Tides are the regular rise and fall of sea level caused mainly by the gravitational pull of the Moon and the Sun on Earth’s oceans. For ships and submarines, tides are not just a background detail because they change water depth, current speed, and safe travel windows. A harbor that is deep enough at high tide may be too shallow at low tide for a loaded cargo ship.
Careful tide planning helps prevent groundings, delays, and dangerous navigation through narrow channels.
Key Facts
- Under-keel clearance = water depth - ship draft
- Safe water depth = ship draft + required clearance
- Tide height above chart datum adds to charted depth: actual depth = charted depth + tide height
- Tidal range = high tide level - low tide level
- Spring tides occur near new moon and full moon and usually have the largest tidal range
- Neap tides occur near first quarter and third quarter moons and usually have the smallest tidal range
Vocabulary
- Tide
- A tide is the periodic rise and fall of sea level caused mainly by the Moon's and Sun's gravity.
- Draft
- Draft is the vertical distance from a vessel's waterline to the lowest part of its hull or keel.
- Under-keel clearance
- Under-keel clearance is the amount of water between the bottom of a vessel and the seafloor.
- Tidal current
- A tidal current is the horizontal movement of water caused by the rising and falling tide.
- Chart datum
- Chart datum is the reference water level used on nautical charts to show charted depths.
Common Mistakes to Avoid
- Using charted depth as the actual depth is wrong because the real water depth changes with tide height. Add the predicted tide height to the charted depth to estimate actual depth.
- Ignoring ship draft is wrong because a ship can touch bottom even when the water looks deep. Always compare actual depth with draft plus required under-keel clearance.
- Planning only for tide height is wrong because tidal currents can make docking, turning, or channel transit harder. Check both water level and current timing before entering a harbor.
- Assuming high tide is always the safest time is wrong because strong currents near some tide stages can create navigation risks. The safest window may be near slack water, when current speed is low.
Practice Questions
- 1 A harbor channel has a charted depth of 9.0 m. The predicted tide height is 2.4 m. A cargo ship has a draft of 10.5 m. What is the actual water depth, and what is the under-keel clearance?
- 2 A submarine needs at least 15 m of water above its highest point and 8 m below its keel while submerged. Its vertical height is 12 m. What minimum total water depth is required for safe operation?
- 3 A large cargo ship must enter a narrow harbor with enough depth and also avoid strong sideways tidal currents. Explain why the captain might choose a time near high tide but close to slack water rather than simply entering at the highest predicted tide.