Re: [PSUBS-MAILIST] gelcoat

["Rob Carlson" <rob.carlson@verizon.net>]

Some surface ships use a sound masking system called a Prarie Masking
system.  It uses a fine stream of air bubles along the hull and I guess acts
as a sound boundary.  Could something like that have a positive aspect on
drag (reduce it)?  A military submarine wouldn't think of using this because
the air bubbles need to collapse eventually causing a micro "bang!"  For a
surface ship that's not a problem because of the proximity to the surface
i.e they don't "pop' but rise to the atmosphere.

So .. for a non-covert sub the noise associated with a system would not be a
problem if it reduced the drag.  The system could be used for "high speed"
transit so as to minimize the use of precious air.  Since most of the subs
we are talking about travel in the sub- 10kt speed does any of this really
matter?

- Rob Carlson

----- Original Message -----
From: "Tim Curtis" <navark@yahoo.com>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, February 27, 2001 7:58 AM
Subject: [PSUBS-MAILIST] gelcoat


> 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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