AN Ch. 7 — Towing Equipment

1. Introduction — varying the towing point and line length (7.1)

Source: HENSEN, Henk. Tug Use in Port: A Practical Guide. 4th ed. Rotterdam: STC Publishing, 2021. Chapter 7 — Towing equipment.

Edital: Anexo 2-B, Área II (Arte Naval / Shiphandling), item 9 (Hensen) → Chapter 7 — Towing equipment · Anexo 2-A, Área II, item 26 (Utilização de rebocadores portuários — equipamento de reboque: cabos, pontos de reboque, ganchos, guinchos, gob ropes, sistemas de escape rápido, cargas de trabalho seguras) com itens 4 (Emprego de rebocadores na manobrabilidade) e 31 (Métodos de utilização de rebocadores empregados no Brasil).

Earlier chapters stressed that the location of the towing point governs both the safety and the performance of a tug. The two are tightly linked: the more safely a tug can work in different conditions, the fewer its limitations. A clear example — the higher the towing point of a conventional tug, the larger the list when towing on a line, because the athwartships force has a longer lever; that raises the risk of girting and so limits performance, above all for conventional tugs but for other types as well.

There are two main ways to improve performance and safety through deck equipment:

• Make the towing point transferable, or fit more than one fixed towing point — this affects both performance and safety.
• Fit a quick-release system — an emergency safety device.

A third need, also raised before, is to vary the length of the towline while assisting — for instance when dynamic forces from waves are high, to cut the counteracting effect of the tug's propeller wash on the ship's hull, or when the tug's manoeuvring space is limited. This chapter covers the deck means of varying both the towing point and the towline length, the value of a good deck arrangement, how a line can be released in an emergency (the quick-release system) and — crucially — the towline itself, the one link between tug and ship.

2. Additional towing points and gob ropes (7.2)

Being able to shift the towing point especially helps conventional tugs. Transferable systems are grouped by how they move the point: along the tug's centre line (fore-and-aft), along a partly circular track, or along a full circular track.

2.1 Moving the towing point along the centre line (7.2.1)

Additional fixed towing points. The simplest way is to provide more than one fixed point. Some combi-tugs carry an extra towing point far aft, which greatly enhances their value as a stern tug and lets them act almost as a tractor tug. Some VS tractor tugs built for escorting have an extra point far aft to cut the steering effort needed to stay in line with the escorted ship; to deliver steering force the original point is used. Tugs may also have hydraulically operated towing pins (Fig. 7.2) that shift the point aft or forward, e.g. for escorting — increasingly seen on harbour tugs (tug Telstar, CRT tug). One caution: towing pins like those on Telstar may crush the towline, increasing wear.

Gob-rope systems. A second, conventional-only method uses a gob rope to move the point fore-and-aft. In the basic arrangement a length of wire is secured to a side bollard, led through a fairlead or small H-bollard on the centreline of the work deck, and ended in a large shackle that slides freely along the towline (Fig. 7.3A). As the towline moves more abeam, the gob rope tightens and relocates the towing point between the original fixed point and the fairlead. An improved version has a separate gob-rope winch (Fig. 7.3C), controlled from the wheelhouse if possible, with the wire led through a central swivel fairlead at the very stern; varying its length can shift the towing point right to the after end, so a conventional tug can do steering or speed control simply by shifting the point — comparable to a tractor tug with its point near the stern. This arrangement is, for instance, compulsory in German ports.

2.2 Partly circular and full circular tracks (7.2.2)

The radial towing hook moves the point along a partly circular track (Figs. 7.6–7.7); the same can be done with a towing winch whose line runs via a fixed fairlead and then a second fairlead moving on a circular rail (Fig. 7.8). Radial systems give smaller heeling angles, so higher athwartships towline forces can be applied — more performance and more safety. The radial hook is currently used only by conventional tugs (the "azimuth friction-free towing point", Fig. 7.9, is a related idea). Some tugs instead have towing staples with an athwartships opening, letting the towrope slide to the low side and create an extra righting moment (and probably extra abrasion). Full circular tracks are the carrousel arrangements discussed in Chapter 2 — the DOT tug and the CRT (Carrousel RAVE Tug).

