Understanding compressed air.

[safrole]

From surfaced to neutral submerged

From surfaced to neutral submerged trim, the displacement of the entire boat never changes. The overall weight of the boat never changes. The displacement IS the bouyant force and it always equals the entire weight of the boat. The boat, as a dynamic system, will seek to correct an imbalance imposed upon it, such as a ballast tank being filled or emptied.

If the boat is submerged at neutral trim and you blow the tank, the weight of the boat has not changed, you've just added 500cc to the displacement. BUT, displacement is constant, so the boat pops 500cc out above the surface to eliminate the "extra" displacement you just added. Then the boat is once again in equilibrium. But nothing ever really changed, not the weight, and not the displacement.

The mass (or weight) of the section that popped out is immaterial, because that part weighed the same in air or water; it's always been pushing down on the boat with the same force. The boat just needs to equalize the displacement that you upset, but the weight of the boat never changed.

Thinking of the boat as "heavier" after taking on water is really not correct, IMHO. The water (while underwater) weighs zero, so only the reduction in displacement should be considered. The system will equalize based on displacement, which is only a volumetric calculation. Converting to force based on water density is misleading. You can convert the entire boat's new displacement to force, and you'll find it's unchanged and still exactly equals the weight of the boat.

This exercise has really changed my mind on this topic. I started making those graphics to prove my earlier statement was correct, but it surely backfired on me.

[gotland]

Thank You very much for

Thank You very much for this statement!

To say it in a german dialekt]http://www.subcommittee.com/forum/icon_biggrin.gif[/img]

Gotland

[JWLaRue]

>> From surfaced to neutral

>> From surfaced to neutral submerged trim, the displacement of the entire boat never changes.

....so, riddle me this]both[/i] a surfaced and submerged displacment?

>>>The overall weight of the boat never changes.

Depends on whether or not we're including the weight of the water in the ballast tank.

>>>I'm not sure what your last point is, Jeff.

Ahhh...sorry. My point is that in my example, the volume of the object doesn't change, but we've changed the weight.

-Jeff

[mylo]

For the sake of every

[color=#000000]For the sake of every R/C submariner....this issue MUST be resolved and clarified. Maybe we need pictures ?

For starters, is this statement true]

[don prince]

Gentlemen,

It's me again.... Cambridge Ocean

Gentlemen,

It's me again.... Cambridge Ocean Technology Series 2 Concepts in Submarine Design, page 28.

"If we regard the outter surface or envelope of the submarine to be its boundray with the sea, then the volume enclosed in this boundary must be sufficient to enable the submarine when surfaced to float with some of this volume above the surface. To submerge, the submarine has to be able to increase its weight until it equals the bouyancy of the total volume. Since the only readily available source of weight is the surrounding sea, the natural thing to do is to flood part of the space within the outer envelope with sea water; to surface this water is then discharged to sea. This space constitutes volumes of the submarine which are termed the Main Ballast Tank (MBT). Other spaces within the outer envelope may be flooded, either wholly or partially, depending on where they are located relative to the waterline. Such volumes are termed Free Flood (FF) Spaces and do not contribute to the bouyancy of the hull."

It's the weight of the water taken into the ballast tank that needs to approximate neutral bouyancy to dive the sub, the Free Flood spaces don't contribute to boubance; only weight.

The question I had about two OTW Type VIIc U-boats, one with a resin hull, and the other with a brass hull. Which will require the greater sized ballast tank. The answer is quite simple; if the waterline is the same. then the ballast tank water intake is the same (weight).

the reason for this statement is because the foam at the hull waterline and the lead in the keel is used to set the waterline and provides some of the lift to which ever tower is heavier. The Ballast tank only has to counter the lift/bouyancy above the waterline.

To demonstrate this principle. Take a small rubber ball and attach a hook on the bottom and clip on top. Use a small square piece of brass and a small square piece of resin (Same physical size). Place the ball in the water and add weights to the hook until the waterline is established with the resin tower. Then add a weight until the tower top is at the water line. Save the weight that was used to dive the ball. Now, do the same experiment with the brass tower. First you must setup the same waterline on the ball, now add the saved weight,,, And there you have it..

I have to agree with Jeff LaRue, it's bouyance and weight. The thichness of the u-boat tower material does not enter into the equation.

