coast, tides can fall below MLLW. A -1.5 ft tide means the actual depth is 1.5 ft less than charted depth. This is a common trap in calculation questions.
MHHW is for bridge clearance
Vertical clearances shown on NOAA charts under bridges are measured from MHHW — the average highest high water. At spring high tide or during wind-driven surge, actual clearance may be less than charted.
Common Exam Question Formats — Tides & Currents
| Question Type | What It Tests | Key Steps |
|---|---|---|
| Tidal height at a given time | Rule of Twelfths | Find range; apply 1-2-3-3-2-1; add to LW |
| Actual depth of water | Datum and tidal height | Charted depth + tidal height above MLLW |
| Subordinate station tide | Time and height corrections | Reference time +/- correction; reference height x ratio |
| CMG and SMG with current | Current vector triangle | Plot boat vector + current vector; resultant = CMG/SMG |
| Course to steer for desired CMG | Ferry angle / crab angle | Steer upstream; use vector triangle to find CTS |
| Spring vs neap identification | Moon phase and tidal range | Largest range = spring (syzygy); smallest = neap (quadrature) |
| Type of tide from description | Semidiurnal / diurnal / mixed | Count highs and lows; note if heights are equal or unequal |
| Tidal hazard identification | Eddies, rips, overfalls | Match description to cause and location characteristics |
Frequently Asked Questions
What is the difference between tide and tidal current?
Tide is the vertical rise and fall of sea level caused by the gravitational pull of the moon and sun. Tidal current is the horizontal flow of water caused by those same forces. They are related but offset in time: tidal currents are strongest at mid-tide and weakest (slack water) near high and low tide.
What is MLLW and why does it matter for navigation?
MLLW stands for Mean Lower Low Water, the average of the lower of the two daily low tides over a 19-year tidal epoch. It is the standard chart datum for NOAA nautical charts in the United States. Charted depths are measured from MLLW, so actual depth equals charted depth plus tidal height above MLLW. When the tide is below MLLW, actual depth is less than the charted depth.
What is the Rule of Twelfths and how is it used?
The Rule of Twelfths estimates tidal height at any point in a 6-hour tidal cycle. The tide rises or falls 1/12 of its total range in the 1st hour, 2/12 in the 2nd, 3/12 in the 3rd, 3/12 in the 4th, 2/12 in the 5th, and 1/12 in the 6th. To find height, multiply total tidal range by the cumulative fraction and add to (or subtract from) the starting tide level.
What is set and drift and how do I compensate for current?
Set is the direction toward which a current flows (expressed in degrees true). Drift is the speed of the current in knots. To compensate, you must steer a course to windward of your intended track by an angle called the ferry angle or crab angle. Use the current triangle: plot your vessel speed vector and the current vector, then solve for the course to steer (CTS) that will result in the desired course made good (CMG).
What are eddies, tide rips, and overfalls?
Eddies are circular current patterns that form on the downstream side of obstructions like points and jetties. Tide rips are rough, choppy water caused by strong tidal currents flowing over shallow areas or meeting opposing currents. Overfalls are especially dangerous standing waves that form where strong currents run over shoal water or against a contrary wind, creating breaking seas even in otherwise calm conditions.
What is slack water and how do I find it?
Slack water is the brief period when tidal current speed is near zero, transitioning between flood and ebb. It occurs approximately at the times of high and low tide, but can be offset by hours in constricted channels. Find slack water times in the NOAA Tidal Current Tables for the reference station nearest your location, then apply the subordinate station time correction for your specific area.
What is the difference between spring tides and neap tides?
Spring tides occur during new moon and full moon when the sun, earth, and moon align (syzygy). The combined gravitational pull produces maximum tidal range: higher high tides, lower low tides, and stronger currents. Neap tides occur at the first and third quarter moons when the sun and moon are 90 degrees apart. Their gravitational forces partially cancel, producing minimum tidal range with weaker currents.
Why does the tide rise faster in the middle of the cycle?
The tide follows an approximately sinusoidal curve. The sine function rises most steeply near its midpoint (where the slope is greatest) and most slowly at its extremes. The Rule of Twelfths captures this: only 1/12 of the range changes in the first and last hours, but 3/12 changes in each of the 3rd and 4th hours. This also explains why tidal currents peak at mid-tide — the water is moving fastest when the tide is changing fastest.
How is set different from heading?
Heading is the direction your vessel's bow points, measured from your compass. Set is the direction a current flows toward — it has nothing to do with your vessel at all. Set is an oceanographic property of the water mass. Your heading may be 045 degrees while the set is 180 degrees, meaning a south-flowing current is pushing you while you aim northeast. The interaction between your heading and the set produces your actual course made good.
What does NOAA mean by "lower low water"?
In areas with a mixed semidiurnal tide (two unequal highs and lows per day), there are two daily lows. The lower of the two is called the lower low water (LLW). MLLW — Mean Lower Low Water — is the average of all such lower low tides over the 19-year National Tidal Datum Epoch. This is the zero reference for U.S. nautical chart depths. When the tide is exactly at MLLW, the water depth equals the charted depth.
Related Study Guides
Tides & Currents Fundamentals
Core concepts, Rule of Twelfths, and current triangle basics for the OUPV exam.
Advanced Tides & Currents
Tidal datums, mixed tides, correction factors, and advanced current calculations.
Chart Plotting Exam Guide
Dead reckoning, set and drift on the chart, and speed/time/distance problems.
Marine Weather
Weather patterns, fronts, and how wind interacts with tidal currents to worsen sea conditions.
Navigation Study Guide
Complete TVMDC, compass, and navigation review for the captain's license exam.
Coastal Piloting
Piloting techniques, current corrections, and passage planning in coastal waters.
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