Tide Types: Semidiurnal, Diurnal, and Mixed
The type of tide you experience depends on your geographic location and how the gravitational forces of the moon and sun interact with the geometry of the ocean basin at that location.
Semidiurnal
US East Coast, eastern Gulf of MexicoDiurnal
Western/central Gulf of Mexico, SE Asia, parts of AlaskaMixed (Predominantly Semidiurnal)
US Pacific Coast — California, Oregon, WashingtonUS Coast Distribution — Quick Reference
Predominantly semidiurnal. Two nearly equal highs and lows per day. Mean tidal range varies from 1–2 ft (south Florida) to over 10 ft (Bay of Fundy approaches). Boston: ~9 ft mean range. New York: ~4.5 ft.
Variable by location. Eastern Gulf (Florida Panhandle to Tampa) is semidiurnal. Central and western Gulf (Louisiana, Texas) transitions to diurnal — one high and one low per day with very small range (1–2 ft).
Mixed tides with significant diurnal inequality. Two highs and lows per day but with pronounced height differences. MLLW is notably lower than MLW. San Francisco mean range ~4 ft but MHHW to MLLW is ~5.7 ft.
Tide Datums: MLLW, MHHW, and Chart Datum
Tide datums are the reference planes from which tidal heights and chart depths are measured. Understanding which datum applies to depths vs. clearances is one of the most tested concepts on the USCG exam.
Average of the lower of the two daily low tides. US chart datum — all soundings referenced to this level.
Exam note: Primary exam datum. Charted depths are above MLLW. Actual depth = charted depth + height of tide.
Average of the higher of the two daily high tides. Used for bridge clearances and overhead obstacle heights.
Exam note: Vertical clearances on charts referenced to MHHW. At high water the clearance is LESS than charted.
Average of all low water heights. Used on Atlantic and Gulf coasts where tides are more symmetric.
Exam note: Atlantic coast secondary stations often use MLW. Know the difference from MLLW for exam calculations.
Average of all high water heights. Legal boundary of mean high water line used in property law and jurisdiction.
Exam note: Relevant for federal navigation channel definitions and regulatory boundary questions.
Average of all hourly water heights over the tidal epoch. Used for land elevations and atmospheric pressure correction.
Exam note: Chart elevations of land (hills, cliffs) are above MSL. Tide predictions use MLLW, not MSL.
Critical Distinction — Depths vs. Clearances
Chart Soundings (Water Depth)
Referenced to MLLW. Soundings on the chart represent the minimum depth you are likely to find — actual depth at the time of your transit is the charted depth PLUS the current height of tide from NOAA tables. Depth increases with rising tide.
Overhead Clearances (Bridges, Cables)
Referenced to MHHW. Charted vertical clearances represent the minimum clearance at the highest normal high tide. At the time of your transit, actual clearance is the charted clearance PLUS the difference between MHHW and the current water level. Clearance decreases at high water.
Spring Tides and Neap Tides
The gravitational pull of the moon is the dominant force producing tides, but the sun also exerts a significant tidal force — about 46% as strong as the moon's. When these two forces combine or oppose each other, they produce spring tides and neap tides respectively.
Spring Tides
Occur at new moon and full moon — when the Earth, moon, and sun are aligned (syzygy). The gravitational forces of the moon and sun add together, producing the largest tidal ranges of the month. Spring tides have higher high waters and lower low waters than average.
- • Highest high water of the month
- • Lowest low water of the month — most exposed shoals
- • Largest tidal range and fastest maximum currents
- • Occurs roughly twice per month (new and full moon)
- • Spring tides lag the moon phase by 1–3 days (priming/lagging)
Neap Tides
Occur at first and third quarter moon — when the Earth, moon, and sun form a right angle (quadrature). The gravitational forces of the moon and sun partially cancel each other, producing the smallest tidal ranges of the month.
- • Lowest high water of the month
- • Highest low water of the month — shoals less exposed
- • Smallest tidal range and slowest maximum currents
- • Occurs roughly twice per month (quarter moons)
- • Best time to transit shallow passages with minimal current
Additional Tidal Phenomena
Perigean Tides
When spring tides coincide with the moon at perigee (closest point to Earth), tidal ranges are exceptionally large — sometimes called king tides. Combined with onshore winds, perigean spring tides cause significant coastal flooding even without storms.
