SUBMERSIBLE DESIGN

General Factors to Consider

There are apparently a moderate sized group of people that contemplate
the design and construction of their own small one or two person subs.
This is evidenced by the growing number of PSUBS subscribers.  Many are
just looking at the factors that are involved and thinking about the
feasibility of such and undertaking.  I read a fair amount of traffic
on this subject from the PSUBS E-Mails sent.

Understanding that there is an interest I thought it would be useful to
some if I published some thoughts to the members on how to and how not
to proceed.  As the builder of a fully functional 6000 pound
displacement, 22 foot long electric-hydraulic powered boat, The
Vindicator, I would like to share some knowledge and philosophy.  My
project spanned a nine year period of which I did not work
continuously.

 It has been the greatest technical achievement of my life but at time
one of the most frustrating.

Since the field of submersible design is still wide open to new
ideas and concepts the designer has a fairly wide range of
opportunities to sharpen the designs that others have developed
previously.  On the other hand there is a lot of existing technology
that is proven and should be considered by the designer.

The designer should have in mind the end-use of the boat and what
it is he is asking the design to do.  In my case it was just a toy
to play with and a way to enjoy the mechanics of submarine operation.
Another designer may wish to take passengers, or to build the boats to
sell, or just to have a one atmosphere approach to underwater
observation.  All uses are legitimate as long as they justify the
effort.

Among the most basic design questions would be hull shape, power,
ballast tank placement, window geometries, and life support.  It is my
desire to address some or all of these areas for those interested.

HULL GEOMETRIES

The first area that I would 
like to cover is hull shape.  The geometry of the pressure hull is of 
utmost importance.  When I attempted to build my first submarine as a 
teenager back in my home town, my hull structure was elegant but 
dangerous.  I used 3/4 inch black pipe welded into a superstructure of 
ribs covered with metal not greater than 1/8 inch thick.  This design 
although not finished would have been fatal for the passenger if 
submerged deeper than 15 feet.  From the outside the hull was nicely
curved and streamlined and in a hexagon shape similar to the shape of a 
benzene ring one might have seen in a chemistry class.

The main problem with the design was two fold.  First, the elongated 
cross section of the hull would have placed terrific bending moments at 
the top and bottom on the support structures and caused collapse at 
minimal depths (much shallower than anticipated).  The second problem 
was the lack of substantial superstructure to support the pressure.  
Most people do not conceive of how much force water can place on a hull 
structure.  For example at a 100 foot depth every square foot of hull 
has a force equal to 6243 pounds pushing on it.  Not only must the hull 
be supported properly but the hull material must take the force without 
compromise.

There are many new approaches to elegant hull design that look more
like a jet ski than a conventional submersible.  Some claim an
operating depth greater than 100 feet but most will have severe
problems at depths much shallower than this.  The problem is that
without building the submersibles with extremely thick hull walls the
force will cause bending at short radius curves.  The best geometries
for resisting water pressure are those that are spherical or
cylindrical.  Probably the very best shape you can use is the sphere.
All force generated by the outside water pressure is normal to the
surface as with all shapes, but in this case the force is directed in
toward the center of the craft resulting in each hull section being
wedged against it's neighbor.  This results in a compression of the
material rather than a large degree of bending torque.  Thus design has
a limit and will crush even if perfectly round.  But, the depth of
crushing should be much greater than non symmetrical designs.
        
Another hull design that is commonly used is the cylindrical (tubular)
hull with elliptical, or better yet, hemi-spherical end caps.  This
design was what I chose for The Vindicator.  Hemi- spherical end caps
are better than the elliptical ones that I used but offer some problems
in certain designs where the end of the pressure hull is not the end of
the sub itself.  I have ballast tanks located beyond the ends of my
pressure hull.

Even with cylindrical or spherical designs much care should be 
employed when choosing the thickness of hull.  Most hulls of the above 
mentioned geometries can withstand a much reduced pressure if they are 
out-of-round by only a small percentage.  For this reason most designers 
use framing inside the hull to stiffen the structure.  Framing could be 
employed outside but in most cases the outside portion of the hull is in 
contact with the moving water and framing would seriously increase the 
drag on the boat through the water.  The use of framing, how much and 
where, along with the hull thickness and end-cap thickness should be 
referred to an engineer qualified to make such determinations.  




