To: baidarka@lists.intelenet.net
Subject: Re: Tank tests about the effects of surface texture?
From: marc_pettingill@juno.com (Marc F. Pettingill)
Date: Tue, 18 Feb 1997 22:23:48 EST
On Tue, 18 Feb 1997 15:49, John Winters wrote:
<snip>
> The critical roughness that can be determined by the following
formula;
> k= 14/v where k is the grain size in mils and grain size is the
projection
>above the normal surface and v is the velocity in feet per second.
>
>5.07 feet per second (three knots) any roughness over 2.76 mils will
>increase resistance. Smooth paint has a surface roughness of 2.0 mils.
>Smooth gel coat 1.0 mils. A rule of thumb used in my sailboat racing
>days was that roughness you could feel with your fingers was bad.
John,
could you go into this a little more?
How do fish scales figure in all this? Fast swimming fishes are not
nearly as smooth as a painted surface.
(Or do they just work up a sweat? ;^)
Some years back the high-zoot racers had some sort of micro etched
film that when applied to a sailboat hullwas supposed to significantly
reduce parasitic drag. Ok, really small etched lines, probably below
the critical roughness for the intended speed.
But what did the etching do? (As I recall, the film was very difficult
to
apply, and then the stuff was declared verboten by the governing bodies.)
As to " roughness you can feel with your fingers," you can feel the
difference between a shiny surface, and a surface that has been
deglossed with 400 grit sandpaper. Does this count as being too
rough?
Thanks,
~marc
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