Sink or Swim

September 7, 2024

SINK OR SWIM?

If you want to see a deep-water shipwreck, you are going to need some sort of submersible.

Submersibles are slightly different from submarines because they are not fully autonomous (they cannot function alone). They usually have an umbilical connecting them to a support vehicle, which provides power and gases both for depth control and life support. It is named after the umbilical cord, which connects a baby to its mother in the womb.

A submarine (or any boat) floats because it is designed to displace (take the place of) the weight of water equal to the weight of the craft. This is called the buoyant force and it works in the opposite direction to gravity. We control the buoyant force to move submersibles up and down in the water.

In a real submarine there are ballast tanks at the bottom of the hull (the watertight body of the boat). They help to stop the craft rolling in the water and have a key role in controlling buoyancy. If they are filled with air, the weight of the submarine is relatively low and it has high buoyancy. If the air is replaced by water, the weight of the submarine is relatively high and it has low buoyancy, so gravity pulls the submarine downwards.

How does a submersible get back to the surface once its tanks are full of water?

Compressed air is vented in to the tanks where it expands and pushes out the water. This increases buoyancy, so the craft rises. To keep the submarine at a stable depth during operation, it must have neutral buoyancy. It is possible to balance the buoyant and gravitational force by having a mix of air and water in the tanks.

This is a greatly simplified version of how the depth of a submersible is controlled. However, it is all you need to know to conduct this brilliant experiment at home:

YOU WILL NEED:

– Empty 500 ml plastic bottle (lid and label removed)

– Sharp scissors

– 24 small coins (e.g. pennies)

– Aluminium foil

– 3 elastic bands

– Duct tape (or modelling clay)

– 2 bendy straws

– Bath/deep sink for testing

Build your own submersible

STEP 1:

Using a pair of sharp scissors, cut three small holes down one side of the bottle. Put one in the middle and the others about 5 cm from each end. The holes do not need to be circular and you can make them bigger later if you want to.

This side will become the bottom of the craft. This way it will fill with water from the bottom upwards.

STEP 2:

Cut three pieces of aluminium foil of about 5 x 10 cm each. Make three stacks of four pennies and wrap each one in foil. This will be your ballast. It will be attached to the bottom of the submersible so that the craft does not roll in the water.

STEP 3:

Place the short end of a bendy straw inside the neck of the bottle and tape it to the upper edge (see picture). Then use more tape to seal over the neck of the bottle. It does not need to be watertight. If you do not have duct-tape, use modelling clay to hold the straw in place and seal the end.

Now gently squash the long end of the second straw and squeeze it into the open end of the first straw. Tape the join firmly together. If your test pool is very deep, you might need to add more straws. We will be using this umbilical to control buoyancy.

STEP 4:

Next add your ballast and secure with elastic bands. Do not cover the holes! This ballast will help to keep the submersible stable in the water. If you don’t have any elastic bands, you can tape the ballast on but this will make it more difficult to adjust later.

The moment of truth!

TEST

STEP 5:

When you first place it on water it floats because it has high buoyancy. It will slowly fill with water causing the buoyancy to decrease. At this point gravity becomes the main force, so it sinks. To make your submersible rise again, you need to increase buoyancy by blowing air into the straw.

EXPLORE

Sea trials

STEP 6:

What happens if you remove all the coins? Why?

What happens if you change the number of coins in each position? Replace 4:4:4 with 4:8:12 and test it. Then predict what will happen if you reverse the arrangement. Test to see if you were right.

Can you achieve neutral buoyancy to keep it at a constant depth? You will need a mix of air and water to achieve this.

EXPERIMENT

Going deeper!

STEP 7:

Use a stopwatch to time how long your unmodified craft takes to sink. Then adjust one factor at a time (like ballast size/position or hole size). Why are the times different? Don’t forget to do the hole-size test last!