Lesson 4
Leeway
When a ship moves sideways through the water under the influence of a strong wind, she is said to be 'Making leeway'. Thus Leeway is defined as 'the effect of wind in moving a vessel bodily to leeward', and Leeway Angle is defined as 'the angular difference between the water track and the ship's heading'.
Note: The word 'angle' will generally be omitted when it is evident that direction is implied.
Leeway effect can be observed on a ship by looking aft and noting the angle between the wake and the ship's fore-and-aft line.
LEEWAY ANGLE
Here we can see that the ships wake does not follow the fore and aft line of the vessel.
In the diagram above the wake appears to tail off to port of the fore and aft line. This would indicate that the vessel is being blown across the water surface to starboard. In other words the wind must be coming from the port side and physically blowing the ship to starboard across the water.
Note that the wind does not change the heading of the ship, only the vessel’s path across the surface of the water.
When the wind causes leeway the leeway angle must be applied to the direction that the vessel is heading in order to obtain the actual path of the vessel through the water surface.
The latter is known as the vessels water track and this is what the navigator must lay off on the chart.
IF LEEWAY IS BEING EXPERIENCED THEN ONLY THE WATER TRACK MUST BE LAID OFF ON THE CHART.
In the above diagram the vessel is steering a course of 070o (T), however a Northwesterly wind is causing 10o of leeway. The vessels track through the water will therefore be 10o different from the heading and will be applied downwind. i.e. the water track is 080o (T).
Application of Leeway and Current
Virtually all candidates for DoT Chartwork examinations tackle current/ tidal stream questions with confidence, but relatively few retain their confidence when leeway is introduced.
This can largely be attributed to the seemingly confusing wording used in many leeway/current questions which leaves a candidate uncertain whether to 'allow it' or to 'counteract it'. Indeed, many students cannot be convinced that there are only two categories into which all leeway/current questions fall, the variety of wording used suggesting, to many, an endless permutation of possibilities.
Having read a question carefully, the problem is to decide into which of the two categories it falls (bearing in mind that the word 'counteract' does not always appear in counteracting category questions).
Leeway can only be applied in one of two ways.
a) the leeway is applied with the wind direction
b) the leeway is applied against the wind direction
There are many ways that the examiner can introduce leeway into a question and there are a variety of terms that can be used when describing leeway and its effect.
e.g. the vessel was allowing 10 of leeway.
the vessel was making 10 of leeway
the vessel was counteracting 10 of leeway
the vessel was experiencing 10 of leeway
However the wording in the question is really irrelevant when it comes to deciding which way to apply the leeway.
THE ONLY THING THAT REALLY MATTERS IS WETHER YOU ARE TOLD THE COURSE BEING STEERED OR YOU ARE BEING ASKED TO FIND THE COURSE TO STEER.
i.e. the leeway is applied going with the wind
If the question asks you to find the course to steer then the leeway is applied in the opposite direction to which the wind is blowing.
i.e. the leeway is applied against the wind
A useful way to remember this is as follows;
Given the course Leeway is applied Going with the wind
Asked the course Leeway is applied Against the wind
You should also note that as leeway is the difference between the water track and the direction the ship is pointing in (True Ships Head), then it must only be applied as follows.
When the course being steered is given then the leeway must be applied only to the true ships head.
When the course is being asked for it must only be applied to the water track.
UNDER NO CIRCUMSTANCES MUST LEEWAY BE APPLIED TO A COMPASS COURSE. IF YOU DO THIS YOU WILL END UP CALCULATING AN INCORRECT VALUE FOR THE DEVIATION OF THE COMPASS.
THIS IS TREATED AS AN ERROR OF PRINCIPLE IN EXAMINATIONS AN WILL RESULT IN A PENALTY OF HALF THE MARKS FOR THE QUESTION BEING DEDUCTED.
Also remember that Beam Bearings are 90° to the True Ship's Head and not the water track, if leeway is involved.
