The present invention provides low profile, thick (“stubby”) longitudinal fins having a cross section which is a parallelogram, trapezoid or a triangle extending from 10% to 100% of a coil pass and comprising from 3 to 45 weight % of a coil in a radiant section of a furnace for thermally cracking one or more of paraffins and naphtha. The fins provide an additional surface through which heat may be transferred to the coil making the coil more efficient reducing greenhouse emissions.