The Under Keel Clearance (UKC) allowance of 10% of a ship’s draft (amount of water between the vessel’s hull and the sea bottom) has been the topic of significant discussion with many groups stating that tankers with just 1.3m under the keel and a deep draft of 13m are allowed to pass through the Second Narrows.
This is not correct.
The under-keel clearance at Second Narrows is closer to 13m between the hull and the sea bottom. The 10% allowance is only applicable at the extreme of the allowable channel width which is not where the ships are when transiting the Narrows. The pilots also use their Personal Pilotage Units to ensure that the vessel remains in the centre of the channel which gives significantly more water under the keel than the 10%. The green line in the diagram below is the required 10% but as can be seen the actual under keel clearance is over 12m.
Under Keel Clearance – Minimum 10% of Draft
Narrowest Section of 2nd Narrows Movement Restricted Area
Basis a depth below datum of 22.0 metres, a tide height of 4.60 metres and a draft of 13.5 metres, the actual Under Keel Clearance below the vessel is in excess of 13 metres. Only the outer edge of the 120-metre channel (the green band) is 10%.