towline weight and catenary moment Practice Questions

1. The weight of a long, heavy wire tow hawser creates a catenary that primarily affects the towing operation by:

• A.Providing a shock-absorbing cushion that dampens surge loads between the tug and tow✓
• B.Increasing the static tension at the towing vessel's stern beyond the line's rated breaking strength
• C.Eliminating all horizontal pull on the tow and converting it to a vertical downward force
• D.Reducing the effective towing force because the catenary weight acts opposite to vessel motion

Why: The catenary (sag) in a wire towline acts as a natural spring, absorbing surge and snatch loads that occur from wave action and speed changes. This cushioning effect protects both the towline and the towing bitts from shock loading.

2. As towing speed increases, the catenary in a wire tow hawser will:

• A.Decrease as the increased tension straightens the line, reducing the shock-absorbing effect✓
• B.Increase because hydrodynamic drag pulls the line deeper into the water
• C.Remain constant regardless of speed because catenary depends only on line weight
• D.Increase proportionally with speed as wave action creates more slack

Why: At higher towing speeds, increased towline tension straightens the catenary. This reduces the elastic (spring) effect of the wire, making the system more susceptible to shock loads from surging, requiring careful speed management.

3. What is the term for the dangerous situation where a tow sheer causes the towline to pull athwartships and the towing vessel cannot steer to escape, leading to potential capsize?

• A.Girting (or girthing)✓
• B.Yawing resonance
• C.Towline snatch
• D.Catenary collapse

Why: Girting (or girthing) describes the situation where the towline comes abeam of the towing vessel, creating a severe heeling moment that can capsize the tug before the crew can respond. It is a leading cause of tug capsizing and is especially dangerous in confined waterways.

4. A wire tow hawser weighs 15 kg/m. At a scope of 400 meters, this hawser has a total weight of 6,000 kg. The vertical component of this catenary weight acts on the towing vessel's stern causing:

• A.Increased stern draft and reduced aft freeboard, lowering the towing vessel's overall reserve buoyancy✓
• B.Increased bow draft as the vessel compensates with forward trim
• C.A reduction in effective GM proportional to the hawser weight
• D.No effect on trim because the catenary weight is balanced by tension at both ends

Why: The downward pull of a heavy wire hawser at the stern increases stern draft and reduces aft freeboard. This effect is especially significant in heavy weather or when running before seas, where reduced aft freeboard can allow water on deck.

5. The moment created by a towline that is made fast above the vessel's center of gravity will tend to:

• A.Heel the vessel toward the tow when the towline is taut and angled upward✓
• B.Improve stability by creating a righting couple opposite to any initial heel
• C.Have no effect on transverse stability because towline forces are longitudinal
• D.Raise the metacentric height by adding virtual buoyancy at the stern

Why: A towline made fast above G creates a heeling moment when the line is under tension and angled athwartships. The moment arm is the vertical distance between the point of towline attachment and G; if attachment is above G, the towline's transverse component heels the vessel toward the tow.