3. Towing bitts, hooks and winches (7.3)

3.1 Method of towing and varying line length (7.3.1)

Varying a towline's length is often hard or impossible, and depends on how the line is secured. On tugs without a winch, towing bitts, bollards and quick-release hooks are used (bitts and bollards also serve when more than one line is made fast even on winch tugs). When ship lines are used they are often put on the towing hook, and the ship's crew must adjust the length — slow, given today's small crews. A tug's own fixed-length towlines (an eye spliced at each end) cannot be varied at all, so such tugs carry two or more of different lengths. A line on a bitt can be re-secured longer or shorter, but only when slack and with time and manpower; in an emergency a line on a bitt is almost impossible to release, whereas a quick-release hook should release without trouble if the system is reliable. The bottom line: only a towing winch lets the length be varied efficiently, and operational safety is involved when a line cannot be slackened or released fast.

3.2 Towing hooks (7.3.2)

Besides the radial hook there are two basic types — the standard hook and the disc hook (Fig. 7.10). The disc hook is a round plate with a hook-shaped opening, developed to absorb the stored energy when a line under tension is released (fibre towlines with large stretch store a lot of energy): instead of a heavy impact on hook and deck, the released energy hugely accelerates the disc itself. Standard hooks may use rubber buffers to soak up impact. Hooks can also have spring shock absorbers to reduce peak forces, and load-monitoring. Classification societies require a quick-release system operable both locally and by remote control from the wheelhouse (Bureau Veritas, for instance, demands emergency release from the bridge with full view and at the hook, identical means at each station, protected against unintentional use, with the procedures communicated to the crew).

Modern designs include the hydraulically locked towing hook and hydraulically locked towing bitt (Brusselle, with URS): the hook or tumbling bitt is held by a hydraulic cylinder, and when release is operated the pressure falls and it opens or tumbles (Note 1: site the valve close to the cylinder, electrically activated, for speed). An automatic release exists too (older Russian tugs in St Petersburg): at a preset heel an iron ball is freed and its weight pulls the wire that opens the hook; modern electronic versions release at a preset angle. Whatever the type, only one thing matters — the system must be fully reliable and trouble-free under all conditions.

3.3 Towing winches (7.3.3)

The towing winch is an essential part of the link, and tug performance and safety depend on it working properly. The operator must have a good view of the winch, or a line trapped between lower layers may be heaved in again instead of paid out. A winch lets the length be adjusted at any time, usually from the wheelhouse and without extra manpower, and makes handling heavy lines faster and easier. Automatic spooling gear is sometimes fitted but, because harbour towlines are short, often absent or unused. Drives are through reduction gears, powered hydraulically or electrically. A frequency-controlled (electric) winch offers smooth stepless control, easy installation, space savings and immediate readiness; its main edge is efficiency — hydraulic systems lose 20–40 % of power, electric only 10–15 % — which matters for continuous use but little for the occasional use typical of harbour tugs.

3.3.1 Winch characteristics (7.3.3.1)

Four characteristics matter for ship assistance:

