Nav Brief - Under Keel Clearance

David Patraiko FNI - Director of Projects, The Nautical Institute

All navigators will be aware of the importance of Under Keel Clearance (UKC) and the basic calculations involved. In its simplest form, static UKC is the space between the bottom of the ship and the seabed. When UKC=0, you are aground – and that’s not usually a good thing.

Calculating the static UKC requires detailed knowledge of chart datum depths through the waterway, tide height at each time and location through the waterway and the ship’s draughts.

All of this is a good start, but how much UKC is actually needed? That is a bit more complicated. Some companies will have a UKC policy, likely included in the Safety Management System (SMS). All navigators on board should be aware of this policy. It may give minimum UKC as an absolute value (e.g. one metre) or as a percentage of the draught (e.g. 15%). It may also vary by situation, such as a minimum 50% of draught in deep water, down to 1.5% alongside. Ports may also mandate a minimum UKC. This should be identified during passage planning.

The story continues. Ships tend to move up and down due to a range of natural forces, all of which need to be taken into account to avoid a grounding. At speed, a ship ‘squats’ deeper into the water, and the resulting difference in UKC can often be measured in metres. There have been many groundings attributed to squat, and squat calculations need to be tailored to your particular ship and speed.

Roll, heel and pitch might also result in some areas of your hull (not necessarily the keel) getting closer to the bottom. Ships can naturally heel due to swell, windage and turning. Even a slight swell can reduce the UKC by many metres given a pitching motion – something to think about when crossing a bar to get into port. Other issues might include reduced depth due to pipelines or other obstructions. Even the nature and stability of the bottom may change, perhaps due to storms or earthquakes.

It is essential that professional navigators understand UKC and how natural forces can reduce the static UKC. These calculations can be complex, but they must form part of the passage plan and the bridge team / pilot exchange.

Today, sophisticated software programs can reduce UKC margins to the safe minimum for commercial purposes. Lowering the draught by even a few centimetres can result in hundreds of tons of additional cargo. Mariners might not be aware of the detail of those calculations, but should always seek to understand the quality of data used, and always check the results against good seamanship.

As with all issues raised in The Navigator, we hope this introduction is the start of a conversation with your bridge teams, in your classrooms and beyond. As always, it pays to learn and share your knowledge through mentoring and collaborative working.