Re: [PSUBS-MAILIST] Re: high speed subs

["Captain Nemo" <vulcania@interpac.net>]

----- Original Message -----
From: <VBra676539@aol.com
"No, that's the wrong direction to head in. The attitude change is forced on
the boat(s) by the prop thrust. I assume some of it is due to side wash,
especially on the big (36 X 36) wheel that we used at Perry; every stern
driven vehicle I ever piloted suffered to some extent with the problem.
Little Aquarius, with a 14" kort nozzle did it, right up to the big PCs
which  were over 30 feet long. The drag and flow problems wouldn't kick in
until things were moving. Mind you, a certain co-op engineer who no longer
speaks to me on the net did some tests on this K of mine many moons ago, and
wasn't any too thrilled with its characteristics as I recall. Don't you have
this problem to some degree with the skinny hindquarters of  your N?  Vance"


(To answer the last question first):  No.  There is no "aft settling"
problem with the NAUTILUS MINISUB whatsoever.  The ballast system enables me
to operate at any attitude: level, vertical, inverted, or any degree
inbetween.   The only time she's been tail-low was when I deliberately
ballasted her down that way to achieve a slightly nose-up attitude during
the first dynamic dive tests: (an aid to recovery in case something went
wrong).  When I apply power, she stays stable; underway, she planes out flat
and drives straight as an arrow.  Good positive dynamic stability; easily
destabilized therefrom to dive aggresively.  The skinny empenage gives no
problem, and actually seems to enhance relative flow to the prop and over
control surfaces in that area of the boat.

Now, getting back to the "aft settling" problem in your K, the Perry's, and
others: OK,  I understand this is NOT a flow / drag condition engendered by
poor hydrodynamic relative flow over the hull while in motion: the boat
settles stern first BEFORE it begins moving, right?  If that is the case,
then I see two, maybe three possibilities:

1. (Weight shift to the rear due to moving water in  partially filled
ballast tank?)  Highly doubtful.  I just threw that one in here to eliminate
it; I really don't think this is the problem, though someone might say it
could be a possibility.  I don't believe this would be the case in all those
subs; or that the people driving them wouldn't be aware of it.  I just
didn't want it to look as though I'm overlooking it; but I think you'll
agree this probably isn't it.

2. ("Side wash" [as you put it] against the hull.)   I'm trying to visualize
this: I think what you're saying is that part of the thrust column is
impacting the hull in such a way that it is either deflected vertically,
creating a downward force on the structure; or maybe creating a flow-induced
zone of lower pressure, down into which the aft-section of the hull is being
drawn?  Yeah, it seems possible; even with Kort nozzles: depending on where
(and upon what) the thrust column is striking the hull.  Between the two of
us, only you know what the application looked like: whatayathink?

3. (P-factor.)  Basically, a destabilizing force produced by unequal
pressure on the spinning blade disc, and transmitted to the structure at a
tangent vector via gyroscopic precession.  (I've was running this one
through my head on the job last night.  Gotta lay down some ground work to
explain it.  Bear with me, OK?)

Let's think aerodynamics for a second.  The advancing blade of the main
rotor on a helicopter in forward flight encounters greater impact pressure
than the retreating blade does: this  causes an imbalance of pressure on the
spinning blade disc, and can contribute to a condition called "retreating
blade stall."  Therefore, modern helicopters incorporate articulated rotor
hubs to compensate for the effects of this unequal force.

In an abstractly similar way, the descending blade of a 36-inch sub
propeller moves into an area of greater depth pressure than the ascending
blade does: (there is about 1.345 PSI difference in pressure over a 3-foot
variable in depth, and thus the top of the spinning blade disc is operating
at that much less pressure than the bottom of the disc is on your 36-inch
prop).  With a disc surface of 1017.36 square inches, that's a total
variable of 1368.3492 inch pounds acting unequally on the spinning blade
disc; or a difference in force of 684.1746 inch pounds acting on the top and
bottom halves of the disc. The actual distributed force will probably be
focused on a smaller portion of the overall blade area (like, say,  one
quarter of the rotational arc), so the destabilizing effect will be
increased in that zone; and the resultant adverse force delivered through
the prop shaft to the structure will also be amplified by leverage focused
at a point located somewhere between the 18-inch blade tip and the shaft
center.  Added to this may be the force generated by the inertial mass of
the prop blades themselves amplified by the energy imparted to them via
rotational movement.  Thus, we can see a potential for the distribution of
unequal force from the prop to the shaft as a result of variations in depth
pressure acting on the surface of the (large) 36-inch spinning blade disc.

Other factors which may effect the angle at which this force acts upon the
submarine structure include (1) torque; (2) hydrodynamic resistance to blade
rotation; and (3) gyroscopic precession.

The result of (what me might call) this "unequal-pressure-generated
prop-factor", combined with torque, and vectored by hydrodynamic resistance
and gyroscopic precession, could be delivering an inertial force to the hull
structure sufficiently powerful enough to physically alter the subs pitch
attitude, (in much the same way as similar forces engender adverse yaw on an
airplane initially accelerating for take off).  Or at least, it seems
theoretically possible to me.

That's about the best I can do without actually seeing the actual
application.  In any case, there's one of those "non-obvious gremlins" at
work here that's causing your boat to drag her butt when she shouldn't be.
There's probably other possibilities of why this occurs that may become
apparent with additional consideration; but as of this morning, that's the
best I can come up with.

Anyway, you know more about the actual design than I do; whatayathink?

Hope this helps.

Very best regards,

Pat Regan
vulcania@interpac.net