Worked Example 7C

Bringing it all together

In this example section we bring together the process of working up a course to steer with calculating hours of tide and tidal vectors. We’ll do this by using questions that are just like those that will appear in the chartwork assessment paper. You’ll get most benefit if you follow the worked example yourself on the paper charts.

Worked example 7C

At 1505 SP (zone-1) on March 1st 2009, the log reads 2456.8 miles and the boat has fixed its position using the GPS which displays 4.2 miles 018°T to a waypoint placed at the centre of the compass rose in position 45°54’.00N 005°55’.25W.

The yacht would like to make a passage to Ensign Bay in position 45°55’.25N 006°05’.40W. The vessel is able to maintain a boat speed of 7.0 knots.

  • What is the magnetic course to steer to the buoy using tidal diamond ‘J’
  • Will it take less than an hour, exactly one hour or more than an hour to get to the waypoint?
  • What will the speed over the ground be?
    With a fresh northerly wind expecting to produce 10° leeway, what is the magnetic course to steer to allow for the leeway?

The fix

Fig 8.20
Fig 8.20

First, we must fix the position of the vessel. We are given the range and bearing to a waypoint at the centre of the compass rose. Always remember that the range and bearing to a waypoint is from the vessel to the waypoint and not the other way around (Fig 8.20).

The fix position at 1505 SP (zone-1) is:

45°57’.80N 005°53’.40W

The proposed ground track

Fig 8.21
Fig 8.21

We can now find the position of our destination and draw a line from our fix representing the ground track or proposed course over the ground towards the destination at Ensign Bay (Fig 8.21).

When we measure the distance from the fix to the destination we can see that it is 7.8 miles. We know the speed of the vessel is 7.0 knots, so should take us about one hour to get there. We therefore need to work up a 1 hour plot.

Remember to draw the proposed ground track a little further than the destination and that measuring the distance from our fix to the proposed destination and comparing it to our boat speed indicates how many hours of tidal stream we need to use on the plot to get the correct course to steer.

Determining the Tidal Stream

Fig 8.22
Fig 8.22

Remember that the tidal diamonds and tidal stream atlas use Victoria as a reference and therefore must consult Victoria tide tables for times of high water and tidal range for the 1st march.

Fig 8.23
Fig 8.23

We must convert our passage time from 1505 SP (zone-1) to 1605 SP (zone -1) into UT time so that we can work with the tide tables etc. We must subtract one hour to convert SP (zone -1) to NP (zone 0) time which is equivalent to UT time. This enables us to work with the Tidal Diamonds which use Victora as the Standard Port.

Our passage time is therefore 1405 UT to 1505 UT

We can see from the tide tables (Fig 8.22) that the nearest High Water to our passage time is 1139 UT and the tidal range is 6.2m.

We can consult the Victoria tidal curve mean tidal ranges box and see that our tidal range is a spring range. (Fig 8.23)

Fig 8.24
Fig 8.24

We can enter the times of High Water in our tidal stream proforma so that we may determine the correct hour of tide to consult the tidal diamonds table (Fig 8.24).

We can now fill in the proforma starting with the time of HW Victoria on the 1st March. Our target time is after high water and shall fill the bottom part of the proforma to assess the correct hour of tide.

We can see from the tidal stream proforma that the hour of tide we are looking for is HW+3

Fig 8.25
Fig 8.25

From tidal diamond ‘L’ on the tidal diamond table we can see that at HW+3 Victoria the tide has a set of 344° True and rate of 2.0 knots. (Fig 8.25)

We now know that the set and rate we need to plot on the chart is

334° True 2.0 knots.

Fig 8.26
Fig 8.26

Now that we assessed the tidal stream that will affect us on our passage we plot the tidal vector line on the chart (Fig 8.26), making sure to draw 3 little arrows.

Remember the tidal vector line is drawn from our starting location, i.e. our original fix.


Determining the heading or water track

Fig 8.27
Fig 8.27

We ca now set our dividers to 7.0 knots which is the boat speed and draw an arc to cross the proposed ground track. To do this place one point of the dividers at the end of the tidal vector and mark the ground track where the other end of the dividers reaches.

We can measure the course to steer which is the same as the bearing of the water track which is 305° True. From the plot we can see that we have a little bit of tide slightly assisting, so the boat will actually travel further over the ground in one hour than it does through the water.

The nearest compass rose indicates to us that the variation is 7°25’W in 2005 and is decreasing by 8’E annually. So in 2009 the variation will be 32’ less than in 2005.

7°25’ – 32’ = 6°53’
which rounded to the nearest degree is still 7°W variation.

We can now use our Nemonic CadET to calculate the magnetic course to steer:

246°T + 7°W variation (2009) = 253°M

The arc crosses slightly passed the destination so we know the passage will take less than one hour. The predicted Speed Over Ground is calculated over an hour in this case. We measure the distance from the starting fix to where the arc crosses the ground tack and not the actual destination. This gives the distance the vessel will travel in one hour which is the same as our SOG over the proposed one hour passage time.

The predicted SOG will be 8.7 knots. We know the Speed of the boat through the water is 7.0 knots , so makes sense that the difference between the two speeds is the push that we get from the tide along this course which is 1.7 knots. The tide is giving us a lift to our destination.

The application of leeway

Fig 8.28
Fig 8.28

The last thing we have to do in this example is to make sure we compensate for the leeway we expect on our passage.

Again its a really good idea to draw a wind arrow beside your plot so that can get a better idea which way it is affecting the vessel and how we need to change the bearing to compensate.

We can see that it is blowing from the north and will push the boat 10° south off course so to compensate for it we must add 10° to our magnetic course to steer.

253°M + 10° leeway = 263°M

We must be careful to apply leeway correctly, if we subtracted it rather than adding it we could be anything up to 20° off course which could have serous implications to our navigation and ability to make our desired passage.

Important Points

  • Both the proposed ground track AND the tidal vector are plotted from the starting position which could be a fix or an EP.
  • Each tidal vector is plotted from the end of the tidal vector for the previous hour.  While drawing in the ground track it is worth extending it well beyond the destination.
  • Boat speed is always ARC’d across the proposed ground track from the end of the last tidal vector and NEVER just joined to the destination.
  • On this course you won’t be examined on courses to steer of passages of more than one hour in duration.
  • The course to steer is from the end of the tide to where the ARC intersects the proposed ground track.
  • Draw a wind arrow near your water track to make it clear which way leeway should be applied.
  • Be careful to use the correct plotting symbols otherwise you might be woken up to explain them to whoever is now on watch!
  • You need to develop plotting accuracy within: 0’.1 of arc (0.1 mile = 1 cable = 200 yards) for distance and 1° of arc for bearing.
  • It is critically important to remember that COG and SOG can ONLY be derived from measurements along the ground track and that the water track plays no partat all in these measurements – EVER!
  • Bearings can go up to 360° and are always written with 3 digits, where there are less then leading zeroes are used to make up the number, e.g. 000°T.
  • Marine bearings are always rounded to the nearest whole number of degrees, e.g. 045o29’T would be rounded down to 045oT and 045o30’T would be rounded up to 046oT.
  • Whenever you are asked to plot a position on a chart in an exercise or assessment, as well as showing us the plotting, you need to demonstrate your understanding and ability by typing the latitude and longitude coordinates in the text of your answer.