What is the formula to float a ship?

Answer 1

When the total weight of the ship is less than the weight of the volume of water (salt or fresh) that the ship displaces, it will float. You can make a ship out of steel or concrete, as long as it weighs less than the volume of water it displaces. How sea-worthy the ship is depends on what type of waves or what the possiblity is of the ship taking on water, unless the exterior is sealed. To calculate this, you will need the exterior dimensions of the hull and weight of the ship. I would use the metric system, because the units are easier to convert for this example. Kilograms (1000-grams) for weight and Liters (1000-cubic centimeters) for volumes. Using a volumetric formula (i.e. length x width x height) to obtain the volume you wish to be below the water line. Take the weight of the ship and divide it by the volume you obtained for the ship at or under the water-line. If this number is less than the specific-gravity of fresh water (lighter than salt water) it will float, provided the center of gravity is on the bottom of the ship. (otherwise it may turn up-side down). The specific gravity of water is ~1.0 kilogram per Liter.

For example if the volume (LxWxH) displaced (below the water line) by the ship is 100 liters and the weight of the whole ship is 60 kilograms, 60/100 = 0.6 which is less than the water specific gravity 1.0, then it should float. Bear in mind that the ~70% of the weight of the ship should be below the water line or the ship will not have stability (it may turn over). This should also take into account added weight to the ship such as cargo or (God forbid) water.