Meteorological Tide
Wind and atmospheric pressure can raise or lower water levels independently of astronomical tides. Onshore winds pile water up against the coast (setup). Strong offshore winds or high pressure systems push water down (setdown). These effects can be 1–3 feet at exposed coastal stations.
Tide Lag
The tide does not reach its maximum immediately after the moon crosses the meridian. It takes time for the tidal wave to travel across the ocean and into bays and estuaries. This geographic lag can range from minutes to many hours depending on the shape of the water body.
Standing Wave vs. Progressive Wave
In semi-enclosed basins (bays, estuaries), the tide may act as a standing wave with nodes and antinodes. At a node, current is maximum but tidal range is zero. At an antinode, tidal range is maximum but current is near zero. This is why some locations experience strong currents with minimal tide range while others have the reverse.
The Rule of Twelfths — Tide Height Calculations
The Rule of Twelfths is the primary method tested on the USCG exam for calculating the height of tide at any time between high and low water. It assumes a 6-hour tidal cycle and a sinusoidal tide curve — the actual NOAA method is more precise, but the Rule of Twelfths is accurate enough for exam problems.
Rule of Twelfths — Hour by Hour
Memorize the sequence: 1 - 2 - 3 - 3 - 2 - 1. These are twelfths of the tidal range per hour.
| Hour After High/Low | Rise/Fall This Hour | Cumulative Change | Notes |
|---|---|---|---|
| Hour 1 | 1/12 of range | 1/12 of range | Slowest rise/fall — near high or low water |
| Hour 2 | 2/12 of range | 3/12 of range | Accelerating |
| Hour 3 | 3/12 of range | 6/12 of range | Fastest — mid-tide, maximum current near here |
| Hour 4 | 3/12 of range | 9/12 of range | Fastest — still near mid-tide |
| Hour 5 | 2/12 of range | 11/12 of range | Decelerating |
| Hour 6 | 1/12 of range | 12/12 of range | Slowest — approaching next high or low water |
Worked Example — Rule of Twelfths
Problem:
Low water at Portland, ME is at 0630, height 0.2 ft. High water is at 1245, height 9.8 ft. What is the height of tide at 0930?
Step 1 — Find the tidal range:
Range = High water - Low water = 9.8 - 0.2 = 9.6 ft
Step 2 — Find one twelfth of the range:
1/12 x 9.6 = 0.8 ft per unit
Step 3 — Determine elapsed time from low water:
0930 - 0630 = 3 hours after low water (tide is flooding/rising)
Step 4 — Apply the Rule of Twelfths for 3 hours:
Hour 1: 1/12 = 0.8 ft
Hour 2: 2/12 = 1.6 ft
Hour 3: 3/12 = 2.4 ft
Total rise = 0.8 + 1.6 + 2.4 = 4.8 ft
Step 5 — Add rise to low water height:
Height at 0930 = 0.2 + 4.8 = 5.0 ft above MLLW
When the Tidal Cycle Is Not Exactly 6 Hours
The Rule of Twelfths assumes a 6-hour cycle between high and low water. If the actual cycle is different (say, 5 hours 45 minutes or 6 hours 20 minutes), NOAA's more precise method uses a cosine curve calculation. On the USCG exam, you will typically be given problems where the half-cycle is close enough to 6 hours that the Rule of Twelfths applies directly. When given an intermediate time in hours and minutes, round to the nearest whole hour for the exam calculation.
Reading NOAA Tide Tables
NOAA publishes annual tide predictions for hundreds of reference stations and thousands of secondary stations. Understanding the table format and secondary station corrections is an essential exam skill.
NOAA Tide Table Column Format
| Column | Content |
|---|---|
| Date | Calendar date of the prediction |
| Day | Day of the week |
| Time | Local standard time (LST) or local daylight time (LDT) of high or low water |
| Height (ft) | Predicted height in feet above chart datum (MLLW) |
| High/Low | H = high water, L = low water |
Reference Stations vs. Secondary Stations
NOAA's tide tables provide full daily predictions for a limited number of primary reference stations. For the thousands of other locations (secondary stations), the tables provide correction factors:
- ▶Time difference: Minutes added to or subtracted from the reference station time to get local high/low water time.
