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Hi. Your design is taking shape and I see it's being refined as time
goes on. I have a couple of ideas I'd like to offer. Jim K. has a large dome
viewport on his sub. I think it's a 39 inch dome with a short tapered section
connecting it to his 48 inch dia. hull. He will be able to advise you on what it
takes to fabricate the dome and hull connection. Jim posted a picture of his new
dome a short while ago and it's available on the psub site. Look at member
projects. I think his dome was about $4K and the size was decided on due to
fabrication limitations. Unless you are building the dome with a
cast-molded method, 39 inch is as large as it gets.
A single cast-molded dome larger than that will be very
expensive. Maybe closer to $20K. I don't see any type of guard or
bumper shown on your sketch to protect the domes from impact. You might
consider that.
If I see the sketch right, you have an
acrylic tube section in your tower. I've heard good things about that type
of window, and cost wise, it seems like a good way to go. Greg Cottrell builds
many of the windows we use, and should be consulted on window design,
fabrication, and availability. He's the guy with the right answers.
On the saddle tanks you have on the sketch. Are they
welded to the hull or are they to be fabricated as separate tanks and then
attached? All the subs I've seen have a way to access the interior of the tanks
so cleaning and re-painting can be done. Look at the pictures on the psub
site at the subs with saddle tanks. They are very stable both on the surface and
while submerged, and provide a large area on top of the sub that's very useful
while surfaced. Dan's blue sub has a flat "deck" and it's easy to climb into.
Very user friendly. His tanks have square access plates that can be opened
so maintenance on the inside can be done. They can also be removed for
painting on the hull. If the tanks are made of stainless or fiberglass, then
inside work is minimized. Stainless will get dented, fiberglass will break. Each
tank needs a way to add/remove air so the more tanks you have, the more
through-hulls and plumbing you need. The larger the tanks are, the more air you
need to carry for blowing them, and when submerged, the more mass you are
dragging around. There's a trade-off here.
The 4 tank design is maybe more than you need. If your
sub is balanced side-to-side, it won't roll much. More important is
front-to-back control. The angle is important because you could possibly
get the sub angled enough to prevent air from releasing and then the situation
gets worse so the sub can actually get vertical. A slight difference in ballast
tank buoyancy can make a big difference in "center of buoyancy" and throw the
sub into a dive angle. Plumbing locations and design are critical elements,
along with the shape of the tanks. The tanks either need to be located at the
very top of the sub, or at least have a point way up high so that even with a
steep angle, the plumbing is always at the top.
Your skids on the sketch could have places where
something like a tree branch or other submerged object could get wedged in
there. Any place where something can get hooked on your sub should be avoided.
Especially on the bottom where a sunken object could trap the sub. It doesn't
take much to hold a sub down. Just a small tree branch or bit of seaweed is
enough. Try to keep the entire surface as sleek as possible. This will help
eliminate snag points, and with a sleek shape, your batteries will last much
longer.
On your trim tanks, they look really big. I
didn't see any drop weight on the sketch. Most of the small subs have fairly
small trim tanks. This is used for fine adjustment of buoyancy. Any major
change like adding a passenger or adding/subtracting a piece of
equipment is compensated by adjusting the on-board lead ballast while at
the surface. If you anticipate picking something off the bottom which would add
weight to the sub, a lift-bag or some type of device to add
buoyancy is an inexpensive approach, and is safer. You can detach yourself
from the object/lift-bag and still surface the sub. Trim tanks usually are not
much more than about 10 gallons. Sometimes there are two. One in front, and one
in back, with a pump to move the water back and forth so the "trim angle" can be
adjusted. This is after the total amount of water is added to achieve
neutral buoyancy, and the trim tanks are valved off from the exterior. The trim
tanks are usually filled while the sub is submerged but still shallow, so a lot
of pressure isn't built up inside the tanks. If you launch the sub in a fresh
water river and then motor out to the ocean, the difference in buoyancy between
fresh and salt water will make it necessary to readjust the trim tank fill
level, but generally the level stays the same so filling is made at the
beginning of the dive. The drop weight and it's release mechanism is a critical
part of your sub. I've always thought that the bigger-the-better approach here
was good. It's the last resort to getting out of trouble while submerged, and
should be designed so it always works, no matter what. It's better to lose an
expensive drop weight than to be stuck and needing a rescue. If you attach a
buoy to the drop weight, you can always go back down and hook a cable to it for
retrieval from a surface boat. Not many places have a sub available to come and
rescue you if you're stuck on the bottom, and flooding and exiting a trapped sub
is a whole can-of-worms that we talk about all the time, but nobody really wants
to go there.
The drop weight is a crucial part of any sub.
I suggest you build some scale models of your sub, and
test them. For your design, a small camping propane tank is very close to the
right shape of the hull. Measure the displacement, and make the hull model weigh
the same amount as your proposed hull will be. Make some ballast tanks that
again weigh as much as the full size sub will. See how big the tanks need to be,
how it all balances out. Copy ( in scale ) the different designs and test them.
I found this method to be very useful, and really a lot of fun too. It saved me
a lot of work and made the design process much easier. Keep it all to scale in
size and weight, and you will get a good idea of how the sub will perform. I
made several ballast tank designs with thin sheet metal, soldered them water
tight, added lead weights where needed, and attached them to the hull shapes I
had made. I learned a lot from the process, and saved a ton of money before
committing to my final design. I even used the models to test how much power it
takes to move it through the water, how much drop weight to use, and how stable
it was when submerged and surfaced. Now that I've started on the real thing, my
grand sons get to play with the models.
This phase of your project is the most fun, and should
actually take the most time. There was a thread here a while back, talking about
"how" to design a sub. My approach was to first decide what I wanted the sub to
do, and with that as a goal, design it so it achieves that purpose. There are as
many designs as there are guys doing it. Many of the individual components are
the same from one sub to the other, while some of the different shapes are
radically different. A good place to start is the K boat plans. Really a bargain
at that price, even if you don't plan on following them very closely. It's a
tested and proven design, and is a good place to start. A small sub probably
starts around $20K for the minimum so $350 for a set of plans that works is
really cheap, even if it's just for the information contained in them.
I'm no expert, but all the good advice I've received
from these guys has helped save me a ton of dough and made the whole process a
lot of fun.
As I said, this part of the build is the most fun. Enjoy
it, and ask lots of questions. These guys are all willing to help. Frank
D.
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