Nayak — Marine Pilotage · Ch. 6: Safe Positioning of a Vessel in the Channel

1. Overview — Safe Positioning in the Channel

Source: NAYAK, Capt. Santosha K. Theory and Practices of Marine Pilotage. Chapter 6 — Safe Positioning of a Vessel in the Channel.

Syllabus: Anexo 2-B, Área II (Arte Naval), item 8 (NAYAK), Cap. 6 — posicionamento seguro no canal; margens; enfiamento; set tolerável. Cross-list Área III, item 6.

Once the vessel is safely inside the channel, the aim of the pilot and the bridge team is to keep her position near the centre line, where the maximum depth is expected and the cross-track error is smallest. Holding that line is the best guarantee that the ship stays well within the safe margins of the channel. Whenever an external force sets and drifts her to one side, the pilot must act so that both bow and stern remain inside the channel limits at the same time.

The reason bow and stern are stressed is that ECDIS or any electronic screen shows only the GPS position — that is, the position of the bridge. It never shows exactly where the bow or the stern lands. Keeping the bridge position within the channel therefore does not automatically place the extremities in safe water. To manage this the navigator must understand the 2D configuration of the channel, the exact location of bow and stern, and the tolerable set the channel allows.

On the ECDIS the marker sits at the bridge — it shows only the GPS position. A vessel that looks centred on screen can still have her bow or stern reaching into unsafe water — the electronic picture never draws the true position of the extremities.

2. 2D Configuration of the Channel — Blue and White Margins

In most places there is no abort point once the vessel is committed: she cannot reverse course into abundant water until she reaches the turning basin inside harbour limits. At all times her bow and stern must stay within the channel. Two zones define what "within" means — the blue margin and the white margin.

Fig. 6-1 Top view of the channel — RBL to port, GBL to starboard, centre line in the middle, blue margin at each edge and white margin further inboard

2.1 Blue margin

The blue margin is a strip along each extreme edge of the channel, running from the hand-marks line inward to an imaginary parallel line a short safe distance inside. It flags water whose depth is not kept to the correct level by regular maintenance dredging, because siltation and over-fall of seabed material frequently reduce it. Its width varies with the channel, but is typically 20–30 m on either side. If the bow or stern enters the blue margin, the vessel is likely to meet smelling of ground and sudden erratic swings of her course.

2.2 White margin

Unlike the blue margin, the white margin is the safe water available on either side of the vessel. Within it she adjusts her position so that both bow and stern stay in safe water. Its width comes from the channel width once the two blue margins and the ship's own beam are removed, and the remainder shared to each side.

$$\text{White margin} = \tfrac{1}{2}\,(\text{Channel width} - 2\,BM - \text{Ship width})$$

2.3 Significance in a narrow channel

When the vessel follows a narrow channel in heavy weather, she takes a considerable set to the lee side. It may happen that her bridge lies on the centre line while the starboard bow reaches the extreme edge of the starboard white margin; on the other tack the port bow reaches the port edge. At that point she is at her maximum tolerable set. Any further set carries the bow into the blue margin.

Once the bow is over the blue margin — where depths are unreliable and earthen fall-overs occur — it may come very close to the bank laterally while the bottom smells the ground or scratches over it. Either of two consequences follows.

Bank cushion pushes the bow away, making the vessel swing violently to the opposite side.
Scratching a shallow patch reduces the vessel's speed suddenly yet significantly.

For deep-draft vessels the danger escalates. When the bow meets a shallow patch its speed drops sharply; sensing grounding, the navigator increases power and puts the wheel hard-over to the opposite side. That brings the stern to the far channel wall — so the bow is already touching bottom while the stern reaches the blue margin, and the whole side from bow to stern grounds along the channel.

3. Determining the Course to Steer and the Position w.r.t the Centre Line

While transiting, the vessel must adjust her heading to make good an appropriate COG that keeps her within the white margins. To reach the required COG at any instant she has to decide, intermittently, what course to steer (CTS). Lateral space is far more restricted than longitudinal space: the vessel advances along the fore-and-aft direction while the athwartship room stays tight, and deep-draft ships may face UKC limits as well.

Finding the position of the vessel relative to the centre line is therefore the constant priority. A little inattention lands her near the banks within seconds; if any part — bow or quarter — comes dangerously close, bank cushion sets in and swings her to the opposite side. To decide the CTS, the navigator must know three things at any moment.

