Douglas Scale Ship Speed

Apart from the wind conditions sea state can also play a part in performance during a voyage. In the Baltime form the words ‘smooth water‘ are used as well as good weather. In the aftermath of severe weather the sea can be quite rough for some time even though the wind has died down. Equally in some parts of the world very large swells can be encountered which will affect performance despite relatively calm weather conditions. The sea state is measured by the Douglas Scale (which actually deals with sea waves and swell separately) and is generally designated at scale 3 and below, slight and light, above which days are not included. If water conditions are not mentioned it is very unusual for them to be considered in the calculation of vessel performance. If owners are aware that the vessel is likely to encounter, say, heavy swell on a single time charter trip it is well worth including a provision to deal with this in relation to performance. The usual source for information about the weather encountered by the vessel is from the ship’s logs and any other recording devices on the vessel. There will also be historical records of weather conditions available from bodies such as the Meteorological Office in the United Kingdom and other national organisations. The weather will, on most vessels, be recorded on a watch-by-watch basis which will show the day’s overall conditions. Very often the master will report daily to owners giving the conditions and distance covered. Since the charterers are very concerned with the ship’s performance they may wish to have a degree of control over the potential consequences of bad weather on the vessel. Since they cannot control the weather they must have some say in the handling of the vessel in response to it. As a result there have grown up what are called weather or ocean routeing companies. They will provide the charterers with forecasting of potential weather and sea conditions that a vessel is likely to encounter on passage and suggest a route that minimises the impact of the weather. One of the best examples of where weather routeing may assist is on Trans-Pacific voyages. Although taking a northerly passage will be shorter, if it is likely that the vessel will encounter adverse conditions for much of her passage then the routeing company may advise a more southerly course which, although longer, may take less time because of more favourable conditions. In some cases the agreement in the charterparty about the use of routeing company advice may stipulate that it is just that, advisory, and that the final decision about navigation is left with the master; on other occasions the clause may provide that the master is to accept and act on the company’s advice as if they were charterers’ orders. The owners may be entitled to an indemnity if the vessel suffers loss as a result of following these orders. Weather routeing companies will also prepare post-voyage or charter analyses of vessel’s speed and consumption using their own data rather than relying directly on the logs. The information that they use may be from meteorological organisations, reports from other vessel’s or experience tables and records. If this is the case and a claim is made based on the company’s data there may be a conflict with the ship’s own recorded material. Unless there is a provision to the contrary in the charter, London arbitrators will generally rely on the ship’s logs as the primary source of the weather records.