Re: [PSUBS-MAILIST] gelcoat

["John Pier" <caribsub@coqui.net>]

The new Russian torpedo, still under development goes 200 miles an hour
under  the water, yes 200 miles / h
however it is kind of a rocket more than a torpedo and  to reduce the huge
drag at this speed , the torpedo releases a "skin" of gas between the water
and its hull. No sand paper here !!!
Herve Jaubert

----- Original Message -----
From: Tim Curtis <navark@yahoo.com>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, February 27, 2001 11:58 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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