The exact location of the vessel with respect to the centre line of the channel.
The effective set, or Dash — that is, Set ± Leeway ± Gyro error, equal to a settled heading minus the corresponding settled COG, in degrees.
The maximum tolerable set of the vessel in the channel.

A correct course to steer is the most important decision a navigator makes every couple of seconds while following a narrow channel. Its weight can be understood from the grounding of M.V. Ever Given in the Suez Canal in March 2021.

3.1 Positioning the bridge relative to the centre line

Depending on the weather, the navigator decides where to place the bridge relative to the centre line, and hence the heading. Three scenarios cover the cases.

Fig. 6-2 Bridge positioning by weather — set ~0 on the centre line; weather from port sets to starboard; weather from starboard sets to port

Scenario	Weather / set	Bridge position	Heading vs general direction of channel
1	Weather negligible; set ≈ 0°	Exactly on the centre line	HDG = COG, both in the general direction of the channel
2	Weather from port; sets to starboard	Starboard of the centre line	Steer a heading less than the general direction, to make a COG parallel to the CL
3	Weather from starboard; sets to port	Port of the centre line	Steer a heading more than the general direction, to make a COG parallel to the CL

4. Sources of Position Estimation — Leading Lights and Cross-Track

While transiting a narrow channel it is critical to ascertain the exact position of the vessel relative to the centre line at any moment. Navigators should use more than one method to cross-check. The available sources are listed below.

#	Source of positioning
1	Leading light
2	Hand-marks lines — RBL & GBL (series of buoys)
3	RADAR & parallel indexing
4	Sector light
5	ECDIS & GPS
6	Virtual buoy series
7	Use of tugs for guidance — last resort

4.1 Leading lights (transit lights)

Leading lights, or transit lights, are a set of two or three lights fixed vertically at known positions. When the vessel finds them in line, she has an accurate position line; the same principle marks the centre line of a channel, telling her whether she is on the centre line or off it and by how many metres.

Fig. 6-3 Leading-lights aspect — vessel to port: lower light to the right; on the centre line: lower below upper; to starboard: lower light to the left

Scenario	Vessel position	Aspect of the lower light
1	Off-centre, on the port side	Lower light seen to the right of the upper
2	Exactly on the centre line	Lower light seen exactly below the upper
3	Off-centre, on the starboard side	Lower light seen to the left of the upper

The three figures show the vessel with three different headings on purpose: the ship may head to port of the light, onto the light, or to starboard of it. The navigator must not confuse the vessel's position with respect to the centre line with her relative heading with respect to the leading lights — position and heading may be on the same side or on opposite sides. Combining the scenarios, the lower light appears to swing pendulum-fashion from starboard to port and back as the vessel moves from port to starboard and back across the centre line.

4.2 Cross-track from the leading-light angle

The geometry links the small angle $A$ that the leading lights make with the vertical, the cross-track $X$ of the vessel from the centre line, and the distance $(d+L)$ of the vessel from the lights. For small angles the arc equals the radius times the angle in radians, i.e. radius times the angle in degrees divided by 57.3, with radius $(d+L)$.

Fig. 6-4 Leading-light geometry for cross-track — X = (d + L)·A / 57.3, where the sight-lines drop from the vertex to the channel edges

$$X = \frac{(d+L)\,A}{57.3}\qquad(A \text{ and } L \text{ variable})$$

The angle $A$ is inversely proportional to $L$, the distance of the vessel from the inward end of the channel. So for a fixed cross-track the leading-light angle grows as the vessel nears the inward end, and shrinks as she moves away. Worked figures from the book, at $A = 2°$ and $d = 2000$ m: with $L_1 = 2500$ m, $X_1 = 4500 \times 1 / 57.3 \approx 78$ m; with $L_2 = 4000$ m, $X_2 = 6000 \times 1 / 57.3 \approx 104$ m. In a well-buoyed channel the buoy distances from the inward end are established, so the pilot reads the cross-track from the leading-light angle alone.

Leading lights are rendered unusable in restricted visibility — they serve only while the lights stay visible with discernible clarity. The exact angle $A$ cannot be measured by any instrument; it is a rough estimate that grows more accurate with experience.

