Stability

Keeping a ship upright

A large steel ship is designed to be heavier at the bottom than the top, often the lower part is made of heavy metal, sometimes with ballast (extra weight added at the bottom) while the upper levels is made of light weight material such as aluminum. Engines and any other heavy items are kept as low as possible.

The weight, or force of gravity is pushing down through the center line of the ship, and the buoyancy is equal at the same point. When the ship tilts a little to the side by some means, the buoyancy is in the new center of contact with the sea, and no longer directly under the center of gravity, it therefore pushes it back up straight. If the center of gravity is too high or can be made by some other reason to move outside of the point the buoyancy force is pressing up, the ship will capsize.

The side to side rocking motion of a ship is caused by the rise and fall of waves, when the water is higher on one side the ships center of contact with the water will be on that side causing the ship to tilt over the opposite way to stay level in relation to the wave. When waves run across the path of a ship this sets up a pendulum effect, when you go directly into waves this motion does not occur but the ship may go up and down in relation to the front and back.

When a ship moves through water it in effect creates mountains in front and a valley behind, so its nose will rise and back will be lower. The faster you go, and therefore the more water you move in relation to the size of the craft, the more marked this becomes. With small speed boats going very fast they can even get to the point that they will do a backwards somersault. Fast flowing water past a stationary craft can have a smaller but similar effect.

Stabilizers

Stabilizers are like small wings that stick out below the waterline. They reduce the side to side motion. They are most effective then the ship is moving forward though the water, large gyros, spinning weights, also will reduce or eliminate the movement.

It is also possible to use the movement of water or another liquid to level up a container within a ship, or to control servos that through hydraulics or other means achieve a stable platform.

Yachts

Sailing yachts often have a center board, a long heavy plate that can be dropped down the center of the boat. This has two effects acting like a stabilizer but mostly by lowering the center of gravity. Sailors hanging off the side of a yacht while racing is also done to move the center of gravity using the weight of the sailors.

Why ships are long and thin

A ship could be constructed far more solidly and would be far more stable if it was wider, but the effort (cost) to make it travel through the water would be greater, it would also be more difficult to get into and out of ports.

Why narrow canoes are less stable than wide canoes

When you sit in a canoe, or rowing boat that is wide you are far more stable than you are with a very narrow one. The reason for this is that the center of the point of contact with the water is further out on a wide canoe, and therefore the center of gravity has to go a lot further over before it will tip over.

If you imagine a line straight up through the center of a ship, or canoe and person, and you imagine a second line up from the center of contact with the water, you get to a point where the two lines cross, this is called the metacentre, the wider the ship or canoe the higher up this will be.

If you were to determine the center of gravity, on the line on the center of the ship, if it is below the metacentre you have a stable craft and if above an unstable one.

Another way to look at it with a larger structure is to look at how much water is replaced on one side (increasing buoyancy) as compared to the other and if it is sufficient for the weight that is over it.

Relative size

When a person steps into a rowing boat it moves quite a lot, when you step onto a cruise ship you have no effect at all. Likewise people moving onto, off or across an island will have no effect on its stability.

Perceived stability

The nearer you are to the water level and center of the ship, the lower the amount of space that you are moved through. if you climbed a mast of a ship or go to the outside of a very tall ship, any movement appears magnified. People unused to boats and those who think the ship could sink easily will always be inclined to tense up and feel the movement more. After a long time on a ship with constant movement, standing on solid land can seem peculiar.

Why oil tankers have slowly got larger, to the point now, where some are very large

More oil can be moved with very little extra effort and only the same crew.

Limitations with ships

Throughout history for centuries the size of the ship you could build was limited by the size of the largest tree you could find, as the keel of the ship had to be built out of a single piece. There was no way, that was able to be found, to produce a strong enough joint, and ships built with joints would break apart in heavy seas.

When a ship is in large waves, the entire ship at times will be supported by one or two points with the rest of the ship bridging the wave. The structure of the ship has to therefore be strong enough to withstand both the weights and the springing actions created. Wood being strong and flexible was the earlier material of choice.

With a large oil tanker or other large ship today, we have to construct it so that it can both withstand the stresses caused by bridging several waves and the metal fatigue effect by constantly being flexed. It has to be remembered that the stresses are caused by the combined weight of the ship and its load.

Ballast and trim

In olden days with sailing ships goods would go one way and when there was no load or the load was light, ballast such as stone or other heavy items were carried, to keep the center of gravity low and to get the ship to sit at the right level in the water. A ship sitting lower in the water has a lower center of gravity it will be more stable. Today many ships are ballasted by having water tanks in the sides, and trimmed front to rear, by having tanks at front and back.

Car ferries capsizing, and moving loads

If a small amount of water gets into the hold of a car ferry or other single compartment ship it will roll over. It is not the weight of the water that causes this, but the ability of the water to move. Tipping to one side, the water runs down to form a level counteracting the action that would normally right the ship. This could be eliminated by dividing the hold into a number of sections vertically down the length and across the width, however this would reduce the flexibility, and it would reduce the number of vehicles that can be carried. With several sections, water on each side of the baffle, or wall would be held back and be lifted, thereby creating a force that will right the ship.

Applying all of this to stabilizing an island

An island can be built stronger if it is wider, as well as long, and has to be designed so the center of gravity is lower than the metacenter, and therefore is stable. As weight increases without any increase in area, the island will sit lower in the water, and providing the weight is kept low or towards the center of the island, then as it is loaded it will become more solid and stable but may need weight added or water tanks to trim the levels.

The stresses on large structures are greater so the island structure has to be formed so the box structures become a single reinforced structure throughout.

The compartments have to be sub divided and form sealed tanks, that are pressure tested, so that in the case of damage, it will not sink or capsize. As long as sufficient compartments are not flooded so as to replace the volume of water equal to the total structure and contents, the island will stay afloat even if damaged. The safety margin is the amount of reserve buoyancy (sealed compartments above the water level).

Even in the worst situation possible and the island was broken in two, the island halves would float, and could be repaired. If an island unit became some how separated from the kingdom and was loose in the ocean, it would stay afloat until it hit land at some point, and would be most likely to survive any impact. We do not believe it would be possible to steer, but could be towed.

If units are produced elsewhere and towed to the kingdom, the differences in floating height will need to be considered between the site of fitting out and the kingdom site. This also provides the opportunity by the design of the hull to make the island shell easier to tow and stable when in place.