Since port construction and maintenance dredging is costly, using maximum vessel draught when transiting harbour channels tends to minimize cargo transportation cost. Thus an economic trade-off has to be made between allowable draught and the dredged water depth in the navigable port area. In addition, safety considerations require that the maximum draught allowed be controlled to avoid vessel grounding.
For a vessel to safely transit a port area she must have adequate clearance under the keel. In the design and operation of ships, as well as in the design and maintenance of ports, the required underkeel clearance (U.K.C.), the term used to define the distance between the ship's bottom and the channel bed, becomes an important economic and safety consideration.
The following paragraphs review previous PIANC work on U.K.C.
The report of Working Group II of the International Oil Tankers Commission (pages 88 to 93 -PIANC -Bulletin no 16, Vol. III 1973) gave available information about channel dimensions and manoeuvring areas for large oil tankers (100.000 to 300.000 tons deadweight) and formulated some recommendations. This same report underlined the need for further research  .
The report of Working Group 4 of the ICORELS (supplement to Bulletin no 35, Vol. 1-1980) in reviewing the relevant recommendations of the 2nd International Oil Tankers Commission came to the conclusion that, without further study and comprehensive site investigations on a comparable basis it was not yet possible to derive a general rule to determine the minimum ships U.K.C. of ships in harbour entrances and in the manoeuvring areas. The report did give an indication of required gross U.K.C. (pages 17 -19) which varied from 7% of the draught (in manoeuvring and mooring areas) to 20% of the draught (in the open sea)  .
In muddy areas the definition of the nautical depth may be difficult. A report, which gives a definition of navigable depth, describes the effects on the manoeuvrability of a ship, and discusses measuring and presentation methods, is published in the PIANC's Bulletin no 43 .
Recognizing that additional work was required on U.K.C., a Working Group was formed under the auspices of the Permanent Technical Committee II of PIANC to define the various factors which influence the vertical clearance under the ship's hull.
This report of the Working Group first identifies all the presently known factors which influence U.K.C.. It includes details for the determination of the ship related factors, but not of the water-level and fairway related factors.
The report also discusses the procedures for the combination of these factors, and methods currently available or under development, for determining the U.K.C.. This information. can be used in the design of channels as part of engineering studies or for the determination, by port operators, of the necessary U.K.C. required bya certain ship in a channel. The methods presented, called deterministic, probabilistic or probabilistic combined with experience factors, should be studied by the user to determine which method is most appropriate for his purposes.
The report is confined to conventional vessel types only such as tankers, bulk-carriers, LNG and container ships as defined by the ICORELS report .