Towing Vessel Operations

1. Towing Gear — Hawsers, Pendants, and Bridles

Commercial towing operations rely on a specific vocabulary for gear components. The exam tests your ability to identify each component, understand its function, and apply safe working load (SWL) principles.

1.1 The Towing Hawser

The towing hawser is the primary line connecting the towing vessel to the tow. Hawsers may be natural fiber (manila — largely obsolete), synthetic fiber (nylon, polyester, polypropylene), or steel wire rope. For offshore and ocean towing, steel wire or synthetic fiber hawsers are standard. For assistance towing of recreational vessels, nylon is preferred for its elasticity and shock absorption.

1.2 Wire Rope Hawsers

Wire rope is used for heavy ocean tows and salvage towing. The construction (e.g., 6x19, 6x37) describes the number of strands and wires per strand. Wire rope does not stretch significantly, making catenary and surge pendants especially important to absorb shock loads. Wire hawsers must be inspected regularly for broken wires, kinks, bird-caging, and corrosion.

1.3 Synthetic Fiber Hawsers

Nylon hawsers are commonly used in assistance towing because nylon stretches significantly (up to 20-30% of its length) under load, storing elastic energy that dampens surge and snatch loads naturally. This built-in elasticity reduces — but does not eliminate — the need for careful towline management. Polypropylene floats but has lower strength and UV resistance than nylon or polyester.

2. Hawser Ratings and Safe Working Load (SWL)

Every towing hawser has a published Minimum Breaking Strength (MBS) — the tensile load at which it is designed to fail. Actual operations must remain well below this figure.

2.1 Safety Factor and SWL Formula

2.2 Factors That Reduce Effective SWL

The rated SWL assumes a straight-line pull on new, undamaged gear. Several conditions reduce the effective strength in practice:

• •Bending around sheaves, fairleads, or bitts — a tight bend can reduce strength by 40% or more
• •Splices — eye splices retain approximately 80% of rope strength; knots only 40-70%
• •Wear, abrasion, and UV degradation in synthetic hawsers
• •Broken wires, kinks, or corrosion in wire rope
• •Dynamic loads from surge and snatch — may temporarily multiply static tension by 3-5x
• •High temperature — nylon loses significant strength above 150°F

2.3 Practical Hawser Inspection

Before every towing operation, inspect the hawser end-to-end. Look for: cuts, abrasion, chemical contamination (oils, solvents), heat damage, discoloration, broken yarns (synthetic) or broken wires (wire rope), kinks, and worn or cracked splices. Any hawser with visible damage should be removed from service immediately.

3. Towline Catenary and Length

Two of the most important variables in tow-astern operations are catenary and towline scope (length). Both directly affect how surge and snatch loads are managed.

3.1 Catenary Defined

Catenary is the natural downward curve of a hawser under its own weight between two suspension points. In towing, the towing vessel and the tow are the two points; the hawser hangs in a curved arc between them. A well-catenaried hawser acts as a mechanical spring:

• •When the towing vessel accelerates or a wave lifts it, the hawser straightens, absorbing energy
• •When tension eases, the hawser sags back into catenary, releasing energy smoothly
• •This buffering prevents sudden load spikes from becoming mechanical shocks

3.2 Increasing Catenary

3.3 Optimal Towline Length

Optimal towline length depends on sea state, vessel size, speed, and navigational constraints. General guidance:

4. Surge and Snatch Loads

Dynamic loads in towing systems can far exceed the static pull required to move the tow at constant speed. Understanding surge and snatch is critical for the exam and for safe operations.

4.1 Static vs. Dynamic Tow Force

The static bollard pull is the steady-state force required to keep the tow moving at a given speed. Dynamic forces add to this whenever relative motion changes — acceleration, wave action, steering inputs, or speed changes. These dynamic additions are surge and snatch loads.

4.2 Surge Loads

A surge load occurs when the towing vessel and tow move apart (in the direction of the hawser) more rapidly than normal, causing hawser tension to spike above steady-state. Common causes:

• •The towing vessel rises on a wave crest while the tow remains in a trough — the relative separation increases suddenly
• •The towing vessel accelerates (throttle increase, wind gust on vessel)
• •The tow decelerates (groundswell hits tow broadside, steering failure causes broach)
• •Course changes that increase the effective distance between tow point and tow

4.3 Snatch Loads

A snatch load is an instantaneous shock load caused by a slack hawser suddenly coming taut. This is the most dangerous dynamic load in towing. When a hawser goes slack (vessels converge) then snaps taut (vessels diverge), the impulsive tension can be many times the SWL. Prevention:

• •Maintain adequate catenary at all times — never let the hawser go fully slack
• •Avoid speed reductions that close the gap between towing vessel and tow
• •Use nylon hawsers or surge pendants to add elasticity
• •Station crew away from snap-back zones

5. Towing Lights and Shapes Under COLREGS Rule 24

COLREGS Rule 24 governs the lights and day shapes displayed by towing and pushing vessels. This rule is heavily tested on the captain's license exam. Memorize every configuration.

