SURFACE FINISH VS DRAG FOR MODEL BOATS

By Richard Ellis

GHMYC

   Much controversy has been generated about this topic. In the end, only rigorously measured data will rule the day. Frank Bethwaite, who wrote the book High Performance Sailing, has to me, done the correct homework. He performed studies on the effects of surface finish on Laser rudders. They are the correct size to mean something for model boaters. They took two identical Laser rudders blades of modified NASA 0009 section, (a non laminar flow profile). Filled and faired them and surfaced them flawlessly and progressed them towards a mirror surface. One blade was left with a 1200-grit finish, quite smooth, but not polished. The other blade was polished to a mirror finish. The polished blade was able to produce about 20% more side force than the 1200 grit blade at the same conditions, which means that flow stayed attached longer on the polished blade and thus the flow energy was converted to side thrust more efficiently. What this data says is that even a non-laminar blade benefits from extremely smooth finishes, which delay transition to turbulent flow.

  Another test done with a typical foil showed that the polished blade had a minimum drag coefficient of around .005 (at zero angle of attack) versus .009 for a rougher blade (say about 300 grit). This difference increases when the blade was deflected, (as a rudder making a turn).  In the case of a laminar blade the minimum drag of the polished blade was .003 versus .008 for a non-polished blade.

  The conclusion is that a super finish reduces drag and improves lift/drag coefficients for appendages and for hulls of model boats. So the extra efforts done to use fine grit sandpaper and then use a polishing wheel will payoff. We may not see the dramatic differences noted above in our model boats, but there will still be difference.

Reference: High performance Sailing, chapter 19 “Foils”, section 19.3, “surface texture” and fig 19.1 and 19.2