Line Handling and Mooring

TL;DR — Ground tackle and mooring lines must be satisfactory for the vessel's size and operating waters, approved by the cognizant OCMI; towlines rigged for astern towing must be free of knots, terminated with a thimble-splice or poured socket, and inspected at least monthly. 46 CFR §184.300 33 CFR §164.74

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What the Rule Says

Mooring Lines and Ground Tackle

Federal regulations require that every vessel be fitted with ground tackle and mooring lines necessary for safe anchoring or mooring. 46 CFR §184.300 The gear provided must be satisfactory for the size of the vessel and the waters on which it operates, subject to approval of the cognizant Officer in Charge, Marine Inspection (OCMI). 46 CFR §184.300

The standard mooring line arrangement consists of five line types: the bow line (leading forward from the bow), the stern line (leading aft from the stern), the forward spring line (leading aft from the bow), the after spring line (leading forward from the stern), and breast lines (leading perpendicular to the dock at the beam). USCG Boatswain Manual Ch. 2 §2.1

Marlinespike seamanship — the use, care, and maintenance of all rope, fiber line, wire rope, and synthetic line aboard ship — underpins all line-handling operations. USCG Boatswain Manual Ch. 1 §1.1 Modern small-vessel operations use predominantly nylon and polypropylene synthetic lines, with wire rope reserved for standing rigging and heavy-load applications. USCG Boatswain Manual Ch. 1 §1.1

Nylon is the most common dock and anchor line material, valued for its high strength, excellent shock absorption, and resistance to mildew. USCG Boatswain Manual Ch. 1 §1.2 Its primary weaknesses are a loss of strength when wet (approximately 10–15%) and stretching under load. USCG Boatswain Manual Ch. 1 §1.2 Polypropylene floats, which makes it useful for certain applications, but it has lower strength than nylon and degrades under UV exposure. USCG Boatswain Manual Ch. 1 §1.2

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Towline Requirements — Towing Astern

The owner, master, or operator of each vessel towing astern must ensure that towline strength is adequate for its intended service. 33 CFR §164.74 The regulation identifies seven factors governing towline size and material selection:

1. Appropriate for the vessel's horsepower or bollard pull. 33 CFR §164.74 2. Appropriate for static and dynamic loads expected during the intended service. 33 CFR §164.74 3. Appropriate for expected sea conditions. 33 CFR §164.74 4. Appropriate for exposure to the marine environment and any chemicals used or carried aboard. 33 CFR §164.74 5. Appropriate for temperatures of normal stowage and service. 33 CFR §164.74 6. Compatible with associated navigational-safety equipment. 33 CFR §164.74 7. Appropriate for the likelihood of mechanical damage. 33 CFR §164.74

Rigging requirements are equally specific. Each towline as rigged must be: 33 CFR §164.74

• Free of knots.
• Spliced with a thimble, or have a poured socket at its end.
• Free of wire clips, except for temporary repair — in which case the towline must have a thimble and either five wire clips or as many as the manufacturer specifies for the nominal diameter and construction, whichever is more.

Condition monitoring requires the owner, master, or operator to: 33 CFR §164.74

• Keep on board (or in company files) a record of the towline's initial minimum breaking strength, as determined by the manufacturer, a classification society, or an approved tensile test.
• If the towline is purchased from another owner/operator or retested for any reason, keep a record of each retest of minimum breaking strength.
• Conduct visual inspections at least monthly and whenever serviceability is in doubt.
• Evaluate serviceability and remove the towline (or any part) from service per manufacturer/class society recommendations or a company replacement schedule that accounts for: nautical miles or time in service; operating conditions; loading history; surface condition including corrosion and discoloration; visible damage; material deterioration measured by diameter and lay extension; and tensile test results if necessary.
• Keep a record of material condition from each inspection. If this record lapses for three months or more (except when the vessel is laid up, out of service, or has not deployed its towline), the owner, master, or operator must retest the towline or remove it from service.

Terminal gear requirements for towing astern include: 33 CFR §164.74

• Terminal gear material and size appropriate for the towline's strength, anticipated loading, and environment.
• Each connection secured by at least one nut with at least one cotter pin or other means of preventing failure.
• Towline lead arranged to prevent sharp bends at fairlead blocks, chocks, or tackle.
• A method — mechanical or non-mechanical — that easily releases the towline without endangering operating personnel.
• Protection from abrasion or chafing by chafing gear, lagging, or other means.
• A winch that evenly spools and tightly winds the towline, except on vessels towing in ice on Western Rivers or those using a towline of synthetic or natural fiber.
• If a winch is fitted, a brake attached to the main drum with holding power appropriate for the vessel's horsepower or bollard pull, operable without power to the winch.

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Towline Requirements — Towing Alongside and Pushing Ahead

For vessels towing alongside or pushing ahead, the owner, master, or operator must ensure that face wires, spring lines, and push gear: 33 CFR §164.76

• Are appropriate for the vessel's horsepower.
• Are appropriate for the arrangement of the tow.
• Are frequently inspected.
• Remain serviceable.

Note that the regulatory requirements for alongside/pushing operations are less prescriptive than those for towing astern, but the obligation to inspect frequently and maintain serviceability is explicit. 33 CFR §164.76

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Towing Astern — Operational Principles

Towing astern connects the tow line to the towing vessel's stern and the towed vessel's bow. USCG Boatswain Manual Ch. 3 §3.1 The catenary — the natural sag of the tow line — absorbs shock loads. USCG Boatswain Manual Ch. 3 §3.1 Longer tow lines increase shock absorption but reduce maneuverability. USCG Boatswain Manual Ch. 3 §3.1

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Why It Matters on the Exam

OUPV and Master 100 GT candidates regularly encounter questions drawn directly from 33 CFR §164.74 and §164.76, as well as from the Boatswain's Manual material on line types and mooring arrangements. The exam tests both regulatory specifics and practical seamanship knowledge.

