By Russ Swinnerton
There’s nothing mystical about chart work: you draw lines on a chart to model your journey, pre-planning courses to steer and distances to paddle. And afloat in unknown territory, even with the limitations of your spray deck as chart table, chart work can help you keep your reckoning up to date, so you can safely find your way. It’s addictive: you ‘shoot up’ unfamiliar marks, you take a fix or two, and pretty soon you’re hooked.
Here are a few terms we need to know, to allow us to do that modelling:
Term – Definition – Chart work convention
course The heading you plan to steer through the water. Direction is written along the line in three digits (e.g. 045). Add M for a magnetic course.
dead reckoning position (DR) A position determined from the course steered and distance paddled (calculated by estimated paddling speed divided by time elapsed). A small line drawn at right-angles to the course, with a time written alongside.
estimated position (EP) A position allowing for the expected effects of wind and stream – that is, the combined effect of the leeway we make through the water and how the water moves over the ground. A small triangle, with a dot in the centre, with a time written alongside.
track The direction that we make good over the ground. On your chart, it’s the line joining successive fixes.
position line A line that passes through your position, determined by observation of a bearing, a transit, or a range (and there are other ways that we can talk about over a drink one day). We represent a bearing as a short line near our position, with an arrow-head pointing away from the object observed. A range line is an arc of a circle, centred on the object, with arrow-heads at each end.
fix A combination of position lines, usually three, but two will do at a pinch if you’re sure of the objects observed and the accuracy of the observation. The symbol for a fix is a circle with a dot in the centre, and a time written alongside.
transit The observation of two objects in line. A position line from a transit can be drawn straight on the chart through the two objects – but if you do take the bearing, you can check the magnetic variation. The shorthand symbol for a transit is ‘φ’.
Nautical mile Equivalent to one minute of latitude. Measured on a north-south meridian of longitude on the side of your chart.
Practical tips for fixing
The golden rule about fixing your position is to be sure of your fixing marks. One certain position line is far more use to the navigator than three dodgy bearings of indeterminate objects. Decide what you want to take before you take the first bearing, and write the names down. Then observe the marks, write down the bearings, and note the time. Plotting the bearings on your chart will take a few minutes on a rolling deck; don’t forget to keep an eye out for dangers while you’re plotting.
Paddling through a chain of islands, or past many points of land, you can identify your next marks by shooting up; taking the bearing of an unknown point when you fix your position, then lay off the bearing from the fix to see what it touches.
When selecting marks, choose near objects rather than far. A small error will have a greater effect at distance. Using the rule that one degree subtends 30 metres at one nautical mile, a one degree bearing error at 10 miles will move your position 300 metres. And one degree is a pretty good observation with a hand-bearing compass in a rolling kayak.
Aim for a 60 degree cut with your bearings. With three position lines, that’s the optimum combination to minimise the effect of observation errors. If two lines cross at a shallow angle, then a small error in the bearings will have a big difference in the resulting position. And 90 is better, we aim for 90 in a fix from a transit and single compass bearing, so that error has less effect. And if you’re sure of your marks, two good bearings 90 degrees apart will give a good position, but that third bearing provides a reassuring check.
Don’t forget ranges and depth contours can be used as position lines. We don’t carry radars, but it’s possible to get a range by measuring the angle between the water line and the top of an object of known height, using a device called a Kamal (I’ll describe how to make and use one in a later article). And we don’t carry echo sounders, but we can usually tell when we’re paddling through a tide race where that big depth change or shoal is.
We use the term ‘cocked hat’ to describe the triangle you get when your position lines don’t quite cross on a point. Generally, you can assume you’ve got small errors in all three lines, and put your fix symbol in the centre of the hat. But if it’s a big cocked hat, and the position you pick makes a difference, then assume the worst case, take the most dangerous intersection of two position lines, and navigate accordingly.
Tools of the Trade
On the kitchen table, you should do your planning with a sharp 2B pencil, you’ll permanently mark your chart with an HB. But take a sharp chinagraph pencil or a crayon to sea for chart-work and note-taking.
Use dividers and compasses to measure distances ashore, but that’s one more thing to stick in your leg in a seaway, so don’t bother with one under the deck bungies.
Courses and bearings are best measured in the kitchen with something like a Linex nautical plotter, it looks like a wide plastic ruler with a 360 degree circular protractor mounted in the centre. You set the angle with the ruler’s centre line, and align the protractor’s north-south lines with a meridian on your chart.
Ashore or afloat, you can use a Portland protractor and a straight edge for bearings. Even better for sea time, put a thin string through the hole in the centre of the Portland protractor, and it becomes ideal for plotting bearings. Wrap the string around your pencil, and you can improvise a pair of compasses for plotting ranges. And the edge is useful for measuring distances.
For your navigation notes – recording fixes, or bearings – in a pinch you can use your deck or your map case, but you’re much better to put a few sheets of paper through a laminator, punch a couple of holes, and bind with two cable ties. Voila, a cheap waterproof notebook, ready for chinagraph pencil notes. And you can also use its edge to measure distances.
Paddling to a pre-determined position – Tips for the Navigation Challenge
If you budding navigators are planning to take part in our Lake Macquarie navigation weekend in May (and who isn’t?), here are a few chart work suggestions to help.
You’ll be in your kayak looking to find checkpoints you’ve plotted on your chart. All the checkpoints will be close to the water, you just need to know where on the shoreline to stop and search. So the shoreline will be one position line, and you just need to add another position line to give you your ‘fix’ on the checkpoint’s location.
If there’s a conspicuous object on the land (such as a charted radio tower, edge of land or lighthouse) in roughly the same direction as you’ll be travelling, you can pre-plan the approach bearing using the line joining the visible object and the checkpoint. If off track, the easy way to know which way to steer to regain the line of bearing, is to remember: “bearing high, steer high”.
You can also find your way using a back-bearing, that’s the line joining a known position astern (edge of land, powerhouse chimney, conspicuous pine trees, and so on) and the checkpoint. Just look over your shoulder to keep the object astern. And a transit astern is of course easier to judge than a bearing, so look at your chart to see if there’s a transit that ‘points’ to your hidden checkpoint, and then steer to get on that transit line to approach the target.
Another strategy is to paddle to one side of the checkpoint, “aiming off”, far enough away to be sure which side you are on and then turn towards and paddle along the shore until a bearing or transit to an object near the beam comes ‘on’, showing you you’re in the right place.
Some checkpoints, especially on shifting sand islands, will plot as if they’re on the water, but that’s because the chart hasn’t kept up with the changes to the land. So you’re going to need a couple of bearings to confirm you’re in the right place.
And don’t forget all bearings will be magnetic bearings, so add east variation to get a true bearing (the CADET rule: Compass Add East = True).