[PSUBS-MAILIST] Stupid Question

["James Frankland" <frankland@cwgsy.net>]

Hi All.

I'm just trying to get my head around a few principles of physics that i havent done since school!  Just wondered if anyone can shed some light for me.   See what you think.

Ok, so ive got an idea/design for an ambient pressure sub.  Pretty basic design, just to get me started.  Cabin will be open at the bottom, with an automatic pressure compensator to pump air into the cabin.  Thats ok i think.

I can calculate the volume of my machine easy enough, to give me a displacement.  But i've no idea of the weight.  Its to be a steel frame with wood covered in fibreglass making up the hull.  As its ambient there is no need for a pressure proof hull.

What i was going to do is just get on and make it "about right", and then take it to the tip where they can weigh it on their weighbridge.  I could then calculate the required ballast i would need to add to the bottom of the keel to make it float at the right point. 

Of course, this doesnt allow me to alter the size of the tanks and i may end up with them being far to big, or to small.

So, assuming i do all that and somehow its about right.  On the surface, the ballast tanks will be empty, with a water inlet valve at the bottom, a vent at the top and a high pressure air inlet somewhere.  

But, this is where im getting confused.

On the surface, with tanks empty, there will still be a certain amount of pressure crushing my fibreglass tanks?  Ok, they will be only just under the water, but still under pressure.

So, i open the inlet valve and water comes in until the pressure is equal.

Open the vent to let air out (inlet still open) and the sub will begin to sink.  Shut off vent when neutral buoyancy is achieved and always leave bottom inlet open.  Adjust buoyancy with high pressure inlet air.  Pressure should stay equal.

Is that lot right?  

Do i have to launch the sub with the inlet valve open and allow water into the tank to keep the pressure equal?  I think probably so.  But thats going to affect the buoyancy calculation.

hmmmm.  Confused.

Anyway, i would appreciate any ideas suggestions.  It might be quite simple?

Many thanks
James











 

 
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