• Brake holding capacity. The holding power of the brake, referred to the first layer on the drum (more layers → lower capacity). For harbour towing it is typically two to three times bollard pull, though policies vary down to one times. There is a key trade-off with towline strength: a low holding capacity with a high-strength line lets the brake slip before the line breaks (protecting and prolonging the line) but can limit performance, since at steep angles towline force exceeds bollard pull and power must be cut to avoid slip; a high holding capacity (e.g. 3× BP) lets the line break before the brake slips unless quick-release is used in time, but limits performance less.
• Maximum pulling capacity. The stall heaving capacity — the maximum line pull at the first layer with control in heave and the line stationary. As the outer-layer diameter is often about twice the first-layer diameter, the pull from the top layer is only about 50 % (a 100-ton brake slips at ~50 tons from the top layer). A "15-ton winch" means 15 tons maximum pull; the pull falls as drum speed rises.
• Rated pull / pull-speed. Pull at a nominal speed, given e.g. as "10 tons × 10 m/min". These matter most in narrow basins and high wind/current: a tug may have to cut power to shorten line, and all that time the ship drifts — so the higher the pull and related speed, the better.
• Slack-line speed. The faster a slack line is retrieved when letting go, the less the risk of fouling a propeller and the sooner the tug can switch to pushing.

Towing winches carry quick-release operable at the winch and from the wheelhouse, which minimises girting risk — but a reservation: quick-release is often too slow for girting, which is generally a very fast process with rapidly rising forces.

3.3.2 Types of towing winch (7.3.3.2)

• Single / double drum. A double drum gives a backup line (if one parts) or different lines for different purposes (harbour vs sea). Drums may be separately or jointly connected to the shaft, each with its own band brake (Figs. 7.11–7.12).
• Waterfall winch. Two or three drums stepped up and back like a waterfall, mainly on anchor-handling and sea-going tugs (Fig. 7.13); the top drum holds the main/sea line, lower drums the working/harbour lines (Note 4: drum capacities vary a lot).
• Split drum winch. Well suited to fibre lines (Fig. 7.14): a single drum with a tension section and a storage section. It reduces crushing of inner layers and the torque on the shaft, because only a few layers sit on the tension drum; somewhat harder to operate, and central-flange damage has been reported in those still in use.
• Double winch. Two independent bow lines at once — on several Japanese reverse-tractor tugs and Panama Canal tugs.

3.3.3 Classified by towline-force control (7.3.3.3)

Winches can carry a self-rendering / tension device that automatically pays out above, and heaves below, a set tension — for shock loads in waves/swell, but not suited to narrow port areas. By force control there are three families:

• Conventional winch (Fig. 7.15). A drum with a brake on one side and a drive on the other; pulling by the drive, paying out by varying the band brake. The drive is generally used only during connection (low load) and disconnected during towing to spare the gears. Critical flaw: with a band brake the required release force rises with towline load, and above a certain load it exceeds the release system's force — so the quick-release no longer works. Typical use: sheltered waters, low assistance speed.
• Constant-tension winch. A large torque motor solidly geared to the drum, holding a roughly constant tension with slowly varying force; but payout speed is limited by motor speed, and as tension rises the veering speed falls. Problematic in emergencies when tension must be lowered fast — possible capsizing. Release speed at full load 10–20 m/min, at low load 50–75 m/min. Typical use: at sea in long waves and slow motions.
• Render-recovery winch. The tug master sets the forces at which it should render and recover well before the operation. Unlike constant tension, the render mechanism is separate from recovery (recovery by the drive/motor, release by a slipping clutch), switching automatically by load — its big advantage being a quick, reliable abort at high speed and high load. Control is mechanical (instant release at overload) or electronic. The reaction delay must be near zero and absolutely reliable. Typical use: exposed conditions, short rough waves, high assistance speeds (escorting).

3.3.4 Requirements for towing winches (7.3.3.4)

Classification societies set requirements (DNVGL example). For the drum: it must be able to be declutched from the drive; the drum diameter must be ≥ 14 times the towline diameter (and never below the manufacturer's bending radius); at least three dead turns must remain; disc sheaves must surmount the top layer by at least 2.5 rope diameters; each drum of a multi-drum winch must work independently; and the inboard end must attach with a weak link that releases at low load. For the emergency release: drum release must be possible in all modes and even in black-out, with brakes back in normal function immediately, acting within a maximum of three seconds, paying out in a controlled manner (no free spinning), with energy for at least three release attempts (or five minutes of held-open brake), operable from all control stands, sited beside the emergency-stop, and taking priority over the emergency-stop.