5. Reading the Buoy Series — Bird's-Eye and Observer's View

Looking at the aspect of the series of channel buoys, an experienced navigator can tell where the vessel lies relative to the centre line. The line of red buoys is called the Red Buoys Line (RBL) and the line of green buoys the Green Buoys Line (GBL). On the centre line the two lines look symmetrical — like a highway with even markings on both sides; off the centre line they look asymmetrical.

Fig. 6-5 Bird's-eye view of the buoy series — starboard side: red line more oblique; centre line: symmetrical; port side: green line more oblique

Scenario	Vessel position	Aspect of the buoy lines (obliquity)
1	On the starboard side of the channel	Red series looks more oblique; green series looks less oblique
2	On the centre line	Red and green series exactly symmetrical when looking ahead
3	On the port side of the channel	Green series looks more oblique; red series looks less oblique

Fig-6.5 gives the bird's-eye view of the channel; Fig-6.6 gives the matching real view to an observer's eye inside the channel. From the obliquity of the series, the navigator on the bridge tells at a glance whether the vessel lies to port or starboard.

The obliquity rule holds only in fair weather, when the buoys stay in place. In a cross wind or surface current the buoys are shifted from their original line as the mooring cable stretches, so allowance must be made for the buoys drifting towards the lee side of the original series.

5.1 Other sources of positioning

RADAR & parallel indexing — effective in restricted visibility if fixed radar-reflective objects (breakwaters, offshore platforms) lie within range. Tolerance is tight: the range must resolve a 10 m change of position, so 0.75 or 1.5 mile ranges work well. Parallel indexing on the channel buoys is possible, but the buoys carry a 30–40 m position error depending on weather.
Sector light — different-coloured sectors mark specific position lines; the point where the colour changes gives a position line, telling the navigator at a glance where he stands. Like leading lights, sector lights are unusable in restricted visibility.
ECDIS & GPS — puts nearly all navigational information on a single screen for real-time decision, but the GPS position always carries a location error of a few metres. That error should be established by the authorities in fair weather with good visual fixes and passed to navigators. ECDIS+GPS is a backup, and comes into its own during dual extremes — extreme weather with poor visibility — when no other source is available.

6. Effective Set (Dash) and Determining Gyro Error

The objective is always to keep the vessel on the centre line, steering a course that makes the vessel follow the general direction of the channel — a track parallel to the centre line — with sufficient margins on the weather-side bow and the lee-side stern. The course to steer must therefore account for the existing set and gyro error.

Fig. 6-7 Heading to steer and the Dash — COG, true course and gyro heading; Dash = ±Set ±Gyro error, Heading to steer = COG ± Dash

$$\text{Heading to steer} = \text{COG} \pm \text{Dash}, \qquad \text{Dash} = \pm\,\text{Set} \pm \text{Gyro error}$$

The term Dash is the realistic value of degrees correction the pilot actually works with. When the pilot boards he usually knows neither the true gyro error — the master's figure is often grossly wrong — nor the exact set, which varies with windage area, displacement, draft and the strength of the weather, and which keeps changing as speed and course change. He gets only a rough idea of the set. The difference between the heading he steers and the COG he obtains is the Dash: the combined result of gyro error and weather-generated set. So for a pilot the working correction is the Dash, not the gyro error or the set taken separately.

6.1 How to determine gyro error while in the channel

In open water an unnoticed gyro error is not very hazardous, but in restricted waters — where accurate course-keeping is the key — it can be dangerous. Masters should confirm the gyro error from multiple reliable sources before entering restricted waters. Three methods help the pilot ascertain it.

Method	How it works
Method 1 — on the transit	Position the vessel on the centre line, then alter slowly to head exactly onto the transit lights. Read the gyro heading $g$; the transit bearing of the channel is $t$. Gyro error = gyro heading − transit bearing.
Method 2 — fixed object	Take the RADAR bearing and the gyro bearing of a fixed conspicuous object (e.g. a breakwater) near right ahead, so the bearing changes little while measured; the difference is the gyro error.
Method 3 — pilot boat	With no conspicuous object, the pilot boat acts as the object, coming close to right ahead with no appreciable change of bearing — achieved with both stopped, or with the boat brought right ahead momentarily, or when another craft is on a collision course.

$$\text{Gyro error} = \text{Gyro heading} - \text{Transit bearing}$$

If the exercise cannot be done from the centre line, the pilot may head the transit lights from any side of the channel within the margins; the error there stays within about 1°, still inside tolerance.

