[PSUBS-MAILIST] gelcoat

[Tim Curtis <navark@yahoo.com>]

The idea behind all of this is to reduce the amount of
flow that is touching the surface in question.  Like
you said, laminar flow is better for as much of the
hull (wing) as possible in terms of drag.  There will
almost always be turbulent flow over the majority of
the hull though.  These turbulent vortices suck energy
from the hull to maintain their motions.  If you were
to place micro-grooves along your hull that were
matched to 80-90% of the diameter of these turbulent
vortices, then you would have a reduction in drag. 
The reason being, is that the turbulent flow actually
touches less of the hull this way - if you picture two
triangles, like two mountains next to each other, with
a huge sphere/cylinder supported by the two points. 
The sphere is too large to fall into the valley
between the mountains, so it only touches at two
points.  If you can get your turbulent flow to only
touch at two points instead of dragging completely
across the body, you will decrease your drag.  This is
probably poorly worded, and I apologize.  It is much
easier to describe with diagrams.

   -Tim




 I seldom post anything but often take a look in to
see what is going 
on.  Now I am truly puzzled.  First, I am not a fluids
engineer, but I do 
know some facts about aerodynamics from years of
studying airplanes.  I 
think you fellows are talking about turbulent boundary
layer.  As air flows 
over an airplane wing the velocity of the air
approaches zero as it comes 
in contact with the wing itself.  In an airplane an
undisturbed boundary 
layer is very important.  This creates laminar flow
down to the wings 
surface and greatly reduces drag.  One reason that the
P-51 in WWII was so 
fast was that they could maintain this laminar flow
well past a third of 
the wings cord.  The British gained advantage with
this effect also until 
they started painting their Spitfires with a rough
dull finish camouflage 
paint which disturbed the laminar flow and caused
greatly increased 
drag.  Everything I have studied suggests that you
want to minimize 
turbulent boundary layer flow to reduce drag.  I have
no doubt that putting 
something like course sandpaper on the hull of a sub
would be a massive 
mistake.
     Perhaps there is some finish that promotes a type
of turbulent flow 
that is benificial but before anyone goes for the
rough look I think you 
need to research this extensively.

G, Boucher


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