4. Towline safety systems — cutters and weak links (7.4)

4.1 The cutter (7.4.1)

If the winch brake will not slip, the line cannot be released or cut in time, and the quick-release fails or is too late, the tug and crew are in a helpless, very serious situation — and the only hope left is the line parting. These are real situations. A line may part because a low-safety-factor line is used, or because the minimum breaking load has fallen with age. But a tug should also be able to assist to its full capability and not be limited by towline strength — so deliberately weak lines are not the answer.

Webtool's portable emergency rope cutter (Fig. 7.19–7.20), first made for tankers to stop the pull-back tug capsizing, is a different solution: being portable, it can sit on the tug or the tanker, in two modules — the cutter and a charged hydraulic power source. A wire version cuts steel wire up to 70 mm within three seconds; a fibre version severs ropes including Dyneema and Spectra. It can be built into towing hardware (e.g. an H-post, swinging out of the way), built into the winch so the rope always passes through it, or hung onto the rope before the fairlead. It is a good last-resort option, but must be kept always ready, because emergencies are never expected.

4.2 The KO-LINK weak link (7.4.2)

Sometimes a deliberate weak link is used — a pendant with a breaking strength below that of the main line. The KO-LINK (Lankhorst, Fig. 7.21) is a ring that joins main line and pennant so as to create a controlled weak link that prevents overload, while replacing the cow-hitch knot (which, if badly made, unequally loads the eyes and causes premature failure). Using a weak link of perhaps twice bollard pull instead of three times means the link parts and the main towline is saved — similarly, a winch with the MBL higher than the brake holding power lets the brake slip with the same effect. Either way the tug may avoid capsize, though the assisted ship loses its assistance. The penalty: the weak link's lower safety factor makes it prone to fatigue under frequent high loads, so it must be carefully analysed and regularly replaced to prevent early parting.

5. Towlines — requirements, types and materials (7.5.1–7.5.3)

5.1 Requirements (7.5.1)

A towline must carry the load between tug and ship and cope with dynamic loads from their relative motion. Four basic requirements follow:

• Tensile strength — enough for the forces of shiphandling.
• Elongation — dynamic loads must be well compensated, by the line or other means, to avoid excessive peaks on line, winch and fairleads.
• Linear density — manageable on tug and ship; flexible enough to handle when no winch is used.
• Life — minimum wear, distortion and loss of strength.

5.2 Steel wire rope (7.5.2.1)

A steel wire rope is strands of wires wound around a fibre or wire core (Fig. 7.23). It is described by strands × wires + core type, e.g. 6 × 36 IWRC (Independent Wire Rope Core). The lay is the twisting of wires or strands:

• Right- / left-hand lay — strands clockwise (right) or anti-clockwise (left) viewed along the rope.
• Equal lay vs cross lay — equal-lay wires (all the same lay length) outlast cross-lay (different lengths, high stress at the crossings → early internal failure), are stronger and resist cyclic loading better; the best is the Warrington/Seale construction.
• Lang's lay vs ordinary lay — Lang's (wires and strands laid the same way) wears better but tends to untwist; ordinary lays them oppositely.

More wires give flexibility and fatigue resistance but less abrasion resistance; a steel wire core resists winch crushing, is 7–8 % stronger and stretches slightly less than a fibre core. Towline grades are usually 1770 N/mm² (180 kgf/mm²) or the stronger 1960 N/mm² (200 kgf/mm²); in the USA, IPS ≈ 1770 and XIPS is higher. Maintenance: inspect regularly (especially at eyes and shackles) for broken wires, corrosion and deformation; OCIMF discards a rope with more than 4 broken wires over 6d, or 8 over 30d (d = diameter).

