2.3 Purchases, Blocks & Tackle

Why Purchases Appear on Master 100

Even on vessels under 100 GRT, masters supervise boat falls, hatch or canopy gear, improvised lifts, and maintenance aloft. Q160 Deck General still asks classic block-and-tackle questions: name the purchase, count parts of line, size the sheave, and keep people alive in a bosun's chair. You do not need a deep-sea bosun's resume—you need the definitions and safety rules the exam repeats.

Blocks, Sheaves, Tackles, and Fairleads

A fairlead is not optional cosmetics: a line that jumps a sheave or saws across a cheek will fail the rope and can explode a block under load.

Mechanical Advantage — Counting Parts of Line

Ideal mechanical advantage of a simple purchase is approximately equal to the number of parts of line supporting the moving block (the lines that share the load).

Counting method used on exams: look at the moving block and count how many parts of rope leave it toward the standing block (including the part made fast there if it supports the load). That count is the ideal MA. If you pull on a hauling part that adds another supporting part when the hauling part comes off the moving block, include it—follow the diagram in the stem carefully.

Worked idea: A gun tackle with two single blocks and two parts supporting the load → pull half the weight on the hauling part (ideal), ignoring friction. To lift 400 lb ideally at MA 4, hauling force ≈ 100 lb before friction.

Friction and real-world MA

Each sheave costs efficiency. Sticky bearings, undersized sheaves, wet manila, and bad leads mean you pull more than W/MA. Blocks must be lubricated and inspected; cracked shells and frozen sheaves are discarded, not painted over.

Sheave Diameter vs Line Size

Bending a fiber line over too small a sheave crushes interior fibers and kills strength. Coast Guard teaching problems often use a rule of thumb for manila:

Minimum sheave diameter ≈ 2 × the circumference of the rope (manila teaching rule).

Example: Manila line with 3-inch circumference → minimum sheave diameter about 6 inches. (If a stem gives diameter of rope instead of circumference, convert carefully: circumference ≈ π × diameter; many exam items state circumference directly.)

Wire rope uses larger ratios and different tables—do not apply the manila rule blindly to wire. The exam's point is: undersize sheaves destroy line and are unsafe.

Reeving, Leads, and Side Loads

• Reeve so the hauling part leads fairly and the blocks do not twist into a foul lead.
• Keep the load line in the plane of the sheaves; side-loading cheeks invites failure.
• Use becks or straps sized for the load; pad eyes and railings not designed as lift points are not "close enough."
• When two purchases share a load, match them and control swing—passenger decks are no place for a free-swinging engine part.

Bosun's Chair and Personnel Safety

A bosun's chair is a seat used to suspend a person for work aloft or over the side (painting, inspecting, clearing a fouled point). Personnel lifts are not casual cargo lifts.

Critical exam/safety point: the chair's bridle is secured with a shackle, and the shackle pin must pass through the eye of the bridle (ride with the pin through the bridle eye) and be properly seized/secured so it cannot unscrew under motion. Using a random hook that can slip off the eye is unacceptable.

Additional practices:

• Inspect chair, bridle, gantline, and blocks before each use
• Tend the hauling part with a competent crewmember—never leave a person aloft on an unattended line
• Keep tools on lanyards; control paint and debris over passenger spaces
• Mind electrical hazards and stack gas if working near the stack or antenna farm
• Prefer purpose-built stages and fall protection policies required by company SMS when available

If the stem asks how the bosun's chair is made fast to the gantline, choose the answer that keeps the shackle pin through the bridle eye and secured—not a open hook resting under the eye.

Safety Around Purchases on Small Passenger Vessels

1. Know the SWL of blocks and line; do not lift engines with dock-line leftovers.
2. Clear the area under the load; passengers stay behind barriers.
3. Use tag lines to control swing.
4. Never stand in the bight of a hauling part.
5. Lower with control—do not depend on a single pair of gloved hands without a turn on a bitt or cleat when loads are heavy.

Master scenario

You must lift a 200 lb pump from the engine room hatch using a double purchase (ideal MA 4). Ideal hauling load ≈ 50 lb before friction; with friction plan for more. Check sheave size against the manila fall, fairlead clear of the hatch coaming, keep passengers off the afterdeck, and treat any person going down the hatch on a chair as a personnel evolution with a locked shackle pin through the bridle.

High-yield Q160 checklist

• MA ≈ parts of line at the moving block
• Gun ≈ 2, double purchase ≈ 4 (typical teaching values)
• Manila sheave diameter ≈ 2 × circumference
• Fairleads prevent chafe and side loads
• Bosun's chair: shackle pin through bridle eye, secured

Master those five and purchase questions become systematic instead of mysterious.