THE ROD AND REEL


Like the actual downrigger, this part of the system is above the water, and therefore, does not experience drag from the water. Analysis of this system is done when the downrigger itself is not examined, because the downrigger acts as a rigid body. The rod can be modeled as a type of spring, giving it some unique physic principles.

When the system is all set up, it is important to reel the slack (increase the tension) out of the line from the rod tip to the clip beneath the surface. This is done to ensure a proper hook up when a fish bites. After the line is triggered from the clip, there is alot of slack in the line. Limiting the slack is crucial; the elasticity of the line and the spring in the rod also aid in this process. This process will be covered in greater detail in the following sections.

For now, our system is in equilibrium.



In the photo, notice the rod is bent. It contains potential energy caused by increasing the tension in the fishing line. Not only potential energy is present, but also a force is exerted on the fishing line.
Potential Energy                                                                    Force
                                                     
                                                    PE = (1/2)kx^2                                                                     F = -kx



In both equations, k is a spring constant which is determined experimentally. For fishing rods a type of spring constant is in the form of rod "weights" or stiffness. The variable x is the displacement from equilibrium. In the case of the rod, it would be the distance the rod tip travels from straight to bent. Keep in mind, these equations are for springs. Fishing rods are different but can be modeled by the equations.

The bending in the rod is directly related to the tension in the line which was caused by reeling in line or shortening the length of the line.


Different rods have different levels of stiffness as portrayed above. Also varying the weight applied to the same rod will result in a similar effect. In our system, the rod exerts a force in the upward direction, and the fishing line is in tension in the downward direction. The rod does not counteract the the tension single handedly; the reel also shares in the force. One important thing to note, is that the rod also acts as pulley- redirecting the tension in the line to the reel as shown below.




This force induces a moment or torque around the center of the reel:

T = r  x  F

Where the torque equals the moment arm r crossed with F or the tension of the line. Our system is in equilibrium, so the internal gears counteract this moment. The net torque is zero. No motion is occurring.





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