Re: [PSUBS-MAILIST] Trim Tank Idea

[MerlinSub@t-online.de (Carsten Standfuß)]

Hi Harry, 

nice idear - a friend and I play with this item some years before 
for a deep diving AUV. It gives a very simple and easy to contol 
just two electric switches dive system which works without compressed
gases and without vales ! And can easy controled by a
computer-autopilot. 

Not impossible but in your own words :  

- "..is a cylinder designed to withstand diving pressure
  containing a piston.." 

And thats is one of the manufacturing problems - a 2 ton heavy double 
seater needs about 20 % of his displacement in two tanks as tank volume 
to get a good freeboard. 20% of 2 tons in freswater is two tanks 
of 100 Liters or 0,1 qubic meters each tank. 

If we say for the first loop that the cylinder tanks is about 
1 meter (3 feet ) long. Which diameter ? 

Volume of a cylinder : 

h = length of a cylinder
pi = natural constant, about 3,1415
D = Diameter of a cylinder (inside)

V = h x pi/4 x D x D with h = 1 meter
V = pi/4 x D x D or 3,1415/4 x d x d seems 
V is 0,7854 x D x D with V is 100 Liter or 0,1 t we can write :

0,1 m3 = 0,7854 x D x D or 
0,1/0,7854 =  D x D 
or square root of 0,12733 = D 

means D = 0,357 meter - or just below one feet. 

Okay - ask the price at the next metal shop around the corner for 
a cylinder with 3 feet length and a diameter of one feet 
made from stainless steel with a mirror surface on the inside... 

- ".. and suitable seal on a threaded shaft.. "  

And ask also for a pressure tight seal of a diameter of one feet 
which can resistant the double of your dive deep. The seal must move 
3 feet fore and aft inside the cylinder. 

The threaded shaft must resistant on a maximun length of 3 feet 
a pressure of :

Pressure area x dive deep 

say your dive deep is 100 meter means (with 1 feet is equal to 0,3048 m) 
about 328 feet. 

The area of the piston is D x D x pi/4 
or 0,357 x 0,357 x 3,1415/ 4 = 0,1 square meters 

0,1 square meters x pressure of 100 meters (equal to 10 atmospheres) 
is a pressure 10 x 0,1 = 1 t (about 1 ts) pressure on a 3 feet long 
threaded shaft - not impossible - but a heavy shaft - and expensive.  

A normal threaded shaft can not be used because this threaded is
designed 
to locked under pressure (of a nut for example). You need something
for example used in a lathe.  

- "..The shaft is cranked in and out of the cylinder rack and pinion
style with  a heavy duty servo.."  

Yes - very heavy servo - has to move the threaded shaft against 
a weight of 1 ts.

Conclusion : This trim tank could generate a very simple and easy to
control little submarine dive system - if your can manufacture the item
and the diameter of the piston is not to big and the dive deep is not to
great. 
Also the threaded shaft moves in and out - means you needs a lengths
for the cylinder and the same additional lenmgths for the shaft. 

The increasing of air pressure inside the sub is not a great deal 
- say inside volume is about 1,5 m3 increase by 0,2 m3 of air is 
equal to 1 atmosphere  raised to 1,13 atmosphere - not a real dive deep
for a scuba decompression computer. 

- somebody should build some of this items - would be 
great as regulator in small subs.. If it work : Sale it to the Psubers ! 
I want two of 5 tons each for a water pressure of 500 meter (1640 feet). 

The first similar drawing of this idear was description by : 
---------------------------------------------------------------
"Inuentions or Deuifes"
Very neceffary for all Generalles and Captaines, or Leaders of 
men, as wel by Sea as by land : Writen by William Bourne 
Anno domine 1578
At London Printed for Thomas VVoodco
dwelling in Paules Churchyard, at the figne of the black (Beare). 
----------------------------------------------------------------

Some of my soures are older..  And this one was the first one 
which descripte a submarine.. 
 
PS : We never build the AUV - just a 1/4 RC model.

see you - Carsten - the idear is still great..  

Harry Spillett schrieb:
> 
> Hi Guys,
> 
> I am still trying to fine tune my understanding of the pros and cons of
> various designs. Can I have your views on the following:
> 
> In my quest for knowledge on everything submarine I have bought a book on
> model submarines. It seems that most employ a "diving cell" as a version of
> MBT. A common method employed by the modellers is a "piston tank".
> 
> As the name suggests it is a cylinder designed to withstand diving pressure
> containing a piston and suitable seal on a threaded shaft. The shaft is
> cranked in and out of the cylinder rack and pinion style with a heavy duty
> servo. It is also common to have a rotary pot fitted to the shaft to
> indicate the volume of water in the cylinder either for auto depth
> correction gear or simply as a bar graph.
> 
> My question is could a similar design be used (on small PSUBS) as an
> accurate "air free" trimming system. You could have one fore and aft and one
> amidships used like a VBT. If a suitable motor was used (electric or
> hydraulic I guess) it could be made to compress the trapped air behind the
> piston (rather than vent into the pressure hull like the models, this may
> help the seal stand up to diving pressure too, I am on a roll now!). Like
> the modellers you could have a sensor on the shaft to feed an  automated
> control system or more likely a relative bar graph display.
> 
> Have I just reinvented the wheel or is this a viable possibility!
> 
> Harry