5.3 Synthetic fibre rope (7.5.2.2)

As bollard pull rose, steel lines grew heavy and hard to handle, so fibre towlines are increasingly preferred for their strength, stretch and light weight — escort tugs especially, with their high indirect-mode forces. Construction runs from yarns to strands or plaits to rope, the strands laid opposite to the yarns to stop unlaying (S-lay = left hand, Z-lay = right hand). Common constructions:

• Three-strand (hawser-laid) — the commonest twisted rope; tends to kink/hockle (which cuts strength); good abrasion resistance.
• Six-strand with core — like wire rope, less prone to hockling.
• Eight-strand plaited (square braid) — four pairs alternately left/right lay, virtually unkinkable, flexible, high energy absorption.
• Twelve-strand (hollow braid) — easy to splice, non-rotating, very efficient; the 12×12 HMPE rope (twelve 12-strand ropes braided together) is much used for towlines, including escort lines.
• Braided / double braid — equal Z and S strands make a torque-free rope that does not twist under load; a double braid is a core rope inside a cover rope, with different fibres tuned for stretch, float, abrasion, etc. With low-stretch high-performance fibres (Spectra, Dyneema, Kevlar) the cover is merely a protective jacket and adds no strength.

Conventional fibres.

• Polyester — heaviest conventional fibre, does not float, most durable, high wet/dry strength, excellent abrasion, low extension, high friction (good for belaying), high melting point.
• Nylon (polyamide) — does not float, strongest man-made fibre after Aramid/Dyneema/Spectra, but wet strength only 80–85 % of dry and loses strength fast under cyclic loading; highest stretch.
• Polypropylene — lightest, floats, weakest of the three, low melting point (fuses under friction), degrades in UV.
• Blends (e.g. polyester/polypropylene) — fall between the pure ropes; the polypropylene fuses first and protects the polyester from fusion damage (Note 10: in very cold areas all towline performance, including modern fibres, changes).

5.4 Characteristics — strength, friction, chafe (7.5.2.3)

Figs. 7.27–7.29 compare elongation, properties and fatigue. A new synthetic fibre stretches significantly more than a "broken-in" one, because working the rope removes the void spaces and aligns the fibres. Elastic (working) elongation is the immediately recovered stretch at a given load; it falls as the rope is used and depends on construction (twisted ropes stretch more than braids). For tension-tension fatigue, Dyneema survives far more cycles to failure at a given load than Aramid, polyester or steel wire — but if peak loads rise, cycles to failure fall fast for Dyneema and Aramid.

Strength definition matters: US practice uses spliced ropes (breaking strengths reported ~15 % higher when un-spliced samples are used); ISO 2307 counts a 10 % reduction for a factory-spliced rope; the pennant-mainline connection may be weaker still — so the MBL of the whole towline is what counts. External coatings and finishes (e.g. polyurethane) add strength and abrasion/UV resistance. Chafe gear (anti-chafing sleeves) protects the strength member and is far cheaper to replace than a pennant.

5.5 Composition of towlines (7.5.3)

A harbour towline can be a single steel wire + messenger; steel wire + stretcher + steel pendant + messenger; fibre + steel pendant + messenger; or fibre with/without fibre pendant + messenger. The messenger must float, be long enough for the highest ships, and be strong and grippable — best braided and torque-balanced (a three-strand messenger rotates and twists the pendant/main line), about 25 mm, in polypropylene (high friction, floats); HMPE messengers slip on the capstan and should not be used. The messenger should join the pendant or main line through a grommet rather than a girth/cow-hitch directly on the eye, which bites down under load and abrades the eye; a swivel may be added against twist.

Towlines, pendants and stretchers. A steel pendant is usually the same construction as the line but smaller diameter (so it breaks first and only it is replaced). Stretchers (about 10 m) absorb dynamic load; nylon stretches most but degrades wet and damages steel-wire towlines torsionally, so polyester or polyester/polypropylene are often preferred; a stretcher should have 25 % higher dry breaking strength than the wire/main rope (OCIMF wants 37 % extra for nylon, because nylon loses 10–15 % wet). For low-stretch Dyneema/Spectra lines, a nylon or polyester pennant adds back some stretch, important for dynamic absorption and to limit peak loads, especially with short lines.

