4.9 Tides: Tables, Datums & Height of Tide

Tides on the Navigation General Module

Q162 Navigation General — Near Coastal tests tides as professional planning tools, not as astronomy trivia. A Master Less Than 100 GRT on coastal or inlet waters decides when a bar is passable, when a dock is dry under the keel, and whether a whale-watch departure still has enough water under the hull at the ramp. Get the vocabulary and the arithmetic right: tide is the vertical rise and fall of the water surface; current is the horizontal flow of water. Mixing the two terms is a common exam and bridge-watch error.

Tide Patterns: Semi-Diurnal, Diurnal, Mixed

Tides are driven primarily by the gravitational pull of the Moon (with a smaller solar component). Over a roughly 24-hour-50-minute lunar day, different coasts show different patterns:

Exam language you must decode:

• High water (HW) / high tide: the local maximum water level of a rising tide cycle
• Low water (LW) / low tide: the local minimum of a falling cycle
• Stand: the brief period at high or low water when the vertical change is nearly zero — the water is "standing" before it reverses
• Range: the vertical difference between a high and the following low (or as defined in the table for that cycle)
• Spring tides: larger ranges near new and full moon (Sun and Moon aligned)
• Neap tides: smaller ranges near first and third quarters (Sun and Moon at right angles)

You do not need orbital mechanics on Q162. You need to read a tide table, recognize the pattern, and apply the height of tide to charted depths and clearances.

Chart Datum and Height of Tide

A nautical chart's soundings (depths) are measured from a chart datum — a reference plane for that chart. On most modern U.S. charts, the sounding datum is Mean Lower Low Water (MLLW): the average of the lower of the two daily low waters over a tidal epoch. That choice is conservative for depth: most of the time the water is at least as deep as the charted sounding (plus any positive height of tide), though weather, setup, and local anomalies can still make water lower than predicted.

Height of tide is the vertical distance of the actual (or predicted) water surface above (or, rarely for negative tides, below) the chart datum. Tide tables and software publish predicted heights of tide at reference stations and apply corrections for subordinate stations.

Depth of water — the formula that appears on exams

A standard working relationship is:

Depth of water ≈ charted depth + height of tide

Example: charted depth 8 feet, height of tide +4 feet → depth of water about 12 feet at that time and place (ignoring wave troughs, surge, and chart uncertainty).

For under-keel clearance thinking, subtract draft (and a safety margin for squat, sea state, and chart error):

Clearance under keel ≈ (charted depth + height of tide) − draft − margin

If clearance is near zero or negative, you do not go — or you wait for more tide. Master 100 passenger ops add one more layer: a grounded COI vessel is both a safety event and a regulatory/commercial disaster. Plan bar crossings and shallow approaches with tide first, ego second.

Height of tide is not the same as "how much the water rose today"

Height of tide is always relative to datum, not relative to "this morning's low." If MLLW is the datum and the table says height of tide is 3.2 feet, the water surface is 3.2 feet above MLLW, whether or not the last low was 0.5 feet or −0.2 feet. When the table shows a negative height of tide, the water is below chart datum — depths are less than charted.

Tide Tables and Predictions

Professional sources include NOAA tide tables / CO-OPS predictions, printed Tide Tables, chartplotter/weather apps that pull official predictions, and local subordinate station corrections from a reference station. For exam and watch use:

1. Select the correct station (or nearest reference + correction).
2. Read times and heights of high and low water for the date.
3. Interpolate if you need height of tide at a time between tabulated highs and lows (rule-of-twelfths is a rough field method; better tools use curves or software).
4. Apply any secondary station time/height differences listed for your inlet or dock.
5. Re-check when weather (strong onshore wind, seiche, river flood) can pile water up or blow it out of a bay — predictions assume average meteorological conditions.

Rule of twelfths (awareness only): in a semi-diurnal cycle of roughly 6 hours of rise, the tide often rises about 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the range in successive hours. It is a rough field estimate, not a substitute for a proper table when planning passenger bar crossings.

Bridge Clearance

Fixed bridges show a charted vertical clearance measured from a stated vertical datum (often Mean High Water or another high-water reference on U.S. products — always read the chart notes). Conceptual exam logic:

• At a higher stage of tide, the water surface is higher → less air gap under a fixed span.
• At a lower stage of tide, air gap increases.

A simplified teaching model when charted clearance and tide height share a consistent high-water relationship taught in exam problems is:

Usable clearance ≈ charted vertical clearance − (height of water surface above the clearance datum)

In plain Master language: more tide under the keel can mean less room over the radar arch. Before you commit a flybridge or mast under a span, know air draft, charted clearance, datum, and today's height of tide. Never assume the number painted on the bridge is good at every stage of tide without checking the reference.

Master 100: Bars, Inlets, and Tide Windows

Near-coastal passenger masters live on tide windows:

• Inlet and bar crossings: breaking seas on a shallow bar are often worst near low water and on ebb against an onshore wind/swell. Many operators prefer higher water and a favorable current window (current chapter next) for passenger comfort and under-keel safety.
• Docks and ramps: low tide can leave a vessel aground at the berth or make the boarding ramp unsafe for elderly passengers.
• Shallow fishing grounds: charted 6 feet plus 1 foot of tide may still be thin for a vessel drawing 4.5 feet once waves trough and you add a margin.
• Haul-outs and bottom work: deliberately use low water for drying work only when the vessel is designed and planned for it.

Decision checklist for the master

1. What is charted depth on the track and at the berth?
2. What is height of tide at ETA and at ETD (return may be lower)?
3. What is draft, plus squat and sea-state margin?
4. Is there a bar or inlet where low water + swell = no-go for passengers?
5. Any bridge on the route that needs clearance at this tide?
6. Document the plan in the voyage briefing so the mate and lookout know why you delayed departure 45 minutes.

Tides are free energy and free risk. Q162 asks whether you can convert tables + datum + draft into a go / wait / reroute decision. That is command, not hobby astronomy.