How to size your WTC ballast tank

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  • trout
    Admiral

    • Jul 2011
    • 3633

    How to size your WTC ballast tank

    I posted this on Facebook, but I wanted to share here. This seems to be one of the top three or four questions I get. I am interested in your thoughts to see if I missed anything or if there are other suggestions.



    Estimate Ballast Tank Size




    A common question is “How to figure the size of a ballast tank?”

    There are several ways to estimate a ballast tank size. I will list several ways, but you will need to try them and see what works for you.

    Archimedes is said to have gotten into a bath and noticed the water rise. This discovery led to what is known as the Archimedes Principle. Out of that came ways to figure buoyancy and to accurately measure the volume of objects.

    So, you created a new sub hull, how big does the ballast tank need to be? You will only be concerned for what is above the waterline.

    Fill a bucket to the brim with water, dunk the parts that will be above the waterline, capture all the water that comes out, that water is the volume you need. You got to love metric measurements, 1 gram of water is equal to 1 cc of volume. Weight the water, then figure out the volume (I.e. That seems like a lot of work and you are not done. We have the volume, but not material the hull is made from. Wood is different than lead. Fortunately a plastic model like Revell’s 1/72 Gato or Mobius 1/72 Skipjack has a specific gravity about equal to water meaning you do not need to adjust your ballast size. If your hull was lead, it would be at least 5 times that of water, a ballast tank would need to be 5 times larger.

    That seems like a lot of work, is there another way?

    Of course, suspend the submarine with fishing line and attach it to a fish scale (it will need to be accurate). Submerge the submarine (you may need to add weight to get it submerged) and record the weight. Lift the sub to waterline and record that weight. The difference is the mass of water needed to lift your sub. Again, 1 gram of weight will equal 1 cc of volume.

    Not happy with that way? How about this…..

    float the sub at the waterline with some stability weight and foam, then add weight at the COG / mid point for the ballast tank. The amount of weight needed to submerge the sub to just the point of the sail to go under water or close to neutral balance is the weight you convert to the ballast needed to support surface trim. 1 gram of weight = 1 cc of volume

    What more choices?

    I simply weigh all the parts above waterline and use that to figure a ballast tank size. It is not the most accurate, it can result in a tank too big. It is better to have extra than not enough.

    More?

    Find someone that has built a successful working sub made of the same materials you have and ask what size they have used.

    What if it is 3D printed?

    In some programs, maybe all, it can calculate the volume of your sub. If it is split at the waterline, then it can tell you the volume that will be used to calculate the volume of the ballast tank.

    For the suggestions offered, add 10 to 20% to your size. It is easy to add weight if it is too large, but harder to get to waterline if it is too small a ballast tank.

    Finally trial and error. Your expertise will develop a knack to guesstimate. David Merriman has built so many subs that he can pretty much eyeball the size needed.

    I have not used each one of these, but others I know, that build great subs, have. You will need to try one that works for you.

    If you have a better way, please share!
    If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.
  • redboat219
    Admiral

    • Dec 2008
    • 3186

    #2
    Do you need to include the WTC in the hull during data collection?

    How about weight of the components that go between the ballast tank, battery, pumps, servos, esc, pitch controller, drive motors, etc.?
    Make it simple, make strong, make it work!

    Comment

    • SubSteve
      Lieutenant

      • Apr 2022
      • 84

      #3
      Hey Tom,

      Good stuff!!
      This is the 2nd (I think) method you talk about. I have great confidence in this process but yikes is it unwieldy with an 82" submarine! The video shows the process of dunking the empty hull in the water suspended by a sling in turn hanging on a digital scale. It is not necessary to have anything in the hull as long as it will sink on its own. Once submerged, zero the scale and lift the hull out of the water to the design waterline. The digital scale reading is the actual weight of the submarine above the waterline and that weight will be the starting point for ballast tank sizing calculations. Made easy as the portion of the model in question is complete and represents the likely weight. This assumes anything you place in the hull is below the waterline, especially the WTC, or equivalent dry area. You can stop the video at about the 5:00 minute mark as the rest is our local Dallas fun float coverage. Note SubEd's USS Shark sneaking in at the beginning to observe the process.

      Comment

      • RCSubGuy
        Welcome to my underwater realm!

        • Aug 2009
        • 1884

        #4
        Originally posted by redboat219
        Do you need to include the WTC in the hull during data collection?

        How about weight of the components that go between the ballast tank, battery, pumps, servos, esc, pitch controller, drive motors, etc.?
        The only components you need to worry about are any that sit above the surfaced waterline of the boat. This may include portions of your actual watertight cylinder, bulkheads, etc. Ignore absolutely everything else.

        Comment

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