and second curved surface member respectively extending from the proximal edge 16 24 to the distal edge 18 26 it is noted that in the alternate embodiment shown in fig7 the planar member 14 a does not define a chord a longitudinal line 38 a however serves the purpose of enabling this description to identify selected angles accordingly it is understood that the u201c chord u201d of the alternate embodiment i e planar member 14 a is a longitudinal line or alternately an infinite chord as shown in fig1 and 7 the first and second member bodies 12 14 or 14 a preferably each include first and second stiffening members 40 42 and 44 46 first and second member respectively the first and second stiffening members 40 42 and 44 46 are disposed at the lateral sides 20 22 and 28 30 of the first and second member bodies 12 14 or 14 a respectively each include first and second stiffening members 40 42 and 44 46 and preferably have a u shaped cross section with the u201c tines u201d of the u shape extending outwardly and generally parallel to the arc defined by the associated member body 12 14 or 14 a each first and second stiffening members 40 42 and 44 46 may include support ribs 48 extending between the u201c tines u201d of the u shaped cross sectional stiffening members 40 42 44 46 the first curved surface member body second distal edge 18 defines a blade 50 the blade 50 may be a thin and or tapered portion of the first curved surface member body 12 alternatively the first and second member bodies 12 14 14 a may be made from a non metallic material and the.