2.2 Marlinespike Seamanship: Lines, Knots & SWL
Key Takeaways
- Manila is natural fiber (holds knots well, loses strength when wet/rot); nylon is strong with high stretch and dangerous snap-back; polypropylene floats and is common for throw lines.
- Right-hand laid manila is coiled clockwise (with the lay) to avoid kinks; flake and figure-eight coils are used for ready running line.
- Core knots: bowline (temporary fixed eye), square knot (reef; like-sized lines, not critical loads), sheet bend (join unequal lines), clove hitch (spar/pile), rolling hitch (haul along a spar or take strain on another line).
- Safe working load (SWL) ≈ breaking strength ÷ safety factor (example: 15,000 lb break ÷ 5 = 3,000 lb SWL).
- Chafe protection and respect for synthetic snap-back are non-negotiable on passenger-vessel decks.
Lines Are Structural Members You Can Hold
On a Master LT 100 vessel, lines secure passengers' safety as surely as bulkheads do: dock lines, towing bridles, lashings, and boat falls. Q160 Deck General tests fiber identification, knots/hitches, coiling, safe working load, and the hazards of chafe and snap-back. Treat rope work as engineering with a safety factor—not folk craft.
Fiber Types: Manila, Nylon, Polypropylene, Polyester
| Fiber | Traits | Typical deck use / caveats |
|---|---|---|
| Manila (natural) | Good knot-holding; stiffens/weakens when wet or rotted; stores dry | Traditional teaching rope; inspect for dark rot and powder |
| Nylon | High strength and high stretch; absorbs shock loads | Mooring and towing; snap-back hazard if it parts under load |
| Polyester (Dacron) | Strong, lower stretch than nylon, good UV resistance | Sheets, control lines, many modern dock lines |
| Polypropylene | Floats; weaker; UV degrades; low melting point | Throw rings, some light-duty floating lines; poor for critical hot-work areas |
Exam contrast: nylon's elasticity is an advantage for surge at the dock and a danger if the line fails—stored energy becomes a whip. Manila does not store energy the same way but loses capacity when wet and from mildew. Poly floats—so it can foul props less if trailed, but it is not the go-to for high-load mooring.
Lay, Coiling, and Ready Line
Most traditional three-strand line is right-hand laid (twisted so strands spiral up to the right as you look along the rope). Coil right-hand laid manila clockwise (with the sun / with the lay) so twists feed out instead of building kinks. Coiling against the lay produces hockles and fouled pays.
For running gear you may flake line on deck or make a figure-eight coil so it can run free without rings catching. Never bury the bitter end under a pile that must pay out in an emergency. On passenger decks, keep coils clear of walkways and scuppers so water and feet do not destroy order.
Knots and Hitches You Must Own
| Knot / hitch | Purpose | Exam note |
|---|---|---|
| Bowline | Temporary fixed eye that does not slip | "King of knots"; easy to untie after load |
| Square (reef) knot | Join two similar-size lines for non-critical use; reefing | Not for life-critical joins; can capsize into two half hitches if mis-tied as a granny |
| Sheet bend (single/double) | Join two lines, especially unequal diameters | Preferred over square knot for joining |
| Clove hitch | Quick hitch to a spar, pile, or rail | May slip under alternating load; back up when needed |
| Two half hitches | Make a line fast to a pile/ring after an eye or around the stand | Common finishing hitch |
| Rolling hitch | Hitch that grips when pulled along a spar or to take strain off another line | Classic for relieving a riding turn or hauling along |
| Figure-eight | Stopper knot in the end of a line | Prevents unreeving |
| Becket bend | Same family as sheet bend when one end is a becket/eye | Join line to a fixed eye |
Selection logic on the exam: need a non-slipping eye → bowline. Join thick to thin → sheet bend. Temporary hold on a piling → clove hitch. Need to slide-grip along another rope → rolling hitch. Do not use a square knot as a substitute for a bowline on a life-safety eye.
Breaking Strength, Safety Factor, and SWL
Manufacturers publish breaking strength (ultimate load at failure under test). Deck operations use a safe working load (SWL) or working load limit well below that value.
Rule used on Coast Guard-style questions:
SWL = Breaking strength ÷ Safety factor
Worked example: Line breaking strength 15,000 lb, safety factor 5:
15,000 ÷ 5 = 3,000 lb SWL
If the problem gives SWL and factor, multiply to recover breaking strength: 3,000 × 5 = 15,000 lb. Safety factors vary by use (often about 4–6 for many fiber-line teaching problems; wire rope and cargo gear may use different tables). Read the stem; do not invent a factor when one is supplied.
De-rate for reality: knots, splices, age, UV, chafe, chemicals, and shock loading all reduce capacity. A knot can cut effective strength dramatically versus a proper splice. Never treat catalog breaking strength as the load you may put on a weathered dock line with a clove hitch in the middle.
Chafe and Fair Leads
Chafe is friction wear where line rubs on chocks, rail edges, rust, or other lines. Prevent it with:
- Fair leads through chocks and fairleads sized for the line
- Chafing gear (leather, canvas, plastic sleeves, rubber) at contact points
- Avoiding sharp bends and overloaded angles on bitts
- Inspecting eye splices and wear points before every departure day
A parted spring line in a surge is often a chafe story, not a mystery.
Snap-Back: Synthetic Line Failure
When nylon (or other high-stretch synthetic) parts under heavy load, the ends snap back along the line of pull with lethal energy. Standing in the bight or in line with a taut towline is a classic killer. On Master 100 decks:
- Keep passengers and crew out of the snap-back zone
- Prefer controlled loads and proper SWL margins
- Inspect for flat spots, glazing, melted fibers, and cut strands
- Remember that a line "humming" under extreme tension is a warning, not a challenge
Working Deck Habits for Passenger Vessels
- Right size line for the job; match SWL to expected load plus margin.
- Coil with the lay; ready lines that will run free.
- Use the knot that matches the job; finish and dress knots so they can be inspected.
- Protect against chafe at every rail and chock.
- Treat synthetic tow and mooring lines as stored energy weapons if they fail.
High-yield Q160 drills
- Manila wet/rot vs nylon stretch/snap-back vs poly floats
- Clockwise coil for right-hand laid manila
- Bowline / sheet bend / clove / rolling hitch purposes
- SWL = break ÷ factor (15,000 / 5 = 3,000)
Those five themes cover most marlinespike items on Deck General.
A line has a breaking strength of 15,000 pounds and is used with a safety factor of 5. What is the safe working load?
Which knot is best for forming a temporary fixed eye that will not slip under load and can usually be untied after loading?
Why is nylon mooring or towing line especially hazardous if it parts under heavy load?