I wanted to start a discussion on ballast tanks for big ships. It seems that the interest in Battlestations is tending toward very large, and very heavy ships, instead of sensible cruisers and destroyers. So I see ballast tanks as the best way to reduce the weight of the ship when you are taking it out of the water and transporting it. So I'll describe what I did for the St. George and hopefully others will contribute their thoughts. I wanted to get as much useable water ballast as possible, so that meant using as much of the hull below the penetrable surface as possible. I was originally going to build the tanks level with the bottom of the windows and run a channel down the middle for the pump. However, since this would be basically building two tanks - one on each side - I opted for a simpler single tank. To keep the water flowing to the pump, the top of the tank is sloped up at each end. In this picture you can see the inside of the tank, made up of the hull bottom, and the cross braces for the tank top. The bars in the middle are 4.5 pounds of ballast. This picture is of the top of the tanks fitted. West Systems epoxy was used to seal all the wood surfaces. A sump was created from the bottom of a junction box and added to the piece in the middle. A 1/8" Clippard fitting was added to the tank top at each end to vent the tank. The next step was sealing the tank, which probably took me two hours. I had put epoxy with fibers around the edges of the tank to seal the gaps, and followed that with a coating of plain epoxy. Using my shop air, I put some pressure in the tank and felt for leaks. Not only did I have leaks around the edges of the tank top, but also around the ribs and on the outside where the 1/32 ply joined to the stringers on the sides. I should mention that I had fiber glassed the whole bottom with 3oz cloth and it was fine. I had to be very careful with the pressure I used for leak checking too. Anything more than a couple of psi would break the tank top free of the cross supports and I had to start all over. Eventually I did get it sealed. Next was putting holes in the bottom. There are 16 holes, one on each side of the keel and between each rib. Because the ribs divide up the interior tank bottom, this was necessary for good drainage. When it was all done, I brushed on a coat of self-leveling polyurethane. Makes it look nice Next was setting up the vent system. Originally, I used a 1/8" line from both vents and jointed them to a Clippard quick connect fitting in the bow. The purpose of this was to have a single fill and vent to make it easier to use. However, when I did my first tests, I ran into a couple of problems. First, I had some water in the line going from the rear vent to the front. The surface tension of the water bubbles in the line was enough that it would not allow the tank to vent. It's very frustrating when you can't get your ship to sink after drilling 16 holes in the bottom! So I finished most of the rest of the ship and thought about my problems. For my next test I disconnected the rear vent line and made sure that it was dry. Sure enough, it vented fine then. I tried a MAV3 instead of the quick connect valve, but it was too restrictive also. So eventually I brought the two vent lines into a X fitting, with the third outlet going to an open 1/8" line for the vent and the forth outlet going to the quick connect fitting for the fill. This setup worked, taking about 3 minutes to completely fill the tank. When I'm ready to take the ship out of the water, I just connect a CO2 bottle to the quick connect valve, use a surgical clamp to shut the vent tube, and blow out all the water. It just takes a few seconds, so I'm very happy with that. Overall, the ship weighs 14 pounds dry (including the 4.5 pounds of permanent ballast), I had to add another 5 pounds of ballast plus a 1.5 pound battery. The ballast tank adds about 13 pounds when full for a total displacement of 34.5 pounds.
Mark, Excellent design work as always. My question was how did you determine the amount of water to add as ballast? Was it the easy way: determining the volume below the penetrable area, calculating the mass of that volume, and making sure you still had weight to work with? Or was it the hard way: equations with Greek letters? (when I say easy way, I mean the shortcut I would tend to take when math is involved) On the vents: are the ends opposite the "X" of the two vent lines in the high spots in the bow and stern of the ballast tank?
I did a rough calculation for ballast by multiplying the available height by the length and width of the ship, then multiply by the prismatic coefficient. Fancy name, but gives the area of the projected shape and it's always about .6 for ships and also coincidentally for airfoils. Then I take the resulting cubic inches and put them in an excel spreadsheet that gives me the water weight. In this case, I calculated that it would be less than 15 pounds, and I knew I needed about 30 pounds displacement, so I just built the maximum size the ship would handle. In a warship with all the guns and support systems you may need to be a little more careful with the ballast size, but in the cargo ship I didn't have to worry about that. By X fitting I mean a 4-way manifold where everything joins together. The vents come off the high points at each end of the tank. This is to ensure that all the air is eliminated from the tank - you don't want an air space that allows water to slosh around.
While USS Georgia is not particularly large, I do want to be mindful of my back Thanks for the tech article, this topic is one that I hadn't seen addressed in warship forums
Yes, I did have it laser cut. It is a 150% version of the hull from my Bogue CVE kit. The superstructure was done in CAD but cut out by hand. An interesting ship, but a little on the large size for a convoy ship.