- ▶Height ratio: A decimal multiplied by the reference station height to get local height. Example: 0.85 means local height is 85% of the reference height.
- ▶Height difference: Feet added to or subtracted from reference height. Some tables use this instead of a ratio.
Secondary Station — Worked Example
Reference station high water: 0815, height 8.4 ft
Secondary station corrections: Time diff +20 min, Height ratio 0.90
High water time at secondary station:
0815 + 0020 = 0835
High water height at secondary station:
8.4 x 0.90 = 7.56 ft ≈ 7.6 ft
Height of Tide at an Intermediate Time — Full NOAA Method
While the Rule of Twelfths is the exam shortcut, NOAA's official method uses a correction table based on the actual elapsed time and the actual tidal range. The process:
- Find the nearest high and low water from the tide tables (for the correct station after secondary station correction).
- Calculate the duration of the tide (time from low to high or high to low in the applicable direction).
- Calculate the elapsed time from the nearest high or low water to your desired time.
- Look up the correction factor in Table 3 (NOAA tide table publication) using duration and elapsed time.
- Apply the correction factor to the tidal range to get the change from high or low water.
- Add to low water height (rising tide) or subtract from high water height (falling tide).
The NOAA method handles non-6-hour cycles correctly. For the USCG exam, the Rule of Twelfths is faster and is the expected method unless the problem specifically refers to NOAA Table 3.
Tidal Current vs. Tidal Height — The Critical Distinction
One of the most commonly misunderstood concepts on the USCG exam: tidal height and tidal current are related but NOT synchronized. Confusing them leads to dangerous passage planning errors and is a frequently tested exam pitfall.
Tidal Height
The vertical position of the water surface — how high or low the water is at a given moment. Tidal height is what NOAA tide tables predict: times and heights of high and low water. Maximum height occurs at high water; minimum height at low water.
Maximum: At the moment of high water
Minimum: At the moment of low water
Zero rate of change: At high and low water (the tide "stands")
Fastest rate of change: Mid-tide (approximately 3 hours from high or low)
Tidal Current
The horizontal flow of water caused by the rising and falling tide. NOAA current tables (separate from tide tables) predict slack water times and maximum current speeds. Current is MAXIMUM when the tide is changing fastest, and ZERO (slack) when the tide momentarily stops.
Maximum flood: Approximately 3 hrs before high water
Maximum ebb: Approximately 3 hrs before low water
Slack water: Around high and low water (varies by location)
Offset from tide: Typically 1–3 hours at coastal inlets
The Inlet Trap — A Classic Exam Scenario
A common USCG exam scenario: "You plan to enter an inlet at high water to maximize depth. Is this also the best time for slack current?" The answer is NO. At most coastal inlets, slack water occurs 1–3 hours after high water, not at the moment of high water. The tide continues to flood into the inlet for some time after the outer station reaches its peak height, because the inlet restricts flow and the interior basin has not yet reached equilibrium. Always consult NOAA current tables separately from tide tables when planning inlet transits.
Tidal Current Terms
Horizontal flow of water toward the shore or up an estuary as the tide rises. Direction varies by geography.
Exam tip: Flood does not mean 'toward the sea' — it means the current associated with rising water. Direction is local.
Horizontal flow of water away from shore or seaward as the tide falls. Opposite of flood.
Exam tip: Ebb current often lasts longer than flood in tidal rivers due to freshwater discharge adding to the seaward flow.
The brief period of little or no horizontal current between flood and ebb, or ebb and flood.
Exam tip: Slack water does NOT occur at high or low tide — it typically lags 1–3 hours behind at inlet passages.
The peak speed of flood or ebb current, occurring roughly at mid-tide when the tidal range is changing fastest.
Exam tip: Maximum current roughly aligns with the 3rd and 4th hour of the Rule of Twelfths — the steepest part of the tide curve.
The direction TOWARD which the current is flowing, expressed in true degrees.
Exam tip: A current with set 090 is flowing toward the east. Wind direction is named for the direction it comes FROM; current set is where it goes TO.
The speed of the current in knots.
Exam tip: Drift is measured in knots, not miles per hour. On the exam, drift is always expressed as a speed of the current vector.
