Economic speed or Optimum speed. Several factors are considered when determining the economic speed of a vessel. This is the speed producing the best possible financial result for owners mainly because at the economic speed the least fuel will be consumed, especially in times when fuel costs are very high, and also produce the least wear and tear on the main propulsion machinery. These factors include:
1. The prices for bunkers in the ports en route.
2. The relation between fuel consumption of the vessel at high and low speeds.
3. Daily operating costs.
4. The net freight per ton of cargo.
5. Operating profit per day.
6. Future employment of vessel, i.e., when the vessel should be ready to load for the next voyage.
7. The state of the freight market: good market means high speed.
8. Maximum design speed.
9. Technical ability of engine to operate at very low speeds.
10. The ratio between days at sea and days in port.
11. Weather conditions.
The best method to obtain economic speed is to draw up calculations in a tabular manner for a certain regular trade at varying speeds, e.g., coal from Hampton Roads to the Continent or Japan or grain from the Gulf of Mexico to the Continent or Japan on the basis of full cargoes being carried on one leg of the voyage and the return voyage being made in ballast. Such calculations facilitate a comparison between the financial results. Apart from the economic speed, the economic size also enters in the picture. It is clear that, broadly speaking, the larger the deadweight capacity, the lower are the costs per ton of cargo carried, but here again there are factors which govern the economic size, such as the limits of port facilities to be observed, requirements of the trades which will constitute a main source of employment, etc.
It was thought that the fuel consumption generally varies as the cube of the speed. With the oil crises in 1973 and 1979 more fuel-efficient engines and better bunker consumptions have resulted. The fuel consumption therefore varies also with the year of build of the ship.
The situation is different for liner operators. Obviously, the necessity for liner operators to offer shippers a transit time, which compares favorably with the transit time offered by their competitors, has an important bearing upon the required speed. Another factor is that a higher speed may be required in order to operate a liner service with sailings at regular intervals with a certain number of ships.