Tide
What causes tides, how to read them, and how they affect your passage
What Causes Tides
Tides are the rise and fall of sea level caused by the gravitational pull of the Moon and, to a lesser extent, the Sun acting on Earth's oceans. The Moon pulls the ocean toward it on the near side of Earth, and inertia creates a corresponding bulge on the far side. As Earth rotates, coastal locations pass through these bulges — creating the tidal cycle.
Most locations experience two high tides and two low tides per day (semidiurnal tide), separated by approximately 6 hours 12 minutes. Some areas — particularly parts of the Gulf of Mexico — experience only one high and one low tide per day (diurnal tide). Mixed patterns with unequal highs and lows are also common along the US Pacific Coast.
The tidal cycle is not exactly 24 hours because the Moon orbits Earth once per month, shifting the timing of high water by about 50 minutes later each day. A tide table is the authoritative source for your specific location — never assume tides follow a predictable 6-hour pattern at a given port without checking.
Why does high tide occur approximately 50 minutes later each day?
A port shows only one high and one low tide per day. This tidal pattern is called:
Spring and Neap Tides
Tidal range — the difference between high and low water — varies with the Moon's phase. When the Sun and Moon are aligned (full moon or new moon), their gravitational forces combine to produce spring tides: the highest high tides and lowest low tides of the month. When the Sun and Moon are at right angles (quarter moon), their forces partially cancel, producing neap tides: smaller tidal ranges with less variation between high and low.
Spring tides can expose hazards normally covered, create stronger tidal currents, and allow access to areas that are too shallow at neap low water. Neap tides are milder and more predictable. A passage that works at neap low water may be impossible — or dangerously fast — at spring tides.
The relationship between lunar phase and tidal range is consistent but delayed: the maximum spring tides typically occur 1–2 days after the full or new moon, not on the day itself. This lag (called the age of the tide) varies by location.
When planning a passage that involves a critical depth clearance, look up the tidal range for that day. Spring tides near new or full moon are when you'll see the lowest low waters of the month — both an opportunity (more water at high) and a risk (less water at low).
Spring tides occur when:
You need to cross a bar with a charted depth of 5 feet at a negative tide. Your draft is 4.5 feet. Which tidal condition is most dangerous?
Reading Tide Tables
Tide tables list the predicted times and heights of high and low water at reference stations for each day of the year. Secondary stations nearby have correction factors applied to the reference station times and heights. NOAA publishes free online tide predictions at any US location; tide tables are also available as apps, almanacs, and publications.
Tide table heights are listed in feet or meters above chart datum (MLLW). A height of 0.0 is chart datum itself; negative values mean water is below chart datum — the lowest expected level is less than charted. Heights above 0 add on top of charted depths.
The rule of twelfths provides a quick estimate of tide height at any time between high and low without interpolation tables: divide the tidal range into 12 parts. In the first hour after low, 1/12 of the range rises. Second hour: 2/12. Third and fourth hours: 3/12 each. Fifth hour: 2/12. Sixth hour: 1/12. The tide rises fastest in the middle two hours.
Low water: 0.5 feet at 0800. High water: 4.5 feet at 1400. Tidal range = 4.0 feet.
At 1000 (2 hours after low): 0.5 + (1/12 + 2/12) × 4.0 = 0.5 + 1.0 = 1.5 feet above datum.
At 1100 (3 hours after low): 0.5 + (1+2+3)/12 × 4.0 = 0.5 + 2.0 = 2.5 feet.
The mid-flood period (hours 3–4) is where the tide rises fastest — 3 feet per hour in this example. Plan depth-critical passages away from these hours if possible.
A tide table shows high water at 4.2 feet and low water at -0.5 feet. What does the negative value mean?
Using the rule of twelfths, in which hours after low water does the tide rise fastest?
Tides and Passage Planning
Timing a passage around the tide is one of the most practical navigation skills. Arriving at high water gives maximum depth for entering shallow harbors, crossing bars, or anchoring. Leaving on the ebb at harbors with strong tidal currents lets the current assist departure. Arriving at low water exposes hazards you can see — useful when exploring a new anchorage at the best possible visibility of the bottom.
Adverse currents can significantly affect passage time. A 3-knot current against a 6-knot boat halves the boat's effective speed over ground. Planning to ride a favorable tide — or at least avoid the strongest adverse period — can shave hours off a coastal passage.
Always plan your critical depth moments — bar crossings, channel transits, harbor entries — to coincide with sufficient tide height. Calculate the needed water depth (draft + safety margin), subtract the charted depth, and determine the minimum tide height required. Then check the tide table to confirm that height will be present at your estimated arrival time.
Your boat draws 6 feet. The bar you need to cross has a charted depth of 4 feet. What minimum tide height above datum do you need (with 1.5-foot safety margin)?
A harbor has a 2-knot tidal current running on the flood. Your engine pushes the boat at 5 knots. What is your speed over ground against a full flood?
Summary
Tides are caused by the Moon's (and Sun's) gravitational pull on Earth's oceans. Most locations have two high and two low tides per day.
High water occurs about 50 minutes later each day because the Moon orbits Earth, shifting its position relative to any location.
Spring tides (new/full moon) have the greatest range — highest highs and lowest lows. Neap tides (quarter moon) have the smallest range.
Tide table heights are measured above chart datum. Negative values mean water is below datum — shallower than charted depth.
The rule of twelfths: tide rises 1, 2, 3, 3, 2, 1 twelfths of its range per hour — fastest in the middle of the cycle.
Key Terms
- Tidal range
- The difference in height between high water and the following low water
- Spring tide
- Tides with the greatest range, occurring at new and full moon when Sun and Moon align
- Neap tide
- Tides with the smallest range, occurring at quarter moon when Sun and Moon are at right angles
- Semidiurnal tide
- A tidal pattern with two highs and two lows per day — the most common type
- Diurnal tide
- A tidal pattern with only one high and one low per day
- Chart datum
- The reference water level from which charted depths are measured — typically MLLW in the US
- Negative tide
- A tide that falls below chart datum — water is shallower than the charted depth
- Rule of twelfths
- A rule of thumb for estimating tidal height at any point in the cycle: 1, 2, 3, 3, 2, 1 twelfths per hour
- Age of the tide
- The delay between the lunar phase (new/full moon) and the resulting maximum spring tide — typically 1–2 days
Tide — Quiz
Low water is at 0600. The next high water is at 1200. Tidal range is 6 feet. Using the rule of twelfths, approximately how much tide has risen by 0800?
What is the primary difference between spring and neap tides?
A tide table shows the predicted low water as −0.8 feet. The charted depth at a bar is 3.0 feet. What is the actual depth at this low water?
Your boat draws 5 feet. A harbor entrance has a charted depth of 3 feet. The current tide height is 2.5 feet. Can you enter safely with a 1-foot margin?
Why does high tide occur approximately 50 minutes later each day at the same location?
References & Resources
Related Links
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NOAA Tides & Currents — Free Tide Predictions
Free NOAA tide predictions for thousands of US locations, plus tidal current data.
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US Sailing — Coastal Navigation Course
Formal certification course in coastal navigation including tides, currents, and passage planning.