castle esc's that say they have a max of 6s can not run 6s for long, use 5s max, BEC is battery eliminator circuit, over a certain voltage you have to cut or remove the middle red wire on the servo plug going to the receiver, then use a external 4 or 5 cell battery pack to power the servos and esc's controller. you dont need the 240 in sport boats, the 120 will be fine, but on 3s2p and a 7xl it would give you some headroom to run a bigger prop, motor life will be cut down a tad, but not to much.