Tide Fundamentals
High Tide
Maximum water level in the cycle. Tidal current is at or near slack (weakest) around high tide.
Low Tide
Minimum water level. Shallowest conditions — most dangerous for draft-sensitive navigation.
Tidal Range
Difference between high and low tide. Ranges from <1 ft (Gulf Coast, diurnal) to 50+ ft (Bay of Fundy).
Tide Types by Location
| Type | Highs/Lows Per Day | Where Found | Characteristic |
|---|---|---|---|
| Semidiurnal | 2 highs, 2 lows | East Coast U.S., most of world | Two similar tides/day; ~6h 12min between |
| Diurnal | 1 high, 1 low | Gulf of Mexico | One tide cycle per day (~24h 50min) |
| Mixed semidiurnal | 2 highs, 2 lows (unequal) | West Coast U.S. | Higher high water, lower low water |
Spring & Neap Tides
Spring Tides
Occur during new moon and full moon when the sun, earth, and moon align (syzygy). Gravitational forces add together.
- • Higher high tides, lower low tides
- • Greatest tidal range
- • Strongest tidal currents
- • Twice per lunar month (~every 14 days)
Neap Tides
Occur during first and third quarter moons when sun and moon are 90° apart. Forces partially cancel.
- • Lower high tides, higher low tides
- • Smallest tidal range
- • Weakest tidal currents
- • Twice per lunar month
Rule of Twelfths
Estimates tidal height at any hour during the 6-hour rise or fall cycle. Assumes a sinusoidal tide and a 6-hour cycle (close enough for exam purposes).
| Hour of Cycle | Tide Rises (or Falls) | Fraction of Range | Cumulative |
|---|---|---|---|
| 1st hour | 1/12 of range | 1/12 | 1/12 |
| 2nd hour | 2/12 of range | 2/12 | 3/12 |
| 3rd hour | 3/12 of range | 3/12 | 6/12 (halfway) |
| 4th hour | 3/12 of range | 3/12 | 9/12 |
| 5th hour | 2/12 of range | 2/12 | 11/12 |
| 6th hour | 1/12 of range | 1/12 | 12/12 (complete) |
Worked Example
Low tide = 1.0 ft at 0600. High tide = 7.0 ft at 1200. What is the depth at 0900?
Tidal range = 7.0 − 1.0 = 6.0 ft
0900 = 3 hours after low (3rd hour complete)
Cumulative rise after 3 hours = 1/12 + 2/12 + 3/12 = 6/12 = ½ of range
Rise = 6.0 × ½ = 3.0 ft
Height at 0900 = 1.0 + 3.0 = 4.0 ft
To find actual depth at a location, add the tidal height to the charted depth (which is referenced to MLLW).
Tidal Current vs Tide
| Concept | Tide | Tidal Current |
|---|---|---|
| Definition | Vertical rise and fall of water level | Horizontal flow of water |
| Strongest at | High and low tide (range greatest) | Mid-tide (between H and L) |
| Weakest at | Mid-tide | High and low tide (slack water) |
| Table used | Tide Tables (NOAA) | Tidal Current Tables (NOAA) |
| Direction | Up and down | Flood (toward land) / Ebb (away from land) |
Flood Current
Water flowing landward (into harbors, up rivers). Generally occurring while the tide is rising.
Ebb Current
Water flowing seaward (out of harbors, down rivers). Generally occurring while the tide is falling.
Set, Drift & the Current Triangle
Key Terms
Set
Direction the current flows toward (not from). A set of 045° means current flows toward 045° (NE).
Drift
Speed of the current in knots.
Course Made Good (CMG)
Your actual path over the ground, accounting for current.
Speed Made Good (SMG)
Your actual speed over ground.
Course to Steer (CTS)
The heading you must steer to achieve your desired track despite current.
Current Triangle Method
Plot your intended course and boat speed vector from point A
From the end of that vector, plot the current vector (set direction, drift speed)
The line from A to the end of the current vector = CMG and SMG
To find CTS: from point A, draw a line of boat-speed length to the current vector — that angle is your CTS
Worked Example
Boat speed 10 kts on course 090°T. Current: set 180°, drift 3 kts. Find CMG and SMG.
Boat vector: 10 kts due East (090°)
Current vector: 3 kts due South (180°)
Resultant (CMG): use vector addition — East 10, South 3
SMG = √(10² + 3²) = √109 ≈ 10.4 kts
CMG = 090° + arctan(3/10) = 090° + 16.7° ≈ 107°T
On the OUPV exam, current triangles are solved graphically with parallel rulers and dividers on the chart.
Using Tide & Current Tables
NOAA Tide Tables
- • List times and heights of high/low tides
- • Reference station + subordinate station corrections
- • Height corrections: add/subtract from reference
- • Time corrections: apply to reference station time
- • Heights referenced to MLLW (chart datum)
- • Free at tidesandcurrents.noaa.gov
NOAA Current Tables
- • List times and speeds of max flood/ebb and slack water
- • Speed in knots (set direction given separately)
- • Reference station + subordinate corrections
- • Speed corrections: multiply by ratio factor
- • Time corrections: add/subtract minutes
- • Critical for passage planning through tidal channels
Exam Strategy
1-2-3-3-2-1 for Rule of Twelfths
Memorize the sequence. The 3rd and 4th hours account for half the total change — that's when current is also at maximum.
Set = direction TO, not FROM
A current with set 270° is flowing west. This is opposite of wind, which is named for where it comes from. Exam questions exploit this distinction.
Slack water ≈ high and low tide
Not exactly, but close enough for exam purposes. Strongest currents occur near mid-tide, not at the tidal extremes.
Frequently Asked Questions
What is the Rule of Twelfths?
The Rule of Twelfths estimates tidal height at any time during a 6-hour tide cycle (from low to high or high to low). In the 1st hour the tide rises 1/12 of its total range; in the 2nd hour 2/12; in the 3rd hour 3/12; 4th hour 3/12; 5th hour 2/12; 6th hour 1/12. This gives the characteristic S-curve shape of a tidal cycle.
What is the difference between tide and tidal current?
Tide is the vertical rise and fall of water level caused by the gravitational pull of the moon and sun. Tidal current is the horizontal flow of water caused by tidal forces. They are related but not simultaneous — tidal currents are strongest at mid-tide (between high and low), while currents are weakest (slack water) near high and low tide.
How do I correct for current when navigating?
Use the current (set and drift) triangle. Set is the direction the current flows toward. Drift is the current's speed in knots. To find your Course Made Good (CMG) and Speed Made Good (SMG), plot your intended course and speed vector, then add the current vector. The resultant vector is your actual track and speed over ground.
Related Study Guides
Practice Tides & Currents Questions
Work through OUPV-style Rule of Twelfths, current triangle, and tidal height questions with instant feedback — free on NailTheTest.
Start Practicing Free