500 ml or 750 ml piston tanks for a XXI? - Need advice from experienced people

[jsl]

Hello people,

Our university midterm project is now underway. Our project goal is to build a microprocessor controlled model submarine.

The backbone of this model will be Engel piston tanks, but we are not sure which piston tank size to use.

At a later point, our test model (essentially a WTC with a small prop) would be build into a Robbe XXI. Our problem is if we should try to install two 750 ml tanks, or if we could get by with the smaller 500 ml tanks. The finished model does weigh about 12 kg and our concern is, that the model may be too sluggish and have too little surplus boyancy if we use the smaller tanks. However, small tanks would be preferred due to their smaller size.

This is where you come in. Any comment from people with experiences in piston tank controlled VIIC's and XXI's are most welcome.

Thanks in advance,
Jacob Skov Larsen
Technical University of Denmark

[Guest]

This sort of question has

This sort of question has been asked many times, and you will find answers if you search for 'ballast tank' sizing.

Personally I would choose a different boat to experiment with.

Andy

[wingtip]

not enough surplus bouyancy?
remember,

not enough surplus bouyancy?
remember, that when fully flooded the boat is neutral bouyant...so any water expelled will make it rise... unless your planning on retrieving something from the depths of the ocean (pool) floor thus making it heavier lol..........

[jsl]

unless your planning on retrieving

unless your planning on retrieving something from the depths of the ocean (pool) floor thus making it heavier lol..........

Do not laugh to heartly, my friend.

This is a university project and we have decided to make the design as versatile as possible. Retrieving things from the bottom..? Why not.

After a few hours searching this forum back years, I found only very few references to which size piston tanks people build into their Robbe VIIC.

We have decided to have neutral bouancy when both tanks are 2/3 waterfilled. This way the microcontroller can adjust the depth and perform any vertical movement desired.

Still... I have not heard any people arguing wheather we should try 750 ml tanks, or if we could get by with two 500 ml tanks. We would like the finished XXI uboat to rise reasonably out of the water when both tanks are empty.

[wingtip]

well if the 750's would

well if the 750's would physically fit i'd get those... you could use your programming to only take in 500 for testing purposes then you could use the full 750 if you needed more.... just a personal opinion...
sounds like a interesting project though, hope you post pics and info here......

[Antoine]

hello

Nice project you have !

hello

Nice project you have ! I wish I could have that kind of projects during my university...

The problem is simpler than it may sound. Choosing the size of your ballast tanks will be directly linked to the water volume displaced by the WTC. If it is a true cylinder the volume will easily be calculated (theorically).
Knowing this volume, you will have to know the dry weight of your ready to run WTC (2 weights will be calculated, one with 2 500ml pistons and one with 2 750ml pistons). Then you will be able to choose between the two options to see what is the best regarding the reserve buoyancy needed.

I do not recommend the use of a XXI hull, (I concure with Andy). Its sailing capabilities are not the best even if it is a cool looking submarine. Try the simplest design first....everything depends of your project goals....

Antoine.

[JWLaRue]

I'm not sure that anyone

I'm not sure that anyone has had an opportunity to try modifying a stock robbe Type XXI kit to provide it an active ballast system.

However, since this new kit is similar in size to the robbe Type VII.........

Folks who have added an Engel piston to this kit have found that even with a (single) 750ml piston, you cannot get the sub from a fully surfaced state to a fully submerged state. It's *possible* that a single 825ml piston would work.

Wingtip said]can[/i] be used to achieve something very, very close to neutral buoyancy by carefully adjusting the position of the piston.

-Jeff

[Antoine]

few XXIs in Germany were

few XXIs in Germany were modified with Engel tanks (2 500ml) successfully. There is room on the tray.