Connections. Cow-hitch or eye-to-eye join two similar ropes without hardware (cow-hitch does not significantly weaken a correctly sized assembly; a grommet leg is assumed at 85 % of rated break force); the synthetic-fibre rope connection (Fig. 7.35, e.g. Lankhorst's) can be made as the strongest connection or as a weak link, and releases easily.

6. Towline strength, length and safety factors (7.5.4–7.5.6)

6.1 Strength reduction — the SRT study (7.5.4)

Towline strength falls over time and jobs, and how the lines are treated matters greatly. A Samson / DSM study on Crowley escort tugs (12-strand Dyneema SK75; pennants used ~1 year / ~600 jobs, main lines ~2 years / ~1,200 jobs, reversed end-for-end after a year) found, on used lines:

• The ends (to 65 m) kept only 61 % of original strength — a loss of nearly 40 %; the midsection 81 %; pennants 63 %.
• Total reduction up to 40 %: abrasion/cutting 5–10 %, drum compression 12 %, line twists (1–1½ turns/foot) 15–20 %.
• Shock loading seemed to have no effect on residual strength, provided the tug is handled in a controlled manner.

6.2 Retirement, inspection, maintenance (7.5.5)

The goal is to retire a towline before it ruptures. Inspection has three levels: deployment (external, by the crew as the line goes out), routine (external + internal of the in-service section, every fixed number of months) and detailed (full length, trained crew and/or manufacturer). Residual-strength testing means cutting a sample and sending it to a facility; OCIMF recommends retirement at 75 % residual strength. Keep rope certificates and a logbook of hours, jobs, maintenance and damage. Key points of attention:

• Small-diameter fairleads — bending HMPE under tension around a small radius causes local heat, melting and sudden failure; the smaller the D:d ratio, the worse.
• Abrasion — still a main cause of failure: external ("fuzzy" surface, shields the inner fibres) and internal (broken/fused fibres within, from repeated bending) — hard to inspect on jacketed or double-braid lines.
• Axial compression / kinking — once thought not to affect HMPE, but the Zarga rope showed yarn kinks along the load-bearing core; remove kinks under no load.
• Line twist, glazed areas, inconsistent diameter, discolouration, stiffness — all reduce strength or signal shock loads, heat, internal damage or contamination; retire if in doubt.
• Storage — clear of chemicals, oil, paint vapours; UV harms polyethylene, polypropylene and Aramid; clean before storing; end-for-end periodically to spread wear.

6.3 Basic towline length (7.5.4 bis)

The master sets the length by experience; it depends on tug and ship size, deck height, conditions, manoeuvring space and ship's speed. A short towline gives faster tug reaction — when the length is doubled, the reaction time roughly doubles (Fig. 7.36). Three advantages of a short line:

• Faster tug reaction (to check a sudden sheer).
• Reduced ship's path width (less time to sheer or drift).
• Less manoeuvring lane for the ship-plus-tugs combination.

But a short line can cut effectiveness through the tug's propeller wash on the hull, so sufficient bollard pull is needed to compensate (higher BP also restores position faster). Where space is very tight (dockyards, narrow bridges), two short towlines on the forward tug give the quickest reaction.

So with a steep line: small vertical tug movements (port waves, passing ships) create high peak loads; the high fairlead/staple friction means heat and wear, shortening the line's life (strong, replaceable pendants are recommended). The pulling effect $P_1$ stays equal to $P$, but the negative propeller-wash effect can still reduce it — least with a tractor tug, whose units are farther from the hull (Note 11: the wash effect is small when longitudinal along the hull, large when transverse). For broadside work (Fig. 7.40) a steeper line raises the righting force and lowers the heeling force, so a very short line is then good for safety; but a forward tug at speed must weigh that against the very small bow-to-tug gap and little reaction time — which is why masters dislike short lines in dense fog or at high ship speed.

