Commercial Marine / Towing / Technical Guide

Towing Pennants in Tug Operations: The Sacrificial Link That Protects Your Mainline

On a working tug, the pennant is one of the least expensive components in the towing assembly — yet it has a major influence on mainline life, connection speed and the reliability of the towing system under load. This guide is written for tug masters, marine superintendents and harbour towage operators: what the pennant is there to do, how the two strength philosophies differ between ship assist and ocean towing, and what two recent UK tug incidents taught the whole industry about inspection and retirement.

Whether you run ASD tugs on ship assist, conventional tugs on barge work, or escort duties at a terminal, the towing pennant sits at the hardest-working point of the whole system: the last few metres between your winch line and the assisted vessel’s bitts. It passes through the ship’s chocks and over deck hardware that is rarely in pristine condition, takes the worst of the chafe and shock, and gets connected and disconnected more often than any other part of the gear. Getting the pennant right is one of the highest-return decisions a tug operator can make across the commercial marine fleet.

What does a pennant actually do in tug operations?

A towing pennant (you will also see pendant in North American usage) is a relatively short length of rope or wire connected to the working end of the tug’s main towline or winch line. On a typical harbour ship-assist job, the pennant is what goes up through the panama chock and onto the ship’s bitts, while the mainline stays on the winch drum and pays out astern.

The pennant has three jobs:

• Absorb the abrasion. Ships’ chocks, fairleads and bitts are often pitted, scored and rusted. Every surge of the tug drags the line across that hardware. The pennant takes that damage so the mainline does not.
• Speed up connection. A lightweight synthetic pennant — particularly HMPE, which also floats — with a large soft eye is faster and easier for the ship’s crew to heave up and drop over the bitts than a heavier mainline, which matters when the pilot wants tugs made fast quickly.
• Act as the planned point of replacement. This is the economic heart of the pennant concept. A pennant is a fraction of the length — and cost — of a full mainline. When it wears out, you replace the pennant, not the drum. Operators who run without pennants end up cropping and re-splicing the expensive mainline instead, shortening it job by job.

That third point is worth stating plainly: the pennant is sacrificial by design — sacrificial in the economic sense, meaning it is expected to wear and be replaced on a schedule, not that it should be deliberately weaker than the mainline (a separate concept covered below). A pennant that has been in service “as long as anyone can remember” is not a sign of good gear — it is a warning sign, for reasons the incidents below make clear.

Stronger or weaker than the mainline? Two philosophies

Ask two experienced towing people whether the pennant should be stronger or weaker than the mainline and you may get opposite answers — because both exist, in different corners of the industry. Knowing which philosophy your towing plan follows is fundamental.

Approach	Where it applies	Logic
Pennant at least as strong as the mainline	Harbour towage, ship assist, escort — everyday tug operations	The pennant wears faster than anything else in the system, so its new strength must be equal to or greater than the mainline’s to keep an adequate margin as it degrades in service. OCIMF’s Static Towing Assembly Guidelines — written for static towage of tankers, but widely referenced across towage — state that the MBL of a spliced synthetic pennant should be not less than that of the main towline, because of anticipated wear and tear on pennants [5].
Deliberate weak link (“fuse”)	Some ocean tows, rig moves and barge tows, where specified by the towing plan or warranty surveyor	A short, cheap fuse element is intentionally sized below the strength of the rest of the assembly, so that if the system is shock-overloaded, the inexpensive fuse parts instead of the mainline — protecting the costly tow wire and simplifying recovery. Not all operators accept this principle, and it must never be improvised.

The danger zone is the middle: a pennant that is weaker than the mainline by accident — through age, wear, or simply being the wrong rope for the job. That is not a fuse; it is an unplanned failure point at the working end of a loaded towline. If your operation intends a weak link, it should be specified in the towing plan with a defined strength. If it does not, the pennant should be specified at or above mainline strength when new, and replaced before wear erodes that margin.

What a UK tug pennant failure taught the industry

The most instructive case study for any tug operator is the UK Marine Accident Investigation Branch’s report into a towline pennant that parted on a harbour tug in Southampton in December 2019, while the tug was working as the stern tug assisting an ultra-large container ship onto its berth [1]. As the pilot called for 50% and then 100% astern thrust, the tug’s winch brake slipped twice under the rising load; moments later the pennant parted close to the ship’s deck and the towline snapped back, shattering a wheelhouse window and injuring five crew inside with flying glass.

