RCABS, bladders and buoyancy - analogy; balloon sitting on top of water

[Guest]

Gents,
Prior post from JWLaRue

"Okay...now I'm a bit confused.

It's not clear to me how a ballast tank (RCABS or otherwise) is able to provide lift for a hull to get to the waterline when the ballast tank is at and above the waterline....at that point the portion above the waterline is just parasitic weight and not contributing to lifting the hull.

The analogy of a ballon sitting on top of the water is not a good one. The balloon sits on the "top" of the water because it takes very, very little of the internal (air) volume of the balloon to lift the weight of the balloon out of the water. In other words, I think it's a misleading analogy."-Jeff

Art here. The confusion here may be that the top of the WTC needs to be just below the waterline for the bladder on top of the WTC to provide lift to the correct waterline. Sorry it took so long for my response, but it took a while for me to get a picture to illustrate my correct balloon analogy. I hope you can see the bladder 2/3 to 3/4 of the way above the surface of the water, and still lifting the top of the weighted WTC almost up to the surface. The second picture is to show the deflated bladder and weighted WTC at the bottom of the pool, prior to inflation and surfacing.






Edited By Art Broder on 1118011215

[anonymous]

Works for me.

Steve

Works for me.

Steve

[JWLaRue]

Okay...but let's not confuse the

Okay...but let's not confuse the issue by claiming that the air in the bladder above the waterline is providing any form of lift.

Whatever lift the bladder is providing is only that portion at or below the waterline. If there is bladder up out of the water it's because the lift generated (by the part below the waterline) is large enough to lift all that weight completely out of the water.

-Jeff

[tabledancer]

Wow,now I am really confused,

Wow,now I am really confused, I thought that I had it all figured out.Could someone please tell me exacty how this system works.It seems to me that you are just moving air from one storage area to another,and somehow this causes the boat to surface.
Show me the lite,
T.D.

[anonymous]

TD,

All it does is exactly

TD,

All it does is exactly that. I have 3 boats now that use Art's system. The air is moved from the WTC using a pump that inflates the bladder and up you go.

There's a servo on a switch that turns the pump on. Swing the servo the opposite direction and it opens a valve and the vacuum inside the WTC sucks the air back to the WTC removing it from the bladder. Back down you go.

I like the system very much. Always works for me.

Here's two videos that show the RCABS system in action.

http://sneill.com/uboat/type7_2.wmv

http://sneill.com/seawolf/seawolf_2.wmv

Steve

[bigdave]

Hi TD,
Jeff and Art

Hi TD,
Jeff and Art are not discussing IF or HOW the system works. They are discussing WHY the system works. Steve explained the basic operiation of the RCABS which is correct. They just have different opinions of why the bladder does it's job. I feel they are both right. If you look at the bladder it looks like it is floating on the water, and it is. But only to the level were it displaces enough water to compensate for the boyancy needed to faise the WTC. I hope you get my drift because I am confusing myself Dave.

[JWLaRue]

...what Dave said!

RCABS works great.

We're

...what Dave said!

RCABS works great.

We're essentially having a physics discussion.

-Jeff

[anonymous]

Reminds me of Crimson Tide.

Reminds me of Crimson Tide. They were both right too! LOL!!

Steve

[Guest]

I agree with Jeff and

I agree with Jeff and Dave. Now we're getting down to semantics, in what has been a gentlemanly discussion. When Jeff refers to the portion of the bladder above the surface of the water as "parasitic weight", and, even though I agree that portion of the bladder provides no lift, I would still like to refer to it in a more positive way, and call it "reserve buoyancy". There may be a time when a few ounces of water collect inside the bottom of the WTC, or a few ounces of weeds or fishing line with weights wrap around the props, and the model needs to be brought up from the depths. Whatever works, works.

[bigdave]

I think we all agree

I think we all agree a little reserve buoyancy is a GOOD THING! The Titanic could have used some BD.

[tabledancer]

ALLRITE GUYS,
T hankyou,Art,Jeff,Steve,and Dave now

ALLRITE GUYS,
T hankyou,Art,Jeff,Steve,and Dave now I know the hows ,whys and ifs of the system .You guys have all been a giant help for me.I just like to ask a lot of questions.And Dave the motors and servos that you suggested arrived the other day also haven`t decided which receiver to get, yet Still working on the hull.
Take care all,
TD

[tabledancer]

One more question, http://www.subcommittee.com/forum/icon_question.gif ,how

One more question, ,how do I trim the boat when it`s time.On the surface with the bladder full or semi fulll Should I set the waterline with a full bladder then deflate it slightly for running deck awash.Will there be this much control over the dive or will I have to run in a slightly down plane mode.
TD

[bigdave]

Hi TD,
I like to

Hi TD,
I like to trim my sub so when the bladder is empty it is just negatively ballasted. Dave.