Re: [PSUBS-MAILIST] Weighing

["Gary R. Boucher" <protek@shreve.net>]

  My thoughts on W&B

    My 23-foot long 6000-pound displacement sub does not use a single dive 
plane.  The angles for ascent and descent are controlled strictly by the 
center of mass of the vessel.  This center can be shifted a small distance 
from front to back using a hydraulic cylinder and a 325-pound weight.  This 
scheme requires strict limits on where the center of mass (CM) and center 
of buoyancy (CM) are to be located.
     To determine the exact position for center of mass of my boat, I took 
a simple piece of heavy angle iron, turning it so that the 90-degree 
pointed edge faced upward.  I then used hoists to lift my boat and set it 
down on the angle.  Repeated balancing efforts allowed me to zero in on the 
exact position of the CM.  Empty, my sub weighs over 5000 pounds and I 
could hold it balanced with one finger on this steel fulcrum.  Of course 
calculations are fine two, but nothing gives you the exact center of mass 
like a fulcrum.  I wish the center of buoyancy was this easy to 
determine.  That had to be calculated with great care.
     To measure the vertical position for the CM I first positioned the sub 
on its fulcrum.  I then allowed the bow to be lowered several inches to a 
specific resting position on a concrete block.  Since I teach physics at my 
university, I "borrowed" some laboratory masses and a strong weight 
hanger.  By strategically placing the masses on the stern at a pre-measured 
point I was able to add mass slowly until the bow began to rise from its 
resting position and angle.  Using a little trig I calculated the vertical 
center of mass.  This is probably not needed by most sub builders who 
utilize very bottom-heavy designs employing dive planes or ballast changes 
to ascend/descend.
     I did not measure the left-right CM.  I just lucked out and it floated 
level!  One must be cautioned about this type of no-planes design.  The 
problem comes in when the sub is surfaced.  Because the CM and CB are so 
close the sub can be unstable when the center section comes out of the 
water, lowering the center of buoyancy.  My sub is a balanced trade off of 
stability in large waves vs the instability necessary to get my (+ - ) 20 
degrees of dive surface angle.  Make no mistake about it though..   It's a 
hoot to dive!
     I plan to do a presentation at the PSUBS convention on my subs design 
and operation.

Gary Boucher