buoyency help

[oracle_9]

Hi,

I will be scratch building a sub, which is not yet determined. I am still continuing my research before I get me feet wet...erm, so to speak.
I understand in order for a sub to float the weight of the sub must be less than to the weight of water for a volume it displaces.

1) But if it the sub is equal weight to the water does it simply float just below the surface of the water and if you have forward motion then you can control its axis of motion??

If the volume of the sub is heavier than water then it will of course sink and depending on the weight it will go to a specific depth (?)

I hope I got this right.

2) How can I figure how much weight is require to sink the sub to a specific depth while stationary?, and not go lower unless you steer it down while in motion

In my priminary plan so far is to have the sub go down to a specific depth of 1 feet with full tank being filled. The periscope will remain exposed over the surface of the water for air to excape and enter for sinking and rising.

3) Lets say we have a sub which has a 1 liter volume. The wieght less than 1kg of weight, the sub naturally floats, but how much for it to sink to "about" 1 feet? (hmm, I guess altiude is a factor here, so lets say sea lever for arguments sake) Is there a way to calculate this or is this very difficult?

[raalst]

let's see.

1) if the

let's see.

1) if the weight equals displaced volume, then the sub will move
anywhere it likes. it can be deep or shallow. sort of like being
weightless. if you are heavier than 0 grammes you stay put on earth,
exactly 0 grammes will enable the wind to blow you anywhere, and
lighter means you're off.

2) as you can guess from above answer, you either float or lie on the
bottom. in practice it is impossible to "hover"just by controlling
your buoyancy. things like currents, changes in water density
(caused by it's composition and temperature) will have effect
on the boat and it will drift away. in general, the lower you get
the harder you sink, due to compressing of the air bubbles that
are trapped under your hull, and maybe compression of your
floating parts (often foam). and the higher you get the faster
you rise.

in practice we try to get as close as we can to that 0-point, but we
accept that fine control is done with the forward motion and dive
planes.

this remaining on a specific depth is called an unstable equilibrium,
and therefore the dive planes are controlled by some fancy
feedback electronics to compensate the drifting away.

there is no such thing as "1 ounce extra per meter depth"

3) see above

no worries, it took me a while to grasp it too, and I sure hope I wrote this down without too much errors....

[oracle_9]

Thank you, it cleared up

Thank you, it cleared up some things.
So I should get as close as to 0 as possible.

I misunderstood the static diving. People say they can sink without moving forward, so I assumed you can control the depth as well withouth horizontally moving. But from what I understand now, in static you basically fill up the tanks to reach as close as that zero mark, which would get water over the decks a bit, and then when you want to sink further you apply down dive plane and forward motion but much less speed then dynamic requires.
Ok, I am getting close?

[raalst]

that's what I do.
ideally

that's what I do.
ideally (for me) the zero mark is equivalent with just a bit of periscope sticking out of the water. at that point the buoyancy is
positive and equal to the weight of a centimeter of plastic periscope.
that is not much at all... one drop of water leaking and buoyancy gets
negative. But my sub is fairly small (trumpeter kilo 1/144 scale).

It is of course possible to flood the tanks of the sub just a bit more so
you sink (because you get below the 0 mark), then the dive planes
have to act against that to stay at the desired depth.

alternatively you *can* sort of hover while not moving forward,
but that means you
1) measure the actual depth,
2) and if you are above you flood the tanks a bit more, sinking
to (more likely past) the desired depth.
3) then when you measure your depth is below the desired depth,
you empty the tanks a bit so you rise to (past) the desired depth.
4) goto 1)

so, while you cannot make sure that you remain on a certain depth
without effort, you can ping-pong around where you want to be with
some electronics and finely controllable (trim) tanks.
these Piston tanks that are mentioned often are able to do this fine control; pressured gas systems are by nature (their gas bubble volume changes with depth) not suitable for fine control.

Real subs have trim tanks and ballast tanks.
The ballast tanks can be full or empty, for roughly the 0-mark or
being on the surface. always works with pressured air.
they use their small trim tanks to finely tune their buoyancy.
but only while not moving because the effects of the dive planes
quickly grow with forward speed, rendering the trim control unable to
compensate for the effect of the diveplanes at greater speeds.
same goes for model subs.

so with increasing complexity these systems can be found in model subs
1) have the dive planes push you under at large speeds (dynamic diving)
2) flood the tanks to get close to the 0-mark and use dive planes at
normal speeds
3) flood ballast tanks, use dive planes at speed and have trim tank for
no-speed control

Phew, hope that made sense...

