Understanding compressed air.

[JWLaRue]

Yabbie1,

...well, I'm not in agreement

Yabbie1,

...well, I'm not in agreement on the volume vs. weight question.

I've always used the weight-based calculation and it works very well.

-Jeff

[don prince]

I agree with Jeff (weight)...

In

I agree with Jeff (weight)...

In addition, which would be most obvious...

#5. The center if gravity is below the center of bouyance. The greater the distance between the two, the more stable the sub will be in heavy Seas on the surface (not likely seen in a model). However, a turn at speed there will be less keel angle with the greater distance.

Regards,
Don_

[safrole]

Don I agree with you

Don I agree with you about the relative stability. Also I was brewing up the following graphic before I read your post, so it's in no way directed at you.

What if you tie a big balloon onto the aft of your boat? Should the tank be more toward the back then? You didn't add any WEIGHT, after all. Well, I guarantee if you leave the tank amidships you'll be riding butt-high after you dive. That, or you'd have to trim it so the balloon is already half under, which would be a ridiculous solution. It's volume that matters.

A lot of people have done very well using weight calculations, and maybe it's just plain easier that way. In my everyday life the earth is flat and it works great for me.



If you see any wrong numbers, just say and I can amend the image.

Its seems from the last picture that the tank should be shifted to the right for best trim underwater, which would be directly under the center (average) of displacement (not weight) of the superstructure. The only misalignment of mass and bouyancy vectors will be during the "decks awash" phase, which is usually short-lived and gives the best compromise. Interestingly, this is the time when real fleet-boat type subs were the most unstable as well, but I think it was because their metacentric height became quite small (like what Don mentioned) during this time.

[yabbie1]

Just an observation]http://www.subcommittee.com/forum/icon_question.gif) then calculations

Just an observation]http://www.subcommittee.com/forum/icon_question.gif[/img]) then calculations based on weight will not be too far wrong. What's more, the error will be in your favour, causing you to provide a slightly larger ballast tank than required, which is obviously preferable to the reverse.

This does not make the logic of the method correct. Materials of considerably greater or lesser density - such as the balloon or lead superstructure - would give very wrong results when calculations are done on the basis of their weight. The inflated balloon superstructure, for example, would suggest a ballast tank of only some 3cc is required.

[tmsmalley]

Wish Art Meyer was still

Wish Art Meyer was still around - he could explain this fully - of course none of us could understand it - but I'm sure he'd be right!

[berserk]

...well, I'm not in agreement

...well, I'm not in agreement on the volume vs. weight question.

Hello Jeff!

Like Gotland and Yabbie1 says on Page 3 of this discussion, which I fully agree btw, it's only the volume what counts on the ballast tank.
The weight of the superstructure is only important for the stability of the boat.

The best way is to build a superstructure thin and light. If you can't have both, prefer a thin superstructure.

They was given a lot of examples to test it by yourself. Feel free and do it, you will see how the wind blows. May your calculations give you values that works. But it makes it not correct.

With kinds regards

Berserk

[JWLaRue]

Tim,

I'm with you on that

Tim,

I'm with you on that one!

-Jeff

(Andreas/Berserk - simply restating part of someone else's point of view does not make it any more or less valid)

[berserk]

Hi Jeff.

Unfortunately you ignore the

[color=#000000]Hi Jeff.

Unfortunately you ignore the facts. It's easily to test it. Take a Styrofoam block with a size of 10x10x10cm (1 litre) and drop it into the water. Then put weight on it. I bet it’s nearly 1000 grams. Now drill a hole with a diameter of 6cm in it. Also put weight on it. But now, the foam submerges approximately at a weight of 490 grams.

And this proves the theory of volume. The Styrofoam without hole weigh 15 grams and with hole about 7 grams.

Don’t blame me, it’s just how the world works.

BTW]

[gotland]

Well Jeff,

I don't want to

[color=#000000]Well Jeff,

I don't want to affront anybody. Really not. But I am sure, if Art Meyer would be arround, he would agree with our point of view.

I agree with Yabbie1, that You can be nearly right with Your method of weight, if Your parts above the waterline are not to far away from water in relation of weight/volume. But this depends a little bit on "good luck". Be happy, that this system worked good for You the last Years.

To out myself]

[don prince]

Hi Safrole,

The three dimension axis

Hi Safrole,

The three dimension axis are yaw, pitch, and roll... The keel angle is the roll axis. Your demo has to do with pitch. The book I referenced does cover trim tanks, etc., to deal with pitch as well as everything else.

I suggest the following web site for some informative reading...

http://www.fleetsubmarine.com/diving.html

Regards,
Don_



Edited By Don Prince on 1128128142

[safrole]

Don, thanks for the link.

Don, thanks for the link. It's very informative.

The author may know a lot about the operation of a fleet boat submarine, but his theory is incorrect.

To submerge, the crew opens vents at the top of the ballast tanks. This lets the air out. The bottom of the ballast tank is open to the sea, so as soon as the air is released water comes in to take its place, the displacement increases, and the boat slips below the surface.

Venting the tanks does not increase displacement, but rather decreases it, which is the real reason the boat dives. He's probably got it backwards because of the point Jeff raised earlier, the larger figure for submerged displacement listed in the stats of all submarines. I sent him an email about the error.

His graphics sure do put mine to shame!

[eloka]

Dear all,

its very easy to

Dear all,

its very easy to understand the Sub-techiques.

Here is a very easy Formel to count

Vu = pressure resistant displacement under the CWL
Vo = pressure reresistant displacement above the CWL
Tu = Ballasttank volume in [m³] under the CWL
To = Balasttank volume in [m³] above the CWL
p = shipweight in [Mp]
y = water desity [MP/m³]

it applies the relationship]http://www.s54-elster.de/smilies/74.gif[/img]

You see, Live can be so easy.
kind regards
Dirk

[berserk]

@ Myles

Consider to use a

@ Myles

Consider to use a trim system without compressed air. Piston tanks, for example, are very reliable. It makes no fun trying to trim the boat with compressed air or rather is impossible.

So you can flood the main ballast tank completely and then trim the boat until it hangs on the periscope or the upper edge of the sail. Now it's possible to emerge/submerge the boat only with the main ballast tank (completely full/completely empty).

[mylo]

Yes,

I am evaluating the

Yes,

I am evaluating the compressed air / piston tank options.

Initially, I wanted the model to function as close to the real thing as possible, which included, pressurized air for ballast tank control.

.....however, I have reconsidered this in exchange for more reliable operation, which, is ultimately what is important. The piston tank design looks very good to me.

A prototypical R/C sub isn't any fun if it doesn't work.

Mylo

[gotland]

Dear Mylo,

as Bersek writes, the

Dear Mylo,

as Bersek writes, the combination of both system is the most comfortable way to drive Your sub. Especially when it's a big one!

There is one big different between the compressed air system in our modells and the system of a real type VII boat.

Both have big diving tanks which are not perfectly closed or "physical open". But the real boat also so called "Regelzellen", which are really closed on the upside and at the bottom. They are working with pumps. So there is no way to compres a left air bubble by diving-pressure.
This makes the boat stabil nearly hoovering.

So if You want to work with compressed air (I love it) make a main central ballast tank of about 4 liters (center of gravity closely to the forward end of the conning-tower) and two small proportional piston tanks (about 1/2 liter each) at both ends of the pressure hull.

have a look at]http://www.schwedenuboot.de.vu/[/url]

at all of these boats the main ballast tank is BETWEEN the box for the periscopes and the pipe/hull.

Regards

Gotland