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Arkmodel Type VII doesn’t submerge

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  • Arkmodel Type VII doesn’t submerge

    I’ve got an Arkmodel Type VII with a 1/72 Gato cylinder.
    The ballast part of the cylinder holds 28 oz of water and I have 20 oz of lead for ballast.
    It’s set up OK but will only submerge 1 1/2”, up to the deck.
    The cylinder , by itself submerges the same amount.
    Pretty frustrating with all the time and effort I put into this boat that all I can get is 1.5” while other boats seem to have no problem submerging.
    Any help will be greatly appreciated.

  • #2
    Have you tried more weight? Once you get it to submerge just barely all the way then you can trim with little bits of foam to what you want.


    • #3
      Be sure it's not surface tension holding the boat up. Those flat decks also like to trap air. If you fully vent the tank, give the model a shake while submerged and see if you free any trapped air and you'll break that tension at the same time. Does it still sit high?



      • #4
        Are you trimmed up by the bow and also noticing air being trapped in the ballast tank? My 1/72 Skipjack submarine used to have a slight up angle trim (about a degree or two), which caused some air in the ballast tank to get trapped since the vent valve is located in the aft end of the ballast tank. I had to push dive on the stern planes while venting to get all that air out. Once I decided to have a zero degree surface trim instead, it was able to submerge statically with no downward angle to fully vent the tank required.


        • #5

          The prior postings make great suggestions. Two builders can build the same sub and there will be differences. There are several hurdles in getting a sub to run well, you are at one of those junctions. Let me begin with 688hunterkiller’s suggestion. Is your boat trimmed correctly in the first place? When trimmed, is your wtc level? Correct that if not.

          Bob is correct about trapped air, it can cause a world of frustration. One time your boat will not dive, the next time it is too heavy. I had my Gato sit in a kiddie pool submerged. It had 1/4 of the tower out of the water. Went and ate lunch came back out and it was at the bottom of the kiddie pool. I had air trapped behind my foam of all things. For air to escape , 1/8” or 3mm hole is necessary. Smaller holes will work the first time you dive, but water will block those holes and submerged air will slowly escape (if at all). My Gato has holes at the bow and aft end and several in the sail or tower. You can disguise the holes.

          Cory’s point is good one, going back to my original statement, two can build the same sub, but have different needs. One such need is weight. If you are certain it is not trapped air, place some weight near center of gravity on your sub, until you reach the depth you want your sub to sit at. Adjust the weights, if needed, forward or aft to maintain level attitude. If you surface your boat, know it will not give you an accurate representation of waterline. To do that you need to place the weight at the bottom of the hull either with rubber bands to externally place or open the sub up and place were you want it.

          you will get through this. As a side note you will be rewarded with an amazing sub on and under the water.

          Last edited by trout; 04-18-2021, 03:21 PM.
          If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.


          • #6
            These are high quality and informative responses to the original poster’s question asked. Nice work to everyone who has contributed.


            • #7
              What is the weight of of the water in the ballast tank and is the weight of the part of the boat that is above the surfaced waterline less than the weight of the water?


              • #8
                Originally posted by type7 View Post
                What is the weight of of the water in the ballast tank and is the weight of the part of the boat that is above the surfaced waterline less than the weight of the water?
                The object of the drill is to work out the weight of water displaced by the above waterline structures. Topside water displaced = amount of water in the ballast tank.

                If the specific gravity of the above waterline structures is 1, then there is a direct correlation between structure weight and ballast tank weight. However, if the structures specific gravity is more than, or less than 1, then you have to plug in the conversion factor (.00001 for feathers, one-zillion for Plutonium). Multiply the weight of the above waterline structure by the conversion factor (GRP and polystyrene are close enough to 1 to be used directly). This is just one way to do it.

                You could also work out the water displaced topside by employing the Archimedean method (messy but fun); or work out the volume of all above waterline structures, employing a little geometry, and get the volume of water you need the ballast tank to hold.

                Plug in 10-15% to the ballast tank volume (because we're all screaming idiots!) and you're good to go.

                Insure that the entire ballast tank is immersed with the boat boat in surface trim -- you lose buoyant force when some of the ballast tank is sticking up in the air.

                Resident Luddite


                • #9
                  you have to plug in the conversion factor (.00001 for feathers, one-zillion for Plutonium)
                  I want to build my next hull out of Plutonium....