HULL REPAIRS

Even with the best planning, skill and care, boats get damaged at sea. From woodpeckers to heated blankets, discover the latest techniques used to assess and repair allweather lifeboat hulls – and how to keep your own boat safe to sail

Put To The Test

We ask a lot of our lifeboats. Crews operate in the roughest conditions to save lives, coming alongside other craft and going into shallower waters to reach casualties. They take great care of the lifeboat entrusted to them, but knocks and scrapes happen.

Our newest all-weather lifeboat hulls, including the Shannon class, have a durable sandwich construction. A thick foam core is set between two thin composite ‘skins’ – a woven fabric layered with epoxy resin. (Most leisure boats have a thicker, single-skinned polyester composite hull, although advanced designs and racing yachts may have a foam core.)

‘It’s more weight-efficient and durable to have a foam core, rather than a single solid skin,’ explains RNLI Senior Naval Architect Pete Sheppard. ‘That strength also limits deflection and bending of the hull, by the considerable forces lifeboats undergo.’

Lifeboats’ spray rails, tow rails and bilge keels are also designed to detach on impact, without damaging the main structure. ‘A bit like a lizard escaping by losing its tail, except our lifeboats can’t grow them back … yet!’ says Pete.

Until we invent self-regenerating lifeboats, repairs will be needed. As the lifespan of lifeboats is extending and more advanced materials are used in their construction, we’re finding that older repairs are needing rework – and a new technique may hold the answer.

MORE THAN SKIN-DEEP

The sandwich construction is incredibly durable, but after an impact, scratches on the surface can run a lot deeper, and true damage is harder to spot than on a single-skinned boat.

‘When you hit any composite – a bit like with your car bumper – they often bend in and then spring back,’ says Michael Colegate, RNLI Safety and

Compliance Engineer. ‘Damage can be barely visible, but the fibres break out on the inside, splitting and separating so you’ve no longer got one strong piece of material. That’s when big cracks can form underneath.’

The risk with all composites, Michael adds, is that damage and fatigue aren’t always obvious: ‘They look perfect but, when that underlying weakness is put under stress, they just snap.’

This hit home in 2014, with the tragic loss of the crew on Cheeki Rafiki. Investigations revealed that a series of previous groundings could have contributed to the yacht’s keel shearing off in the Atlantic.

Since the investigation, the use of non-destructive testing (NDT) has become more common in boatyards, as insurers check that dangerous hidden damage or incomplete repairs don’t remain after a boat grounds.

WHAT’S THE DAMAGE?

Weak areas in any composite can be tricky to spot without causing more damage to the boat itself.

NDTs developed for aviation composites are now being used to assess lifeboat hulls before repairs and refits. We use x-rays to check the deck is still well bonded to the hull, while ultrasound can give good indications of damage within thick laminate.

Thermography, first used on aeroplane wings, applies heat to an area and then maps how it disperses through the laminate to highlight weak or missing layers. In shearography, a laser scans the surface as a vacuum is applied to it. The laser picks up any deflection of the surface, indicating weakness in the structure.

One technique predates the age of flight, but still holds water today. ‘You may see your surveyor tapping your boat with a little hammer or a coin. A dull, hollow thud indicates the extent of any damage,’ says Pete. ‘It’s like detecting a rotten bit of wood in your windowsill by tapping it, however good the paint on top looks.’

Now the RNLI uses an automated tapper or ‘woodpecker’, which creates a digital picture of the damage from the resonance of each tap. Surveyors can check that structural elements are still attached to the hull, without squeezing into small spaces inside the boat.

PREPREG OR WET LAYUP?

Once damage has been assessed, repairs are specified. Traditionally, composites are repaired with a ‘wet layup’ technique, where a mat of fibres is wetted with resin manually, then put onto the side of the boat to form the repair. It can be messy, heavier due to excess resin, more wasteful, and a rush