Tim Coleman is a big advocate of large worms with jig heads. He uses black and red soft plastic worms on a 1 ounce jig and he does pretty well with them.
I think the 18 inches might be a bit much... But if you trolled one behind a boat, put a stinger hook on the back. Let's see, hollow out the worm into more of a tube like device and hang a clamworm on the back stinger hook - you'd probably do pretty good....
