The Physics of Fishing Line

Home the rod, the reel, the fish.
The fishing line used for halibut fishing must be considered for its ability to resist shear and tensile forces that are put on it by the fish. For the reason that halibut can put extreme tensile forces on the line and can shear the line with their sharp teeth, halibut fishing requires innovative technology to increase the fisherman's ability to land fish and keep from losing gear. Most sport fishing requires fishing line that is not very strong such as the mono filament lines used for freshwater and smaller saltwater sport fish that most people are used to. For halibut fishing special braided lines have been designed to hold up to extreme tensile forces while maintaining a small diameter so that a large amount of line can be placed on the reel. The equations that explain shear and tensile forces on the line are: F=S(∆x/L₀) and F=Y(∆L/L₀) in these equations the Y represents Young's modulus for the line that is being used on the reel and represents the force needed to break the line by changing its length. The S in the equation regarding shear forces describes the change in the shape of the line due to the teeth of the halibut. This is why a heavier piece of line is usually used at the end where the hook is attached. This makes it even more difficult for the fish to change the shape of the line enough to cause it to break. The line is weakest at the point where the knot attaches the line to the hook this is the most likely spot for the line to break when fighting the fish. The tension on the knot can be reduced by using a swivel to attach the hook to the line. The swivel allows the line to rotate and absorbs some of the tension that the fish is putting on the knot attaching the line. To see an example of how tensile line strength and knot strength are different click on the picture of the line below. Photo Credit: http://www.pennreels.com/01_products/line/pro.htm