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NAU 102

Lesson 12

Time, Speed, Distance

Calculation of Time, Speed and Distance

A daily task for a navigator.

Calculation requires accuracy and speed.

When will I get there?

How fast should I go?

Speed

Unit is the Knot (kt. or kn. or knt.)

From the knotted speed log.

Speed

Unit is the Knot (kt. or kn. or knt.)

1 knot = 1 nm/hour

Speed = Distance/Time

Calculation

Several Methods

Mental Calculation

“6 Minute Rule”

Logarithmic Scale

Nomogram

Mathematical Formula

Mental Calculation

Solve using simple ratios of time units.

e.g. ½, ⅓, ¼

Your speed is 20 kts. How far will you travel in 30 minutes?

You travel 6 nm in 20 minutes. What is your speed?

10 nm

18 kts

“6 Minute Rule”

6 minutes is 1/10th of an hour.

Every 6 minutes, you travel a distance equal to 1/10th of your speed.

Move the decimal point 1 place to the left or right.

“6 Minute Rule”

E.g. If your speed is 15 kts., how far will you travel in 6 minutes?

15.0

1.50 nm

“6 Minute Rule”

E.g. What is your speed if you travel 2.7 nm in travel in 6 minutes?

2.7

27 kts.

“6 Minute Rule”

You can use multiples of 6 minutes as well.

6 minutes = 0.1 hour

12 minutes = 0.2 hour

18 minutes = 0.3 hour

Etc.

“6 Minute Rule”

E.g. If your speed is 15 kts., how far will you travel in 18 minutes?

15.0

1.50 nm x 3 = 4.5 nm

Logarithmic Scale

On some charts and radar plotting sheets.

Nomogram

On some charts and maneuvering boards.

Nomogram

E.g. How far will you travel in 16 minutes at 13 kts?

Nomogram

E.g. How long will it take to travel in 2.5 nm at 16 kts?

Math Formula

Distance = Speed x Time

D = S x T

S = D/T

T = D/S

ETA

Estimated Time of Arrival

What time will I get there?

For short voyages:

Transit Time = Distance/Speed

Transit Time + Departure Time = ETA

ETA

Example

You depart Point A at 0800 and steam for 60 nm to Point B at 13 knots. What is

your ETA at Point B?

Transit Time = 60 nm/13 kts = 4.6154 hrs

= 4 hrs + 0.6154 hrs * 60 min/hr

= 4 hrs 37 min + 0800 = 1237

ETA

For longer voyages, we need to know about Time Zones.

Time Zones

Earth is divided into 24 time zones.

Time differs by an hour from neighboring zones.

Everyone in the zone keeps the same time.

Time Zones

Each time zone is 15° of longitude wide.

360° / 24 = 15°

Zones are numbered (“zone descriptions”)

0 through -12 in east longitude

0 through +12 in west longitude

Time Zones

Time Zones

First, calculate the central meridian of a zone

E.g. ZD +4

Zones extends 7.5° east & west of the center.

Zone Boundaries

15° x zone description

4 x 15° = 60° W

Time Zones

Time Zones

To find the zone of a particular location:

•Divide by 15

•Round to the nearest whole number

•Convert longitude to degrees & tenths

Time Zones

ZD = -9

141.5°/15 = 9.43

141° 30’ = 141.5°

E.g. What is the ZD of 141° 30’ E

(- E/+W)

ZD = -10142.66°/15 = 9.51

142° 40’ = 142.66°

E.g. What is the ZD of 142° 40’ E?

Introduction to Navigation

Questions?