A special thanks to everyone who is still reading my numerous Coastal Skipper revision posts in preparation for the Ocean Yachtmaster exam. I have certainly dredged up some things from the depths of my brain that had long been forgotten.
I am flitting around with topics all over the place, which is just typical of my personality. So while I am doing a bit of Meteorology here, and a bit of Navigation there, what I am really doing is procrastinating about having to study the Celestial Navigation stuff again. I figure if I leave that bit till last then it will be fresh in my mind when I do actually sit the exam.
So if you are a bit rusty on your navigation skills, or if you are just beginning, then hopefully you will enjoy this post. Lets go right back to the beginning:
Longitude and Latitude
To be able to describe your position anywhere in the world, you can use Latitude and Longitude. These numbers relate to a grid pattern on the earth.
Latitude is described in degrees either North or South of the Equator – the Equator is 0º and the poles are 90º North or South.
Longitude is described in degrees either East or West of Greenwich – Greenwich is known as the Prime Meridian. East covers most most of Europe, Africa, Asia, Australia, New Zealand and half the Pacific. West covers most of the UK, Spain, the West Coast of Africa, the Atlantic, The Americas and the other half of the Pacific.
Longitude lines are meridians and great circles – i.e. each one directs the earth in half. Whereas Latitude lines are like slices of the earth. You can read more about great circles and their relevance here.
Latitude is always written first. The position is given in degrees, minutes and decimals of a minute.
Here in Christchurch New Zealand we are located at 43º53.2’S, 173º63.6’E.
Speed & Distance
1 Nautical Mile = 1852 metres. A nautical mile is 1 minute of Latitude. Latitude is marked on the left and right hand side of nautical charts. If you are taking distance measurements then you must remember to do this on the left or right hand side of the chart and not the top or bottom. The top and bottom of the chart is Longitude and longitude degrees do not equal 1 nautical mile.
A knot is 1 nautical mile per hour.
- Time = Distance ÷ Speed
- Speed = Distance ÷ Time
- Distance = Speed × Time
When working out time, it is best to use the 24 hour clock. To convert 3.75 hours in to minutes:
- 3.75 hours = 3 hours + (0.75 x 60) mins
- = 3 hours 45 mins
To convert 45 minutes in to hours + decimal
- 0 + (45 ÷ 60) hours
- = 0.75 hours
Or 2 hours 15 minutes in to hours + decimal
- 2 + (15 ÷ 60) hours
- = 2.25 hours
Steering a Course
When reading a compass or describing a course, use three figures – i.e. 045º as opposed to 45º
- (C) means Compass, direction, course or bearing as per your compass reading
- (M) means Magnetic – means the compass reading once deviation has been accounted for
- (T) means True – meaning True north as per the chart once deviation and variation has been accounted for
- (R) means Relative
Did you know that Magnetic North moves around? The amount that it varies depends on your position on the earth, and it changes from year to year. So your magnetic compass points to Magnetic North, but all your charts are drawn with the aspect of True North. Variation is the difference between Magnetic North and True North. The Variation figure can be found on the compass rose on a chart.
All courses should be converted to True courses before being plotted on the chart.
In the example below. The Variation in 1984 was 19º08’E with an annual increase of 4′. So the difference between 2016 and 1984 is 32 years. 32 x 4′ = 128′. Divide that by 60 to get degrees and what ever is left over is the minutes so that is 2º 8′ and add that to the 19º08′ so the Variation now should be 21º16′
You can figure out whether to add or subtract the Variation from the compass reading with the wee rhyme below:
- Variation West, Compass Best
- Variation East, Compass Least
So that really means you add the Variation to the compass when it is East and you subtract it when it is West.
Deviation is another compass error but this is caused by magnetic influences from the boat. You can read more about deviation and how to calculate it here.
So if you have a course you are steering on your compass you need to convert it from Compass to Magnetic and from Magnetic to True to get the True course and vice versa.
To remember the order in which to apply the Deviation and Variation, I remember it with the mnemonic: “Cadbury Dairy Milk Very Tasty”
You can then do this in the opposite direction to convert a True course to a course to steer on your compass.