A vessel steering is 046o (C) and is allowing 10o of leeway for a SE wind.
If the deviation for the direction of the ship’s head is 4o W and the variation at that position is 14o E, determine the vessels water track.
NOTE: the course the vessel is steering has been given !!!!!
Compass Course 046o (C)
Deviation 4o W (Dev’n West, Compass Best)
Magnetic Course 042o (M)
Variation 14o E Var’n East, Magnetic Least)
True Ships Head 056o (T)
Leeway 10o (Apply Going with the wind)
Water Track 046o (T)
Example
A vessel has to make a water track of 230o (T) and is experiencing 14o of leeway due to a NW wind.
Assuming that the deviation will be 6o E and that the variation is 3o W determine the compass course to steer to make good the required water track.
Water Track 230o (T)
Leeway 14o (Apply leeway against the wind)
True Ships Head 244o (T)
Variation 3 o W (Var’n West, Magnetic Best)
Magnetic Course 247o (M)
Deviation 6 o E (Dev’n East, Compass Least)
Compass Course 241o (C)
Applying Leeway and Current
The following layout serves as an aid to memory when dealing with leeway and current to follow the correct sequence for each of the two categories, as well as being a reminder to label all directions correctly:
“Allowing”
Ship's head (Course steered/to steer) °C
Deviation °E/W
Variation °E/W apply on
Ship's head (Course steered/to steer) °T
Leeway °
Write the above in the answer paper
Plot Water track °T
on Current/Tidal Stream °T
Chart Ground track °T
“Counteracting”
Notes
Only apply leeway to a true direction. Never apply it to a Compass course as an incorrect deviation for the direction of the ship's head would be obtained from the deviation card.
Apply leeway on the answer paper, not on the chart; never draw the ship's head on the chart in a leeway question. Vector triangles represent movement, i.e.
Movement of ship through the water
Movement of water over the ground (current)
Resultant movement of ship over the ground
It is thus necessary to draw on the chart the Water Track, but to also add the course steered (ship's head) only creates confusion, particularly in running fix questions.
Leeway questions frequently include beam bearings as a test of principle. As stated previously, Abeam is at right angles to the fore-and-aft line of the ship, hence on the answer paper apply 90° to the direction of the ship's head True to obtain the beam bearing, then plot the resultant on the chart as a normal bearing. The Beam Position is where this bearing cuts the Ground Track.
RULES FOR DEALING WITH QUESTIONS INVOLVING LEEWAY AND CURRENT
These types of questions generally are the ones that candidates find the hardest to deal with.
Basically the wording of the question will be such that unless the candidate has a through knowledge of the principles involved then they are likely to make an error of principle with disastrous results in an examination.
The two types of questions are shown below.
a) Questions where the initial position of the vessel is known, the course being steered is given and the navigator has to find the vessels position at the end of a specified period of time.
b) Questions where the navigator is given the vessels initial and final positions and is then asked to find the course to steer to get his vessel to the final position.
In type a) questions the course being steered is given therefore the first thing to do is to find the vessels water track..
In this case the leeway must be applied to the true ships head to obtain the water track. Once this is known then the water track can be plotted from the initial position to find the DR position after the required time.
From the DR position the tidal stream is then laid off to find the estimated position at the end of the given time period.
DEAL LEEWAY FIRST AND CURRENT SECOND
In type b) questions the navigator knows what course the vessel is required to make good.
His first job is to find the true course to steer to counter the effect of any tidal stream, which may be acting on his vessel in getting from initial position to the destination.
This will enable him to calculate the required water track to counter act the current. Once this has been found the course to steer can be found and this is where the leeway must be applied.
DEAL WITH CURRENT FIRST AND LEEWAY SECOND
These two problems will be looked at in greater detail in the next few lessons.
Lesson 4.doc Leeway DGR1999
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dariusz.lipinski