Why tides matter for skippers
Tides govern depth, current, bridge clearances, and whether you can berth safely. High water (HW) and low water (LW) are the anchor points — between them you plan time windows for harbour manoeuvres, passages over shoals, or anchoring.
This article covers the basics. For the in-app calculator: Tides & tide calculator.
HW and LW — what the terms mean
| Term | Meaning |
|---|---|
| HW (high water) | Highest water level in the tidal cycle |
| LW (low water) | Lowest water level |
| Tidal range | Difference between HW and LW |
| Tidal period | Time between two successive HW (approx. 12 h 25 min for semidiurnal tides) |
On charts and in tide tables you often see heights above chart datum — not always “above mean sea level”. Always read the legend for your source.
Planning time windows
Typical questions before departure:
- Is there enough depth at LW in the channel?
- Does the bridge clear at HW?
- When is current strongest (often between HW and LW)?
Combine tides with ETA planning and, where relevant, current allowance.
Offline vs. live tides
NauticCalc offers two approaches:
- Offline: harmonic prediction for ~1,000 stations (focus USA/Pacific)
- Live: WorldTides with your own API key for the Mediterranean, North Sea, and worldwide
Details in Offline tides vs. live.
Common mistakes
- Wrong station selected (the bay next door has a different range)
- Time zone forgotten on longer passages
- Official tide table replaced by app values — for critical manoeuvres, always verify the official source
- Confusing tides with weather-driven storm surge
Summary
Reading tides means knowing HW/LW, deriving time windows, and choosing the right data source. With offline or live data in NauticCalc you can plan harbour and passage legs in a traceable way.
Further reading: Tide calculator · Offline navigation · Sailing without internet