# Under-Keel Clearance: Navigating Safely Over Shoals
Planning a passage does not end with drawing a course on the chart. For the coastal navigator, one of the most critical calculations is the under-keel clearance: knowing at what time there will be enough depth to cross a critical point without grounding.
# What is the chart sounding and how does it relate to the tide?
On nautical charts, depths (soundings) are referenced to Chart Datum. This level is, by convention, the lowest level the tide reaches under normal conditions. The actual depth your boat will find at any given moment is the sum of that sounding plus the tide height at that instant, calculated using the Rule of Twelfths.
# The elements of the calculation: Draft and Margin
To use the under-keel clearance calculator professionally, you need to know two technical details about your boat and one personal decision:
# How does the passage window work?
The calculator determines the exact moment when the water level will rise above your draft plus the chosen safety margin. From that minute, the passage window opens. If the tide is ebbing (falling), the tool will indicate until what time transit is permitted before the water level drops below your safety limit.
This planning is vital for sailing boats with deep keels wishing to enter ports such as those in the Rias Baixas in Galicia or river ports in Andalusia, where tidal variation can make the difference between a comfortable entry or being stranded for six hours waiting for the next cycle.
# Factors affecting the actual passage
| Factor | Effect | Recommendation |
|---|---|---|
| Swell | Reduces water under the hull | Increase safety margin significantly |
| Squat effect | Increases draft through suction | Reduce speed in critical areas |
| Sandy bottom | Depth variable with storms | Keep active sounder and extra margin |