[PSUBS-MAILIST] Bouyancy

[swaters via Personal_Submersibles]

James,
I totally agree with you on the 1/2 inch vents being way to small. Eventually I am going to switch mine out to 3/4"
Thanks,
Scott Waters




Sent from my U.S. Cellular© SmartphoneJames Frankland via Personal_Submersibles <personal_submersibles at psubs.org> wrote:Hi All,
 
A member has asked me to post a description of what was going on with bouyancy on my dive.  Which was as follows:
 
I'd asked the crane driver to be prepared for the first dive to hold me just below the surface as i was expecting to be a little out of control. 
 
At first, i got a bubble stuck in the forward tank.  Ive put quite a large lip on the inside of the tanks in an attempt to cope with Guernsey seas.  I could see the bubble stuck through the forward dome and i squished myself right up into the front to try and force it a bit more level so it could escape out of the vent.  It didnt make any difference and then all the lead i had inside, slid along the deck right to the back, making the entire situation worse.  Closed vents, blew mains and re-surfaced.
 
Next attempt, i got the bubble stuck in the aft tank.  This wasnt as bad because the motor cut away allows air to escape so i got under a little further, but not enough.  I flooded the variable in an attempt to get under.  Still no good, so i asked the divers to shove on the back.  This got the boat under, but then because of the total lack of air in any tanks, i sank to the bottom like a stone.  Depth was 6m (20ft).  I expected to stop on the crane ropes, but the driver must have misunderstood me and before i knew it, the bottom was in sight and then i was down with a gentle bump. 
 
I sat for a bit talking to my brother on the radio and then shut the variable vent and pumped a little air into the tank.  I slowely started to rise.  Flooded it again and started to sink.  Really quite straight forward. 
 
Blowing the VBT completely brought me up fairly quickly until i seemed to be just awash with the surface.  Filling the mains then brought me right out. 
 
I did several more test dives after that and as long as i evened out the venting of the main tanks to avoid the bubbles, i could dive without the divers assistance. 
 
I next got the boat neutral just off the bottom and then used the motors to drive up and down. I was not aware at the time that one motor was not working but it still seemed quite responsive and i could drive all the way up to the top or down to the bottom, stopping anywhere in between.
 
I need to add some extra vents to the main tanks to prevent the air pocket from occuring again, and also those half inch vents are far too small.  It takes forever to vent the main tanks.  Thats one for a winter modification.
 
I think that was it.
Regards
James
 
 
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