5.2 Seamanship Knots, Bends, Hitches & Block/Tackle Rigging

Seamanship Knots, Bends, and Hitches

The ability to tie the correct knot swiftly and securely in darkness, driving rain, or heavy seas is the hallmark of an able seafarer. On board merchant vessels, ropes must be fastened reliably so they hold without slipping under massive dynamic surges, yet can be untied cleanly and rapidly without jamming once tension is eased.

Basic Cordage Terminology

Before examining individual knots, a GP Rating must master basic cordage terms:

• Standing Part: The main, long, stationary part of the rope leading from the load or attachment point.
• Working End (Bitter End): The free end of the line used to tie the knot, bend, or hitch.
• Bight: A curved loop or bend formed in any part of a rope between its ends without crossing the parts.
• Loop (Turn): Formed when the working part crosses over or under the standing part.
• Round Turn: Passing the rope completely around a post, spar, or ring through a full 360° circle so both parts lead in parallel directions.
• Knot Efficiency: Every knot or hitch forces rope fibers into tight radius curves, creating high stress concentrations. Standard knots reduce rope breaking strength by 40% to 55%. For example, a rope with a breaking strength of 10 tonnes will part at roughly 5 to 6 tonnes when tied with a bowline.

Fundamental Categories

1. Knots: Fastenings tied in a rope itself (e.g., stopper knots or decorative knobs).
2. Bends: Fastenings used to temporarily join two ropes together (either of equal or unequal diameter).
3. Hitches: Fastenings used to secure a rope to a solid spar, ring, bollard, stanchion, or another rope under tension.

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1. Essential Seamanship Knots & Hitches

       BOWLINE                     FIGURE-OF-EIGHT              REEF KNOT
    (Non-slip Eye)                  (Stopper Knot)            (Binding Only)
        .---.                           .---.                     .---.---.
       /     \                         /  |  \                   /   / \   \
      |   O   |                       |  / \  |                 |   |   |   |
       \  |  /                         \  |  /                   \   \ /   /
      ==+=+=+==                       ===+=+===                   '---'---'

Comprehensive Master Reference Table

Operational Deep Dive: The Rolling Hitch & Rope Stopper

Transferring a mooring hawser from a warping drum to bitts requires a suitable stopper and a coordinated team. A rolling hitch illustrates axial grip, but it is not the mandatory method for every mooring rope. COSWP prefers the double-tail West Country stopper for fiber moorings.

1. Select a stopper suitable for the line and load. Use natural fiber on natural-fiber moorings, and a compatible synthetic stopper on synthetic moorings; do not use polyamide (nylon) as the synthetic stopper. Check its condition and secure attachment.
2. Apply the stopper by the ship's approved method. With a double-tail West Country stopper, alternate the two tails around the hawser in the prescribed opposing pattern. The responsible supervisor checks the rig before load transfer.
3. Keep everyone clear of bights and recoil paths. Agree clear signals with the winch operator; ease the drum only when the stopper is ready to take the strain.
4. Transfer the line onto the bitts using the approved lead and securing pattern; single turns or figures of eight may be used. Keep hands clear of trapping points.
5. Ease and remove the stopper only after the line is secure on the bitts and the supervisor confirms the transfer is complete.

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2. Anatomy and Types of Pulley Blocks

A pulley block (or maritime block) is a portable or fixed mechanical device housing one or more grooved wheels (sheaves). Blocks change the lead of a line, minimize frictional wear, and compound lifting force into mechanical advantage.

               ANATOMY OF A PULLEY BLOCK
                     [Head Fitting] (Eye / Hook / Shackle)
                           |
                      +----+----+
                      | [Cheek] |
             [Swallow]  (SHEAVE)  [Swallow]
                      |    |    |
                      |  [Pin]  |
                      +----+----+
                           |
                        [Becket] (Bottom Eye)

Components of a Block

• Shell: The outer frame or housing that encases the sheaves. Manufactured from cast steel, drop-forged alloy, galvanized mild steel, or seasoned hardwood (elm or ash).
• Sheave: The grooved wheel over which the rope or wire passes. Fabricated from cast iron, phosphor bronze, or cast steel. The sheave groove must match the rope diameter: if the groove is too narrow, it pinches and crushes the rope; if too wide, the rope flattens under load.
• Pin (Axle): The central hardened steel bolt passing through the cheeks and sheave hub. Equipped with an internal grease duct or Zerk grease nipple for lubrication.
• Cheek: The solid side plates of the shell enclosing the sheave.
• Strap: A continuous internal or external forged steel band that binds the shell, carrying tensile load between the head fitting, pin, and becket.
• Swallow: The open throat between the cheeks above the sheave where the rope enters and exits.
• Becket: A small forged eye, thimble, or lug at the base of the block used to anchor the standing part of the rope.

