Lifeboats are emergency craft carried by ships and some offshore vessels so people can leave safely when staying aboard becomes too dangerous. Davits are the lifting arms, winches, wires, and release gear that hold a lifeboat at the deck edge and lower it to the water. These systems matter because evacuation must work in rough seas, poor visibility, listing conditions, and high stress.
A good launch system controls motion, prevents falls, and keeps people clear of the ship’s side.
Key Facts
- Weight of a loaded lifeboat: W = mg, where m is total mass and g = 9.8 m/s².
- For a steady lowering speed, cable tension is approximately T = W if air resistance and friction are small.
- For accelerated lowering, net force follows ΣF = ma, so W - T = ma downward.
- Gravitational potential energy before lowering is E = mgh.
- Average free-fall lifeboat impact speed from height h is approximately v = √(2gh), ignoring drag and ramp friction.
- Davits use mechanical advantage, brakes, and release hooks to control a heavy lifeboat during launch and recovery.
Vocabulary
- Lifeboat
- A lifeboat is a small survival craft carried on a ship to evacuate people during an emergency.
- Davit
- A davit is a crane-like support system that holds and lowers a lifeboat from a ship into the water.
- Winch
- A winch is a drum and gear mechanism that winds or unwinds cable to raise or lower a load.
- Release hook
- A release hook is a locking device that disconnects the lifeboat from its lifting wires at the correct time.
- Free-fall lifeboat
- A free-fall lifeboat is an enclosed survival craft that slides down a ramp and enters the water using gravity.
Common Mistakes to Avoid
- Assuming the davit simply drops the lifeboat is wrong because davits use brakes and winches to control the lowering speed.
- Ignoring the mass of passengers is wrong because the total load changes the weight, cable tension, braking force, and launch energy.
- Releasing the hooks before the lifeboat is waterborne is wrong because the boat can fall suddenly or swing into the ship’s side.
- Thinking free-fall lifeboats are uncontrolled drops is wrong because their ramps, hull shape, seats, and restraints are designed to guide motion and protect occupants.
Practice Questions
- 1 A lifeboat with passengers has a total mass of 8500 kg. What is its weight in newtons using g = 9.8 m/s²?
- 2 A davit lowers a 6200 kg lifeboat at a constant speed from a height of 18 m. What is the approximate cable tension, and how much gravitational potential energy is lost?
- 3 A ship is listing to one side during an evacuation. Explain why davit angle, release timing, and keeping the lifeboat clear of the hull become more important than in calm upright conditions.