Determing Ballast Tank Volume

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  • redboat219
    Admiral
    • Dec 2008
    • 2735

    Determing Ballast Tank Volume

    How do you determine the volume of the ballast tank need? Is it the same with boats with a separate WTC and those with a dry hull?
    Make it simple, make strong, make it work!
  • coryhenry
    Lieutenant Commander
    • Jul 2020
    • 156

    #2
    Check out this thread

    Comment

    • coryhenry
      Lieutenant Commander
      • Jul 2020
      • 156

      #3
      Ha, didn't notice you already posted and replied on sub committee. Oh well, better to get to much answer than not enough :)

      Comment

      • Monahan Steam Models
        Captain
        • Apr 2020
        • 755

        #4
        Click image for larger version

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        • redboat219
          Admiral
          • Dec 2008
          • 2735

          #5
          Originally posted by coryhenry
          Ha, didn't notice you already posted and replied on sub committee. Oh well, better to get to much answer than not enough :)
          Sometimes it pays if you widen your search area.

          Actually I have post from HWSNBN from this thread 10 years ago, https://forum.rc-sub.com/forum/gener...ations-correct

          "Ballast tank water weight is equal to the weight of the displaced water pushed aside by the above waterline structures.

          Here's a way to figure ballast water weight without having to work out topside structure volume (what submarine is a collection of simple geometric shapes, anyway!? ...).

          Let's start with a simple example:

          If the items above waterline have a specific gravity (density, relative to fresh water) of 1, i.e. those structures have the same density as fresh water, then there is a direct coloration between the weight of the above waterline items and the weight of the water the ballast tank has to hold to pull those structures down till their buoyant force is counteracted by the weight of water taken on.

          Simple.

          But, as a practical matter, the upper portion of the hull, above the waterline, is all to often attached to inseparable portions that extend below the waterline. And the materials from which those structures are made is likely to posses different densities. What to do?

          When you weight the assembly you'll have to figure the ratio of above waterline structure to below waterline structure and subtract lower structure weight from total weight to get the weight of the above waterline structures. Math time. Welcome to planet Earth!

          Some more reality check here:

          GRP and polystyrene materials have a specific gravities a bit greater than 1, they are a bit denser. Anywhere from 1.04 to 1.8. So, what do you do when the structures above water line are denser or less dense than fresh water?

          You identify each type material (polyurethane resin, white metal, brass metal, GRP, polystyrene, Acrylic, etc.) work out the weight of each group, multiply the weight times the specific gravity of the material. That gives you the weight of the fresh water that group displaces. Add 'em all together and you have the weight of fresh water all the above waterline elements displace, and that number represents the weight of water the ballast tank has to take on in order for the boat to achieve near neutral buoyancy in submerged trim.

          Good practice to take that number and plug in an additional ten-percent -- It's easier to fix a boats trim with a ballast tank holding too much ballast water on board, than too little. But, don't overdo it: excessive ballast water puts an unnecessary burden on your means of getting the water out and the additional volume takes up valuable internal real-estate!"


          I want to know if things changed, has anyone else has their own technique of determining BT volume.
          Last edited by redboat219; 02-26-2021, 10:56 PM.
          Make it simple, make strong, make it work!

          Comment

          • Bob Gato
            Captain
            • Feb 2019
            • 826

            #6
            You can determine the volume and displacement several ways-Mathematics is one way and calculating volumes and the materials densities that make up the structure, archeologists calculate the volume of ancient sculls by filling them with rice and then measuring the volume of the rice-but that only gives volume not buoyancy (pos or neg) of the structure, the best way is in the medium that the sub will travel in-either compare the weight difference between submerged and waterline height at a constant waterline ballast condition by pushing it under with a push scale OR the easiest-by comparing the weight at a constant submerged ballast weight to its weight when pulled out of the water to waterline with a pull scale (my choice) The ounce reading is easily then calculated into water fluid ounces required to raise the boat to that height..BG

            Comment

            • Subculture
              Admiral
              • Feb 2009
              • 2119

              #7
              Calculation isn't easy, especially with fibreglass hulls, as they can vary quite a bit in both density and thickness compared with an injection moulded hull. Hulls with complex shapes are more tricky still. An injection moulded hull of revolution e.g. Skipjack kit is quite easy to calculate, but a Type VII might prove more challenging. This empirical method is worth repeating.

              How to calculate your ballast tank volume

              Build your boat to the extent that everything is done except the ballast tank itself. Put it in your test
              tank and you'll almost certainly discover that it's light. Add whatever weight is necessary wherever
              necessary to get it to JUST submerge - not sink like a rock - and sit as level as possible. Something
              like just enough weight to have it float with just the top of the conning tower out of the water and
              then add just enough to get it to completely submerge. At that point, a couple of quarters would
              probably suffice.
              Now add square or rectangular polystyrene foam blocks under the hull until the boat sits at its
              proper waterline. Since it's a round bottomed boat, you'll have to start by building a cradle (use
              foam) so the foam blocks won't just roll out from under. Make that cradle and the foam blocks you
              put under it approximately 4" - 6" long. When the boat is at its proper waterline, determine the
              cubic volume of all the foam (including the cradle) by multiplying length X width X thickness.
              Then convert that to the volume of a cylinder (3.14 X radius squared X length).
              Another benefit to this method is that by sliding the foam fore and aft, you can determine the
              EXACT location for the tank.
              Yes, when all is said and done, you'll have to remove weight equal to the weight of the ballast tank
              itself but that's a small price to pay. Plan ahead when you're adding the weight to begin with - don't
              make it all permanent.

              Skip Asay

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              • redboat219
                Admiral
                • Dec 2008
                • 2735

                #8
                Thanks Subculture.
                Make it simple, make strong, make it work!

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