Section 5

Plotting Basics

Distance equals speed multiplied by time.

If two variables are known the third can be found.

The formula is: 60 x D = S x T which is expressed as:

and can be remembered by the address: 60 "D" Street.

Time is entered in minutes, speed in knots and distance in nautical miles (the same formula will work for statute miles and kilometres). The use of "60" ensures that time is calculated in minutes rather than in hours and tenths of an hour.

The DST Formulas

To find distance, if speed and time (or duration) are known:

D = (S x T) ÷ 60

To find time, if distance and speed are known:

T = (60 x D) ÷ S

To find speed, if distance and time are known:

S = (60 x D) ÷ T

If a boat's speed is 5 knots, how far will it go in 3½ hours?

60D = ST
D = (S x T) ÷ 60.
D = (5 kts x 210 min) ÷ 60
D = 1050 ÷ 60
D = 17.5 nm

If the same boat continues at a speed of 5 knots, how long will it take to cover 6 nm?

60D = ST
T = (60 x D) ÷ S
T = (60 x 6) ÷ 5
T = (360) ÷ 5
T = 72 min or 1h 12m

How fast is a boat cruising if it covers 14 nm in 1hr 45m?

60D = ST
S = (60 x D) ÷ T
S = (60 x 14) ÷ 105 (minutes)
S = (840) ÷ 105
S = 8 kts

Working with Time

In the nautical world the 24-hour clock system is used: 1:15 p.m. is expressed as 1315 and 8 o'clock in the morning is 0800. This is nautical time. Note that there are no spaces between the numbers.

For hours and minutes, as in elapsed time, the abbreviation h is used for hours and m or min for minutes: eleven hours and twenty-five minutes is abbreviated to 11h 25m or 11h 25 min.
Time should be rounded to the nearest minute.
Minutes are divided by 60 and expressed as hours and minutes: 257m becomes 4h 17m.