You have to understand and harness buoyant force, the force that causes a thing to float on or sink in the water, if you want to build a toy boat capable of carrying heavy weights. The key to this lies in a principle discovered by a guy named Archimedes, who was born in 287 BC. He said that buoyant force equals the amount of fluid displaced by a floating object. This discovery made him famous twice, once for discovering the principle and once for shouting "Eureka!" meaning, "I found it!" when he made the discovery.
Weigh the object you wish to carry in your boat. This is the number of pounds of water you must displace with your toy boat's hull. Converting the weight of the water you need to displace to the size of the hull you'll need for your toy boat to be able to accomplish this involves some simple arithmetic that you can do with or without a calculator.
Divide the weight of the object you wish to carry by 62.4 if you'll be playing with your boat in fresh water, like a lake. If you'll be playing in salt water, like at the beach, divide the weight of the object by 64.3. One cubic foot--a cube that's 1 foot by 1 foot by 1 foot--of fresh water weighs 62.4 pounds and 1 cubic foot of salt water weighs 64.3 pounds. For example, if you plan to sail your boat in salt water, carrying a weight of 25 lbs., divide 25 by 64.3: 25 / 64.3 = 0.388, meaning that a little more than a third of a cubic foot of water must be displaced by the hull of your toy boat.
Multiply 1,728--the number of cubic inches in a cubic foot--by 0.388, the amount of water your hull has to displace. The answer, 670.464 cubic inches, is how much space your hull has to take up in the water to be "neutrally buoyant," meaning it won't sink or rise in the water unless something changes: if you put less weight--less than 25 lbs.-- on your boat, or increase the size of the hull, it will be "positively buoyant" and float well.
If you decide to build your hull with 680 cubic inches of space inside its length, width and depth, you can use any combination of numbers that, when multiplied together, equal 680. Most power boat hulls are five times as long as they are wide. Sailboat hulls are three times as long as they are wide.
Pick a number for the depth for your hull. If you chose a hull depth of 4 inches, then divide 680 by 4: 680 / 4 = 170. If you want a sailboat hull 8 inches wide and about three times as long as it is wide, then divide 170 by 8 (the width of the hull) and the answer, 21.25, is the length of the hull. Since your plan says the hull is 8 inches wide and 4 inches deep, multiply those dimensions together to see if you have a hull large enough to carry the 25 lb. weight: 21.25 x 8 x 4 = 680. Since 680 is the number of cubic inches of hull required for the project, that's how large your toy boat must be.
The higher you make the sides of your boat and the deeper you make the hull of your toy boat, the more water you can displace and the more weight you can carry. Even so, add some extra height to the sides of the boat. Though the extra high sides may not displace more water, they'll keep water from splashing in and sinking your boat.
Even if you can do the calculations yourself, have an adult handy before you start your shipbuilding career, to help you with tools that can hurt you.