Striper Talk Striped Bass Fishing, Surfcasting, Boating - View Single Post

Mr. Sandman · 06-21-2008, 12:28 PM

Your plug is rolling because of a low "BG". BG is the distance from the center of buoyancy to the center of gravity. This distance produces the restoring moment and "stiffness" or resistance in roll.

Plugs that wiggle side to side is a different but interesting hydrodynamic effect. Basically you have a controlled unstable situation of flow separation off the lip...it kites to one side to a certain point angle where the flow can not stay attached any longer then the flow separates sheds a vortex and it goes in the opposite direction and so on... The amount of wiggle is driven by the rotational moment of inertia and the size of the lip. You may notice if you put a weight in the rear that it does not wiggle as much (but casts better) Also you may notice that when a weight is forward it wiggles better (but doesn't cast as well). This is because the mass is far away from the center of rotation (the pivot point, which is about 25-40% or so from the nose depending on the plugs shape) . When the rotational inertia is high and it tends to wiggle less when it is low it wiggles easily. (somewhat akin to the old figure skater who spins and pulls in her arms (reducing the rotational inertia and radius of gyration and then spins really fast) If you can concentrate the mass center near the center of rotation it will wiggle a lot and with ease. It is a balancing act to get it the way you like.

Basically if you want it to roll less, put a weight along the center-line low in the plug belly, this will increase BG and if you want it to wiggle more keep it forward.

I will post an interesting plug I made this winter that exploits the best of moving masses. It is really cool and casts a mile. (and catches too!)

06-21-2008, 12:28 PM	#4
Mr. Sandman Registered User iTrader: (0) Join Date: Sep 2001 Posts: 7,649	A little submarine/ship design science for you... Your plug is rolling because of a low "BG". BG is the distance from the center of buoyancy to the center of gravity. This distance produces the restoring moment and "stiffness" or resistance in roll. Plugs that wiggle side to side is a different but interesting hydrodynamic effect. Basically you have a controlled unstable situation of flow separation off the lip...it kites to one side to a certain point angle where the flow can not stay attached any longer then the flow separates sheds a vortex and it goes in the opposite direction and so on... The amount of wiggle is driven by the rotational moment of inertia and the size of the lip. You may notice if you put a weight in the rear that it does not wiggle as much (but casts better) Also you may notice that when a weight is forward it wiggles better (but doesn't cast as well). This is because the mass is far away from the center of rotation (the pivot point, which is about 25-40% or so from the nose depending on the plugs shape) . When the rotational inertia is high and it tends to wiggle less when it is low it wiggles easily. (somewhat akin to the old figure skater who spins and pulls in her arms (reducing the rotational inertia and radius of gyration and then spins really fast) If you can concentrate the mass center near the center of rotation it will wiggle a lot and with ease. It is a balancing act to get it the way you like. Basically if you want it to roll less, put a weight along the center-line low in the plug belly, this will increase BG and if you want it to wiggle more keep it forward. I will post an interesting plug I made this winter that exploits the best of moving masses. It is really cool and casts a mile. (and catches too!)