Re: [PSUBS-MAILIST] Weighing

[Ian Roxborough <irox@ix.netcom.com>]

Hi,

I've been using the same calculation in a computer
program I've been using/writing. I've posted a screen
shot at http://www.prismnet.com/~moki/subfiles.html.

Something I've been wondering about is the point mass
vs the properties of the 3 dimentional shape in reality.
Do anyone think that there is a big difference between
assuming everything is a point mass?

There does seem to be a lot of work involved in dividing
each object up into small cubes and calculating the CM
of all the small cubes.  Although, I would imagine a
big advantage when performing CB calculation with parts
of the submarine out of the water.

At some point I plan on release the submarine design
application as open source (i.e. free).  But I still
think it has a lot of work left to do before then.

Ian.

On Sun, 04 Aug 2002 19:15:09 -0700
"Gary R. Boucher" <protek@shreve.net> wrote:

> Mike,
> 
>      I developed that spread sheet years ago with some ancient software.  I 
> don't think I can even call it up anymore.  But, what you need to do is 
> simple.  Create two spread sheets, or you can do it in one if you 
> want.  You need one for weights and the other for buoyancies. You will have 
> the following columns:
> 
> Name of Component
> X-position
> Y-Position
> Z-Position (optional)
> Weight (buoyancy for second sheet)
> X moment
> Y moment
> Z moment
> 
> The direct center of the hull is usually X=Y=Z=0
> 
> The X,Y,Z positions are the distances in inches from the direct center to 
> each component in their respective directions.
> 
> To get the X moment for a part you multiply the X position of that part 
> times the weight of that part.
> The Y's and Z's are done the same way.
> 
> If the component is behind the direct center, it has a negative X value, if 
> below a negative Y, if to the left a negative Z.
> 
> Add up all the X moments with respect to sign.  Do the same with the Y and 
> Z moments to yield three sums.
> By dividing each sum by the total weight of the boat you will get the 
> location of the CM or what some call the center of gravity.
> 
> Lets say you have one simple submarine weighing 4000 pounds including all 
> components.  You have a hull that is cylindrical, a 200-pound motor located 
> at X = -60 inches and a 300-pound battery in the forward end of the boat at 
> X = +50 inches.  Lets see where the CM is.
> 
> X moment for Hull =      (weight of hull)     x (position) = (4000)(0)   = 0
> X moment for Motor =   ( weight of motor) x (position) = (200)(-60)  = -12,000
> X moment for Battery = (weight of Batt)    x (position) = (300)(+50) = +15,000
> 
> Sum of the moments = 0 - 12,000 + 15,000 = 3,000
> Position of CM in the X direction = (total moment for X) / weight = +3,000 
> / 4000 = .75 inch forward of center
> 
> Does this help your understanding?
> 
> G. Boucher
> 
> 
> At 04:04 PM 8/4/2002, you wrote:
> >On Sun, 04 Aug 2002 11:23:32 -0700 "Gary R. Boucher" writes:
> >
> > >  The Center of Buoyancy is different.  The CB is usually
> > >calculated.  I had a spread sheet that I used where I listed EVERYTHING
> > >that went into the water.  For example; even a steel bar used for
> > >support had it's volume calculated.  It was given an X, and Y position.
> >Each
> > >volume multiplied by the weight density of water gave a small upward
> > >force.  Each force was summed as a torque about the X=0 and Y=0 axis
> > >of my sub.  The result is the CB.  The CB MUST be located above the CM
> >or
> > >the sub  will roll inverted.  This is good for excitement, but not good
> >for the
> > >crew that has to right the mistake.
> >
> >Can we see the spreadsheet?     Is it on line?
> >
> >
> >
> >
> >Mike H.
> >
> >
> >
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> 


-- 
The moon ate the sun on Monday and it was an awsome sight to see.