[PSUBS-MAILIST] Bouyancy

Thu Jun 12 11:03:55 EDT 2014

Scott,  
To clear the lines, I screwed a T onto each of the through hulls that feed air to the MBTs.  (on the open end under the MBTs)  Then I fashioned a piece of copper tubing that directs a stream of air right into the MBT vent line but not actually connected to it.   I put a restriction in the other end of the T to create enough back pressure so the tubing emits a pretty good blast of air directed at the vent port.  

To vent the MBTs, I open the conning tower vent valves then give the tank blow valve a turn till I see the water blow up and out the vents.  That seems to clear the lines and then it vents properly. 
 It's not a great fix but it works.
  ----- Original Message ----- 
  From: swaters via Personal_Submersibles 
  To: Personal Submersibles General Discussion 
  Sent: Wednesday, June 11, 2014 10:37 PM
  Subject: Re: [PSUBS-MAILIST] Bouyancy

  Dan,
  I have the exact same problem with hydrolocking in the vents. I have to open the hatch and blow the water out with my mouth. Pain in the butt.
  Thanks,
  Scott Waters

  Sent from my U.S. Cellular© Smartphone

  "Dan H. via Personal_Submersibles" <personal_submersibles at psubs.org> wrote:

  If I were to do mine over again, I wouldn't vent it as it is in the plans. Instead of running piping from the MBTs to the conning tower, I would run linkage, and have the actual valves mounted on the MBT.  

  Besides slow venting, my biggest problem is getting water in the piping that won't escape to let the air come through when venting.  A homemade mushroom valves made from bathroom sink drains controlled by linkage going to the conning tower, or anywhere easy to reach in the hull, would solve all the problems.  

  My thought, Dan H.
    ----- Original Message ----- 
    From: via Personal_Submersibles 
    To: personal_submersibles at psubs.org 
    Sent: Wednesday, June 11, 2014 9:53 PM
    Subject: Re: [PSUBS-MAILIST] Bouyancy

    Scott, Alec, Vance, and all,

    A 3/4" vent is going to be about 2.25 times as big in cross section as the 1/2" vent.  A 1" vent would be about 4 times as big as the 1/2". What are your thoughts/comments on 3/4", 1" or any other size vent vs another?

    Thanks,
    Jim  

    In a message dated 6/11/2014 5:45:10 P.M. Central Daylight Time, personal_submersibles at psubs.org writes:
      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© Smartphone

      James 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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