The investigation’s laboratory findings should change how every operator thinks about pennant condition:

• The pennant parted at roughly half its original breaking strength. The pennant — a three-strand polyester load-bearing core inside a braided protective jacket — had an MBL of 229.5 tonnes, three times the tug’s full astern bollard pull. Yet destructive testing of the recovered line recorded failure at 119 tonnes, and the MAIB concluded the load at parting was likely equal to or less than that: around 52% of original MBL [1][2].
• The damage was internal and effectively invisible. Specialist examination found two damaged areas in the load-bearing core, including short-wavelength Z-kinks associated with axial compression fatigue in the yarns — degradation the MAIB judged would have been difficult for the tug’s crew to detect [1][2].
• Shock loading was the trigger. The report found it much more likely the pennant parted under the elevated shock loading that followed the brake slippages — snatch loads a fatigued pennant could no longer absorb [1][2].

The investigation’s conclusion echoed the point made above: pennants are a sacrificial element and should be replaced frequently — certainly after damage or high-load occurrences [2]. The report also examined whether the pennant in use was one recommended by the rope’s manufacturer — a question every operator should be able to answer for their own gear [2].

And this was not a one-off. In July 2024, a tow rope on another UK harbour tug at Bristol parted after around 800 uses. The parted line was rotated end-for-end and returned to service without any residual strength testing — and in February 2025 the same tug’s towline parted again under load, this time recoiling into the wheelhouse and injuring two crew. The MAIB’s chief inspector subsequently wrote to the operator warning that towline parting risked becoming “normalised” in its operations [3][4].

For a working tug fleet, the practical lessons are threefold: retire pennants on time and event criteria, not appearance; log shock events (brake slips, snatches, girting near-misses) against the pennant in service, and treat any such event as grounds for replacement or residual strength testing — never for casual reuse; and use pennants matched to the mainline by the rope manufacturer, rather than pairing whatever is in the store with a high-performance winch line.

Choosing the pennant material: HMPE, polyester, nylon or wire?

Many modern tug fleets running synthetic mainlines pair them with HMPE pennants, but each material has a defined place.

Matériau	Strengths in tug service	Watch-outs
HMPE (e.g. Super-12)	Strength-for-weight comparable to steel wire at a fraction of the weight; floats; fast, safe handling and hook-up; low stretch for precise ship-assist work	Like other fibre ropes, susceptible to internal fatigue under cyclic loading that visual inspection cannot reliably detect; needs chafe protection at chocks; low heat tolerance on drum ends and bitts
Polyester	Excellent abrasion and UV resistance; moderate stretch takes the edge off snatch loads; cost-effective	Heavier and does not float; larger diameters needed for equivalent strength. Note: the pennant in the MAIB Southampton case was polyester-cored — internal fatigue is not unique to any one fibre [1]
Nylon (as stretcher/spring)	High elasticity absorbs dynamic loading — used as a shock-absorbing spring in tow assemblies rather than as the chafe pennant itself	Stores significant energy under load — high recoil if parted; loses strength when wet
Steel wire	Most durable against cutting, chafe and heat; still specified in some ocean-tow and emergency towing arrangements	Heavy and slow to handle — a genuine manual-handling risk at the ship’s rail; fishhooks; sinks; corrodes

The honest comparison matters. HMPE has transformed tug work — crews connect faster, handle lighter gear and work safer — and the MAIB case is a reminder that applies to every synthetic pennant, whatever the fibre: some of the most significant degradation can be internal and cumulative, with little or no obvious external indication. The right answer is not to avoid synthetic pennants; it is to run them with the replacement discipline they are designed for.

Specifying a tug pennant: what to nail down

• Strength. As above: at or above mainline MBL for everyday tug work, unless your towing plan formally specifies a fuse. Final ratios against bollard pull and mainline strength should be set by the towing plan, class or warranty surveyor for your operation.
• Length. Long enough that the mainline never reaches the ship’s chock through the working range of the tow — typically a short pennant for harbour ship assist, longer for coastal and barge work. Too long, and you carry unnecessary weight and cost in the sacrificial element.
• Construction. A single-leg 12-strand pennant with spliced eyes is a common arrangement in harbour towage. Grommet (endless-loop) pennants offer high strength for their size, but a grommet spliced in only one leg is stiffer on that side — under cyclic loading the imbalance increases movement at the connection, so specify balanced construction for cyclically loaded tug work [5]. If your crews splice their own eyes, our splicing guide covers 12-strand class techniques.
• Chafe protection. Non-negotiable on synthetic pennants: sacrificial covers or sleeving positioned where the rope bears on chocks and bitts, inspected as consumables in their own right.
• Connection to the mainline. Cow-hitching the pennant to a mainline eye is quick but concentrates wear at the hitch; hardware-free connections using soft shackles avoid introducing steel at height, while conventional shackles remain right for some assemblies. Whichever you use, the connection method should come from the rope manufacturer’s guidance for that specific rope pairing — mismatched connections were among the factors examined in the MAIB investigation.

