WANTED : 2 1/4" x 15" Piston Tank

[mylo]

Anybody know where I can get my hands on a 2 1/4 " diameter piston tank ? The length has to be at least 15".

Thanks,

Mylo

[Guest]

You want a 1 litre

You want a 1 litre piston tank?

Any special reason for it being 2 1/4" diameter- seems an odd size, and a 15" long tank will take a fair while to flood. Probably better to split it into two tanks.

Engel will make a special tank upto 1 litre in size, but the piston size is 75mm (about 3").

There are a couple of folks in Germany who make custom piston tanks. Not sure if they will be exactly 2 1/4", but they can probably get close.

Andy

[mylo]

Andy,

There is a reason

Andy,

There is a reason why I need it 2 1/4 " in diameter as well as close to 1L volume. I'm going to have two piston tanks in my sub (in addition to the main ballast tank). The piston tanks are going to be mounted fore and aft for fine trim control. I have completed the design of my torpedo tube mounts. The fore piston tank goes on top of the torpedo tube mounts with it's primary function to be to offset the bouyancy change after a torpedo launch. In any event, I need the piston tank to be 2 1/4" so that it will physically fit in the hull (I'm quickly running out of room once my torpedo tubes are installed), with the 2 1/4" allowing me to mount it low to the torpedo tubes, putting the piston tank at waterline. The 4 torpedo tubes displace about 500ml in total, so I would like the piston tank to be able to accomodate that volume in order to maintain trim in the event the tubes are emptied (and possibly have air trapped inside them from the launch). Since I would like the piston tanks to be very close to 1/2 full during "normal" trim (to allow for control of both negative and positive bouyancy), the fore piston tank needs to be able to displace the 500mls ( 1/2 L) of the torpedo tubes. The aft piston tank will be mounted the same distance relative to centre of the sub, as well as the same height at water line so that the effect of trimming the two tanks is very similar. The whole installation of the tanks has been designed down to the mm.

...... short answer, ...... my design calls for 2 1/4" diameter piston tanks. Like everything else on this sub, I'll likely resort to building my own.

Mylo

[skip asay]

Mylo -

I think your estimated

Mylo -

I think your estimated requirement for size is way off. First, the tubes themselves will be free flooding, right? If so, that reduces your requirement. Second, the torpedoes themselves will only be slightly buoyant which then reduces your requirement quite a bit more. And as for the air trapped in the tubes after firing, just drill a hole in the aft most upper corner of the tube and then angle the tubes down a slight bit. Any air trapped will vent through the hole. I would guess that you would accomplish your goal with a tank that’s maybe only about 3 - 4 inches long, if that.

Skip Asay

[mylo]

Skip,

No disagreement here. I

Skip,

No disagreement here. I spent some considerable time doing some recalculating, and I would have to agree with what you are saying in terms of the bouyancy changes needed from a piston tank to compensate for torpedo operation.

I don't want to put any vent holes at the back of the tube because I am counting on the expanding air to propel the torp out of the tube, giving it a little boost and get spinning a little on a straight trajectory before it takes off on it's own. I suspect that trapped air inside the tubes will be minimal, and if it is significant enough to actually change the trim of the boat, the air will bubble out the ends of the tubes when trim has a slightly bow up attitude.....if it should get that far.

In realizing that the torps aren't going to have as big of an effect on trim as I initially thought, my use / implementation of the bow/stern piston tanks is more for fine trim control, both surfaced and submerged. I was simply thinking that if I had room, to stuff as big a piston tank fore and aft as I could.....the theory being more volume = more bouyancy to play with. I have redesigned my bulk heads / control rods to allow for the space to put a 750 ml. Engel 3" piston tank, both fore and aft. I wanted to mount the tanks as low as I could, but could only get it so that they will be sitting with 1" of the tank below waterline, the other 2" above.

Thoughts, suggestions, .... comments ? She's all new to me.

