Re: [PSUBS-MAILIST] Center of gravity Vs. Center of Bouyancy

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

All,

I haven't been on Ping's site in years, and I just noticed that the version
of SIMPLIFIED SUB DESIGN MATH I directed you to is an old one lacking some
of the improvements in terminology I made in recent years.  The one that's
up now will work, but there are some terms that could cause confusion.  I
just emailed Ping to straighten it out.

Basically, the idea behind SDM is:  if the design is bottom-heavy,
top-buoyant, and bilaterally symmetrical enough to assure positive dynamic
stability about the roll axis, what remains to be determined is the location
of the Longitudinal Hydrostatic Center of Gravity (pitch axis) when the boat
is submerged in a state of equilibrium, immobile, and at neutral buoyancy.
Doing this involves: calculating the Longitudinal Center of Weight (LCW) ;
calculating the  Longitudinal Center of Buoyancy (LCB); and averaging  the
two variables to find Longitudinal Hydrostatic Center of Gravity (LHCG).

Keep in mind, SDM does not address hull crush depth or hydrodynamic
stability considerations.

Yes, any shift of weight (occupant movement, for example) sufficient to
change the LCW; and/or shift of buoyancy (ballast trim adjustments, etc.)
sufficient to change the LCB; will also change the LHCG, which is derived of
both variables.

And yes, the vertical component of buoyancy can change depending on the
design of the tanks.  But again, a small  sub can be designed top-buoyant
and bottom heavy enough to nullify the effect.    For example, my NAUTILUS
has four wraparound tanks, but the Vertical Center of Weight is so low that
the boat remains dynamically stable in roll when the tanks are dry, or with
any amount of water in them.

Other popular designs (like the Kitteredge 250) have main tanks front and
rear that can be adjusted for buoyancy and trim, but again, these are placed
high and the vertical center of weight is low in comparison, so instability
about the roll axis during ballast operations doesn't seem to be a factor.
The K-250 also has a smaller internal ballast tank; low and amidships, that
the pilot sits on.  I've not driven a K-250, but this looks like it allows
fine vertical trim without disturbing the LHCG.  Dale or Vance could say for
sure.

Pat