Tides Practice Questions

1. The chart datum used on NOAA nautical charts for depth soundings is:

• A.Mean High Water (MHW)
• B.Mean Sea Level (MSL)
• C.Mean Lower Low Water (MLLW)✓
• D.Lowest Astronomical Tide (LAT)

Why: NOAA uses Mean Lower Low Water (MLLW) as the chart datum for soundings in US waters. Depths on the chart are referenced to MLLW — the average height of the lower of the two daily low tides. This means actual depths at high tide are greater than charted, providing a safety margin.

2. The total tidal range at a port is 12 feet. Using the Rule of Twelfths, how many feet does the tide rise during the THIRD hour after low water?

• A.1 foot
• B.2 feet
• C.3 feet✓
• D.4 feet

Why: Rule of Twelfths: 3rd hour = 3/12 of range. Range = 12 ft. Rise in 3rd hour = (3/12) × 12 = 3 feet. Hours: 1st=1ft, 2nd=2ft, 3rd=3ft, 4th=3ft, 5th=2ft, 6th=1ft (total=12ft).

3. What is the height of tide formula for finding actual depth of water at any time?

• A.Depth = Charted depth + Height of tide✓
• B.Depth = Charted depth − Height of tide
• C.Depth = Draft + Height of tide
• D.Depth = Charted depth × Tidal range

Why: Actual depth = Charted depth + Height of tide above datum. Since chart soundings are referenced to MLLW (near the lowest expected water level), adding the current height of tide above that datum gives the actual available depth.

4. At a mixed semidiurnal tidal station, how many high waters and low waters typically occur each day?

• A.One high water and one low water
• B.Two high waters and two low waters of equal height
• C.Three high waters and three low waters
• D.Two unequal high waters and two unequal low waters✓

Why: A mixed semidiurnal tide has two highs and two lows per tidal day, but the heights are unequal — a higher high water, lower high water, higher low water, and lower low water. This pattern is common on the US West Coast and Hawaii.

5. What does Mean Lower Low Water (MLLW) represent?

• A.The average height of all low waters at a given location
• B.The average height of the lower of the two daily low tides over a 19-year tidal epoch✓
• C.The lowest tide ever recorded at a location
• D.The average height between high water and low water

Why: MLLW is the average of the lower low waters observed over the National Tidal Datum Epoch (a 19-year period), used as the reference datum for chart soundings in US coastal waters. It is not the lowest possible tide, but the average of the lower low tides.