The mariner planning a transoceanic passage can select either the shortest route, or the quickest route at a given speed, or the most suitable route from the point of view of weather or any particular requirements.
Climatic condition, however, such as the existence of currents or the prevalence of wind, sea or swell form certain directions, may lead to the selection of a longer climatological route along which a higher speed can be expected to be made good.
Weather routing is done by collection of oceanographical and meteorological data, and data received from weather satellites and good forecasting techniques. The mariner’s first resources for route planning in relation to weather are the Pilot Chart Atlases, the Sailing Directions (Planning Guides), and other climatological sources such as historical weather data tables. These publications give climatic data, such as wind speed and direction, wave height frequencies and ice limits, for the major ocean basins of the world. They may recommend specific routes based on probabilities, but not on specific conditions.
Weather routing makes use of the actual weather and the forecast weather in the vicinity of the anticipated route. By using weather forecasts to select a route, and then modifying the route as necessary as the voyage proceeds and decide the optimum route.
Optimum ship routing is the art and science of developing the “best route” for a ship based on the existing weather forecasts, ship characteristics, ocean currents and special cargo requirements. For most transits this will mean the minimum transit time that avoids significant risk to the vessel, crew and cargo. Other routing considerations might include passenger comfort, fuel savings or schedule keeping. The goal is not to avoid all adverse weather but to find the best balance to minimise time of transit and fuel consumption without placing the vessel at risk to weather damage or crew injury.
The ship routing agency, acting as an advisory service, attempts to avoid or reduce the effects of specific adverse weather and sea conditions by issuing initial route recommendations prior to sailing. Adverse weather and sea conditions are defined as those conditions which will cause damage, significant speed reduction, or time loss.
Weather routing can be classified as follows
- Climatological Routing
- Strategic Routing
- Tactical Routing
A significant advantage of weather routing accrues when:
- the passage is relatively long, about 1,500 miles or more;
- the waters are navigationally unrestricted, so that there is a choice of routes; and
- weather is a factor in determining the route to be followed.
Ship and cargo characteristics have a significant influence on the application of ship weather routing. Ship size, speed capability, and type of cargo are important considerations in the route selection process prior to sailing and the surveillance procedure while underway. A ship’s characteristics identify its vulnerability to adverse conditions and its ability to avoid them.
Ship performance curves (speed curves) are used to estimate the ship’s Speed of Advance (SOA) while transiting the forecast sea states. The curves indicate the effect of head, beam, and following seas of various significant wave heights on the ship’s speed.
Each vessel will have its own performance curves, which vary widely according to hull type, length, beam, shape, power, and tonnage.
With the speed curves it is possible to determine just how costly a diversion will be in terms of the required distance and time. A diversion may not be necessary where the duration of the adverse conditions is limited. In this case, it may be better to ride out the weather and seas knowing that a diversion, even if able to maintain the normal SOA, will not overcome the increased distance and time required.
Based on input data for environmental conditions and ship’s behaviour, route selection and surveillance techniques seek to achieve the optimum balance between time, distance, and acceptable environmental and seakeeping conditions.
Although speed performance curves are an aid to the ship routing agency, the response by mariners to deteriorating weather and sea conditions is not uniform.
Environmental factors of importance to ship weather routing are those elements of the atmosphere and ocean that may produce a change in the status of a ship transit. In ship routing, consideration is given to wind, seas, fog, ice, and ocean currents.
Optimum routing is normally considered attained if the effects of wind and seas can be optimised.