5.1 Towing Astern — Towing Vessel

5.2 The Vessel Being Towed (Tow)

5.3 Pushing and Towing Alongside

6. Tow Astern, Alongside, and Pushing — When to Use Each

6.1 Towing Astern

Towing astern is the standard open-water tow configuration. The tow trails behind the towing vessel on a hawser. Advantages include: excellent speed capability, ability to tow in heavy seas, simplicity of connection, and flexibility in tow length adjustment. Disadvantages include: reduced maneuverability (long tow creates large turning radius), risk of overtaking on the tow on speed reduction, and hawser management challenges in heavy weather.

6.2 Towing Alongside (Breasted Tow)

In alongside towing, the tow is secured directly to the side of the towing vessel with spring lines, breast lines, and bow and stern lines. This configuration provides excellent maneuverability — the combined unit can be steered by varying thrust — making it ideal for docking and undocking disabled vessels, restricted harbor maneuvers, and short transits in protected water. Limitations include: sea state sensitivity (waves can slam the two vessels together), speed limit (typically 4-6 knots), and limited use in open water.

6.3 Pushing (Ahead Tow)

Pushing is the dominant tow configuration on U.S. inland waterways and Western Rivers. The towing vessel is positioned astern of the tow (barge or barge fleet) and drives it ahead. Wire rigging (push wires, face wires) connects vessel to tow. Pushing offers: maximum thrust efficiency, excellent steering control, ability to manage very large barge fleets, and optimized hydrodynamics in river currents. It is not suitable in open ocean conditions because the towing vessel cannot easily separate from the tow in an emergency.

7. Towing Speed Considerations

Towing speed is constrained by several factors. Exceeding safe towing speed risks gear failure, tow instability, or structural damage to the tow.

7.1 Hull Speed of the Tow

Displacement vessels have a theoretical hull speed determined by their waterline length. At hull speed, the tow sits in its own bow and stern waves and cannot efficiently exceed that speed regardless of towing force applied. Forcing beyond hull speed dramatically increases resistance and drag, stressing all gear components.

7.2 Sea State and Towing Speed

As sea state increases, safe towing speed decreases. In heavy seas, reducing speed also reduces surge loads, improves steering control, and reduces the risk of the tow yawing or broaching. The general rule: reduce speed in proportion to sea state. In very heavy weather, maintaining steerage may be more important than maintaining forward speed — minimum steerage speed is the floor.

7.3 Acceleration and Deceleration

Any speed change by the towing vessel must be gradual. Rapid acceleration increases hawser tension sharply. Rapid deceleration (emergency stop) risks the tow overrunning the towing vessel, colliding with the stern, and parting the hawser from the tow side. Always use gradual, deliberate throttle changes and communicate intentions to the crew managing the towline.

8. Western Rivers Rules

The Western Rivers are defined in the Inland Rules as the Mississippi River, its tributaries, South Pass, Southwest Pass, the Tennessee-Tombigbee Waterway, Warrior River, Tombigbee River, Canal connecting the Tennessee and Tombigbee Rivers, Coosa and Alabama Rivers, Cahaba River, Black Warrior River, and related tributaries, plus the Red River of the North. These waters have special rules that differ from general Inland Rules.

8.1 Passing Signals on Western Rivers

On Western Rivers, passing agreements are conducted by sound signal (whistle) differently than on other inland waters. The signals propose and confirm the intended passing arrangement:

8.2 Right-of-Way: Descending vs. Ascending

On Western Rivers, the downbound vessel (traveling with the current) has the right of way over the upbound vessel. The downbound vessel proposes the passing signal. The upbound vessel must comply and acknowledge. This is opposite to the general rule in some situations and is a frequent exam topic.

8.3 Barge Tow Configurations on Western Rivers

Western Rivers tows typically consist of multiple barges arranged in a rectangular fleet pushed ahead by a towboat. A typical configuration may be 3-4 barges wide and 4-6 barges long. Individual barges are 195 feet long and 35 feet wide (standard inland barge). A 15-barge tow (3x5) is approximately 105 feet wide and 975 feet long — nearly 1/5 of a mile. Navigating such tows requires: anticipation of turns well in advance, knowledge of the river, use of flanking rudders, and radio communication with approaching traffic.

9. Inland Towing Rules

The U.S. Inland Navigation Rules (33 USC Ch. 34) apply on all U.S. internal waters not covered by Western Rivers rules or the International Regulations for Preventing Collisions at Sea (COLREGS). Key provisions relevant to towing:

9.1 Inland Rule 24 — Towing and Pushing

Inland Rule 24 mirrors COLREGS Rule 24 for most towing light requirements but adds provisions specific to U.S. inland waterways, including the composite vessel rule and specifics for inconspicuous, partly submerged objects being towed.

