Can used tires be used as ballast in sailing ships?

Answer 1

Wind tries to push the sailing ship over. Ballast, as you know, tries to keep the sailing ship upright. Here's how:

Floating boats have buoyancy, water pushing the ship up, gravity pushing it back down. If buoyancy is greater than the pull of gravity, the ship floats. But, because the ship has weight, it sinks a little into the water, making some of the water move away. The sailing ship is said to displace that water. By the way, the amount of water that is displaced (moved away) will weigh the exact same as the weight of the sailing ship! So, people will refer to the ship's displacement rather than how much it weighs.

Because of the upward push of the water is greater than the downward pull of gravity, we have what is called the centre of bouyancy. Usually, it is quite high, inside the hull of the sailing ship. But gravity's pull also has a centre, too. That is usually lower inside the hull of the sailing ship. In fact, the lower that centre of gravity is OR the higher the centre of buoyancy is, the better. The distance between the two centres is called the sailing ship's metacentric height. The larger the metacentric height, the more a sailing ship will "want" to stay right-side-up.

Rubber (which tires are made of) does not weight very much compared to lead (the metal.) 100 pounds of tires will weigh the exact same as 100 pounds of lead. The thing is, however, 100 pounds of tire (tyre) will probably take up as much space for about two tires. One hundred pounds of lead, on the other hand, will probably fit in your hands... yes, still weighing 100 pounds!

So, if you only have 3 or 4 cubic feet to put ballast in, you want to get as much weight in that volume as you can. Two tires can fit into 4 cubic feet which, as I said, is about 100 pounds (in a small sailing ship, eh?) However, in 4 cubic feet of lead, that should be in the thousands of pounds! That will, of course, bring the centre of gravity way down near the bottom of the sailing ship. This will be much further away from the centre of buoyance and, therefore, give a much bigger metacentric height (the two centres will be further apart, vertically.) The larger the metacentric height, the more stronger the sailing ship will be in remaining upright.

Yes, you can use tires (tyres) as ballast, but lead would be very much better. (Lead is also more expensive than rubber. Ship builders have been known to use steel plates, steel nails, or even, sometimes, concrete! Both are much more efficient at getting the centre of gravity down low compared to rubber. But the best is lead.