primary tube 20 would be covered by a solid fin collar turning to fig4 an alternate embodiment of the fin collar of the present invention is shown fin collar 40 has a rectangular shaped slit 42 the slit 42 is defined by a bottom wall 44 and opposed side walls 46 and 48 the bottom wall 44 may extend to a point approximately 0 02 u2033 from the underside of the fin 12 the collars 40 may have a curvature such that they have a convex shape on the side that faces the tubes 20 in fig5 another alternate embodiment of the fin collar of the present invention is shown fin collar 60 is elongated in the longitudinal tube axis direction the collar 60 has a plurality of slits 62 defined therein the slits 62 also have a rectangular shape and are defined by a bottom wall 64 and a pair of opposed side walls 66 68 the bottom wall 64 may extend to a point approximately 0 02 u2033 from the underside of the fin 12 in fig6 and 7 the fin collar 30 of the present invention is shown with arrows 70 representing air flow around the collar 30 during use the shape of the fin collar 30 provides interruptions around the circumference of the fin collar 30 perpendicular to air flow the interruptions will provide turbulence which is indicated by curved lines 80 in the boundary layer of air along the fin collar 30 which will increase the rate of heat transfer between the air and the tube 20 the increased turbulence will also occur around the area near the base.