They are not specified in the current rules. Many are cargo ships which have a wide variance between full and empty weights.
Just ballast it to waterline per the plans, if you have them. If not, eyeball it to something reasonable.
The best way to build a convoy weight wise is to put just enough weight so that is sinks. Start your build as light as possible. When it’s ready to float and drive around add just enough weight that it sinks. Any heavier and you’re giving away buoyancy and making yourself sink faster. Add the weight so your ship sinks level side to side and aft to stern. The flip side to this is you need to make sure the prop(s) are far enough under water that there is no cavitation. When the props are close to the surface they can suck air into the water. This slows the ship down. Until it get enough water into the ship that the props go under far enough to not suck in any air. When that happens your ship is illegally fast. Like everything it’s a balancing act that you’ll need to play with.
If the weight rules are so loose my follow up question is, how do I know where the water line should be before I float it? To tell where the bottom of the windows are.
There’s no minimum weight for any ship. BUT the less weight you put in it the more penetrable area you have. On warships that’s bad. On convoys it’s not. Convoys need to take as long as possible to sink. How to find the bottom of the window depends on the ship. Something small like an LST you cut the sides down to the 45deg mark. Bigger ships have more solid on the bottom. Ive always found the easiest way to build is ask someone who has one what they did. That’s how I’ve found the bottom window on most of my ships. what are you working on? If no one has that ship float it so it looks right and sinks right before you cut it. After it’s all done but before you sheet it. Put duct tape over the windows and float it. Drive it around. Sink it. see where it sits. Cut out more window as needed.
I’m making a liberty ship with 3D printed parts. In the same way you would with a wood hull. I like the idea of using duck tape to get it to float to mark the water line. I’ll have the sub deck done today and just have to finish the ribs and keel. I’m actually very happy with it so far.
With a Liberty ship I’d guess the windows are going to be pretty tall. 4”-5” of balsa needed to sheet it. if you put a small enough battery in it it should float with no skin on it. don’t be tempted to place the prop high in the stern. Put it as low as you can. This will help with cavitation I talked about earlier.
Remember that your transport gets a small pump, so freeboard is not a major concern. weight distribution, handling, and water channeling is more important. You don't want to be too heavy, or the ship won't accelerate. You don't want to be too light, or the ship will be tippy, and there won't be room for water channeling. If you look at historical photos and at plans, it looks like you want the waterline right around the rudder post, back aft. Liberty ships were generally loaded evenly or low astern. For an even load, measure the height of the rudder post and then mark the waterline the rest of the length of the ship at the same height. For a low-astern load, subtract 1/8in from that measurement at the 1/3 mark, 2/3 mark, and bow. I would love to see some photos of this build.
The Washington Cascade Column (his local group) does not follow IRC convoy rules. We created our own rule set for convoy that frankly is a lot more fun. Convoys ships should be something that carried some form of cargo and are allowed a small single half unit pump. They sail around buoys to earn points while the two teams attack/defend before their eventual sinking. To keep things fair, on the second sortie the convoy is allowed to patch completely and changes teams. It brings everyone together on the pond, provides a change of pace, and makes a great way to recruit new captains, and its fun! For your build, I would plan to have the hard sides a touch high and then add weight to lower her deeper in the water. I don't think anyone local is going to get too picky about a cargo ship as long as it looks like you tried to follow the rules.
