DIY Cardboard Boat That Won’t Sink

• Design your boat to be 18-48 in (46-122 cm) wide and 18 in (36 cm) tall. Make your boat long enough to fit the crew members you’ll be sailing with.
• Multiply the total volume of your boat design by 62.4 (1000 if you’re using metric measurements) to see if it’ll support the total weight of your crew.
• Build your boat with corrugated cardboard to give it a sturdier frame.
• Finish off your boat with latex paint and varnish so it’s ready to hit the water.

Designing the Boat

Check the rules if you’re entering your boat in a contest. As a general rule of thumb, stick with corrugated cardboard, cardboard blocks, cardboard tubes, duct or masking tape, glue, latex paint, and varnish for your boat. Most regattas have a pretty strict list of materials that you can and can’t use, so you don’t want to get disqualified before the race even starts! 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.

Build a flat-bottom boat to prevent it from tipping in the water. A flat-bottomed boat offers lots of stability, which is great if you’re racing across a pool or lake. Boats with V-shaped bottoms have a greater chance of capsizing once you’re in the water.

Design the boat to be 18-48 in (46-122 cm) wide and 18 in (36 cm) tall. If you’re sailing solo, 18-30 in (46-76 cm) gives you enough wiggle room along the width of your boat. If you’re planning on sitting side-by-side with someone in your boat, give yourself 48 in (122 cm) of breathing room. Then, plan for your boat to be about 18 in (36 cm) high—this gives you enough space to paddle in the boat while kneeling or sitting down.

Make the boat between 8 and 12 ft (2.4 and 3.7 m) long. Factor in your crew size when you’re finalizing the dimensions of your boat. An 8 ft (2.4 m) long boat might be best if you’re sailing by yourself, while a 12 ft (3.7 m) long boat would work better for a larger crew. 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.

Give your boat a pointed bow if you’d like it to go faster. When your boat has a squared-off bow, the water hits the very front of the boat and creates some water resistance. When your boat has a triangular front, you run into far less of that water resistance—this makes you cut faster through the water! 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. Plus, a square-shaped boat works well with certain themes and decorations, like a school bus or fire truck.

Build a horizontal block across the boat to give it extra support. Plan on making a “support block” that spans horizontally along your boat. You can either position this piece so it separates the hull from the crew compartment, or put it in the center of the boat to create 2 separate crew compartments—just be sure to balance the weight in each.

Multiply the boat’s volume by 62.4 (28.3 metric) to ensure it can hold your crew. Find the volume of your boat by multiplying the width by the height and length. Then, multiply this number by 62.4 (26.3), since water weights 62.4 lb per cubic foot (28.3 kg per cubic meter). This number is the total amount of weight your boat can support without sinking. 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 ft (0.819 m). Multiply 30 cu ft (0.819 m by 62.4 lb/cu ft (1,000 kg/m), 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.

Build and test a small, to-scale model of your boat design to make sure it works. Grab a manila folder or piece of thin cardboard—this will be sturdy enough to make your mini boat. Fold and cut the material into your design so you can see if the structure seems viable or not. 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!

Constructing the Boat

Build your boat with big, flat sheets of corrugated cardboard. Stop by your local pharmacy, recycling center, or big-box store and see if they have any spare cardboard boxes they’re tossing away. Grab as many sheets of corrugated cardboard from these places as you can—this type of cardboard is much sturdier and will give your boat much more structure. 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.

Cut or fold the pieces of cardboard together to shape your boat. Reference the sketch and model you made earlier to guide your work. Using a box cutter, carefully slice any smaller pieces or components from a larger sheet of cardboard. To fold the cardboard, just crease it with the blunt bottom of your box cutter (make sure it’s retracted first!). You can also use a tool like a screen roller to crease the cardboard.

Paint the cardboard with multiple coats of latex paint and let it dry. Pull out your paint color of choice and go over all of the cardboard you’d like to paint. Latex paint takes around 4 hours to completely dry, so make sure your cardboard has enough time to air-dry between coats. House paint works really well for cardboard boats. Any type of water-based paint will work for this project.

Glue the pieces of cardboard together and clamp them in place. Grab a bottle of carpenter’s glue or Liquid Nails—these are pretty heavy-duty and will give your boat a lot of structure. Thoroughly coat 1 of the joints or pieces of cardboard with an even layer of glue and stick it to the adjoining piece. Then, secure the pieces with clamps to make sure that the cardboard doesn’t move or come apart. Wait for the glue to dry completely before removing the clamps. Check your glue’s packaging to see how much time it needs to dry and cure. Never use hot glue for this—it could melt in the hot weather when you’re outside.

Add an extra layer of cardboard on top of the first layer for reinforcement. Once you’ve built a basic framework for your boat, glue an extra layer to the outside of your boat with polyurethane adhesive. An extra layer of cardboard help make your boat’s hull extra sturdy. Clamp the pieces of cardboard together to help them dry in the correct shape. Layer the sheets of cardboard so the corrugations in each sheet run in perpendicular directions. Give the bottom of your boat some extra security with 2 extra layers of cardboard. Make sure that the painted pieces of cardboard are always facing out.

Cover the seams with reinforced paper tape. Go over both the inside and outside of each seam with several pieces of tape to make sure that they’re watertight with no visible cracks or crevices. Duct tape or clear tape can work in a pinch since you’ve already painted your boat. 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.

Add a coat of varnish to make your boat waterproof. Go over both the inside and outside of your cardboard boat to make sure that it’s totally waterproof before the regatta. Check the varnish label to see how long to dry and cure.

Decorate the boat so it matches your theme. If you’re racing in a cardboard boat regatta, there’s a good chance that you and your crew have chosen a theme for your boat. Now, it’s time to have fun decorating and jazzing your boat up! Just make sure that your additions won’t mess with the structural integrity of the boat or break the rules. 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.