Reading NOAA Current Tables
NOAA publishes separate current tables for major waterways. These are distinct from tide tables and predict the speed and direction of tidal currents rather than water heights.
Current Table Format
| Column | Content |
|---|---|
| Date/Day | Calendar date and day of the week |
| Slack Water Time | Time of minimum current (near zero) — listed for each slack of the day |
| Maximum Current Time | Time of peak flood or ebb current velocity |
| Maximum Current Speed | Speed in knots — positive = flood, negative = ebb (or labeled F/E) |
| Direction | Direction of maximum current flow in true degrees |
Current at Intermediate Times
Similar to tide height calculations, you can estimate current speed at any time between slack and maximum using NOAA Table 4 (a cosine-based interpolation table). The Rule of Twelfths does NOT apply directly to currents — currents use a different curve (more sinusoidal in shape but with a different relationship to time than the tide height curve).
For exam purposes: current is approximately zero at slack, builds to maximum at mid-point, then falls back to zero at the next slack. A rough approximation for the fraction of maximum current at a given fraction of the slack-to-slack interval is sin(elapsed fraction x pi).
Current Secondary Stations
Like tide tables, current tables have reference stations and secondary stations. Corrections include:
- ▶ Time difference for slack water (added to reference slack time)
- ▶ Time difference for maximum current
- ▶ Speed ratio for flood maximum (multiply reference speed by ratio)
- ▶ Speed ratio for ebb maximum
- ▶ Direction of flood and ebb at the secondary station
Set and Drift — Computing and Correcting for Current
Set and drift describe the actual current affecting your vessel. Knowing how to compute them from observed positions and how to correct your course and speed to counteract them are essential navigator skills and common USCG exam problems.
Computing Set and Drift from DR vs. Fix
The Method
- Mark your dead-reckoning (DR) position for a specific time on the chart.
- Mark your actual position (fix by GPS, cross-bearing, or running fix) for the same time.
- Draw a line from the DR position to the fix position.
- Measure the direction of that line in true degrees — this is the SET.
- Measure the length of that line in nautical miles.
- Divide the distance by the elapsed time in hours — this is the DRIFT in knots.
Physical Interpretation
Your DR position is where you WOULD BE if there were no current — based only on your compass course and known speed through the water. Your actual fix is where you ACTUALLY ARE. The difference is entirely due to the current (assuming no leeway from wind). The current vector carried you from where you thought you were to where you actually are.
Course to Steer — Vector Triangle Method
Once you know the current (set and drift), you can calculate the course to steer through the water to make good your desired course over the ground.
Setup:
Desired course made good (CMG): 000T (due north). Boat speed through water: 8 kts. Current: set 090T, drift 3 kts.
Step 1 — Draw the intended track:
Draw a north-south line representing the desired track (000T).
Step 2 — Draw the current vector from the departure point:
From the start, draw a vector of 3 nm toward 090T (east). This represents where the current will push you in 1 hour if you stay stationary.
Step 3 — Strike an arc:
From the tip of the current vector, draw an arc of radius 8 nm (boat speed). Where this arc intersects the intended track line is the end of your water track vector.
Step 4 — Read the course to steer:
The direction from the departure point to where the arc meets the track is the course to steer through the water. In this example, it is approximately 338T (northwest) to counteract the easterly current.
Step 5 — Speed made good:
The length of the track vector (from departure to the arc-track intersection, measured on the chart scale) is the speed made good in knots. Here approximately 7.4 kts.
Combined Current Vector (Multiple Currents)
When multiple current sources affect your vessel simultaneously (tidal current plus wind-driven surface current, or two tidal streams meeting), you must combine them into a single resultant vector before solving the course-to-steer problem.
Method: Draw each current vector head-to-tail on the chart. The vector from the start of the first to the end of the last is the resultant (combined) current. Use this resultant as the single current in your vector triangle. The exam will specify each current separately and expect you to combine them graphically or use component arithmetic (north/south and east/west components).
Depth Calculations — Actual Water Depth at a Given Time
Calculating the actual water depth over a shoal or bar at a specific time is one of the most practical and frequently examined tidal applications. It requires combining a charted sounding with the predicted height of tide from NOAA tables.
