Take 10: Issue 26 Under Keel Clearance

01 Feb 2021 The Navigator

Unclear about Under Keel Clearance? These top ten tips will help answer many of the most pressing questions

1 Nearest hazard 

Charts and radar can be seen as depicting shipping in a twodimensional format. However, the nearest navigational hazard for ships is often under the keel. 

2 Safe navigation 

Understanding how to predict and manage Under Keel Clearance (UKC) is critical to safe navigation. All navigators must be aware of their UKC, how UKC can change and any company or port limitations. 

3 Static 

UKC Quite simply, how much water is left between the bottom of the ship and the seabed in calm static waters. This is easily calculated using known draughts, salinity, height of tide and chart datum. 

4 Dynamic draughts 

Ships don’t always stand still. A ship’s draught can change due to features like squat, roll, pitch and heave. 

5 Squat analysis 

A known physical property where the draught of a ship can become deeper (by metres) with an increase in speed and the proximity of shallow water and narrow channels. Squat can be estimated by tables and/or calculations. It should always be specific to your ship (hull design) and should be assessed at the passage planning stage. 

6 Roll, pitch and heave 

Even slight movement by sea swell can deepen the draught by many metres. This can be particularly important when entering a port across a shallow bar, where an ocean swell is present. 

7 At fixture 

Even navigators can’t control everything! The issue of UKC for individual ports should be addressed at the stage where a vessel is contracted or ‘fixed’ for charter. This will save a great deal of trouble and possible litigation – but it is still important to check, for every port, every time. 

8 Management plan 

UKC can and should be managed. When making a plan, gather the most up-to-date information on depths and bottom contour, ship and company UKC policy, and know when a reduction in speed is necessary to ensure safety. 

9 Be wary 

Not all underwater hazards are charted. Be wary of the possibility of changing depth due to environmental conditions or obstructions such as anchors and debris. The date of the latest hydrographic survey may also be an indication of risk.

10 Complexity Understanding 

UKC and how to manage it can be complex. It is well worth discussing these issues with your bridge team and taking the time to mentor those who are starting out.