STEWART KNOWITALL: How do boats float when they are heavy?

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Let me start with a simple answer and then give you some details. A boat can float because the weight of the boat and everything in it is less than the weight of the amount of water that the boat displaces. So, what does this mean?

Why don’t we try an experiment together. Say your mom or dad has a piece of scrap wood from a building project, maybe a 2x4 that is about 1 foot long. The size of that piece of wood is 1.5 inches by 3.5 inches by 12 inches long. If you multiplied those numbers together, you would find the volume of that piece of wood. In this case, the volume is 63 cubic inches. Let’s take that piece of wood to a bucket filled completely with water and and hold the wood so that it is completely under water. The amount of water that would spill out should be the same as the volume of the piece of wood and you would say that the wood “displaced” 63 cubic inches of water.

If you could catch the water that spilled out of the bucket and weigh it, you could calculate what we call the “weight density” of the water, which is about 0.036 pounds per cubic inch. The 63 cubic inches of water that was displaced would weigh about 2.268 pounds. As long as the wood weighs less than 2.268 pounds, then it will float in water. If we weighed the piece of before starting our little experiment, we would have found out that it weighed about 1.17 pounds, therefore it should float.

If you fill your bucket back up to the very top and then just placed the wood in the bucket without holding it under, then you would have a very simple boat. Some water will still spill out of the bucket, because our little “boat” will displace some water, but not as much as when held under. If we divide the weight of the wood (1.17 pounds) by the weight density of water (0.036 pounds per cubic inch), we would get an answer of 32.5 cubic inches. This means that our “boat” will displace about 32 cubic inches of water, which is about half the volume of the whole “boat.” Therefore, about half of the wood will be above water.

What about the real, full size boat that your family and friends can go tubing or fishing in? As long as the weight of the boat, all of the people, and the gear loaded in the boat are less that the volume that the boat takes up in the water, the boat will float.

Most boats have large amounts of empty space filled with air (which is very light); even big cruise ships. That empty space helps keep the volume of water displaced large as compared to the weight of the boat, allowing the boat to float.

Brian Brady

Associate Professor of Mechanical Engineering Technology at Ferris State University

Brian Brady has a masters in Mechanical Engineering from University of Illinois at Urbana-Champaign, and a bachelors of Mechanical Engineering from GMI Engineering & Management Institute (Kettering University).

He has over 15 years in automotive component manufacturing and special machine tool design/manufacturing and is a member of the American Society of Mechanical Engineers and the American Society of Engineering Educators.