Mylo

[skip asay]

Mylo -

The amount of volume

Mylo -

The amount of volume you will need is MUCH less than a 3” Engel tank will supply, especially since it will be at the extreme ends of the boat. I’d guess that something on the order of 1 1/2” diameter with a 2” stroke would suffice. This will give you just about 2 ozs. of weight change which, as I pointed out, is at the extreme ends. At the SubComEast get togethers, I used to take an awful lot of razzing when I would reach into my pocket for change and then drop a dime, nickel, or quarter into my boat for fine trimming. “Hey Skip, you have to pay to run that thing?”

Another thing to think about is that the smaller tank should be able to fit below the surfaced water line. With 2” of the Engel tank being above the water line (X2), your main ballast tank size would be enormous.

Skip Asay

[mike dory]

Torpedoes and trimming

I'm assuming you are going to use gas powered torpedoes correct? if that is the case, I've found, in my years of firing gas powered torpedoes, that in reality no trim tanks are needed for the little extra weight. If, like in my Type XX1 you load all six tubes, I just click up one click on the rear diving plane from center, till they are fired. It does look neat when the torpedoes race away from the boat with a trail of air coming out of them. A real crowd pleaser. My torpedoes go between 40 and 55 ft. depending on out side air tempature. We all look forward to seeing your boat in action. All our best wishes Mike Doryl

[mylo]

Another question;

Could I effectively

Another question;

Could I effectively be able to trim the sub with only one trim tank mounted aft ? With the absence of any torpedo tubes (not putting a functional aft tube in), I am able to get a 3" piston tank completely below water line.

If a 3" piston tank was exactly at waterline with 1/2 the tank below, and 1/2 the tank above, does that basically make it neutrally bouyant ? In other words, would it it demand main ballast tank bouyancy to get it to sit there, or would the piston tanks own inherent bouyancy provide enough ? To put it yet another way, if you just set a an empty 3" piston tank in a tub of water, how far does it sink down ?

Skip, am I wrong in thinking that, if I have the room, to put as big of a piston / trim tank in as I can ? I'm thinking more along the lines of, instead of putting lead weight here and there, or foam here and there, in order to get the sub trimmed to waterline just right, why not just have a big trim tank that has much more adjustablility ? It would serve this function, instead of just being responsible for minute adjustments in trim from a torpedo launch.

Mike, Yes, I like the gas powered torps for the bubble wake. Have you been able to run your torps at 'close' to scale speed ? I don't want some kind of water misile going Mach 10. I was thinking that simply having a small exhaust hole could make this happen. (This topic is a whole other thread).

Questions, questions..... I've entered into a whole other phase of the project. Learning curves, .....do they ever go DOWN hill ?

Mylo

[hakkikt]

Mylo,

think of the main ballast

Mylo,

think of the main ballast tank as an empty tube closed at both ends. If you put that on the water, it will not sink much, maybe a few millimeters, depending on the weight of the material (plastic? alu? brass?).

Now imagine you can hang small weights on that tube. These weights represent your boat... hull, servos, motor, battery, you name it. With every weight, the tube will sink a little more, until it is almost entirely below the water, but still on the surface. The weight that you have now hanging on the tube is the weight that your ballast tank can move up to the surface - if you push the tube below the water, it will come up.
Obviously, it will take more weight to make the tube almost disappear below the water than to make it submerge only halfway.

That additional weight is what you lose in the equipment of your boat. So if you use a main ballast tank that will be half out of the water anyway, you either have much less weight available for your boat, or you are wasting a lot of space, because to carry only so much weight, a smaller tube would be perfectly ok.

-Harald

[JWLaRue]

Are folks saying that the

Are folks saying that the volume of ballast tank that is *above* the water contributes to buoyancy? That does not seem correct.....

-Jeff

[mylo]

In order to avoid murking

In order to avoid murking the waters on this topic, I'll try and clarify what I'm up to.

