Current, Leeway, Tidal stream, Set, Drift ,H.S.A. ,V.S.A. .

 

Current, Leeway, Tidal stream, Set, Drift

Current

Is the movement of the surface of sea water due to seasonal causes such as wind, gradient, salinity, etc. In open sea, the direction and rate of current is fairly constant over long periods and over long distances. However, near land, it can vary drastically due to changes in the shape of the coastal features.

Leeway

Simple explanation of leeway

Leeway is the angle between the course steered and the angle of progress of the ship.

Tidal stream

In coastal waters, a combination of current and tide is called a tidal stream. This changes in strength and direction every hour and can be computed, at a given location, for a given time, by use of tidal atlases.  

Set & drift of current

Set & drift of current is the offset from the DR Position to the fix at that time. See accompanying diagram.

Drift is expressed in nautical miles and is the value for the whole day. If you need the rate of current experienced in knots, divide the drift by the period – 24 hours in this case.

Effect of Wind and Current on Course and Distance Made Good

You can understand this better by an example – see accompanying diagram.

Suppose you are walking in a direction 090° (T).  Another person pushes you gently, firmly and steadily towards your right. You will be facing 090° (T) but actually progressing to the right of 090°(T), say 100°(T). If you were a ship, your course would be 090°(T) and your ‘leeway course’ would be 100°(T). The leeway is then said to be 10° to the right. The force pushing your ship to the right is the wind.

Facts & assumptions regarding leeway

1. Fact: Wind is named by the direction from which comes. If you tossed a small piece of paper in the air, and it blew towards SW, the wind must be coming from NE. The wind is named NE.  
2. Fact: Wind from the port side would push the ship to starboard. Leeway is then termed ‘Right’. Conversely, wind from the starboard side would push the ship to port. Leeway is then termed ‘Left’.
3. Fact: Leeway is expressed in degrees right or left of the true course steered.
4. Assumption: Wind does not affect the speed of the ship. In reality, strong wind from ahead would cause an appreciable decrease in the speed made good, whereas, wind from astern would not cause much increase in the speed made good. Since the effect of wind on the speed of the ship is not predictable, as there are far too many factors affecting this, we assume that wind does not affect the speed of the ship. 
5. Fact: In open sea, you can estimate the leeway by looking at the wake of the ship. Wake is the trail left astern by the ship. Comparing the wake & the direction of the keel of the ship would give the amount of leeway.

Example of application of leeway

Your ship is steering 240° (T).  Wind direction is NW. Inspection of the wake indicates a leeway of 5°. Calculate the leeway course. 

Reasoning: Co 240° (T), wind from NW (315°). Since the wind is from the starboard side, ship will be pushed to port – the leeway will be to the left. See accompanying diagram.

Course to Steer Allowing for Tidal Stream or Current or Wind

Use of appropriate symbols

Whenever any line or point is plotted on a chart, appropriate symbols should be marked to indicate what does each line or point represents. This helps in avoiding confusion. These symbols are as follows ---

Line or Point to be identified	Symbol
A fix obtained by terrestrial or celestial observation.	Point with a circle.
DR position obtained by applying course and distance to a fix.	Point with a cross.
Estimated position obtained by applying current direction and rate to the course steered (CS) and distance covered in a given time interval.	Point with a triangle.
CS or course to steer (CTS).	A single arrow marked on the course line.
Course made good (CMG).	Two arrows marked on the course line.
Current.	Three arrows marked on the current direction line.
Bearing line of a terrestrial object.	No symbol.
Azimuth line of a celestial object.	The line is plotted from the ship’s position towards the celestial body, and a single arrow is marked at the end of the line.
Position line obtained by celestial observation.	It is drawn perpendicular to the Azimuth, for a short distance, with single arrows marked at both the ends of the line.
Transferred Bearing line.	It is drawn parallel to the original Bearing line, for a short distance, with single arrows marked at both ends of the line.
Transferred Position line.	It is drawn parallel to the original Position line, for a short distance, with double arrows marked at both ends of the line.

Applying current to CS to obtain CMG

Applying current to CMG to obtain CTS

Example of counteracting current and leeway

Refer to the accompanying diagram. At 0800, in position A, you desire to pass 5 NM off Point B. A current is estimated to set 225º(T) at 2 knots. Leeway is 3º for a S’ly wind. If your ship’s engine speed is 12 knots, find the compass course to steer, if Variation is 4º(W), using the deviation card shown herein. At what time and distance off will Point B come abeam?

