When measuring speed there are two main parameters involved, time and distance, excluding weather, sea depth, currents, tide etc.
By measuring distance with respect to time the slope of the graph indicates trial speed attained. (see Ref. 1)
Since ship speed is measured in knots (1 nautical mile/hour), by measuring the time required for the ship to run from one point to the other, the speed is immediately obtained directly in knots.
This method prevailed up to now but contains map relative accuracy, observational mistakes, etc. so whenever a ship was to be delivered, it was driven up and down a measured mile distance at various RPM and her corresponding speeds thus determined.
Trial procedures are well described and will not be analysed here. (see
Table 1 with references)
Meanwhile trial procedures have improved by using D.GPS etc. (see Ref. 1 and 2) but not always employed. However, MARIN (see Ref. 3) has pointed out that time is important and should not be less than 5’ nor more than 10’ in duration.
Now time becomes more important and distance less. Bearing in mind the advances in measuring methods, there is no need to use the single nautical mile any more. The minimum(5’) distance required for trial speed measurement greatly exceeds the single nautical mile, at today’s trial speeds.
Table 2 illustrates time needed to complete the single nautical mile run. Only a trial speed of 12 knots corresponds to the measured mile distance for 5 minutes , and 6 knots for 10 minutes (see insert of
Table 3.
On the contrary, Table 3 illustrates MAX/MIN trial duration at various trial speeds likely to be met today. According to MARIN such a duration ensures more accuracy and the measured mile method may finally begin to fade away.
