Safety Depth is calculated to ensure a vessel has sufficient clearance to navigate safely in a waterway. You solve for Safety Depth by using the following formula:
Safety Depth = Maximum Draft (static) + UKC (Company's Policy) + Squat (Maximum) – Height of Tide
Here's a breakdown of each component:
-
Maximum Draft (static): This is the deepest point of the vessel below the waterline when it is stationary in calm water. It's a fixed value determined by the loading of the vessel.
-
UKC (Under Keel Clearance) (Company's Policy): This is the minimum required distance between the bottom of the ship's keel and the seabed. This value is often dictated by the company's internal policies and considers factors such as the type of seabed, accuracy of charts, and acceptable risk level. A common UKC might be 0.5 meters or more, depending on these factors.
-
Squat (Maximum): Squat is the decrease in under keel clearance caused by the vessel's movement through the water. As a ship moves, it creates pressure changes around the hull. This results in the ship settling lower in the water (squatting). Squat is greatest in shallow water and at higher speeds. Estimating maximum squat requires calculations or approximations based on the vessel's speed, block coefficient, and the water depth. There are various formulas available, such as the Barrass formula. It is crucial to estimate the maximum squat expected.
-
Height of Tide: This is the predicted height of the tide at the time of passage. It represents the amount of water available above the charted depth. We subtract the height of the tide from the other values in the formula because the charted depth is based on a low water datum. A higher tide increases the available depth, thus decreasing the required safety depth relative to that charted depth.
Example:
Let's say:
- Maximum Draft = 10 meters
- UKC (Company Policy) = 1 meter
- Maximum Squat = 0.5 meters
- Height of Tide = 2 meters
Then:
Safety Depth = 10 m + 1 m + 0.5 m - 2 m = 9.5 meters
This means that, according to this calculation, the minimum required depth of water (charted depth plus tide) for safe passage must be at least 9.5 meters.
Important Considerations:
-
Accuracy: The accuracy of each input is crucial. Ensure accurate draft readings, reliable tide predictions, and realistic squat estimations.
-
Real-time Conditions: Always be aware of real-time conditions that could affect safety depth, such as unexpected changes in tide, weather conditions, or inaccurate charts.
-
Chart Datum: Understand the chart datum used on the nautical chart. Charted depths are usually referenced to Lowest Astronomical Tide (LAT) or a similar low water level.
Calculating Safety Depth is critical for safe navigation, ensuring a vessel avoids grounding and minimizes the risk of damage to the vessel or the environment. Always consult with experienced navigators and utilize approved resources for accurate calculations.
