Re: [PSUBS-MAILIST] Hull Calculator

["Sean T. Stevenson" <cast55@telus.net>]

On March 18, 2011 09:38:10 you wrote:
> Sean, Cliff,
> 
> Using the hull calculator spreadsheet it appears that ribs can take on
> numerous dimensions to achieve similar results.  For example, with a .5
> inch hull I can get very similar numbers for the Pa of inter-stiffener,
> stiffener, and longitudinal calculations by using both .25 inch thick
> webs and .5 inch thick webs.  The thicker web requires a shorter length
> and flange to get the same (or very closely similar I should say)
> results as the .25 inch web.
> 
> Is there any specific reason or rule that the web thickness must match
> the hull thickness?  It appears that in this case the same end result in
> water depth can be achieved by using the lighter .25 inch web material.
> 
> Jon

No such rule - as long as the stiffener geometry passes the stiffener tripping 
criteria, and the welds are full penetration, sections with the same moment of 
inertia will perform identically.  A thinner web will require less weld 
deposition for a full penetration weld (double bevel), which is an important 
consideration, as is the fact that moment of inertia (second moment of area) 
increases with the cube of section height, but only linearly with section 
width, making a tall stiffener more efficient than a thick one.  The design 
constraint which determines the geometry you use is the effective internal 
diameter of the hull (i.e. inside the stiffeners), as this is determined by 
your operational requirements.  Anything you can do to make the outer hull 
diameter smaller will decrease displacement significantly, decreasing weight 
and power requirements, which is obviously advantageous.  Thus, even though a 
tall stiffener is more efficient per unit material volume, using a wider one 
could enable you to have a smaller overall hull diameter, with is a larger 
advantage.  Similarly, you can get the same performance using a greater number 
of smaller stiffeners, but the tradeoff is the additional welding and weld 
inspection which must be done.  The optimum solution is dependent on the costs 
of welding, inspection, raw materials, labour, and of course the implications 
of vessel displacement on other costs as you vary hull diameter.

That software hull optimization algorithm I was working on had to take a back 
seat to more pressing matters in my life, but I got it as far as providing a 
few discrete solutions:  1) Lowest overall steel volume required for 
fabrication, 2) lowest vessel displacement, and 3) lowest weld deposition 
volume.  Each of these are the result of a single-variable optimization.  The 
actual best solution for any particular vessel is not going to be any one of 
these, but rather the solution to a multi-variable optimization (which is 
slightly more complicated math), which is highly dependent on the designer's 
priorities, and on the relevant costs, which are difficult to estimate for 
general applicability.  That is one of the reasons I hadn't pursued that 
project more vigorously - there really is no "one size fits all" solution.

Having said that, there are a few practical issues that may guide your chosen 
design.  In particular, if your stiffeners are to be fabricated from material 
cut from plate, it makes sense to make all components of identical thickness 
in order to maximize usage of the plate you purchase.  Also consider warping - 
if you have a thick web, the amount of heat you have to put into it to weld it 
to the hull will tend to deform the hull in a direction which (for internal 
stiffeners) decreases strength.

-Sean



************************************************************************
************************************************************************
************************************************************************
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 53
Weare, NH  03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************