The Formula
Actual Depth = Charted Depth + Height of Tide
Where height of tide is the predicted water level above MLLW from NOAA tables (can be negative)
Example 1 — Simple Addition
Charted depth at shoal: 5.0 ft
NOAA predicted height of tide at transit time: +3.2 ft
Vessel draft: 4.5 ft
Actual depth = 5.0 + 3.2 = 8.2 ft
Margin = 8.2 - 4.5 = 3.7 ft under keel
Vessel can transit safely.
Example 2 — Negative Height of Tide
Charted depth at bar: 6.0 ft
NOAA predicted height of tide: -0.8 ft (spring low tide)
Vessel draft: 5.5 ft
Actual depth = 6.0 + (-0.8) = 5.2 ft
Margin = 5.2 - 5.5 = -0.3 ft (NEGATIVE)
Vessel will ground — do NOT transit.
Overhead Clearance Calculation
For overhead obstacles (bridges, power lines), the formula works in reverse:
Actual Clearance = Charted Clearance - (Current Water Level - MHHW)
Or equivalently: Actual Clearance = Charted Clearance - (Height of Tide - Height of MHHW above MLLW)
Since charted clearances are referenced to MHHW, and the tide is usually BELOW MHHW, actual clearance is usually MORE than the charted value. At spring high tide (which can exceed MHHW), actual clearance may be LESS than charted. Always check whether you are transiting at a time close to high water if you have limited vertical clearance.
Hurricane Storm Surge and Meteorological Tides
Storm surge is the most deadly and destructive aspect of tropical cyclones. Understanding its mechanics is tested on the USCG captain's license exam and is critical knowledge for any mariner in hurricane-prone coastal areas.
What Creates Storm Surge
Wind Setup
Sustained hurricane-force winds pile water against the coastline for hours. Onshore winds create a surface slope — water accumulates on the shore side. The longer the fetch (distance over which wind acts) and the shallower the water, the greater the surge.
Inverted Barometer Effect
Low atmospheric pressure at the storm center allows the sea surface to dome upward. Each 1 millibar reduction in atmospheric pressure raises sea level by approximately 1 centimeter (0.4 inch). A Category 4 hurricane with central pressure of 930 mb (normal ~1013 mb) produces about 0.83 meters (2.7 ft) from this effect alone.
Coastline and Bathymetry Amplification
Funnel-shaped bays concentrate surge. Shallow continental shelves allow surge to build higher. The Texas and Louisiana coastlines are especially vulnerable because of their extremely shallow offshore shelf.
Storm Surge and Navigation
Total Storm Tide
Total storm tide = storm surge + astronomical tide. If surge is 10 ft and astronomical high tide is 4 ft, total storm tide is 14 ft above MLLW. Always consider the astronomical tide phase when assessing storm threat — a hurricane making landfall at spring high tide compounds the danger.
Reverse Surge
After the storm passes, offshore winds on the trailing edge can drive water away from the coast, causing abnormally low water levels. This reverse surge can expose shoals and make harbor approach dangerous for vessels drawing significant draft.
Tide Gauge Reading
Tide gauges show the combination of astronomical and meteorological tide. During a storm, gauge readings can show surge as a dramatic departure from the predicted tide curve — sometimes several feet above or below prediction hours before landfall.
Tidal Rivers — Freshwater Influence and Salinity Gradients
Where rivers meet the sea, tidal and riverine forces interact to create complex current patterns, salinity stratification, and seasonal variations that affect navigation, anchoring, and passage planning.
Estuarine Dynamics
Salt Wedge
Dense, salty ocean water underlies less-dense freshwater river discharge in a wedge-shaped intrusion that extends upstream beneath the surface flow. During flood tide, the salt wedge moves upstream. During ebb, it retreats. The interface between salt and fresh water is called the halocline or salt front. Vessels moving through a halocline experience abrupt changes in water density, which can affect propeller efficiency and echo sounder readings.
Asymmetric Currents
In tidal rivers with significant freshwater flow, the ebb current lasts longer and runs stronger than the flood current because river discharge adds to the seaward tidal flow. An inlet with 6-hour equal flood and ebb tides becomes a river mouth where ebb may last 7–8 hours and flood only 4–5 hours. This asymmetry affects transit planning: you can ride the flood up and the ebb down, but the timing windows are not equal.