Regards,
Don_

[safrole]

One can argue that it

One can argue that it still counts because it's inside the hull, which is saying the some of the water the boat displaces is being displaced with the water in its ballast tank - not perfectly fair. But I think this would be mostly a matter of convention and not a strict fluid dynamics definition. Plus having the two figures tells you how much reserve bouyancy the boat has at a glance, which is handy.

The boat does not "weigh" any more by taking on water. When you are in the swimming pool and you blow the air out of your lungs, you sink. But don't you technically weigh a little bit LESS? You sure do. But your volume (displacement) has been reduced so you lose that bouyancy and down you go.

The ballast tank does one of two things. It either adds to the displacement or it does nothing at all. It cannot pull the boat down, even though its physical arrangement on board may suggest it. This calls for a graphic.



Scene 1 the ballast tank is adding to the disp of the boat, exerting an upforce equal in grams to it's cc's of water.

Scene 2 it is no longer doing that. The boat does not weigh more; it displaces less. But as the tower goes under, the displacement goes back up to the original amount.

Scene 3 cheap labor has allowed the tank to break loose, illustrating how under water the ballast tank is close to weightless. Even if it's "inside" the WTC, it's the same thing.

This is at the heart of our miscommunication I think. The calculations require ballast to be "lost displacement" and not "added weight".

The fact that weight does not change is illustrated more clearly with the piston tank, since escaping bubbles (which weigh nothing) may cloud the issue with gas ballast.

Here's one of a piston tank, which compresses the air behind it. Again there's no weight change here, even though it appears to "draw water on board". It doesn't matter if the piston face is open to the water or attached to the water by a tube, so I've drawn it both ways.

[mylo]

So there we have it.

So there we have it. A ballast tank is simply a way of adjusting a subs displacement.

Empty tank = more displacement = float.
Full tank = less displacement = neutral.

Gentlemen, do we agree on this principle ?

The diagram depicting the poorly Chineese constructed ballast tank that broke away from the sub shows clearly that the ballast tank, when full, does not add weight to the sub. A full ballast tank is basically water surrounded by a shell. An empty one however, displaces water, thus floats, thus floats when it's attached to a sub, thus wants to push the sub out of the water and it does exactly that, pushing a weight equal to the weight of the water that the ballast tank displaces, above the water line.

...........correct ??

[gotland]

Ok Don,


You describe a very

[color=#000000]Ok Don,


You describe a very good experiment]

[safrole]

Mylo, the upforce is indeed

Mylo, the upforce is indeed equal to the grams of water the tank displaces. But that upforce is not what dictates how much boat comes out of the water.

The tower coming out of the water is not counteracting the upforce of a dry tank. It is restoring balance to the overall displacement of the boat.

Remember little uboat crewmembers carrying lead from the keel up into the conning tower will not make their boat sink any lower, though the tower would indeed be heavier. Building a heavier tower onto your boat means you trim it with less lead in the keel. But only the tower's displacement matters for designing the variable displacement, the ballast tank.

As shown on the earlier graphic with the solid metal tower, a tank can "lift" even more than its own weight of water out of the water, because it's not really lifting, it's adding 500cc of displacement. The boat then pops up 500cc of tower out of the water, even though the tower weighed 800g. If little crew carried some of this metal down into the keel of the boat, it would ride no higher in the water.

[safrole]

http://www.modelbrass.com/images/BallastTankStudy5.jpg

[mylo]

Bing !!! *the light comes

Bing !!! *the light comes on*.

Ahhhhhhhhhhhhhhhhhhhhhh......yes.............of course.

I knew pics would help.

Thanks.

[don prince]

Hi Safrole,

Just as the book

Hi Safrole,

Just as the book said [Cambridge Ocean Technology Series 2 Concepts in Submarine Design] ISBN 0-521-55926-X... It has everything you would ever need to know about submarine structural design. I got it from Amazon.

By the way, your graphics are excellent.

Regards,
Don_

[JWLaRue]

Don,

Many thanks....that certainly helps nail

Don,

Many thanks....that certainly helps nail it down!

-Jeff

[yabbie1]

All quiet on the weight

All quiet on the weight vs volume front.... does this mean agreement has been reached?

If that is the case, I think that the practical consequences should be summarised for any budding builder of model submarines too lacking in time or application to follow all that has been written previously. Dare I do that? Well, here goes]http://www.subcommittee.com/forum/icon_lol.gif[/img]

[safrole]

Yabbie, your point #4 is

Yabbie, your point #4 is an excellent extrapolation, and I agree it's the center of the displacement of the superstructure under which the tank should be centered. The point is mostly academic, but I think you're right.