POWER FOR PROPULSION

Nuclear is best but who can spare the uranium.  Laying the jokes aside,
there is more than one approach to powering a submersible.  I actually
envisioned a "Thermite Engine" when I was younger that used the
compound thermite to generate heat for boiling water and producing
thrust.  The problem with thermite is that once ignited it burns at
6000 degrees F and can NOT be extinguished.  I had thought of canisters
of the material that could be ignited periodically to produce a
superheated chamber of steel that water could be injected into.
Probably a potentially fatal idea.  Who wants to ignite an uncontrolled
chemical chain reaction in a closed vessel under water and hope that it
stays contained.  This was the closest thing I could come up with at
age 16 to mimic nuclear power.  Oh well, please do not do this at
home.  Bad idea!

A less radical approach to energy production under water would be 
the application of an air breathing engine such as a diesel that runs on 
compressed air.  Here are some problems with this approach;  First, you 
could only carry a limited supply of air for the engine that would 
result in limited use.  Probably only a few minutes of underwater power. 
 
Second, the underwater engine would produce gasses that would have to 
be released through the hull and bubble to the surface, much the same as 
the old chemical torpedoes we used in WWII.  Depending on the depth 
there would be a possibly substantial back-pressure for this off- 
gassing.  

The third potential problem, and probably not the last, would 
be the danger of having flammable liquids on-board a closed air filled  
vessel.  Diesel would probably be much better than gasoline!
    
The idea of a reciprocating engine to be used in a submarine would 
be enhanced if the engine did not produce off gas products like CO2 or 
CO.  The use of pure hydrogen and pure oxygen has merit.  Both can be 
maintained under pressure and mixed at the point of cylinder injection.  
A modified carburetor could be used with some kind of control system to 
adjust the flow.  High tech to say the least as this engine would 
require some development.  The benefits would be little or no off-gas 
product.  The two fuels would produce water in vapor form that could be 
cooled by the infinite heat sink of the outside water.  The condensed 
water would weigh the same as the two fuels before burning, thus 
creating no ballast problem.

The EXTREME danger is obvious.  That is the escape of hydrogen into the
hull compartment.  Hydrogen at levels of 1 or 2 percent in pure oxygen
is explosive.  In air, levels of about 4 percent are explosive.  An
explosion on-board would be fatal in most cases.  One interesting
approach would be to fill the engine compartment with an inert fluid
that was not flammable and would not allow the unnoticed release of
either gas.  With such a non- compressible fluid in place, any
off-gassing would result in a sudden fluid pressure increase and could
shut off all fuel supply.  How about that, just came up with that idea
while writing.


BATTERIES

Getting back to more practical and state-of-the-art working 
propulsion systems the designer can choose battery power.  This is the 
accepted norm for most all subs of this class.  The variations come from 
the type of batteries and their placement.  I chose lead-acid golf cart 
batteries for my design, six of them.  Mine are connected in series to 
provide 36 volts at 220 amp-hrs.  There has been a lot of talk about 
gel-cell batteries for use in subs.  I have a Gel-cell in my airplane 
and love them.  They work upside-down and right-side up.  However you 
must remember that in general the amp-hour rating of the gel- cell is not 
that of the cheaper lead-acid type and the expense is much higher.  
Nothing much is cheaper per amp-hour than the old fashioned lead-acid 
battery.  This battery has been around since the early 
1900's.

There are several problems with the lead-acid battery.  For one 
thing they are full of approximately 25% sulfuric acid.  This acid is 
very corrosive as anyone knows who has had to clean car battery 
terminals.  If these batteries ever explode they can throw sulfuric 
acid.  This has been known to blind people in accidents where they were 
splashed with the acid and were not able to flush it quickly.

Another problem with the lead-acid battery is the off- gassing of
hydrogen.  This occurs when the battery is charged, discharged and to a
lesser extent when the battery just sits not being used.  As mentioned
earlier the hydrogen is not just flammable but explosive in almost any
quantity.  The charging cycle of the battery produces by far the most
hydrogen.  This is why you have to take special precautions with jumper
cables on cars.  When a jumper cable is being used the battery can take
on excessive charge current and produce large amounts of hydrogen.
When the user disconnects the cable the arc can ignite the gas and
produce an explosion.  It is difficult for me to imagine what would
happen if such an explosion occurred while submerged inside a pressure
vessel.