Seasonal Variation
During spring snowmelt and heavy rain, high freshwater discharge can suppress tidal influence, reducing upstream tidal range to near zero and producing a strong, nearly constant ebb current. During late summer drought, river flow drops and saltwater can intrude many miles upstream — affecting water supplies and drastically changing current patterns. NOAA current predictions for tidal rivers include seasonal factors, but actual conditions depend heavily on recent rainfall.
Navigation Implications
Anchoring in a tidal river requires considering current reversal timing and strength. During a strong ebb, anchored vessels swing hard downstream. The flood may be much weaker, so a vessel that seemed well-anchored on flood may drag on a subsequent strong ebb. Always check weather and river flow forecasts when anchoring in a tidal river, not just astronomical tide predictions.
Practice Questions — USCG Exam Format
Work through these problems using the methods described above. Each question mirrors the format and difficulty distribution of the actual USCG captain's license exam.
Question 1 — Rule of Twelfths
MediumTidal range at a station is 12 feet. High water is at 0600. Using the Rule of Twelfths, what is the approximate height of tide at 0800?
Show AnswerHide Answer▶
Question 2 — Depth Calculation
EasyCharted depth at a shoal passage is 4 feet. NOAA tide tables show the height of tide at your planned transit time is +2.5 feet. Can a vessel drawing 6 feet transit safely?
Show AnswerHide Answer▶
Question 3 — Set and Drift
HardYour DR position at 1200 is 41-30N, 070-15W. Your 1200 fix by GPS puts you at 41-30N, 070-12W. The time elapsed since your last fix was 1.0 hour. What is the set and drift of the current?
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Question 4 — Course to Steer
HardYou want to make good a course of 180 True at 6 knots through water. Current is setting 270T at 2 knots. What is the course to steer and what is your speed made good?
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Question 5 — Current Timing
MediumA coastal inlet has maximum ebb current of 4 knots. You plan to enter with 20 feet of water under your keel. The tidal range is 6 feet and the cycle is 6 hours. When after high water is maximum ebb current likely to occur?
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Question 6 — Secondary Stations
MediumNOAA tide tables show high water at a reference station at 0800 with height 6.2 ft. The secondary station you need has a time difference of +35 minutes and a height ratio of 0.85. What is the predicted high water time and height at the secondary station?
Show AnswerHide Answer▶
Pro Tips — Exam Strategy and Common Mistakes
These tips address the most common errors on USCG tides and currents exam questions. Internalize these before exam day.
The exam almost always uses the Rule of Twelfths for tide height problems. Memorize the sequence 1-2-3-3-2-1.
Set is where the current goes TO. Wind direction is where wind comes FROM. They are opposite conventions — the exam tests this distinction.
Slack water lags high and low tide by 1–3 hours at most inlets. Never assume slack water at high tide without checking current tables.
Spring tides occur at new and full moon (when sun and moon align, increasing gravitational pull). Neap tides occur at first and third quarter moon (when sun and moon are at right angles, reducing pull).
On current vector problems, always construct the vector triangle on paper: water track, current vector, course made good. Do not try to do these in your head.
MLLW is used for soundings (depth). MHHW is used for overhead clearances. Mixing these up on the exam is a common error.
When the height of tide is negative (below MLLW), actual depth = charted depth MINUS that value. Negative height of tide means water is lower than normal — subtract it.
Tidal river ebb currents run longer and stronger than flood because freshwater discharge reinforces the ebb. Factor this into your upstream transit planning.
Frequently Asked Questions
What is the Rule of Twelfths and how is it used on the USCG exam?
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What is the difference between tidal height and tidal current?
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What is MLLW and why is it used as chart datum?
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What are set and drift, and how do you calculate them?
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What is the difference between semidiurnal, diurnal, and mixed tides?
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How do you calculate the actual depth of water at a given time?
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What is a storm surge and how does it affect navigation?
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How does freshwater river discharge affect tidal rivers?
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Related Study Guides
Chart Plotting Exam Guide
Dead reckoning, current triangles, running fixes, and vector problems on the chart.
Navigation Study Guide
Comprehensive navigation fundamentals: compass, bearings, piloting, and electronic aids.
Marine Weather Routing
Reading weather systems, gale warnings, routing around storms, and barometer interpretation.
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