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

[Wheelerdealer]

Hi Mylo, if you want

Hi Mylo, if you want two piston tanks to act as fore-aft trim then you will need 2 piston tanks. You will need a suitable controller like the Engel Tmax controller that will effectively shift water ballast between the two tanks whilst keeping the total combined volume in the two tanks the same. The futher the separation between the tanks, the greater the effect. For fine trim to maintain neutral buyancy you either need 2 piston tanks as above or one centrally mounted piston tank. If you only have 1 piston tank for fine neutral buoyancy trim then you'll need another method ataining fore-aft trim such as a shifting weight (eg servo used to move a battery or some lead).
If any part of your piston tank is above the waterline when surfaced, your main ballast tank needs to be increased by the same volume that is above the waterline to compensate. If you are going down the two piston tank route, I would move them closer to to the centre of the boat (reduce separation between the tanks) to give you finer control (greater resolution - bigger piston movement required for the same effect) and also to allow you to move them below the waterline by clearing the top of your tubes/torpedoes. As has already mentioned , fore-aft trim is effected by relativley small moment forces.

[Wheelerdealer]

ignore

ignore

[mylo]

Skip, Wheeler,

Thank you for

Skip, Wheeler,

Thank you for that. A light has suddenly come on, bigger does NOT equal better because of such small, precise changes that are being dealt with. As well, mounting them at the extreme ends does NOT equal better, but in fact is worse for the reason you point out Wheeler. Instead, a piston tank much smaller mounted closer to center, to deal with these small volumes will allow for more precise control, which is FAR more important than having 'so/so' control over a large volume mounted at the far ends. This has an added bonus in that I have room both fore and aft to mount these smaller trim tanks completely below waterline. Got it. ......back to the design table. This is the 3rd complete set of bulk heads that I'll be throwing in the garbage. Better to turf them at this stage......I guess.

Thank you all,

Mylo

[skip asay]

Mylo -

I don’t know what

Mylo -

I don’t know what you’re planning on using for main ballast (bladder, Propel, compressed air, pump, etc.) but if you’re planning on using either Propel or compressed air why not try this.
Main ballast tank with a small piston type tank INSIDE to allow you to ‘find’ neutral buoyancy. One small tank somewhere ahead of center with another somewhere behind center. Now you can either use an airplane fuel pump to move the water back and forth or you can have 1 of these a small piston tank. The key is that they’re connected so that the overall weight stays the same, it just gets shifted.

Stay tuned. I’m taking pictures and writing up my version of a “poor man’s’ proportional control for small piston tanks.

Skip Asay

[mylo]

Skip,

I plan on having

Skip,

I plan on having a low pressure blower (ask David Meriman about it, it's a design he's working on....but it sounds like an air pump idea to me) that will purge the main ballast tank by basically pumping air into the tank from a vent hose located high in the tower, with the water of course being purged out of holes in the bottom of the ballast tank leading into the free flood area. Once the tower breaks the surface, the blower can start pumping air in causing the sub to gain bouyancy until it reaches the proper waterline, which will be an empty main ballast tank. In addition to this, there will be a compressed air resevoir for emergency blow and purge control of the main ballast tank while the sub is totally submerged. There should be very little need to use the compressed air as the dive planes will control submerged depth since the sub should be as close as possible to being neutrally bouyant with a full main ballast tank. Essentially, it will function like the real boat does. .....which brings me to why I REALLY like your pumping water to/from fore/aft trim tanks method, as this too is how the real boat functions to control trim.

If I understand what you are saying, I could have the actual pump mounted inside the WTC with the water lines from the fore/aft trim tanks running to it, which would be much simpler (which means less breakdown and maintenance) and cheaper than having a somewhat complex piston tank setup outside the WTC to run the trim tanks. Yes.....the pumping water thing is a good idea, I like it.

Looking forward to your article.

Mylo

[skip asay]

"If I understand what you

"If I understand what you are saying, I could have the actual pump mounted inside the WTC with the water lines from the fore/aft trim tanks running to it"

The only thing is, for safety's sake, I would mount the pump OUTSIDE the WTC. And, depending on which pump you use, put a restrictor in the (water) line to slow the pumping action down. Don't forget you'll have to run a hose between the tops of the 2 tanks to balance the air pressure.

I would also think seriously about using a small piston tank inside the main ballast tank. This will prevent your buddies from asking "hey, you have to pay to run that thing?". I would guesstimate around a 1 1/2" dia. X 2" -3" length.

