ABSTRACT Consideration is given to the drag of cylinders in fluid flow as influenced from the very large and lumpy roughnesses of marine growths. A means of characterizing the effective diameter is based on the Strouhal number of the roughened cylinder. This can be applied even to cylinders covered with thick accumulations of kelp, or heavy encrustations of mussels or barnacles, or any combinations. Thus, the effective diameter can be a function of Reynolds number. The effective roughness can be evaluated from force measurements. This paper presents results from measurements on laboratory models of marine growth roughened cylinders. INTRODUCTION A considerable fund of evidence has been developed on the force transfer coefficients for smooth and sand roughened circular cylinders in steady and unsteady ambient flow. In particular, the offshore industry and the U. S. government have sparked a considerable effort on the very complicated problem of determining wave and current forces on platforms at sea. It would be futile in the space alloted here to try to review the large number of reportings of others on this subject. Suffice it to say that the recent works of Borgman, Chakrabarti, Dalrymple, Dean, Garrison, Hudspeth, Keulegan and Carpenter, Morison et al, Sarpkaya and Wiegel (arranged in alphabetical order) and many others, have contributed greatly to the understanding of cylinders in the drag, drag/inertia and inertia regimes. Only the work of others that has direct bearing on a particular subject in question will be referenced in this paper. Even so, much work of others will be omitted and no doubt even overlooked. I apologize if anyone feels slighted and say at the outset that applicable work not mentioned herein and later brought to my attention will be much appreciated. This paper presents the results of some studies on heavily roughened cylinders in steady flow. A relatively smooth cylinder was tested and a sand roughened cylinder in order to establish a "ground truth." This is part of a larger study presently in progress at Oregon State University of macro-roughened cylinders in waves and in oscillatory flow. However, testing is still in progress at this writing and results are not yet available for publication. (References, etc. at end of paper). Heideman, et al (2) show a plot of the post-critical steady flow drag coefficient as a function of relative roughness, k/D, where k is the height of the roughness elements and D is the smooth cylinder diameter. It is reproduced here as Fig. 1 to illustrate that the drag coefficient still increases with increasing k/D at the limit of the plot, where k/D has a value of about 0.05 or so. However, it is known that marine growth thickness in the Santa Barbara channel (for at least one place) can be greater than one foot in the near surface levels on structural members greater than, say, two or more feet in diameter. One 3/8" steel wire rope, observed by the author and his diving partner, supported growth that had an approximate circumference of 2.2 feet.