Forwarded Entry

[Jonathan Wallace <jon@whoweb.com>]

From: "Gary R. Boucher" <protek@prysm.net>
To: <personal_submersibles@psubs.org>
Subject: Some thoughts...
Date: Tue, 14 Oct 1997 20:39:11 -0500
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I have been thinking about sharing some design thoughts with those
interested in personal submarines.  I have do not know if this will be
useful to most of the members therefore I have not finished my thoughts in
all areas.  I have attached a text file that you may wish to read if
interested.  Let me know if more will be useful.  My web page with lots of
photos is:

http://www.prysm.net/~protek/index.htm

And my direct email address is:

protek@prysm.net

Let me know...     

Gary Boucher
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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.
   =20

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.
    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. =20
    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.
    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. =20
    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 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. =20
    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.



If there are interested readers I can continue this as a series =
discussing window design, weight and balance, and control options.
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