The main ballast tank is going to be the center section of my WTC. The WTC is 6" in diameter by 36" long with most of this volume 'available' to me in the form of a main ballast tank (since the electronics and motors really aren't going to take up much space). The WTC is mounted right in close to the bottom of the hull so that this 'tube' can provide me with sufficient bouyancy to get this beast to water line. The keel will have weight to stabilize the boat. The main ballast tank is not where I have the questions.

The question is concerning my trim tank, which is going to be a piston tank. I want this tank for remote control fine trim of the sub while operating. The diameter/volume has yet to be determined, but I have room for a 3" tank mounted both fore and aft (if I want). The fore tank sits on top of the torpedo tubes, there simply is no other place to put it with all the tubes and mounts in the way below water line. Sitting on top of the tubes, 1" is below water line, 2" is above. The question here is, will this demand a lot of ballast tank bouyancy to get it there while the sub is surfaced, ...or, ....will the bouyancy of the piston tank itself basically get it to that level ? (It really depends on how bouyant the tank itself is, which is what I would like to know). I can mount an aft 3" piston tank either exactly at waterline with it being quite far astern, giving it more leverage, ....or..... totally below, but closer to the sub's center, giving it less leverage. The question with this is, would a single aft mounted piston tank be sufficient for fine trim control of the entire sub, the theory being.... lower the stern, bow comes up. Raise the stern, bow goes down, in any event, the entire trim is effected by raising/lowering the stern alone......in theory. If it is, I'll only install one piston tank aft. I talk of a 3" piston tank because that is the largest diameter I have room for. Obviously, a smaller diameter one will fit as well.

I really appreciate the feedback. I'm at kind of an important engineering step right now in the design of these bulk heads......would hate to **** it up.

Mylo

[JWLaRue]

...trim tank...ballast tank.....the question remains

...trim tank...ballast tank.....the question remains the same.

I don't see how the portion of the ballast/trim tank that is above the waterline will not provide any buoyancy. The air in the tank is the same density as the surrounding air....net effect is zero.

Now, when submerged, that would be a different story.

-Jeff

[mylo]

Jeff,

...... and herein lies the

Jeff,

...... and herein lies the engineering challenge, what with how the dynamics of bouyancy change from surfaced to submerged operation. (why do I feel this topic has been beat to death in the past ?).

I guess I'll sum it up nicely ;

.... where's the best place to put a trim tank ?


Mylo

[JWLaRue]

That's the perennial question when

That's the perennial question when building r/c subs......

I can verify Skip's comments about his use of small change/coins to effect trim changes. Some of SubComEasters now do the same thing! The effect of a very small amount of weight change at the extreme ends of the sub can be remarkable.

In your case, with your sub being as long as it is I'm thinking that very, very little weight change will actually be needed to affect the pitch. The numbers that Skip provided ( "...something on the order of 1 1/2” diameter with a 2” stroke would suffice. This will give you just about 2 ozs. of weight change...") is just what you are looking for. Just place the piston so that the top of it as no higher than the waterline. Lower is okay.

-Jeff

[skip asay]

Mylo -

Many years ago, I

Mylo -

Many years ago, I did the internals for Bud Lederer’s 32P Type VII which is only slightly smaller than yours. In those days, the accepted practice was to build 2 watertight compartments - 1 forward and 1 aft of the centerline. If I remember correctly, there was a separation of about 12” - 14” between them which was used for the ballast tank. I then built 2 small (3” X 2” - again, if I remember correctly) tubes with end caps located fore and aft within the separation between the watertight compartments. These were used very successfully as fore and aft trim tanks using an airplane fuel pump to pump water between the 2 tanks. Believe me when I tell you that it really didn’t take too much water movement to drop or raise the bow! Now taking into account that the sparation between these 2 tanks was a foot, at most, you’ll see that mounting any size tank at the extreme ends of the hull will certainly have an extreme effect on the trim. That’s why I DON’T recommend using the Engel tanks. Add to that the fact that having better than 75% of each tank above the waterline will only increase the required size for your main ballast tank....substantially.

Skip Asay