How Does A Concrete Boat Float?

A rock can’t float, so how does this work?

Ed Dutko, the builder and operator of the 144 foot catamaran named CaboRey, built his  boat out of concrete because he read somewhere it would work. This paper laughed at him, calling it Eddie’s Folly for the entire three years he was building it. Just to show we learned our lesson, we’re passing on the knowledge of concrete boats to you, so you don’t make a fool of yourself laughing at a concrete boat builder.

If a boat weighs 1,000 pounds it will sink into the water until it has displaced 1,000 pounds of water. Provided that the boat displaces 1,000 pounds of water before the whole thing is submerged, the boat floats.

If the material you’re making the boat from is steel or concrete, you can’t make a block of this material, you need to shape it like a boat, that is, scooped out with air in the middle of it. A good portion of the interior of any boat is air (unlike a cube of steel, which is solid steel throughout). The average density of a boat -- the combination of the concrete and the air -- is very light compared to the average density of water, so very little of the boat actually has to submerge into the water before it has displaced the weight of the boat.

Dutko tells us he used concrete to build his boat because there is more concrete around here than steel or wood. (Concrete comes from rocks, and we’ve no shortage there).

He also told us his biggest challenge was obtaining the workforce, let alone workers who were experienced in building boats. He had to bring men over from the mainland and then train them and find housing for them. He said there was a lot of attrition as they got homesick.

To this day engineers drop by just to check it out and see if they can learn something from it. And the old gentleman is directing the building of one of the largest private yachts in Baja. He’s not crawling around on boats anymore, but his vision is carried out by a naval engineer he works with.