11.3 Pilotage Plans, Clearing Bearings, Leading Lines & Soundings

11.3 Pilotage Plans, Clearing Bearings, Leading Lines & Soundings

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The Core Principles of Pilotage

When a vessel approaches an estuary, harbor mouth, or archipelago, navigation fundamentally shifts from passage-making to pilotage:

1. Proximity of Danger: Hazards (drying mudflats, granite rocks, groynes, mooring buoys, commercial shipping) are measured in metres rather than nautical miles.
2. Time Compression: Reaction times collapse from hours to seconds. An errant heading or delayed helm alteration results in grounding within minutes.
3. Lookout Orientation: Navigators must keep their eyes outside the boat. Staring down into a chart table or fixing one's gaze on an electronic chart plotter screen destroys peripheral vision and situational awareness.
4. Pre-Memorization & Simplicity: A pilotage plan must be simple enough to be executed from memory or referenced at a single glance in rain, darkness, and spray.

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Preparing Cockpit Pilotage Cards & Sketches

A standard admiralty chart is far too large, delicate, and cumbersome to unfold in an exposed cockpit during harbour entry. Standard seamanship dictates transcribing relevant pilotage data onto a Cockpit Pilotage Card (a waterproof, laminated, or notebook sketch map).

Structural Anatomy of a Pilotage Card

• Schematic Format: Drawn either as a simplified plan view (bird's-eye) or a progressive linear 'rolling roadmap' showing sequential legs.
• Large, Bold Lettering: Readable from the helm under a dimmed red headtorch at night.
• Labelled Courses: Show the planned tracks and courses to steer clearly. Convert magnetic headings to the steering compass using deviation before giving a compass course to the helmsman.
• Distances in Cables: In pilotage waters, distances are expressed in cables ($1\,\text{cable} = 0.1\,\text{NM} = 185.2\,\text{metres}$).
• Visual Daymarks & Shapes: Cones, cans, towers, church spires, and beacon topmarks drawn schematically.
• Light Characteristics: Printed in full (e.g., Q.R, Fl(2)G.6s, Iso.W.4s).
• Depth Fences: Mark the selected charted limiting contour, and calculate the corresponding sounder reading using tide and instrument offset.
• Communications: Port operations VHF channels (e.g., VTS Ch 12, Harbour Master Ch 11, Marina Ch 80).

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Visual Transits & Leading Lines

A Transit (or Leading Line) is formed when two charted, stationary landmarks or purpose-built navigation beacons line up visually one behind the other as seen from the vessel.

   Rear Beacon (Tall, Elevated)  ▲
                                 │
                                 │  Visual Transit Line
                                 │  (Dead Straight Track)
                                 │
   Front Beacon (Lower)          ▲
                                 │
                                 ▼
                         [Vessel on Track]

The Immense Value of Transits

1. Zero Compass Error: A transit is a physical optical alignment across the surface of the Earth. It is completely immune to magnetic variation, compass deviation, and instrument calibration errors. If the two marks are in line, the vessel is physically situated on that charted line of position.
2. Immediate Drift Detection: If a cross-current or wind leeway pushes the vessel off track, the two marks immediately "open out":
   • When facing the marks, if the rear mark opens to the right of the front mark, the vessel is to the right of the line.
   • If the rear mark opens to the left of the front mark, the vessel is to the left of the line.
   • When approaching the marks, move towards the front mark’s side of the opening to regain alignment, using a safe course and allowing for the current.
3. Leading Lights by Night: By night, leading lights form an identifiable pair with charted characteristics; they may be fixed, flashing or synchronized. Keeping the rear light above the front holds the charted leading line; check that the planned segment has sufficient depth and clearance.

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Clearing Bearings (Clearing Lines)

A Clearing Bearing is a pre-calculated limiting magnetic bearing taken on a prominent, charted landmark that defines the exact boundary separating safe, deep water from an underwater hazard (such as an offshore ledge, drying reef, or sand spit).

Operating Principle

Rather than steering along a single narrow line, a clearing bearing gives the vessel a broad, safe maneuvering corridor:

• "Keep Bearing More Than (NLT - Not Less Than)": For example, when entering an estuary with a dangerous rock ledge to starboard: "Keep St. Mary's Light bearing MORE THAN 085°(M)." As long as the hand-bearing compass reads 086°, 090°, or 100°, the vessel remains in deep safe water. If the bearing drops to 085° or 080°, the vessel is at or beyond the limiting line on the rock ledge’s side; the bearing alone does not establish its heading.
• "Keep Bearing Less Than (NMT - Not More Than)": Used when the danger lies on the opposite side of the line of bearing.

Practical Seamanship Execution

In pilotage channels, the skipper or dedicated pilot stands on the companionway or windward deck with a damped hand-bearing compass, continuously observing the landmark while the helmsman concentrates strictly on steering the vessel.

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Back-Bearings, Depth Contours & Wheel-Over Points

Back-Bearings

A Back-Bearing is a compass bearing observed looking directly astern toward a prominent landmark or navigational buoy that the vessel has just passed.

• If the planned track outbound from a harbor pierhead is $240^{\circ}(\text{M})$, the back-bearing on the pierhead beacon must read exactly the reciprocal: $240^{\circ} - 180^{\circ} = 060^{\circ}(\text{M})$.
• If the back-bearing begins reading $065^{\circ}(\text{M})$, the vessel is being set to starboard by cross-tide and the helmsman must immediately compensate to port.

Bracket Soundings & Depth Contours

The echo sounder provides an independent depth check in darkness or poor visibility. Know whether it displays depth below the transducer, keel or water surface:

• Contour Comparison: First convert the reading to depth below the water surface using the installed offset, then subtract height of tide to obtain depth below chart datum. Compare that value with the chart, allowing for survey uncertainty and similar depths elsewhere.
• Depth Fence: Select the charted limiting contour from draught, UKC and expected tidal height, and calculate its equivalent displayed depth. Set an appropriate alarm and plan an escape into checked safe water. If the limit is approached, slow or take the planned avoiding action; an unqualified5.8m display does not automatically prescribe a90° turn.

Wheel-Over Points

Vessels do not turn on a dime. When helm is applied, a vessel experiences advance (distance traveled forward along the original course) and transfer (distance traveled perpendicular to the original course) before settling on the new heading.

• A Wheel-Over Point is marked on the pilotage card at a predetermined distance (e.g., 0.5 to 1.0 cable) before the actual track junction or turning mark.
• Applying rudder at the wheel-over point ensures that the vessel turns smoothly into the center of the next channel leg rather than overshooting into shallow water.

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Harbour Regulations and Control Signals

Before entry, obtain the harbour’s current directions and notices: check speed/wash limits, reporting points, VHF working channel, movement permissions and traffic signals. Put these instructions in the pilotage plan and maintain the required listening watch.

Signals require their published local meaning. For example, Dover’s instructions say that three red entrance lights mean do not proceed without specific Port Control permission. A light showing an open entrance does not remove any requirement to obtain clearance or avoid other traffic. If the signal, radio instruction or intended movement is unclear, remain safely outside the restricted movement and ask Port Control.

Source: Dover VTS small-craft instructions.

Pilotage Techniques Master Comparison