[oracle_9]

Wow, you spent a lot

Wow, you spent a lot of time to write these up for me, I must say thank you you very much for the help!!!! I learnt a lot.

[aeroengineer1]

something that might help to

something that might help to understand as well is to consider you sub and a just some arbitrary object. When you fill the ballast tank and and the boat submerges, there are two possible conditions that you have achieved. The first it that your mass is that of the mass of water displaced, in which you are said to be neutrally buoyant. The second case is if you fill a little too much and the mass of the sub is greater than that of the water displaced.

Looking at the static forces for each case and you will see that for the first case there is the force of gravity and a buoyant force equal to that force of gravity. From newton's laws we know that the net force acting on the sub is mass times acceleration and if the velocity (speed) remains at a constant value, then acceleration will equal zero. This means that with the forces being equal, if you give a small push to your boat downward (and the forces are equal) it will keep going down till it hits the bottom or some other force acts on it.

In the second case with the mass of the boat greater than the water displaced, then the force of gravity will be greater than the force of buoyancy. (Net Force = mass x acceleration = force of buoyancy - force of gravity) the only way to have a solution to this is if there is an acceleration in the downward direction) and hence your boat will continue its descent and speed up while doing it.

The only way to make your boat hover is to constantly be making small changes in your ballast.

I am sorry if this got too engineering for you, but I do know that there are some that like to have a lot of detail.

Adam

[oracle_9]

Dont worry about it sounding

Dont worry about it sounding too engineering, what you described is understandable, I took some physics in highschool. So all is good.

Also well written aero!

In the second case, I guess thats where the trim ballast raalst described comes into play.

[aeroengineer1]

Yes and no. It depends

Yes and no. It depends on your desire of complexity and system design. For trim tanks to work properly to hover you would need to have a electronic controller to monitor movement and provide damping to the system.

Adam

[Wheelerdealer]

The mass of the sub

The mass of the sub never changes as you dive. The mass of the sub on the surface is the same as the mass of the sub when in is at neutral buoyancy, which is the same as when the sub is sitting on the bottom. The only thing that changes as a sub dives is its displacement or volume. See my rather poor drawings below.


You should be able to work out that the mass of the sub is the same when the ballast tank is empty and its when its full of water. But when the ballast tank is full of water the volume of the physical sub has been reduce as the piston has drawn in. The water in the ballast tank is part of the water in the lake and not part of the sub and is not weighing the sub down. So basically to dive, you need to reduce the buoyancy of the sub (reduce its volume by filling the ballast tank) by the same amount of positive buoyancy exerted by the part of the submarine that is above the water line as it submerges below the surface. If you reduce the buoyancy of the sub by more than this, you plumet to the bottom as the downward force is greater than the upward force.

[aeroengineer1]

The only thing that differs

The only thing that differs from the explainations that have been put forth is where you define you control volume. I define my control volume as the whole exterior of the watertight compartment, and that this control volume has a water intake/exhaust.

The latter defines the control volume as any area which the water touches.

The only difference is that there are calculation simplifications that can be made with the first. With the first you are dealing with a Constant Volume situation that has a nonhomogeneous mixture of air and water and electronics. The advantage to this approach is that you are dealing with density changes.

The other approach necessitates the knowledge of certain volumes which are rather hard to calculate at times, and the calculation of the overall effect of the density is difficult to take into account.

Adam

[Wheelerdealer]

Yes I was just simplfing

Yes I was just simplfing it with a diagram, so its easier to get you head around without working out the maths. Also its easier to understand that the displacement of your sub is more important than mass when it comes to designing ballast tanks and such.
Oh and BTW, Congratulations!

Ramesh

[Guest]

Rolf Harris eat your heart

Rolf Harris eat your heart out!

[mylo]

......let's not forget the "wetsub

......let's not forget the "wetsub / drysub" situations. Didn't mean to make things more complicated.

Wheelerdealer......FANTASTIC graphic. What CAD program is that ? ...is it Freeware ?

Mylo

[Guest]

Wheelerdealer......FANTASTIC graphic. What CAD program

Wheelerdealer......FANTASTIC graphic. What CAD program is that ? ...is it Freeware ?

Mylo

did you notice he left the prop out of shot, cos it s top secret new design

[aeroengineer1]

The best information out there

The best information out there that really has a definitive answer to all this is a book called Concepts in Submarine Design. It will answer all the questions on buoyancy and that overall displacement and weight are a big part of the equation. They also use the control volume of the whole submarine. It is rather good reading on the topic.

Adam