lead core info from they who makes it, Interesting
Cortland uses two sizes of lead insert, one size for their 14# test, and a larger diameter for 18# -- 60#. This means that except for 14#, the weight of the lead remains the same for all line strengths. The weight of the line (core and case) is measured in grains (gr) per 10 foot section: 14# - 46 gr, 18# - 110 gr, 27# - 114 gr, 36# - 118 gr, 45# - 124 gr, 60# - 131 gr. The minor differences in total weight are attributable solely to the weight of the casing (stronger, thicker, heavier). One (1) ounce equals 437.5 grains. Therefore 10 feet of 36# test line weights approximately 1/4 ounce. For you trivia buffs a grain is an old English standard said to be the average weight of a "grain" of wheat taken from the center of the head.
What does all this mean to you, the leadcore line angler? You should select the minimum strength line you feel comfortable with (generally 18# -- 36#), and it won't have any impact on maximum depth. Keep in mind that stronger line, besides being more expensive, is thicker. Thicker line uses up reel capacity, thus limiting the total amount of the line you can spool. You need at least 100 yards of leadcore, 30 to 40 feet of leader and 50 or more yards of backing.
According to Cortland, extensive testing by charter captains in the Northwest has lead (no pun intended) to the following rule of thumb. For 18# test and greater, at trolling speeds of .5 to 1.8 MPH, with a 10' mono leader and a light weight spoon, the line goes down 1 foot for every 6 let out. Put another way, the line drops 5 feet per color (Kerplunk changes color every 10 yards). As in every trolling situation, water resistance increases with length, and eventually additional line out does not give you additional depth. It's reasonable to assume a full 100 yards of leadcore line takes you 40 to 45 feet down and 3 colors (30 yards) of "bird-core" (a leadcore line segment, used in place of a lead weight, for added depth behind a side planner) puts you 15 feet down. According to Gudebrod, another manufacturer of leadcore line, after 25 feet of depth thermocline effects the sink rate ( the water density increases). They suggest 3 colors 15 feet down, 5 colors 25 feet, 8 colors 30 feet and 9 colors 35 feet.