An accurate knowledge of the service speed of a merchant vessel is becoming increasingly important. Service speed and average speed (the former traditionally implying a long-term average, the latter a short-term average) are both derived by dividing the sum of a number of observed distances by the relevant time elapsed. In this way fluctuations in observed distances arising from random fix errors and the effects of change of draught, weather and variable currents are smoothed. Service speed may be used as a basis for determining log errors, engine efficiencies, hull surface condition, liner schedules and charter party speeds. It is therefore relevant to consider possible systematic errors which may be introduced in the calculation of these speeds.