Block Classifications

1. By Number of Sheaves:
   • Single Sheave Block: Houses 1 sheave.
   • Double Sheave Block: Houses 2 sheaves rotating independently on a common central pin.
   • Treble Sheave Block: Houses 3 sheaves rotating independently on a common central pin.
2. The Snatch Block:
   • A heavy-duty single-sheave block featuring a hinged cheek or gate that swings open on one side.
   • Seamanship Advantage: Allows the bight of a rope or wire to be inserted directly into the sheave without threading the bitter end through the swallow. Once the bight is seated, the hinged cheek is closed and locked with a spring-loaded toggle or safety pin.
   • Widely used on deck for altering the lead of mooring lines, warping wires, and cargo runners.

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3. Mechanical Advantage and Tackle Purchases

A tackle (pronounced "tay-kle" in maritime parlance) is an assembly of ropes (falls) reeved through one or more blocks to gain mechanical advantage (MA), multiplying the hauling effort of the crew or winch.

Tackle Terminology

• Falls: The entire rope reeved through the blocks of a tackle.
• Standing Part: The end of the falls permanently secured to the becket of a block.
• Hauling Part: The free end of the falls upon which the crew or winch pulls.
• Running Parts: The moving intermediate lengths of rope passing between the sheaves.
• Fixed Block: The block anchored to a static overhead structure (deckhead, derrick head, or mast).
• Moving Block: The block attached directly to the load, moving with it.

Rigged to Advantage versus Rigged to Disadvantage

A purchase can be rigged in two opposite configurations, altering its pulling power:

   RIGGED TO DISADVANTAGE                  RIGGED TO ADVANTAGE
   Hauling Part from Fixed Block           Hauling Part from Moving Block
         [FIXED BLOCK]                           [FIXED BLOCK]
         /     |     \                                 |     \
      (Haul)   |      \                                |      \
        v      |       \                               |       \
               |        \                              |        \
         [MOVING BLOCK]                          [MOVING BLOCK]
               |                                 /      |
             [LOAD]                           (Haul)    |
                                                ^     [LOAD]
   Effort pulls opposite to load.          Effort pulls in direction of load.
   MA = All supporting parts.              MA = All supporting parts,
                                          INCLUDING the hauling part once.
   Inverting the SAME tackle adds one supporting part; do not count it twice.

• Rigged to Disadvantage:
  • The hauling part leads away from the fixed block.
  • The crew pulls in the opposite direction to the load movement (e.g., pulling downward to raise a load upward).
  • Theoretical $\text{MA}$ equals the number of parts of rope supporting the moving block: $\text{MA}_{\text{disadvantage}} = \text{parts supporting load}$
• Rigged to Advantage:
  • The hauling part leads away from the moving block.
  • The crew pulls in the same direction as the load moves (e.g., pulling upward to raise a load upward).
  • Because the hauling part itself directly helps support the load, the theoretical mechanical advantage increases by 1: $\text{MA}_{\text{advantage}} = \text{MA}_{\text{disadvantage}} + 1 = \text{total parts supporting/moving load}$

Master Tackle Reference Table

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4. Tackle Friction Calculations: The 10% Seamanship Rule

In real-world seamanship, theoretical mechanical advantage overestimates lifting capacity because friction in sheave bearings and bending resistance of the rope consume significant energy.

The Standard Seamanship Friction Formula

The international maritime standard rule of thumb accounts for friction by adding 10% (one-tenth) of the original load weight for every rotating sheave in the purchase:

$\text{Friction Allowance} = n \times 0.10 \times W$

where:

• $W$ = weight of the load to be hoisted.
• $n$ = total number of rotating sheaves in the entire tackle system.

The Total Effective Load ($W_{\text{eff}}$) including friction is:

$W_{\text{eff}} = W + (n \times 0.10 \times W) = W \times (1 + 0.10 \cdot n)$

The Hauling Effort ($P$) required on the hauling part is:

$P = \frac{W_{\text{eff}}}{\text{MA}} = \frac{W \times (1 + 0.10 \cdot n)}{\text{MA}}$

where $\text{MA}$ is the theoretical mechanical advantage (determined by whether the tackle is rigged to advantage or disadvantage).

Worked Calculation Example

Problem: A deck maintenance crew must lift a spare lubricating oil pump casing weighing 800 kg through an engine room skylight. They rig a Two-Fold Purchase (consisting of two double blocks, totaling $n = 4$ sheaves). The purchase is rigged to advantage (hauling part leads from the moving block, so $\text{MA} = 5$). Calculate the required pulling force on the hauling part.

• Step 1: Calculate the friction allowance $\text{Friction} = 4 \times 0.10 \times 800\text{ kg} = 0.40 \times 800\text{ kg} = 320\text{ kg}$
• Step 2: Calculate total effective load ($W_{\text{eff}}$) $W_{\text{eff}} = 800\text{ kg} + 320\text{ kg} = 1,120\text{ kg}$
• Step 3: Calculate required effort ($P$) $P = \frac{W_{\text{eff}}}{\text{MA}} = \frac{1,120\text{ kg}}{5} = 224\text{ kg}$

Analysis: In a frictionless theoretical system, the required effort would be $800 / 5 = 160\text{ kg}$. Sheave friction adds $64\text{ kg}$ of resistance, requiring an actual hauling force of $224\text{ kg}$.