r 3 and t bars 6c 39 similar to those in the prior art and t bars 6c 34 having a dovetail groove in their upper surface are alternately mounted on the main body at a pitch l 1 39 furthermore a tube 6d expansible by hydraulic pressure is disposed in the dovetail groove of the t bar 6c 34 the dewatering blade 6b 39 defines a t shaped groove similar to that of the heretofore known blade to accommodate t bar 6c 39 and a box shaped groove to accommodate the expansible tube 6d each blade 6b 39 is extractible in the widthwise direction of the main body similarly to the blades in the prior art at the time of operation the dewatering blade 6b 39 is supported at two locations namely on the upper surface of the t bar 6c 39 and on the upper surface of the expansible tube 6d the t bar 6c 39 is located directly beneath the land of the dewatering blade 6b 39 and the expansible tube 6d is spaced from the land of the blade as can be seen in fig1 3 each dewatering blade has a surface intersecting the land at an upstream terminal edge thereof at an acute angle so as to form a rake contacting the wire 1 in fig1 when a center line of a land of the blade 6b 39 coincides with a.