Watchkeeping (Art of Navigation)

Q. How do you monitor the position being on the course line, without plotting it?
This is done by parallel indexing technique. While coasting, on every course a target must be identified for doing the monitoring by parallel indexing. On chart, the distance to pass a target is found by finding the nearest distance to pass, by drawing, an arc from the nearest tip of the target on course line.

On the radar screen, an independent EBL is drawn parallel to the ship’s course tangenting the VRM circle of the distance at which to pass the shore object. As the ship moves, the VRM must move ahead with the shore object maintaining tangency on the EBL.

The target moving shipwards of the EBL would indicate that the ship is drifting sideways towards the shore.

The target moving on the non-shipward of the EBL would indicate that the ship is drifting sideways away from the shore.

Q. In the middle of ocean, how would you find out the ship’s approximate latitude? You just have an old almanac.
The declination of the Sun usually remains constant with respect to the calendar dates. This is because the tropical period of revolution is taken into consideration when making calendar. Thus, if the ship’s latitude is around 20oN, and the declination for the day is say 15oS, the Sun at meridian passage would bear south and the altitude will be around 55o. Thus, if the altitude is 54o55’ at merpass, the MZD will be 35o05’ and the observed altitude would be 35o05’ – 15o = 20o05’N.

Q. How can the station pointer be used for finding out the ship’s position in coastal waters?
The horizontal angle between the lights is also the angular separation between the arms of a station pointer. This way if the tips of the arm are placed on the respective light houses on the chart. The centre of the central disc of station pointer is the position of the ship.

Q. You just have a harbor chart, and the only navigational equipment you have is the sextant. Can you find the position?
The sextant can be used to find the horizontal angles between the lights. The horizontal angle principle is used to get two position circles. The intersection is the position. 

Q. How do you get the position circle?
Suppose the horizontal angle between one of the sets of light houses is θ, the lines are drawn to the shipward side of line joining the two light houses. The angle subtended should be 90-θ at both the lighthouses. To make an isosceles triangle. The point of intersection is also a circumcentre to draw the position circle.

Q. What is the fastest way to find the compass error while in harbor or coasting close to land?
A quick way of finding out compass error is by taking a transit bearing. A duty officer must be prepared to take such bearing before hand during pilotage, etc.

Q. Is there any other terrestrial method of finding the compass error?
In a harbor, while alongside a berth, ensuring that the ship is parallel to the wharf, the direction of wharf and the ship’s head must be same, thus, giving compass error. While approaching or leaving a port, the leading lights also can give the compass error.

Q. What is the main purpose of the leading lights?
The main purpose of the leading lights is to detect a cross drift which may drift the vessel out of channel. Thus, while approaching or leaving these lights must appear in the same vertical line.

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