Rope used in boating is durable and expensive and is often handling heavy loads, e.g., when berthing, mooring, towing another vessel, preparing for a storm, or managing sails. The emphasis, therefore, is on safety, reliability, and convenience. In contrast to the fishing knots, value is also placed on being able to use the rope repeatedly and untie each knot without difficulty.
Mooring lines on large ships are nearly always made of a high-modulus polyethylene (HMPE) such as Vectran® or Dyneema®. These ropes float and their minimal elasticity reduces risk of injury due to "snap-back" in the event of a breakage. Each line serves a specific purpose. On large vessels two lines often run in parallel ("doubled up") for safety. The following diagram shows a typical arrangement:
A. Head Line
B. Head Line
C. Forward Breast Line
D. Forward Spring
E. Aft Spring
F. Aft Breast Line
G. Stern Line
H. Stern Line
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