9.2 Inconspicuous Towed Objects

9.3 Sound Signals for Inland Towing

On inland waters other than Western Rivers, vessels use the same sound signals as for other power-driven vessels meeting, crossing, or overtaking. Vessels engaged in towing are still power-driven vessels for purposes of Rules of the Road and must comply with all applicable meeting and overtaking signal requirements. The key difference: towing vessels are often severely restricted in their ability to maneuver — a vessel towing astern is constrained by her ability to deviate from her course.

10. Master of Towing Vessels — Commercial License Requirements

The Master of Towing Vessels (MTV) credential is issued under 46 CFR Part 11 and authorizes the holder to serve as master of a towing vessel engaged in commercial towing operations — including pushing barges, towing freight, and assistance towing. This is a standalone credential distinct from OUPV, Master (less than 200 GRT), and the Assistance Towing Endorsement.

10.1 Application Requirements

10.2 TOAR — Towing Officer Assessment Record

The TOAR is a Coast Guard-approved logbook of specific towing tasks that must be completed under the direct supervision and assessment of a qualified Towing Officer. The assessor must sign off on each task after observing satisfactory performance. TOAR tasks cover:

• •Making up a tow (rigging, connecting, securing)
• •Towing astern: open water, restricted water, at night
• •Pushing ahead: single barge, multiple barges
• •Towing alongside (breasted tow)
• •Docking and undocking a tow
• •Emergency procedures: tow breakaway, man overboard with tow in tow
• •Communication with vessel traffic service and other traffic
• •Logkeeping and record maintenance

10.3 Route Endorsements

MTV credentials are endorsed for specific routes: Oceans, Near Coastal, Great Lakes, Inland, or Western Rivers. Sea service must be accumulated on the routes for which endorsement is sought. A mariner with a Western Rivers endorsement is not automatically authorized to tow in coastal waters without appropriate sea service and endorsement.

11. Tow Watch Responsibilities

Maintaining a proper watch during towing operations is critical. The tow watch officer and crew have specific responsibilities beyond those of a normal navigation watch.

11.1 Watch Officer Duties

• •Maintain a proper lookout by sight, sound, and all available means — including periodic visual checks on the tow
• •Monitor hawser tension — observe catenary periodically and report changes
• •Monitor tow heading and yaw — a yawing tow increases hawser tension and can lead to sheering into the towing vessel's wake
• •Check chafe gear condition whenever sea state changes
• •Monitor weather and sea state — adjust speed or alert master when conditions deteriorate
• •Maintain a navigation watch and plot position to ensure tow stays in safe waters
• •Log condition of tow and hawser at each watch handover
• •Ensure no crew are in snap-back zones during the watch

11.2 Tow Yaw and How to Correct It

A tow that consistently sheers to one side (yaws) places asymmetric loads on the hawser and is harder to control. Causes and corrections:

12. Emergency Tow Procedures

12.1 Taking a Vessel in Tow — Step by Step

12.2 Breakaway Procedures

A tow breakaway occurs when the hawser parts or the connection to the tow fails. Immediate actions:

13. Salvage Towing Basics

Salvage is the voluntary service rendered to a vessel in peril on navigable waters when the assisting party had no prior obligation to assist. A successful salvage gives rise to a salvage award — a legal right to compensation from the vessel and cargo owner. The exam tests the distinction between salvage and simple assistance towing.

13.1 Elements of a Valid Salvage Claim

13.2 Salvage vs. Assistance Tow

An assistance tow is a tow performed under a prior contract (e.g., a Sea Tow membership agreement) where the towing company is already obligated to assist. Because there is a preexisting duty, the tow is not a salvage — the towing company is paid per the contract, not a salvage award. This distinction is critical:

13.3 Special Compensation — Environmental Salvage

Modern salvage law (Lloyd's Open Form and international treaty) includes special compensation provisions for salvors who prevent or minimize environmental damage — particularly oil spills. Even if the salvage attempt fails (vessel is lost), a salvor who prevented an environmental disaster may still be entitled to compensation for efforts and expenses. This modified the traditional “no-cure, no-pay” principle.

13.4 Salvage Towing Gear Considerations

Salvage towing often involves vessels in degraded condition — flooding, structural damage, or grounding damage. Key additional considerations:

• •Assess the tow's stability and freeboard before connecting — a waterlogged vessel may capsize during the tow
• •Use emergency tow wires pre-rigged on the tow if available (required on certain large vessels)
• •Coordinate with Coast Guard on salvage plans, especially for vessels with pollution potential
• •Consider pumping or patching operations concurrent with towing to reduce risk
• •Use heavier hawsers and shorter scope in coastal or harbor areas where maneuverability is limited
• •Brief the crew on escape routes in case the tow capsizes or sinks suddenly

Frequently Asked Questions — Towing Vessel Operations

14. Towing Operations Exam Study Checklist

Use this checklist to verify you have covered every testable topic before your USCG exam.

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