Many regattas ban materials like wood, styrofoam, plastic sheathing, metal, and fiberglass. Coated cardboard is often off-limits as well, along with caulk, staples, clamps, and screws. Basically, your boat can only be made with cardboard, paint, and some type of basic adhesive. Contest rules also limit the types of boats you can make. Raft- and canoe-style boats make the cut, while surfboard-style boats typically aren’t allowed. [2] X Research source

Your boat’s length plays a big role in how easy (or hard!) it is to control. Boats that are less than 10 ft (3. 0 m) long are difficult to keep pointed in one direction, while boats that are more than 12 ft (3. 7 m) long can be difficult to turn.

This isn’t to say that there’s anything wrong with a simple, squared-off bow! Plenty of people have entered regattas with square boats. [7] X Research source Plus, a square-shaped boat works well with certain themes and decorations, like a school bus or fire truck.

For instance, if the boat is 10 ft (3. 0 m) long, 3 ft (0. 91 m) wide, and 1 ft (0. 30 m) tall, the volume is 30 ft3 (0. 819 m3). Multiply 30 cu ft (0. 819 m3 by 62. 4 lb/cu ft (1,000 kg/m3), which gives you 1,872 lbs (819 kg). In other words, your boat can support 1,872 lbs (819 kg) of combined weight without sinking. Believe it or not, a 1 by 1 by 3 ft (0. 30 by 0. 30 by 0. 91 m) box can support someone who weighs up to 187 lb (85 kg) without sinking. If your boat is large enough to physically hold you, it’s likely sturdy enough to float. [10] X Research source

An easy way to scale down the dimensions of your boat is to use the same number of smaller units. For example, if your finished boat will be 10 by 3 by 1 ft (3. 05 by 0. 91 by 0. 30 m), switch the units to inches to make the boat smaller but keep the model proportionate. In this case, you’d make your model 10 by 3 by 1 in (25. 4 by 7. 6 by 2. 5 cm). Stick a couple of coins or rocks into your boat to represent you and your fellow crew members. Don’t be discouraged if your first model doesn’t work out. It’s better to work out the kinks in the planning phase than to have a huge chunk of cardboard that doesn’t float properly!

Grocery stores, fast food restaurants, bookstores, coffee shops, and schools might also have leftover boxes they’re no longer using. You also check out sites like CraigsList and Freecycle to see if people in your community have some extra cardboard lying around.

You can also use a tool like a screen roller to crease the cardboard.

House paint works really well for cardboard boats. [17] X Research source Any type of water-based paint will work for this project. [18] X Research source

Check your glue’s packaging to see how much time it needs to dry and cure. [21] X Research source Never use hot glue for this—it could melt in the hot weather when you’re outside. [22] X Research source

Layer the sheets of cardboard so the corrugations in each sheet run in perpendicular directions. [25] X Research source Give the bottom of your boat some extra security with 2 extra layers of cardboard. [26] X Research source Make sure that the painted pieces of cardboard are always facing out.

Duct tape shrinks down when you paint over it, and clear tape tends to melt beneath a coat of paint. Don’t use either of these tapes if you plan on painting your boat some more. [29] X Research source

Check the varnish label to see how long to dry and cure.

For instance, if you want your boat to look like a pirate ship, add a mast and sail, Jolly Roger flag, cannons, anchors, and a crow’s nest.