AWT Weather Routing/Routeing: Applied Weather Technology

How often does good weather exist?

By George Schlinkert

The answer to this question lies in the definition.

Under the NYPE 1946, the description of a vessel states: “...capable of steaming, fully laden, under good weather conditions about ... knots...” The criteria for ascertaining good weather was left open and various assignments were made based on particulars of a vessel or voyage, although Force 4 was the favored standard.

Under the updated NYPE 1993, the description of vessel states: “Speed about .... knots, fully laden, in good weather conditions up to and including maximum Force ... on the Beaufort wind scale, ...” The criteria for good weather was tied to the Beaufort Scale.

More and more clauses and restrictions have subsequently been added such that the speed description of a vessel may sometimes reflect a sea trial result rather than what a vessel can accomplish in realistic and measurable conditions. The following clause depicts this trend:

ABT 19.0 KNOTS ON ABT 36.5 MTS IFO PLUS ABOUT 2.5 MTS MGO AT DESIGN DRAFT OF 7.6 M BASIS CLEAN AND SMOOTH BOTTOM, EVEN KEEL, DEEP AND CURRENTLESS WATER/SEA WITH A TEMPERATURE OF MAX 28 DEGREES CELSIUS, WIND MAX BAUFORT 2 AND SEA NOT EXCEEDING DOUGLAS SEA STATE 2...

From the point of view of the weather, it is instructive to address what impact the additional restrictions place on the chance of good weather prevailing along a route. The following samples show the percentage that the winds/waves are less than or equal to the terms for 12 consecutive hours, where the height of the wave is the significant combined wave height.

	BF4/2m	BF4/1.25m	BF3/1.25m	BF2/1.25m
October
BZ/ARG – Cape	32.2	4.0	2.1	0.5
Cape – Indonesia	16.4	0.7	0.5	0.1
Gibraltar – USG	48.2	18.3	9.6	2.3
40S-55N worldwide	41.3	16.5	10.4	3.2
January
BZ/ARG – Cape	48.4	10.7	5.8	1.2
Cape – Indonesia	52.9	9.5	6.7	2.0
Gibraltar – USG	22.1	8.6	3.8	0.9
40S-55N worldwide	35.8	13.8	7.5	1.9
July
BZ/ARG – Cape	14.6	1.3	0.8	0.2
Cape – Indonesia	1.6	0.1	0.8	0.2
Gibraltar – USG	69.9	45.6	23.8	5.8
40S-55N worldwide	42.2	19.0	11.0	3.1

The Douglas Sea State has been the subject of various interpretations. One can readily see the affect that tighter definitions and interpretations of good weather terms have on the number of good weather days. The above figures do not take into account any additional limitations from ocean currents, sea surface temperature, ocean currents, or vessel loading condition.