Skip Asay

[mylo]

Skip,

I sincerely thank you

Skip,

I sincerely thank you for your feedback on this topic. You are one to know.

I understand the need to have lines running from the tops of the trim tanks to equalize pressure...... I've been staring at my bulk heads looking for a place to run them.

For 'safety sake' I take it you mean if a line bursts/comes off the pump, you don't flood your WTC with the water in the trim tank system ? Good idea.

I do like the idea of the piston tank inside the main ballast for fine bouyancy adjustment........ it would save me from packing around change too. Can a guy buy these small piston tanks somewhere ?

Mylo

[Wheelerdealer]

Main ballast tank with a

Main ballast tank with a small piston type tank INSIDE to allow you to ‘find’ neutral buoyancy.

No fair, I thought I was the first to think about this. Watch out for an Alfa with this very system

[mylo]

.......... I don't recall anyone

.......... I don't recall anyone saying anything about fairness. Bring your best boat to war, .....winner take all. (wait a minute, I'm putting a type VIIc/41 up against an Alfa ........ not smart).

Mylo

[cstranc]

That ballast method is very

That ballast method is very cool. What a great idea.

If it is sucking surface air and pumping it into the ballast tank would it matter if you pumped while submerged? It would just take water from outside the ballast tank and dump it in the ballast tank...

That's assuming the pump could handle both air and water.

[mylo]

Chris,

I have wondered the

Chris,

I have wondered the same thing. I'm thinking that the pump is for air, and that running water through it wouldn't be a great idea ...... BUT ....if it did happen, the sub itself wouldn't be in jeopardy because as you point out, you are filling water with water for no change in bouyancy (so....no unexpected crash dive to the bottom or something). It would be fantastic if a pump could be found that would not be damaged if water was put through it.......in the event it accidently happened. If these little air pumps are expensive and damage easily with water being put through them, I could see the need for some sort of float valve at the top of the vent tube so that the pump can not pump if the vent tube is underwater (causing the float valve to be actuated.......you know what I mean). Adds a bit of complexity (which I'm sure you find appealing), but could be worth while to save the pump from being damaged. I like the idea of the system, it really reduces the amount of on board compressed air being used. A fill should last an entire patrol season.

Mylo

[raalst]

I do like the idea

I do like the idea of the piston tank inside the main ballast for fine bouyancy adjustment........ it would save me from packing around change too. Can a guy buy these small piston tanks somewhere ?

these can be made easily from a large syringe (if the ones in the pharmacy are too small, check the local vet) and a servo.

I have build it in a trumpeter kilo, works like a charm for at least 2
years now, if a bit slow. but for trimming slowness might be actually a
good thing.

like this ]http://home.vianetworks.nl/~raalst/kilo/kolben.jpg[/img]

after a few tests I used limit switches and fixed the variable
resistor in "neutral". giving a bit of stick up fills the syringe,
a bit of down empties it. this continues as long as you keep
the stick out of neutral position, or until an end switch is activated.

[mylo]

Ronald,

...... hmmmmmmmmmm ........

I do believe

Ronald,

...... hmmmmmmmmmm ........

I do believe slow would be well suited to an application such as fine bouyancy control in a main ballast tank.

I'm sure I'll be able to design/build something.

Mylo

[raalst]

just for completeness :

the

just for completeness :

the servo needs to be converted to 360deg. rotation, check google for
"servo hack". robot hobbyists do this all the time in order to convert
the servo into a motor.

I built the limit switches into the circuit (wires) of the variable resistor.
the resistor itself has the value for "neutral", say 2 Kohm
one limit switch shorts out the resistor -> 0 ohm resistance
the other limit switch breaks the whole circuit -> infinite resitance
the servo electronics then think the objective is reached (or even
passed over) and stops or reverts the motor.
just test which switch goes at which end ! it's easier to test than to
think it out beforehand (with my amount of brain cells at least)

the plus of this approach over killing the motor directly is that
you are still able to command the motor to reverse. if the switch kills the motor, then the whole thing gets stuck. if you know what i mean...

[cstranc]

For modifying a servo:

Easy

For modifying a servo:

Easy Version: http://www.uscoles.com/kapcontinuous.html ( I did this, it took 10min and it worked great)

Less Easy Version]http://www.acroname.com/robotics/info/i ... nuous.html[/url]

Ronald,
Thanks for the pic on the. It looks like the brass rod extends through the servo housing (that's why the pot had to move). Where is the thread that would convert the rotation to a linear motion on the piston?

PS,
Mylo you are so correct. I love this stuff, I just don't get much time.