6.4 Strength and safety factors (7.5.5 bis)

How static forces grow with the vertical line angle (Fig. 7.41): up to 40° the rise is modest, but beyond it the force climbs fast — at 60° it is already twice the towing force; at 45–50° (a large angle when towing on a line) the static force is about 1.5 × the towing force. Dynamic forces add to this — from accelerations, wrong manoeuvres, waves and swell — and vertical accelerations (waves, passing-ship wash) cannot be controlled by careful handling; a longer, more elastic line absorbs them better. Towline forces over twice bollard pull are not uncommon, especially with low-stretch steel or HMPE lines without a stretcher.

(Note 13: bollard pull is the main factor for harbour tugs because mass and underwater area relate closely to tug size and power; for escort tugs the indirect-mode forces far exceed bollard pull, so a separate criterion is needed, though escort lines are nearly horizontal.)

6.5 Ship's mooring lines as towlines (7.5.6)

7. Towline handling (7.6)

As tug power rises, especially with steel lines, towlines get harder to handle, while ship crews have shrunk — often without appreciation of the workload of arrival/departure line handling. This drives the development of alternative attachment systems.

7.1 Safe handling aboard ships (7.6.1)

The rules (OCIMF "Effective Mooring", UK Code of Safe Working Practices) are mostly about keeping people clear of a line whose load they cannot judge:

• Have enough safe heaving lines of proper length and strength ready in good time.
• The tug's towline condition and load are unknown to the ship's crew — stay well clear of it at all times.
• The person in charge must watch the operation so no load comes on before the line is secured or while casting off.
• Never let a tug go until instructed from the bridge; do not act on the tug crew's directions.
• Heave the eye past the bitts to get slack, stopper off, then put the eye on; never manhandle a line onto a bitt with too little slack.
• Do not stand on a slack line to hold it. When letting go, heave in with the messenger and stopper off before taking the eye off; take turns of the messenger around the bollard to control lowering; make no attempt to handle a line with weight on it.

7.2 Methods for passing and securing (7.6.2)

Heaving lines are an old system, often misused — a recurring problem, especially for the forward tug — so alternatives have been proposed. Tugs can carry a crane with a hydraulic clamp to deliver the line (some CH Cates reverse-tractor tugs), though lighter high-strength lines make these nearly redundant. The Aarts Autohook automated hook-up uses a wheelhouse-controlled crane ("manipulator") to place a ball connector into a hook-up point on the ship — no crew on either deck, fast and at higher ship speed — but it needs ships fitted with hook-up points, is hard to aim at night, in poor visibility or in waves (the connector moves most), and is considered impractical even in small waves (though motion-compensating systems might extend it). The connection problem is central to the growing discussion of autonomous and remote-controlled tugs.

8. SWL of the ship's towing equipment (7.7)

Harbour-tug bollard pull has risen from about 20–30 tons to 70–80 tons (100+ for terminal tugs) in 30–40 years, with three consequences for shiphandling:

• The strength of ships' bitts, fairleads and supporting structures did not keep pace, so fittings have been damaged or torn from their foundations — there is no clear relation between tug power and ship-fitting strength.
• Tugs must therefore reduce power so the towline force stays within the fittings' SWL.
• Pulling at reduced power raises the risk to the ship — if two 70-ton tugs cannot deliver against wind pressure, the ship may drift or an extra tug is needed.

