How do lifeboats self-right?

Monday 8 November 2021

In wild winds and stormy seas, volunteer crews depend on their lifeboats to keep them safe while out on a rescue – even if it turns upside down. Many lifeboats in the RNLI’s fleet can bring themselves the right way up if the powerful elements cause them to capsize. Let’s find out how.

An engineer’s blueprint drawing of an all-weather Shannon class lifeboat with self-righting design.

What do we mean by self-right?

RNLI engineers and boat designers have built many classes in our lifeboat fleet to be ‘self-righting’. In other words, if a mighty wave throws a lifeboat sideways, or a full 180 degrees upside down, the lifeboat can bring itself the right way up again – and the crew can carry on with their mission to bring someone back to safety.

Every lifeboat in the RNLI’s all-weather fleet has the capability to self-right: the Shannon class, the Tamar class, the Severn class, the Trent class, and the Mersey class.

The inshore B class lifeboat also has similar self-righting abilities – it’s designed with an airbag onboard that the crew can inflate if the boat is turned upside down, and this helps bring the lifeboat back over again.

How can a lifeboat right itself?

‘There has to be a balance between weight and buoyancy for a lifeboat to be the right way up,’ explains Iain Wallbridge, RNLI Senior Engineer. 'The balance is different for every lifeboat and always needs to be correct, so that volunteer crews can continue with a rescue no matter how bad sea conditions are.’

‘We use software to simulate a lifeboat turning upside down to prove, using calculations, that the balance is always right on our lifeboats. This ensures that, no matter how far the boat has been tipped, it's able to return to an upright position.'

What self-righting looks like

An illustration of a self-righting lifeboat, showing the weight and centre of gravity

The weight (W) is the force of the boat pushing the water down through the centre of gravity (G). The buoyancy (B) is the force of the water pushing up through the centre – and this doesn’t change, even when moving in waves. A boat floats upright when these forces are balanced.

An illustration of a self-righting lifeboat, showing a boat tilting over

As the boat begins to capsize, the underwater shape changes and B shifts. The horizontal gap (H) between the G and B changes. This creates a force trying to push the boat back upright.

An illustration of a self-righting lifeboat, showing the effect of wind and waves on the boat

When the power of the wind and waves begin to push the boat over horizontally, B moves even further as the underwater shape changes.

An illustration of a self-righting lifeboat, showing the boat tilted on its side

Because it’s sealed shut, the wheelhouse adds to buoyancy of the lifeboat as it is submerged under water. The engines start to shut down to protect them from damage.

An illustration of a self-righting lifeboat, showing the lifeboat capsized

When the lifeboat is nearly upside down, the wheelhouse buoyancy continues to help it float. The boat is now ‘unstable’ and flips back upright. All the while, the crew are strapped into their shock-absorbent seats to keep them safe.

A world-class design

When an RNLI all-weather lifeboat is experiencing a capsize, there are certain systems and features onboard that detect what’s happening. They carry out some key actions to protect the lifeboat – and its crew – to keep a rescue mission running:

The engines become idle until the lifeboat is the right way up again.
The radar (positioned outside on top of the wheelhouse) stops spinning.
The lifeboat’s shock-absorbent seats come into their own by keeping the crew safe and protected from sharp and sudden movements.

Volunteer Crew Member and RNLI Senior Naval Architect Ed Davies explains these features further in the video below.

RNLI lifeboats are designed and built in-house – and powered by the generosity of supporters like you. This Christmas, volunteer crews up and down the coast will rely on these dependable workhorses to face all sorts of weather conditions and rescue the next person in need. Your kindness today can help ensure they’re able to launch every time, and save every one who needs them.