High-frequency exam areas include:

• The prohibition on knots in a rigged towline and the required termination method (thimble-splice or poured socket). 33 CFR §164.74 Examiners frequently present a scenario where a knot has been used and ask whether this is acceptable — it is not.
• The wire-clip exception for temporary repair: five clips minimum, or the manufacturer's specification, whichever is more, always with a thimble. 33 CFR §164.74
• The minimum inspection frequency for towlines: at least monthly and whenever serviceability is in doubt. 33 CFR §164.74
• The three-month record lapse rule: if inspection records lapse for three or more months (with the stated exceptions), the towline must be retested or removed from service. 33 CFR §164.74
• The winch brake requirement: must be operable without power to the winch. 33 CFR §164.74
• The winch exemption: vessels towing in ice on Western Rivers, or those using synthetic or natural fiber towlines, are not required to have a winch. 33 CFR §164.74
• The five standard mooring line types and their orientations. USCG Boatswain Manual Ch. 2 §2.1
• Nylon's shock-absorption advantage and its wet-strength loss (~10–15%). USCG Boatswain Manual Ch. 1 §1.2
• The OCMI approval requirement for ground tackle and mooring lines. 46 CFR §184.300

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Common Pitfalls

1. Confusing "free of knots" with "no terminations allowed." The rule prohibits knots in a rigged towline but requires a proper termination — a thimble-splice or poured socket. 33 CFR §164.74 Candidates sometimes read the prohibition too broadly.

2. Misapplying the wire-clip rule. Wire clips are prohibited in normal service. They are permitted only for temporary repair, and even then, a thimble is required along with five clips or the manufacturer's specified number, whichever is greater. 33 CFR §164.74 Candidates often forget the thimble requirement or undercount the minimum number of clips.

3. Assuming the winch requirement is universal. The winch requirement does not apply to vessels towing in ice on Western Rivers or to those using synthetic or natural fiber towlines. 33 CFR §164.74 This exception is a common distractor.

4. Confusing spring line directions. The forward spring runs aft from the bow; the after spring runs forward from the stern. USCG Boatswain Manual Ch. 2 §2.1 Candidates frequently reverse these. Remember: the name refers to the vessel end where the line originates, not the direction it leads.

5. Overstating nylon's wet-strength loss. Nylon loses approximately 10–15% of its strength when wet. USCG Boatswain Manual Ch. 1 §1.2 Candidates sometimes exaggerate this figure or confuse it with polypropylene's UV degradation characteristic.

6. Forgetting the three-month record lapse rule. If inspection records lapse for three months or more, the towline must be retested or removed from service — unless the vessel is laid up, out of service, or has not deployed the towline. 33 CFR §164.74 The exceptions are narrow and specific.

7. Applying astern-towing standards to alongside/pushing operations. The detailed rigging and monitoring requirements of 33 CFR §164.74 apply to towing astern. Alongside and pushing operations are governed by the simpler, four-criterion standard of 33 CFR §164.76. 33 CFR §164.76 Do not import the astern requirements into alongside scenarios.

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Quick Check

Q1: A towline rigged for towing astern has a bowline knot at its bitter end. Is this acceptable under federal regulations?

No. 33 CFR §164.74(a)(2)(i) requires each towline as rigged to be free of knots. The proper termination is a thimble-splice or a poured socket at the towline's end. 33 CFR §164.74

Q2: Under what conditions may wire clips be used on a towline, and what additional requirement applies?

Wire clips are permitted only for temporary repair. When used, the towline must also have a thimble, and the number of clips must be either five or the manufacturer's specified number for the nominal diameter and construction of the towline, whichever is more. 33 CFR §164.74

Q3: How often must a towline be visually inspected at a minimum?

At least monthly, and whenever the serviceability of the towline is in doubt. 33 CFR §164.74

Q4: What happens if the towline inspection record lapses for three months or more?

The owner, master, or operator must retest the towline or remove it from service. Exceptions apply only when the vessel is laid up, out of service, or has not deployed its towline. 33 CFR §164.74

Q5: Which vessels are exempt from the requirement to fit a winch for towing astern?

Vessels towing in ice on Western Rivers and vessels using a towline of synthetic or natural fiber are exempt from the winch requirement. 33 CFR §164.74

Q6: Name the five standard mooring lines and describe the lead of each.

Bow line — leads forward from the bow. Stern line — leads aft from the stern. Forward spring — leads aft from the bow. After spring — leads forward from the stern. Breast lines — lead perpendicular to the dock at the beam. USCG Boatswain Manual Ch. 2 §2.1

Q7: What is nylon's primary advantage as a mooring line material, and what is its primary weakness?

Nylon's primary advantage is excellent shock absorption combined with high strength and resistance to mildew. Its primary weakness is loss of strength when wet (approximately 10–15%) and stretching under load. USCG Boatswain Manual Ch. 1 §1.2

Q8: What four criteria must face wires, spring lines, and push gear meet for a vessel towing alongside or pushing ahead?

They must be appropriate for the vessel's horsepower, appropriate for the arrangement of the tow, frequently inspected,