To a large extent the danger of explosion can be reduced by using 
hydrogen catalyst battery caps on all cells of the lead acid battery.  
These can be purchased from Hydrocap, 975 N.W. 95 Street, Miami Florida, 
33150,(305)696-2504.  I use a cap on every cell of my system.  When 
charging the cells get hot as the battery approaches full charge and the 
hydrogen is generated at a faster rate.  The hydro-caps also tend to 
remedy another problem, that of adding water to the cells.  The caps 
return combined gasses back into the cell as pure water.  This company 
also has catalator elements that can be placed in battery boxes to 
reduce the chance of stray hydrogen generating an explosion.

One major choice is where to put the batteries, inside with the 
operator or in separate pods outside the main hull structure.  I chose 
to have mine with me inside but this is probably more hazardous from the 
above mentioned points.  A friend in Texas who built a sub opted to have 
his in two separate pods external to the main hull.  This is good since 
he has had one majoy hydrogen explosion.  He did not have hydro-cats. 
    
One other consideration that I had during thinking my sub design out 
was the possibility of electric shock if the hull is flooded with water. 
This would probably be worse with salt water and higher voltages like 
my 36 volt system.  I am told that lead- acid batteries also produce 
chlorine if exposed to exposed electrodes.  For this reason I carry 
breathing protection.


SIZING CURRENT DEMANDS

One common problem in designing the electric propulsion system is 
most people do not know that the amount of current that a battery system 
can supply is limited.  Often times a car battery is called on to 
deliver 600+ amps to a starter system.  This is OK for short periods of 
time but excessive current draw for extended periods of time will harm a 
battery.

A 220 amp-hr battery may supply 1 amp for 220 hours, or 22 amps for 
10 hours, but if asked to deliver 220 amps the battery will flounder 
after far less than one hour.  Thus, the amp-hr capacity of a battery IS 
a function of the current draw.  One battery specialist recommended to 
me that I limit the current draw to no more than 1/2 of the amp-hr 
rating of the battery.  I do have about 176 amps of current draw on 220 
amp-hr batteries if I kick the engine in "Hyper" mode.  This does not 
seem to cause a problem no more time than I have been operating the 
system in high speed.  My sub moves too fast even in lowest power mode 
so I find flank speed unnecessary. 


MOTIVE POWER

My propulsion is furnished by a 6.7 horsepower motor driving a 
hydraulic pump.  The pump furnishes hydraulic fluid under high pressure 
to a control system and then through the hull interface to a hydraulic 
motor which turns the prop.  This works well but requires more energy 
due to loss than a direct motor-propeller system.  

Most people will chose to use an electric motor to drive a propeller
directly.  Each horsepower produced is equivalent to 746 watts if there
is 100% conversion from electrical to mechanical energy.  Motors are
less than 100% efficient, more like 70 to 85 percent.  So the total
electrical power can be as much as 746/(.70) or 1066 watts.  Since
power in watts is equal to motor voltage times motor current in amps, a
12 volt one- horsepower motor will require about 89 amps.  A 10
horsepower motor will require 890 amps if 12 volts is the operating
option.  That is why most higher horsepower DC motors operate on higher
voltage.  Remember the limitations of the 220 amp-hr battery?  This
takes some planning or the designer will paint himself into a corner.
I do not recommend even 5 horsepower as this will push most small subs
like a torpedo.


SPEED CONTROL

Speed control is also an engineering problem.  There are several 
approaches.  My approach is simple.  I have "Stop", "Low Power", "Medium 
Power", and "Full Power".  I accomplish this by placing either four 
6-volt batteries, five 6 volt batteries, or six 6 volt batteries in 
series to the motor.  This can be accomplished by switch, but I have 
large current relays that open and close.  Problem here!  If a relay 
sticks then there will be a short on one or two batteries.  For this 
reason I use 220 amp electric vehicle fuses purchased from Grainger.  I 
have never had this problem but if not fused you may enjoy the Fourth of 
July early and at depth.  My system is controlled by computer and the 
computer handles the switching.  There are safeguards to prevent closure 
of the next relay in the system if the previous relay sticks. 

Another way to control motor current and thus power is to use pulse 
width modulation.  This is a great way to control power but the higher 
the current the more difficult it is to find the equipment to do the 
job.  One can get into high priced MOSFETs and need a lot of expertise 
in electronic design to accomplish this.

The rheostat or (variable resistor) can be used but this generates a 
great amount of power in the resistor itself sometimes more than the 
actual motor power.  Cooling and bulk almost rule this approach out 
totally.