Check out my new site at Catfishing.tvNow this is a neat little rig I designed to be able to bring those big flathead catfish outta the rocks without losing the fish due to hanging the sinker in the rocks and the fish break the hook out of the Dropper Loop ... now if you're not useing light line you don't have to mess with tieing the shock line on. I use light line 14# to 17# test and you have to use a shock line to throw large sinkers on light line.
If you're using a spincaster with about 25# test line and throwing a 5 oz sinker you won't need to use the shock line ... just tie your main line onto the 150# test trotline nylon (I use the black tarred nylon you are able to find at most walmarts) with a hook placed on the 3 foot to 4 foot of nylon line tie loops in each end so you can tie the mainline to one end and the breakaway line and sinker to the bottom end. then put the hook where ever you want it like in the middle and tie your dropper loop ... for flathead the longer the dropper loop the better but you must realize that the longer the dropper loop the more it's going to helecopter when casting so if you have to cast way out use a shorter dropper loop.
On the breakaway line use the lightest line you can throw ... I can't throw with anything less than 40# test if I'm swinging the sinker to cast using centrifugal force but if I just cast like a normal cast ... just throwing it without swinging the sinker I can use 25# test pretty easily to throw my 6oz weights. A big flatcat or blue can break 40# test but if it's just a 12 to 15 lb fish in calm water they most likely can't break the 40# test line until the rub it on the rocks a while ... don't worry cause the fish can't get loose by breaking the 150# test dropper loop line. It'll eventually break the breakaway line and it normally doesn't take too long if the rocks you're fishing are like Keystone's bottom beyond the pylons to the east cause most all those rocks are pretty jagged with sharp edges. This doesn't hold true every time though so that's why you need to use as light a breakaway line as you can throw according to distance you're throwing and how much umph you're putting into the cast.
Now the size of the hook is just a matter of preference ... I've been using this rig lately with 2/0 eagle claw baitholders using shadheads for bait and catching quite a few flathead catfish and blue catfish like this. The water is low at night when they turn off the turbines at the dam ... mostly 4 to 6 feet all the way accross and nothing but big rocks line the bottom so you can see why this type rigging is needed when you tie into a 20 lb or bigger fish. If you're using whole live perch you should use a large trotline hook like 6/0 or 7/0 or similar.
The size of the sinker doesn't matter ... if you're throwing 3 oz then use 3 oz ... same if you're using 5's ... you don't have to do anything different there.
If you want to use lighter line like I do but want to throw large weights you will have to use a shock line ... the size of the shock line should be 10 lbs for every ounce of weight your sinker is that you're wanting to throw ... like 4 oz should be 40# test. This is what the distance casters go by ... I use 40# to throw 6 oz weights though and it works pretty well but here's how you do it .... spool your reel with the lighter line and then tie on the shock line using the Albright Knot and then crank the reel handle spooling up line until the shock line makes at least 3 wraps on the spool and then pull the line down to the bottom eye closest to the reel and cut it ... if you're using a 14' pole the shock line will be about 22 feet long approximately then put your hook on the end of the shock line and slide it up and then tie your sinker on ... put the hook up about 2.5 feet above the sinker and tie your dropper loop for the hook or use the flathead rig shown above and you're ready to bait up and let it fly.
Using this rigging with the Sinker Sacrificer would be a better idea most likely as it's a more sure way of breaking off in the proper place. I came up with this rigging when fishing that flood water behind the dam ... you had to fish in the rocks anyhow and I was getting large fish to bite but they'd just bust the hook right out of the dropper loop and then you have to reel in and retie everything ... this flathead rigging stopped all that noise and when the fish would break something it would always break the sinker off leaving you free to reel his junky butt into the bank.
Ya'll let me know if you have more questions on this... I'll try to explain it better if I can.