Inspection and retirement: run pennants like consumables

Because the killer damage is internal, a pennant regime built only on visual inspection will eventually be caught out. A robust approach for a working tug fleet combines four layers:

• Pre-job checks of the working length — external chafe, cut strands, fused or glazed areas, stiffness, and the condition of eyes, covers and connections.
• Recorded service history per pennant: date into service, jobs or hours worked, and every known shock event. A pennant is relatively inexpensive; maintaining a basic service-history log requires very little.
• Hard retirement triggers: a set service interval appropriate to your duty cycle, and immediate retirement after any snatch, brake-slip under load, girting near-miss or visible damage — regardless of how the rope looks.
• Residual strength testing of retired pennants when practicable. Destructive testing of time-expired pennants is the most direct way to learn what your duty cycle actually does to the rope, and it turns your retirement interval from a guess into data.

This is the same direction of travel the wider industry has taken with mooring lines under OCIMF MEG4 — documented line management, defined retirement criteria and residual-strength awareness — applied to the towing deck, where the loads are more dynamic and the consequences of a parted line at least as serious. For operators running mixed towing and lifting and towing gear, one consistent inspection culture across both is easier to train and easier to audit.

Foire aux questions

How often should a tug’s towing pennant be replaced?

There is no single interval — it depends on duty cycle, rope type and loading history. What matters is that replacement is scheduled on time and events, not appearance: set a service interval appropriate to your operation, and retire the pennant immediately after any shock event or visible damage. Investigators in a recent UK tug incident were explicit that pennants are sacrificial components to be replaced frequently.

Why do pennants fail below their rated breaking load?

Because in-service strength is not new strength. Cyclic loading, abrasion and internal axial compression fatigue progressively reduce residual strength, often with little external evidence. In MAIB Report 15/2022, destructive testing of the recovered pennant recorded failure at 119 tonnes — around 52% of its original 229.5-tonne MBL — and the MAIB considered the actual load when the pennant parted was likely at or below this level, with the critical damage hidden inside the load-bearing core where the crew could not see it.

Should the pennant be stronger or weaker than the mainline?

For everyday harbour towage and ship assist, industry guidance is that a synthetic pennant’s strength when new should be at least equal to the mainline’s, because the pennant wears fastest. A deliberately weaker “fuse” pennant exists in some ocean-tow arrangements, but only as a formally specified element of the towing plan — never by accident or age.

Are HMPE pennants better than steel wire for tug work?

For most modern tug operations, yes: HMPE offers wire-comparable strength at a fraction of the weight, floats, and dramatically improves connection speed and crew safety at the rail. Wire remains more resistant to cutting, chafe and heat, which is why it persists in some ocean-tow and emergency towing arrangements. The trade-off with any synthetic is that some critical degradation can occur internally and may not be apparent during visual inspection — which is exactly why disciplined replacement matters.

Is it “pennant” or “pendant”?

Both. “Pennant” is standard in UK and international towing usage; “pendant” is common in North America. They describe the same component: the short sacrificial line at the working end of the towing assembly.

References and further reading

1. Marine Accident Investigation Branch, Report on the investigation of the failure of a towline pennant and injury to the crew on board the tug Svitzer Mercurius in Southampton, England on 22 December 2019, Report No 15/2022, December 2022. Full report (PDF, gov.uk)
2. Maritime Journal, “Report Examines Towline Pennant Snap”, February 2023 — trade summary of MAIB Report 15/2022, including the pennant construction, 229.5t MBL, 119t destructive test result and the axial compression fatigue findings.
3. Marine Accident Investigation Branch, Failure of a tow rope and injury to crew on board the tug Svitzer Avon, Royal Portbury Lock, Bristol, 4 February 2025, Preliminary Assessment 2/2026. Published assessment (gov.uk)
4. The Maritime Executive, “MAIB Calls for Closer Attention to Snapback Safety”, 2026 — reporting the July 2024 rope parting after ~800 uses, its reuse without residual strength testing, and the MAIB chief inspector’s warning on the normalisation of towline parting.
5. OCIMF, Static Towing Assembly Guidelines (STAG), First Edition, 2020 — guidance on pennant strength relative to the main towline, wire vs synthetic pennant durability, chafe protection and grommet splice balance.
6. OCIMF, Mooring Equipment Guidelines, Fourth Edition (MEG4), 2018 — line management planning, inspection and retirement criteria referenced for the wider fleet line-management approach.

This article is general guidance for commercial operators and is not a substitute for your towing plan, class requirements or manufacturer’s instructions. MAIB reports are published for safety learning, not to apportion blame. This article was produced with the assistance of AI and reviewed and edited by Michael Clark, Southern Ropes UK