Speed selection and cruising speed

A ship can operate at a speed slower than its design speed and thus significantly reduce its fuel costs. However, a ship must maintain a minimal speed to assure proper steerage and safe operation of main engine, etc. For most cargo vessels, the bunker fuel consumption per time unit is approximately proportional to the third power of the speed. Thus, reducing the speed by 20% reduces the fuel consumption (per time unit) by about 36%. When bunker fuel prices are high, the cost of bunker fuel may exceed all other operating costs of the ship. Thus, there may be a strong incentive to steam at slower speed and reduce the operating costs. A fleet operator that controls excess capacity (e.g. line operators), can reduce the speed of the vessels and thus reduce the effective capacity of the fleet, instead of laying-up, chartering-out or selling vessels.

In addition to cost and schedules, short-term cruising speed decisions should take into account also the impact of the destination port operating times. If the destination port is closed over the weekend (or at night) there is no point arriving there before the port opens. Thus reducing the cruising speed and saving fuel makes sense. In the case where cargo-handling operations of a vessel that started when the port was open continue until the vessel is finished, even after the port closes, it may be worthwhile to speed up and arrive at the destination port to start operations before it closes. There are a variety of tactics that may be used to take advantage of more appropriate vessel scheduling (MariEMS 2017).