In this type of problem the navigator has to find the position of the vessel after a period of time given certain information.
Usually they will be given the initial position of the vessel (or the means of finding this out) and the course and speed that the vessel is steering.
They will also be given information on any external forces such as tidal stream and wind that are acting on the vessel.
These problems may be broken down into steps as follows:
1) Measure the length of the ground track and divide this by the time interval to determine the speed made good.
2) From the course given apply any compass or gyro errors to obtain the direction of the true ships head. (Make a note of the compass error at this time)
3) Apply any leeway to the true ships head to obtain the water track of the vessel.
4) Determine the initial position of the vessel. (Remember to correct any compass bearings to true bearings using the errors mentioned in 2) above.
5) Plot the initial position on the chart
6) From the initial position lay off the vessels water track, using the speed given and the length of the time period to determine the length of the water track vector.
7) At the end of this vector mark a DR position.
8) From the DR position lay off the tidal vector appropriate to the time interval stated.
9) At the end of the tide vector mark an estimated position. (EP)
10) Join the initial position and the Estimated Position to determine the ground track of the vessel during the time interval given.
A vessel observes light A on a bearing of 018(T) at a distance of 5 nm.
The vessel is steering a course 059o (T) at a speed of 10 knots and a tidal stream is setting 205o (T) at 2 knots. A Northerly wind is estimated to cause 8o of leeway.
Find the vessels estimated position after 1 hr 30 mins.
See diagram below
Method of solution
a) Lay off the bearing through light A
b) draw an arc radius 5 miles from A and determine the vessels initial position
c) Determine water track of vessel.
True Ships Head = 059 (T)
Leeway (N Wind) = +8
Water Track = 067 (T)
d) Calculate distance steamed in 1.5 hrs i.e. 1.5 x 10 knots = 15nmls
e) Lay off water track for a distance of 15 nmls and mark as the DR position
f) from the DR position lay off the tidal vector i.e. 205 (T) x (1.5 x 2) nmls and mark as the Estimated Position.
g) Draw a line joining the initial position with the estimated position to obtain the ground track.
h) Using the compass rose on the chart obtain the direction of the ground track to obtain the course made good during the 1.5 hour interval.
i) Measure the distance along the ground track and divide by 1.5 to obtain the speed made good.
This gives a ground track of 077.5 (T)
And a ground speed of 13 / 1.5 knts = 8.66 knots
Note the speed over the ground is less than the speed given in the question. This would be expected because the tidal stream is working against the direction the vessel is travelling in.
A vessel steering a course of 060 (C) is steaming at 6 knots. The vessel is initially at position “A” on the chartlet below. A tidal stream is setting at 120 (T) at 2 knots and a southerly wind is causing 10 of leeway.
Find the vessels position after two hours.
(Assume that the deviation is 6 E and the variation is 10 W)
a) Convert compass course to True course
Compass Co = 060 (C)
Deviation = 6 E
Magnetic Co = 066 (M)
Variation = 10 W
True Co = 056 (T)
Leeway = -10
Water Track = 046 (T)
b) Lay off water track from position A for a distance of 12 nmls and mark as a DR position.
c) Lay off tide vector in direction of 120 (T) for 4 mls
d) Mark position of EP at the end of the tide vector.
e) Draw a straight line joining A with the EP to obtain the ground track.
f) Using the compass rose obtain the direction of the ground track
g) Using dividers measure the length of ground track off the latitude scale
h) Divide the distance by 2Hrs to get the speed made good over the ground.
Lesson 5.doc Estimated Position DGR1999
5
dariusz.lipinski