Transits and clearing bearings

Two objects in line

In the last two lessons you took bearings of landmarks with a hand-bearing compass and turned them into position lines and fixes. Every one of those lines carries the compass’s baggage: the variation you have to apply, any deviation, and the couple of degrees you can’t avoid when sighting from a moving deck. There is a way to get a position line with none of that — and it needs no instrument at all.

When two charted objects appear exactly one behind the other, they are in transit (or simply in line). You must then be somewhere on the straight line that runs through them and on out to sea, so draw that line on the chart and you have a position line. The line itself is called a transit. When the two objects are not in line they are open, and you say which way one appears from the other: “the mast open to the left of the lighthouse”.

Two objects in line give a position line with no compass, no variation and no deviation — just your eyes.

Leading lines and leading lights

Where a harbour approach must be followed closely, a transit is often built on purpose: a leading line, marked by a pair of leading lights (by day, the structures themselves, or painted marks, do the same job). Keep the two in line and you are on the safe track.

Port Kerran’s leading lights stand behind the head of Kerran Bay. The chart prints the line as Ldg Lts 000° Oc 4s: the lights lead in on 000°T, and both show Oc 4s, an occulting light (on for longer than it is off) with a 4-second period — Module 3 decodes light characteristics. As with every bearing on a chart, the figure is true and is given from seaward: it is the direction you look to see the lights in line.

The front light is 6 m high and the rear one 14 m (heights, as always on land, above MHWS), 3.5 cables behind it. Because the rear light is higher, when you are on the line you see it directly above the front light. The charted line runs from seaward of Nos 1 and 2 buoys towards the front light, straight up the middle of the buoyed channel.

Which way to steer

How sharply a transit warns you depends on its geometry. On the Port Kerran approach, one cable (about 185 m) off the line, the two lights appear 1.5° apart when you are 1 M from the front light, 0.4° at 2 M, but only 0.2° at 3 M. A transit is most sensitive when you are close to the objects and they are well apart.

A transit shows your track

A compass tells you which way the boat is pointing; a transit tells you where it actually is. That makes a transit a superb way to see the effect of a tidal stream (Module 4) and, later, leeway.

Suppose a tidal stream is setting east across the Port Kerran channel. Steer 000°T and the boat is carried east of the line: within a minute or two the rear light opens to the right, while the compass still shows exactly the heading you are steering. To stay on the line, point the bow a few degrees up-tide — towards the west — and adjust until the lights stay in line. Working out that heading in advance is the course-to-steer problem of Module 5.

Natural transits

Purpose-built leading lights are a bonus. Any two charted, fixed objects that line up will do: a lighthouse and a spire, a beacon and a hilltop, the edge of a headland and an island beyond it. Lay a straight edge across the chart and look for pairs that line up over the water you will be sailing.

On the practice chart, the church spire lines up with the radio mast on 333°T. Anywhere south-south-east of Kerran Bay, if you see the mast directly behind the spire, you are on that line. Both can be seen from there: the mast stands above the whole island, and along this line the spire is only 1.8 M inland.

The practice chart gives no heights for the land, so this course keeps it simple: the radio mast and Beacon Hill (87 m) stand above the rest of the island and can be seen over it, but lower objects — the church spire, the old tower and the lighthouses — show only over land close in front of them, within about 2 M.

• Use fixed objects. Buoys swing round their moorings, and can drag or be moved, so a transit using a buoy is only approximate.
• Identify both objects. Two spires or two hilltops are easily confused. Check shapes against the chart, and check the bearing of the pair with the hand-bearing compass.
• Measure from seaward: from the nearer object to the farther one, the way you look at them.

A transit in a fix

A transit is a position line like any other, so cross it with a bearing and you have a fix — often a better one than three compass bearings, because one of its two lines carries no compass error at all.

Checking the compass on a transit

Along a transit you know your bearing exactly, so it is the ideal place to check the steering compass. Steer straight along it with the two objects in line dead ahead (or dead astern), read the compass, and compare that with the transit’s magnetic bearing. The difference is the deviation on that heading. Choose a time with little cross-tide, so that the bow really points along the line rather than a little up-tide of it.

Leaving harbour with the lights in line astern, you are heading 180°T, which is 183°M. If the compass reads 180°C, the deviation is 3°E — and the card gives 3°E for 180°C.

Clearing bearings

Often there is no transit where you need one, but there is a danger near your track and a conspicuous landmark beyond it. A clearing bearing is a bearing of that landmark which keeps you clear of the danger as long as you don’t cross it. The line it stands for on the chart is a clearing line.

Drawing a clearing line

1. Find the danger, and a conspicuous charted landmark beyond it — roughly ahead of you (or astern).
2. Draw a line from the landmark that passes the danger with a safe margin, on the side you mean to pass.
3. Measure its bearing with the plotter, from seaward: the direction you will look to see the landmark.
4. Decide whether it is a maximum or a minimum (below), write it along the line — for example NLT 290°T — and hatch the dangerous side.
5. Convert it to magnetic for the hand-bearing compass, and write that too: NLT 293°M.

NMT or NLT?

NMT means not more than and NLT not less than. To decide which, look along the line towards the landmark and imagine drifting towards the danger. If the landmark’s bearing would grow, you need a maximum: NMT. If it would shrink, you need a minimum: NLT.

How big a margin?

A clearing bearing is only as good as the bearings you take. On a moving yacht a hand-bearing compass is rarely better than a couple of degrees, and the 1 in 60 rule turns that into distance: 1° off is about 1 M sideways for every 60 M away. So 3.4 M from the church spire, 2° is about 1.1 cables. Choose a margin that covers that error, the size of the danger and how well you know your position; this lesson’s examples use 4 cables.

Transits as clearing lines

A transit makes an even better clearing line, because it needs no compass. From the east, the radio mast stands just behind Gull Point lighthouse on 290°T — almost exactly the clearing line for The Teeth, passing 4 cables south of them. Keep the mast open to the left of the lighthouse and you are south of the line, clear of The Teeth. If the mast appears to the right of the lighthouse, you are standing into danger.