It's called shaft packing (
http://www.skidim.com/mobile/searchprods.asp), there are two sizes (1/4, 3/16) confirm which one you have, there are two types (paraben, gore tex) the gore tex can have a slower leak because it heats up less, but costs more. Dig out the old packing, cut three strips of packing to circle the shaft exactly once, cut it on a bias, pack it in so the cut areas are offset. Take it to the lake and tighten the packing nut until you get the drip rate you want. Then lock it with the second nut.