Re: Psub Design Steps

[protek@shreve.net]

Psubbers,

    I wish I were a little more organized like Doug.  But I have never
designed anything using such a structured method.  Now, please do not take
this letter as advice because I feel like Doug's approach is probably more
efficient overall.  But, I would like to throw in my 2 cents worth here.
My approach is more organic and less structured but there are some factors
that influence this approach for me.  I will discuss.
    First of all, Doug is right in saying that one should look at their
desires and motivations to see if they really want to go to the trouble of
building a sub.  As he mentioned, most on the list have passed that point
by now.  I will say that you have to be committed (or maybe should be
committed!) if you are going through with a project like this. Most people
just have no idea what it is like to go through this process.
    All design is an iterative process, held inside certain boundries by
physical and logical constraints.  You need to analyze what you want in a
sub, but you also have to analyze what are the design constraints.  For
example; suppose you want to build a 2-person sub.  This is in general
going to be a larger sub in size and weight.  What happens with the towing
when the sub is 6000 pounds and the trailer is 2000 pounds.  You can not
pull that with a car.  Do you have a 3/4 ton truck with a 10,000 pound
hitch, a towing package, and a 6 liter V-8? What are you going to do with
propulsion, ballast tanks, viewports, etc etc?
    The way I design is, I develope a mental view of what I "think" the sub
should be.  Then I start asking questions about feasability.  The design
changes.  I rethink the overall picture.  I test the new design by applying
crude numbers and then do it again.  Later the numbers can be greatly refined.
    One big problem "most" anyone will have is related to procurement of
components.  If you have unlimited funds you can specify almost anything
you want to enter into your design.  If you want an internal ballast tank
that is 14 inches in diameter with a wall thickness of 0.35 you can have it
fabricated.  It may be expensive!  If you are limited by finances you go to
a pipe distributer and find a small section of pipe (know what type of
pipe!) that you can buy for a fraction of the cost.  But the only pipe you
can find in your local is 15 inches in diameter with a wall thickness of
0.45 inches.  So what do you do?  Factor it into the big picture of your
design.  You will not have this problem once or twice.  You will have it
many times.  How do you get around this?  I allowed certain space and
weight for systems that I had not designed at the time.  Is this the best
way to design a sub?  Maybe not, but try the other approach where
everything is designed totally before you begin.  You will find that you
will be faced with the problems above.
    When I designed my pressure hull, I only knew in general what the
systems were and where the systems were going to be located.  I allowed for
certain space and weight that I knew I could live with and continued my
construction.  When the hull was finished, including ballast tanks and
fiberglass cowlings I weighed it and then turned my attention to design of
the internal systems.  Once again, is this the best approach?  Few
industrial designers would do it this way, but consider their ability to
have fabricated what they need.  Much of my design depended on what a local
scrap metal company had in stock at the time.  Now, I am NOT talking about
structural components, or components that are related to safety.  I am
talking about things like 0.25 inch aluminum plates used for hydraulic
system component mounting, or motor compartment housing etc.
    Also, when you begin construction, you will see better approaches to
what you designed earlier and need the flexibility to change your approach.
 When it is a one-person design-construction this is not difficult, but
with each change you have deviated from the plan.  Since I do not have to
communicate my designs with others I can use a more organic approach.  I
have photographs of my shop during sub construction.  One would never think
anything workable would come out of that mess but my designs work and
usually pretty well.
    One last thought; technical judgement is the key.  You will make
thousands of decisions in a design.  You will be faced with decisions from
what kind of wire to use for a control line to how thick do I need to make
the hull.  You will paint yourself into corners on occasion.  How you use
judgement to get yourself out "safely" is a big issue.  THE BOTTOM LINE IS
BEFORE YOU TEST ANY SUB, HAVE A QUALIFIED ENGINEER LOOK AT ALL OF YOUR
DESIGN IN FINISHED FORM!  Remember, balance in the design approach is
critical.

Gary Boucher