7. Tolerable Set and Maximum Tolerable Set

The tolerable set is the maximum effective set a vessel can have while transiting a narrow channel safely, keeping bow and stern within the white margin. It is visualised with the vessel on the centre line: the bridge stays central while the bow sits off-centre within the white margin on either side. When the bow reaches the extreme edge of the white margin, the vessel is at her maximum tolerable set; any further set carries the bow into the blue margin.

Fig. 6-8 Tolerable set — vessel angled between the white margins; Tolerable set = tan⁻¹(White margin / LOA)

$$\text{Tolerable set (degrees)} = \tan^{-1}\!\left(\frac{\text{White margin}}{\text{LOA}}\right)$$

Fig. 6-9 Maximum tolerable set — the beam is taken from the bridge with W = 2 × white margin; Max tolerable set = tan⁻¹(W / LOA)

$$\text{Max tolerable set (degrees)} = \tan^{-1}\!\left(\frac{W}{\text{LOA}}\right),\qquad W = 2 \times \text{White margin}$$

7.1 Worked example

Channel width 150 m, vessel LOA 300 m, beam 50 m, blue margins 20 m each side. The effective safe lateral width is 110 m (= 150 − 2·BM).

Step	Value
White margin = (150 − 40 − 50) / 2	30 m each side
Tolerable set from CL = tan⁻¹(30 / 300)	5.7° either side of the centre line
W = 2 × white margin	60 m
Maximum tolerable set = tan⁻¹(60 / 300)	11.3°

Before entry, the bridge team uses these figures to decide whether to enter or avoid the channel. While transiting, if the navigator senses the weather-generated set is nearing the maximum tolerable set, he must reduce it by all available means — increasing speed or calling external assistance — to prevent shallow-water effects and consequent grounding inside the channel.

8. The Phenomenon of 'Bow Kissing the Buoys'

Bow kissing the buoys is an effect of bad weather in which the vessel steers a course that brings her bow very close to the buoys on the weather bank of the channel. Two phenomena combine to produce it.

Fig. 6-10 Bow kissing the buoys — under wind the weather-side buoys shift into the channel while the bow, steered to weather, closes dangerously on them

The buoys shift. Weather pushes one series of buoys towards the channel by a few metres — sometimes 20–30 m — while the seabed holder stays put; the opposite series moves away by the same amount. Visually the whole buoyed channel appears to shift to the lee side, though the dredged channel stays where it was. In a channel only about 100 m wide this visual shift matters greatly.
The ship steers to weather. The same weather sets the vessel, so to make a COG in the general direction she must steer a heading a few degrees towards the weather side. This brings her weather-side bow closer to the buoys line on the weather side.

The combined effect brings the weather-bank buoys dangerously close to the weather side of the bow. The scare is worse on large vessels in light condition, where the blind zone at the bow makes the visuals even more alarming.

The instinct is to alter away from the buoys, towards the lee side — but that creates more drift to the lee bank. The vessel then moves bodily off the centre line and the stern comes dangerously close to the lee bank; if the stern smells the ground, the vessel behaves unusually and heaves suddenly on her course. Seasoned navigators stay calm and hold the course even though it looks scary.

9. Bridge Procedures for Safe Keeping of the Vessel in the Channel

Some good practices help the bridge team keep the vessel safe while following a buoyed narrow channel.

Maintain sufficient speed to reduce the effect of external factors and keep the vessel within channel limits.
Keep to the speed-control plan and never exceed it because of poor course-keeping, except momentarily in an urgency with higher engine power.
If safe keeping becomes impossible, or set and lateral drift are excessive despite adequate speed, call external assistance (tugs) well in advance.
Continuously cross-check engine and helm orders to avoid a wrong command; master and duty officer cross-check each other.
Complete the closed loop of communication between the issuer and receiver of every order, and brief the bridge team on it.
Correct helm orders and good steering are crucial to safe keeping.
The pilot must ascertain any gyro error in the steering compass as early as possible after boarding, by his own calculation.
Confirm safe positioning by external source, using at least two available means.
Treat electronic positioning as a backup to the visual checks, referring to it only to confirm the exact location.