Step 1: Choose the plotting interval. 

Seeing the scale of the chart, one hour seems appropriate.

Step 2: Plot the starting fix and time on the chart

In this case, the starting fix is at A on the chart. Mark 0800 next to it.

Step 3: Lay off the course to make good.

The distance to pass off B is 5 NM. Centre B, radius 5 NM, draw an arc. From the starting fix draw a tangent to this arc. This is the course to make good. It is 097º(T) in this case.

Step 4: Obtain the true course to steer

At the starting fix A, draw the set and rate of current for the chosen interval – 225º(T) 2 NM in this case - and call that point Q. Centre Q, radius = 12 NM, cut off an arc to cut the course to make good. The point of intersection is the EP at the end of the chosen interval, 0900 hours in this case. The direction from Q to the EP is the true course to steer to counteract current. In this case it is 088º(T).

Step 5: Counteract leeway

True course to steer to counteract current	088º(T)
Leeway 3º Wind S, hence a/c to starboard	3º(R)
True course to counteract current and leeway	091º(T)

Mag. Co. to steer = T. Co. + Var. (W) = 091° T + 4° W = 095° M

From the Deviation graph following data is derived ---

C. Co. 090° --- Dev. 3° W --- Hence M. Co. = 087°

C. Co. 135° --- Dev. 1° W --- Hence M. Co. = 134°

Correction to obtain the deviation for M. Co. 095° = (95 – 87)  (134 – 67)  (3 – 1) = 0.3°

Dev. = 3.0° – 0.3° = 2.7° W

C. Co. = 095° + 2.7° W = 097.7°

Step 7: Obtain estimated speed made good

The distance from the 0800 fix to the 0900 EP is the estimated speed made good, 10.5 knots in this case.

Step 8: Compute the beam bearing and distance

Ship’s head	091º(T)
Port beam	-90º
Port beam bearing	001º(T)

The point where the beam bearing cuts the course made good is the beam position. Measure off the abeam distance which here is = 35 NM.

Step 9: Obtain ETA at beam position

Interval to steam = Distance to steam ÷ speed MG = 35 ÷ 10.5 = 3.33 hrs = 3h 20m.

Time of starting fix	0800
Interval to beam bearing	0320
ETA beam position	1120

Step 10: State your answers

Compass course to steer to counteract current and leeway = 098º(C); Beam position 5 NM off B will be at 1120 hours.

Distance off by Vertical Sextant Angle

Definition

A Vertical Sextant Angle (VSA) is the angular altitude of the top of a terrestrial object above its base, normally sea level. 

By ignoring the height of the observer above sea level, we have a simple plane right angled triangle, as shown in diagram 13. Knowing the height of the object, and by observing the VSA, you can calculate the distance of the ship from that object.

Distance of object (Miles) = Height of object (m.)  (1852  Tan VSA°)

If VSA is very small, say less than about 2°, then: 

Tan VSA = VSA in radians.

Hence Tan VSA = VSA in degrees/57.3

So formula becomes:

Distance of object (Miles) = (Height of object (m.)  57.3)  (1852  )

Procedure to obtain VSA: 

• Always check for error of perpendicularity, side error and index error, each time before use. 
• Set the index at zero and look at the top of the object. Then bring its reflected image down to coincide with the waterline directly below it as illustrated in the following diagram. 
• Readymade tables are available in nautical tables where you can enter with height of object & VSA & obtain the distance off right away.

Ship's Position by Horizontal Sextant Angle

• Identify three terrestrial objects X, Y and Z, which are easily visible from the ship, and are horizontally well separated from each other.
• Using a sextant measure horizontal angle  between X and Y, and  between Y and Z. These angles can also be calculated by taking compass bearings of the three objects as ,  and . Then , and 
• Draw a line joining X and Y, and another line joining Y and Z.
• Calculate   and 
• If , then construct angle  at X and Y on the line XY, and draw two lines towards sea. 
• If , then construct angle  and draw the two lines towards land.
• Intersection of these two lines is centre of ship’s position circle, which will pass through X and Y.
• If , then centre of the line XY is centre of position circle.
• If , then the ship lies on the line XY.
• Similarly construct another position circle of ship using angle  and objects Y and Z.
• Intersection of the two position circles is the ship’s position.