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Re: [PSUBS-MAILIST] How heavy can I go?
Hi Greg,
I'll take a shot at an answer. You need to know what the displacement
volume of your ballast tanks (BT), the volume in the cabin and the weight of your sub.
To fine tune the numbers you will need to know the displacement of
everything else like fiberglass, fins, motors, scuba tanks... but the
above three items will get you a working number.
Two ways to find volume of you ballast tanks and cabin are:
1. Fill with water. Measure when you scoop it out.
2. Measure how long it takes to fill a five gallon bucket with water
from a hose. That will give you gallons/time rate of fill. Then fill
each ballast tank and cabin. Measure the time for each to fill. Then calculate
how many gallons each will hold by:
gallons = time to fill * gallons/time rate
Then convert gallons to cubic feet with
1 cubic foot = 0,028 m^3 or 28 liter
1 US gallon = 3,785 liter
In formula:
cubic feet = gallons * 3.785 liter/1 US gallon * 1 cubic foot / 28 liter
Then convert cubic feet to displacement depending on whether you are
using fresh water or salt water by using the correct density conversion:
Fresh Water 62.3164 lb/ft^3 at 20 degrees C
Sea Water ~64 lb/ft^3 at 20 degrees C
So the displacement would be (for example in salt water):
displacement = 64 lb/ft^3 * cubic feet
So far you now know the displacement for each ballast tank, cabin and
the weight of your sub. You can weigh the sub at a public scale. Once
with sub and trailer and then just the trailer. The difference would
be the weight of your sub. At our dump the scale is good down to 20 pound
incriments.
In order for your sub to float, positive bouyancy, the displacement of
your ballast tanks and cabin needs to be more then the weight. In
order to sink, negative bouyancy, the displacement of your cabin alone
needs to be less then your weight. You don't count the ballast tanks
since they are completely flooded and do not contribute any bouyancy.
Problem with negative bouyancy is that you will go straight to the bottom.
Now the tricky part. You need a way to adjust that negative bouyancy to
neutral bouyancy. At neutral bouyancy you displace just as much water
as you weigh. The effect is that you sub will float weightless in the
middle of the water column. Some where between surface and on the bottom.
This boyancy adjustment is done with a variable ballast tank (VBT).
With the SportSub type of subs the VBT is the cabin.
The water level in the cabin, and thereby the air volume of the cabin,
is controlled by the Electronic Buoyancy Control (EBC) system. As the
sub goes deeper the EBC lets in more air into the cabin to compesate for
the air that is being compressed in the cabin. As you surface the
EBC lets air out of the cabin to get rid of the excess expanding air.
Your design is going to have a dry cabin and yet an ambient design.
Somehow you are going to have to figure out how to add air to the
cabin as you decend and remove air as you surface to keep the cabin
pressure at ambient.
The VBT will have to be a seperate or at least segragated tank.
that can be adjusted. It can be a hard tank that is adjusted at the
beginning of the dive left alone or it can be a soft tank that
is continually adjusted.
Perhaps you can keep the cabin dry, at ambient pressure and adjust
the VBT all at the same time like this:
Hatch
------------
|---- -----|---------|
| c-- A -- |
| | | | |
| Cabin -|-|- B |
| | | | |
| | | HPA |
Viewport | / | VBT | |
| / | | |
| / |wwwww| |
----------| / | | |--------------------
| | / | | | |
| | ------ | | | |
| BT | Seat | | | BT |
| | | | | |
| | --|-- | |
| |--------------------|----------| |
d
BT = ballast tanks, piping and valves not shown for clarity
VBT = variable ballast tank
HPA = High Pressure Air, second scuba stage
A = valve to bleed air off
B = valve to let air in
c = open pipe reference, see text
d = open pipe refreence, see text
wwww = water level, see text
On the surface the BTs and VBT are empty. Close the hatch.
NOTE: Remember that your sub is an ambient pressure design. There will not be a
pressure diffential, like with a 1atm sub, to force the hatch against any
seals. The hatch mechanism has to be beefy enough to firmly hold the hatch
against its seals or you will flood.
Flood the BTs.
Air pressure in cabin and VBT through pipe "c" is at ambient. Open valve
"A" to let air out and the VBT will flood through pipe "d". With the water
level "wwwww" rising in the VBT.
NOTE: If valve "A" is kept open too long you will flood the cabin through
pipe "c". The VBT must be sized that with both BTs flooded the combined
Cabin and VBT displacement will keep the sub afloat but with VBT also
flooded the sub is negatively bouyant. When the VBT is filled at some
midpoint the sub will be neutrally bouyant.
Once you are neutually bouyant or better yet just slightly positively bouyant
close valve "A" and use your motors to
drive the sub down. As you discend the ambient water pressure will
force the water level "wwwww" to rise and the air in the VBT and Cabin
to compress. To compesate open valve "B" to let more air into the Cabin
and via the pipe "c" to the VBT. This both keeps the air pressure at
ambient and the amount of water displaced constant.
As you surface the ambient water pressure will lessen and the higher air pressure
in the Cabin and VBT will force water out of pipe "d". This will make the sub
more bouyant so valve "A" needs to be opened to let air pressure out of the Cabin and
VBT and maintain the a constant water level "wwwww" in the VBT.
Now this constant ajustment of the air pressure would be a pain. Some kind of
electronic contro, like SportSub's EBC, controlling valves "A" and "B" would be
a good idea. The trick will be adjusting it to keep the water level "wwwww"
at some measurable height. Maybe some kind of float switch.
The opening to pipe "c" would have to be high up in the cabin above the water level
when the sub is surfaced or when you open the hatch water the VBT will fill up and
overflow into the cabin sinking your sub. If the opening is above the water level
then the pipe will only fill up to that point. An optional valve on pipe "c"
can be closed when surfaced just to make sure.
Oh well, so much for a "short" answer.
Regards,
Ray
> From: "Gregory Snyder" <snyde032@umn.edu>
> To: <personal_submersibles@psubs.org>
> Subject: [PSUBS-MAILIST] How heavy can I go?
> Date: Mon, 8 Apr 2002 18:28:45 -0500
> X-Priority: 3 (Normal)
> X-MSMail-Priority: Normal
> Importance: Normal
> X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2600.0000
>
> Dear Shipmates -
> I am rebuilding the modified ambient sport sub (www.ivccorp.com
> <http://www.ivccorp.com/> ) with a top hatch.
> I want to seal off the back swim-in area and make the cabin area dry.
>
> My question is:
> How do I calculate the weight allowable for the sub
> which will allow it to sink when the ballast is blown and more
> importantly,
> Re-surface when the ballast is re-filled?
>
> Thanks -
> Greg