Ships built ≥ 2024 (MSC.1/Circ. 1175/Rev.1). Two methods are now defined — normal towing (manoeuvring in ports and sheltered waters) and other towing (by another ship or tug, e.g. emergency, a different definition from the original circular):

• Minimum design load for supporting structure: 1.25 × maximum towing load for normal towing; the ship's own towline minimum breaking load (based on the Equipment Number EN) for other towing; the greater of the two for combined fittings — and never more than twice the line design load in any direction.
• TOW (safe towing load): must not exceed 80 % of the design loads, marked by weld bead; combined fittings carry both SWL (mooring) and TOW (towing). For bollards the line attachment is taken at not less than 4/5 of the tube height.

A crucial open question remains: which maximum towing load (bollard pull) to base TOW on, and for which ships.

9. Emergency towing equipment, escort and pull-back (7.8)

Emergency towing equipment has little to do with harbour towage but matters for emergencies — towing a disabled tanker out of danger to prevent pollution after loss of propulsion or steering (it may also suit an escort tug's connection).

The IMO "Guidelines for Emergency Towing Arrangements for Tankers" cover oil, gas and chemical tankers. The major components (Fig. 7.47) are pick-up gear, towing pennant, fairlead, strongpoint, roller pedestal and chafing gear:

• Deployment time — at least one arrangement pre-rigged and deployable in harbour conditions in ≤ 15 minutes by one person with no power; a non-pre-rigged one in ≤ 1 hour.
• Strength — working strength (half ultimate) at least 1,000 kN (20,000 to < 50,000 dwt) or 2,000 kN (≥ 50,000 dwt), at all relevant angles (up to 90° each side, 30° down).
• Pennant length — at least twice the highest ballast freeboard at the fairlead plus 50 m.
• Main parts — a strongpoint, a fairlead, a towing connection (chafing chain + Dyneema/Spectra pennant, optionally a nylon shock absorber, or a steel pennant on a drum) and pick-up gear (messenger + pick-up line with light buoys, or a floating messenger with marker buoy).

Deployment is usually manual or by an air rifle that shoots a pick-up line; some systems are remote-controlled even after the crew has left.

Escort and pull-back. OCIMF recommends using the SOLAS emergency arrangements also for escort/pull-back (without impeding them), with separate strongpoints and chocks designed for a minimum of twice the SWL rating (see also the STAG guidelines).

10. A new emergency towing concept — EVATS (7.9)

SOLAS requires a ship-specific emergency towing procedure on all ships since 2012, but emergency towing equipment only on tank vessels ≥ 20,000 dwt — although bulk carriers, container and ro-ro ships can also become disabled. Several prefabricated systems meet IMO requirements; one new system aims to serve all ships.

The Emergency Vessel Attachment and Towing System (EVATS™) — by Glosten with Samson, for the Alaska Maritime Prevention & Response Network — is a synthetic HMPE system that connects a towline to multiple attachment points on a disabled ship. It is deployed from the ship into the water, where the towing end is recovered from the surface; it can be mobilised ashore, delivered by helicopter, carried aboard, or deployed from a towing vessel. The two standard designs (300 MT and 411 MT) exceed the SOLAS strength requirements. It consists of:

• An HMPE floating hawser with two tubular floats and an Orkot® (composite) thimble at the towing end, for attaching synthetic to wire ropes with a shackle.
• A retrieving line with a small high-strength sea anchor, two marker buoys and a lighted strobe; the sea anchor pulls the line and hawser taut on the surface upwind of the drifting ship, creating distance from the bow before connecting (the tug's line then shackles to the Orkot® thimble).
• A bridle with port and starboard legs, joined to the hawser by a Glosten metal "bushing" that self-positions in the bight so both legs share load equally even when rigged asymmetrically — sized to pass through standard chocks and fairleads, tested to over 400 tonnes.

The bridle passes through the centreline chock forward and attaches to multiple deck fittings to spread the load, so individual bitts are less likely to be overloaded; a textured fibre gives grip, chafe protection guards contact points, and the legs are belayed in a figure-8 so they can be set to length and released under tension. Deployment works even in a blackout with no deck power, and two to four crew can rig and deploy it. The system is patent-protected in the USA and internationally.