Re: formulas

[VBra676539@aol.com]

David and Ray and all--are you guys talking about a 1-atm boat or a wet-sub? 
The former doesn't have internal displacement numbers (just pounds on pounds) 
and the latter does. Fussy? I'm so fussy you wouldn't believe it. And the 
engineers figure it right down to the last ounce, including an estimate for 
chicken sandwiches and a pee-bottle!  The VBT is for fine-tuning your trim 
once submerged.  You build in a reasonable payload, which would include the 
errant psubber AND his chicken sandwich, and you prepare for dives by 
figuring the current payload, adding or subtracting fixed ballast as required 
and going in the water within about half of your VBT or trim capacity.  This 
keeps you right on top of what shape the boat is in, and makes you stay in 
control of things.  And yes, displacement figures are VERY carefully done, in 
just the manner you suspect, plus several pages you may not suspect.  Every 
cubic inch of displacement is taken credit for, and every pound must be 
accounted for and compared to the displacement figures.  It's one of those 
fundamental deals like CG/CB and Metacentric Height and stress load and sheer 
calculations.  You can groan and gnash your teeth all you like, but you have 
to do the basics. Not doing them will get you killed.
Vance
In a message dated 5/5/99 7:42:13 PM, ray.keefer@eng.Sun.COM writes:

<<Hi David,

> I've been reading some very old (try March) p-subs messages, and came
> across a couple discussing how to figure displacement -- and from there,
> how much the thing has to weigh to sink. I'm wondering how fussy you people
> (those of you who have done this; those who have something to sink) got
> with this: what about all the various junk inside, which takes up volume
> and weighs more than air? What about gizmos which protrude on the inside?
> Do you stick them in a carefully-calibrated bucket of water and measure the
> water level rise? Do you weigh every little thing before installing it? Or
> do you just figure ballpark: the volume of this cylinder plus this cylinder
> plus these hemispheres -- and then allow for heavy enough trim weights or
> big enough trim tanks to compensate for the little stuff?

To get really acurate you have to take in accout all the above. As you build
you have to weigh everything, including your welding rods!

Also keep in mind that the stuff external to the hull, like planes or soft
ballast tank structures, also displace water. So they are actually lighter in
water the in air.

Still you will not account for every static weight so you design light
and put in hard ballast. 

For the variable weights, to account for air usage (air has weight), depth
compression on hull, temperature, salinity of the water, cargo (people and
their stuff) you will need a VBT. How big despends on how much of a variance
in the variable weights you design for.

Regards,
Ray>>