[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PSUBS-MAILIST]
In a surface ship, the center of buoyancy (CB) is usually below the
center of gravity (CG). Buoyancy acts upward through CB, and weight
acts downward through CG, so with the ship in an upright position, these
forces directly oppose each other in a vertical line. If these points
were immobile with respect to the hull, any deviation from the perfectly
upright condition would act to capsize the ship, as both weight and
buoyancy would act together to create a rotational moment rolling the
ship away from upright. In reality, as the ship rotates away from
upright, the shape of the portion of the hull below the waterline
changes, shifting the center of buoyancy. In a stable ship, the hull is
shaped such that the buoyancy force acting through the shifted CB always
acts in rotary opposition to the weight acting down through CG, creating
a moment that tends to return the ship to the upright condition. This
is how something like a container ship with most of its weight high
above the waterline can be stable. In a submarine, this is possible,
but more difficult to accomplish because of the hull shape constraints.
In the extreme case - a simple cylinder - any rotation of the hull
doesn't change the position of CB at all, and the result is zero
stability. Hence the need to either incorporate a hull which does shift
CB as it rolls in the same manner as a surface ship, or to design the
sub so that CG is lower than CB at all times. In this latter case, with
the buoyant force acting upward and the weight acting downward through
these respective points, they will at all times act to right the ship.
This is absolutely necessary with the sub in the submerged condition,
since rolling underwater doesn't do anything to change the position of
CB. Some military submarines act like surface ships on the surface,
with CG higher than CB, but with a stable hull design for surface
running, and then as they dive, CB rises to a position higher than CG,
making the sub stable underwater. During this transition, CB will pass
through CG, but this usually happens so quickly that there isn't
sufficient time for this temporary instability to permit the ship to
roll. For Psubs, and in fact any smaller sub design, it is desireable
to maintain CG lower than CB at all times - even when surfaced.
Obviously, this more freeboard your sub is designed to have while
surfaced, the more difficult this is to accomplish. I may be mistaken,
but I think the ABS rules require this. I'll have to look into that and
post later. In the meantime, you need to consider the position of CG
and CB in your sub. To illustrate with a simple case - a cylinder with
two endcaps, with a conning tower of some sort on top. For a cylinder
alone, CG would be in the center of the boat. With the addition of the
conning tower and associated superstructure, this material raises CG a
bit higher. This needs to be countered by adding mass below the
centerline to compensate. Submerged, you don't have a problem since the
additional buoyancy in the conning tower raises CB above the centerline.
The bottom line is that all of the heavy stuff should be as low in the
boat as possible. In larger subs, there are many opportunities to use
light materials in the top half of the boat and heavier materials in the
bottom. In smaller psubs, shifting CG is accomplished predominately
through the use of lead ballast installed as low as possible. Note that
if you incorporate jettisonable weight (for emergency buoyancy) into
your design, you still need to have sufficient vessel stability, both
submerged and surfaced, once this weight has been released. Ergo, it is
a good idea to permanently install lead ballast to satisfy all of the
stability requirements first, and then add jettisonable lead to make up
the difference to get the boat neutral accordingly.
-Sean
Iain and Ianthe Gibson wrote:
Folks,
I have not come very far in my design process except for working throu
the hull design and structural engineering factors.
I am wondering how you maintain a low centre of gravity so that you
stay upright all the time whether with full ballasts or empty, when
the ballast tanks on the photos I see seem to be placed anywhere from
above centre or below?
Regards, Iain.
************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003. Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.
If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
removeme-personal_submersibles@psubs.org
Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of
our server receiving your request.
PSUBS.ORG
PO Box 311
Weare, NH 03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************