rim 31 and is provided with two parallel threaded studs 51 projecting perpendicularly outwardly from it the other leg 50 is perpendicular to the rim 31 and defines a vane of uniform width the central part of the pad 30 is held to the bowl shell 29 by means of bolts 53 and nuts 54 the latter elements are of flush head configuration extending through the base sheet 36 with their heads 55 countersunk into the inner wall of the base sheet as a result the heads 55 of the nuts do not interfere with the adjacent sheet 37 and the threaded openings 56 of the nuts face radially outwardly of the pad the base sheet 36 being made of fiberglass has adequate strength to hold the nuts 54 which are bonded in place and is better suited for this purpose than if made of rubber like the remainder of the pad when the pad 30 is placed within the dished central portion 33 of the shell 29 the nuts 54 are positioned in registry with openings 57 through the wall of the shell this provides access to the threaded openings 55 of the nuts so that the shanks of the bolts 53 can enter the nuts and hold the pad to the shell with nuts carried.