A Compass Bearing is the direction of the object taken from the boat when using a hand bearing compass. You assume that there is no deviation.
A Relative Bearing, is relative to the boat. So dead ahead is 000ºR and dead astern is 180ºR. If you take your True Course (that you are steering) and add the relative bearing then you get the true bearing.
To convert a relative bearing to a True bearing – add 360 and then take off the True heading.
A Transit Bearing is when two objects are seen exactly in line with each other. This means that your vessel lays somewhere on this line. Transit bearings are the fastest most accurate way of determining a position line as no calculations are necessary (Yay!!) Leading lights are a good example of transit bearings. When a pair of leading lights appear in transit, they form a line along which the vessel should travel.
- Always use the method that uses the greatest accuracy
- Try combining different methods, radar, navigational lights, depth soundings, bearings, & GPS
- Use objects closest to you
- Ensure you correctly identify the landmarks you are using
- If two or more position lines are obtained at the same time, the boat is located on the intersection of the two lines.
- To identify a fix. Circle it on the chart and put a dot at the fix and the time beside it.
- An ideal angle of cut is 90º, a poor angle is 30º
- A third position line provides an accuracy check. If a cocked hat appears, then the boat is somewhere within the triangle. This could be caused by a plotting error, calculation error, an incorrectly identified landmark, boat movement, instrument errors or chart errors.
- If the triangle is large, then the boat should be assumed to be in the most dangerous position and a new fix needs to be obtained ASAP.
- When sailing during the day visual bearings are best combined with radar, transits, depth soundings and vertical or horizontal sextant angles.
- At night raising and dipping lights, radar ranges and depth soundings can be used.
- Maintain regular intervals between position fixes i.e. every 10 or 30 minutes. This makes your boat speed calculations easier to do. However if an opportunity arises for fixing your position, don’t pass it up.
- If you are close to the coast or in poor visibility, position fixes should be more regular.
Expressing Your Position
When you are describing your position to someone over the radio, you can express it in Latitude and Longitude or Bearing and Distance.
When using a bearing you should express your position to or from that landmark in True.
For example – if we were aproaching the Lyttelton Harbour entrance, we would say that we were two nautical miles North East of Godley Head (as opposed to expressing it as a distance & bearing from your boat – where Godley Head would be two nautical miles South West of your position) Expressing your location in this way means that people can go straight to the landmark on the chart and then plot your position from there, as opposed to trying to do it backwards…
Reading Chart Depths
Chart Datum is the level on the chart at which depths are measured from. It is usually the level of the lowest astronomical tide. So generally, depending on the tide, there is usually always more water above the level on the chart – so it is really a worst case scenario level of water.
Overhead obstacles like bridges are measured from a different level – mean high water springs – or the highest astronomical tide. This means that there is usually less water (or more clearance) available, depending on the tide.
To work out the height of the tide click here.
Charts provide all sorts of valuable information for sailors. One of the very important things is detailing hazards below the water that you can’t necessarily see. They are indicated on the chart as follows:
Rocks that are either just below the water line, or that cover and uncover with the tide are marked as follows:
- + = dangerous rock depth unknown
- + (4.5) = dangerous rock 4.5m below chart datum
- + with spots around the outside = a rock awash at chart datum (so usually just under the water at low tide)
- * (1) = a rock which covers and uncovers and drys at 1m above chart datum. (so if its low-low tide its usually protruding 1m above the water, but if there is 2m of tide above it, then it would be below the water)
- • Rk(3) = this rock doesn’t cover. It is at a height of 3m above MHWS (so at least 3m of it shows above the water at high tide)
Lighthouses are described on the chart. Their different flashing signals allow you to identify which light you are looking at. You can use lighthouses to take bearings from, raise or dip them over the horizon and during the day use them for measuring vertical sextant angles.
With GPS being so easy to use these days, it is really easy to not use these traditional navigation techniques. Do you still use paper charts and manually plot your course?
Tune in next time when I talk about position lines!