Decide
What does Archimedes’ principle say about a floating boat?
The lesson in plain words
- Weight down
- Everything aboard has weight: hull, engine, keel, water, fuel, crew.
- All that weight acts as if it pulled down through one point, the centre of gravity, called G.
- When you move, add or remove weight, G moves too.
- Weight low down keeps G low.
- Buoyancy up
- The water pushes up on the underwater hull. This push is buoyancy.
- Archimedes’ principle says the push equals the weight of the water the hull pushes aside.
- The boat sinks until that water weighs exactly as much as the boat.
- The boat’s weight is called its displacement.
- The centre of buoyancy
- Buoyancy acts as if it pushed up through one point, the centre of buoyancy, called B.
- B is the centre of the underwater shape of the hull.
- When the shape of the underwater hull changes, B moves.
- Upright and at rest
- A boat at rest floats with G and B on the same vertical line.
- Weight pulls down, buoyancy pushes up, and they cancel.
- If you add weight, the boat sinks a little until buoyancy matches again.
- That is why a loaded boat sits lower and has less freeboard, the height of the deck edge above the water.
- Salt and fresh water
- Salt water is denser than fresh water.
- A boat needs to push aside less salt water to float.
- So she floats a little deeper when she motors from the sea into a river or lake, roughly 2–3 % more volume under water.
Key data
- Archimedes
- Buoyancy = weight of water displaced
- Floating
- Displacement (boat’s weight) = buoyancy
- Sea water
- About 1.025 t per m³
- Fresh water
- About 1.000 t per m³: the boat floats slightly deeper
Key points
- G is where all the weight acts; it moves when weight moves.
- B is the centre of the underwater shape; buoyancy acts up through it.
- Afloat at rest, the boat’s weight equals the weight of water displaced.
- Upright and still, G and B lie on the same vertical line.
- Extra load means a deeper boat and less freeboard.
Check what you have read
What is the centre of buoyancy, B?
True or false?
Decide each statement before you see the answer.
G and B are fixed points marked by the builder.
B is the centre of the underwater shape; buoyancy acts up through it.
Extra load means a deeper boat and less freeboard.
A heavy boat floats because it is made of light materials.
Practice case & exam keys
Situation
After loading friends and stores, your yacht’s waterline stripe is submerged and freeboard is much lower. A friend suggests moving everything onto the coachroof. Is that a good fix?
Show the answer
No. The extra weight requires more displaced water, so the boat floats deeper. Moving stores onto the coachroof, the cabin roof, does not remove weight; it raises G. B is the centre of the new underwater shape. Keep heavy stores low and central, check the load limit, and remove excess load.
Exam keys
- Archimedes: buoyancy equals the weight of water displaced.
- G = centre of gravity (weight acts down). B = centre of buoyancy (buoyancy acts up).
- At rest upright: weight = buoyancy, and G and B are on the same vertical.
- Fresh water is less dense than sea water: the boat floats deeper.