Ship Speed and Weather

The weather and sea conditions that a vessel encounters are some of the most important aspects of how she performs. High winds and adverse seas can soon change a voyage that has been properly performed in accordance with the ship’s warranted speed and consumption into one where there is a substantial shortfall in speed and increase in bunkers consumed. As a result of these potential difficulties the description figures are usually only warranted ‘in good weather conditions’; but what does this mean? If the charterparty is silent on what constitutes good weather it is generally accepted that it applies to sea conditions of Force 4 on the Beaufort Scale and
below. The definition for Force 4 is: Wind Speed 11-16 knots; moderate breeze; small waves becoming longer;
fairly frequent white horses; mean height of waves 5 feet. In conditions above Force 4 the vessel is likely to be adversely affected by the weather conditions with a resulting decrease in speed and greater consumption as a result of the slower passage. Therefore when the vessel’s performance is measured at the end of the voyage or charter only performance during good weather days is taken into account. That does not mean that the vessel’s performance in bad weather is ignored entirely as will be shown later. Of course, it is open to the parties to agree what constitutes good weather in the description or performance clause by stipulating the wind speed which is to count. In the case of some vessels, such as fully laden large oil tankers performing at relatively low (‘economical’) speeds, the effect of wind conditions may well be less and therefore the agreed good weather range might be increased to Force 5 or, possibly, even Force 6. On the other hand, for a moderate-sized vessel which has to perform at relatively high speeds, such as a reefer vessel, Force 3 may be agreed as the limit of good weather.