"he days when wood was the automatic choice of material from which to build a lifeboat have long gone - today aluminium, steel and a new generation of composites are providing the solutions to technical problems. James Paffett explains ...Six years ago an account of the materials used in building lifeboat hulls ('What are lifeboats made of?' - THE LIFEBOAT, Autumn 1983) set out the relative merits of timber, steel and glass reinforced plastic (GRP), and remarked on the possibility of using aluminium.

Since that time development has been rapid. The Institution has gained considerable experience with GRP in the Arun and Brede classes, and the Mersey class has come into service - the first boats of this class having aluminium alloy hulls. There is also an experimental version of the Mersey with a fibre reinforced composite (FRC) hull under evaluation, so it can be seen that the Institution now has boats built in five different materials - wood, steel, aluminium, GRP and FRC. Is there, one might ask, some method behind all this diversity?There is indeed some method, and we shall try to bring the 1983 story up-to-date and to explain some of the thinking behind the Institution's choice of materials.

An important factor underlying RNLI lifeboat development in recent years has been the Institution's decision that all future lifeboats shall be 'fast' -that is, capable of 18 knots or more in calm water. The need for speed, combined with all the other RNLI requirements - selfrighting, range, survivor capacity and so on - has placed a severe burden upon the boat designers, and to get the necessary speed and performance they have been obliged to make great efforts to save weight in the hull structure while maintaining the strength and robustness essential in a lifeboat.

The strength requirement has virtually ruled out timber, and wood will be phased out of the RNLI fleet with the last of the eight knot lifeboats.

Steel hulls have proved entirely satisfactory in the Waveney, Thames and Tyne classes - combined with aluminium in the deckhouses and, in some boats, with aluminium decks. There is also a single steel Arun class in service.

Apart from the first three boats, which are wood, the rest of the Arun class hulls are of GRP. The GRP hulls have stood up well in service, though in some boats there has been superficial blistering in the underwater hulls. Remedial measures for this, and one boat whichsuffered from weight growth due to water penetrating into foam-filled void spaces, have proved effective.

The Brede class, with a commercially designed hull, caused some concern when material samples removed for survey showed that some of the GRP in the hulls fell short of Lloyd's stringent standards for strength and stiffness. This represented no great hazard since the hulls had been built with reserves of strength ample for commercial service. However, to be on the safe side, the boats affected are being carefully reinforced with extra layers of high-performance FRC on the hull to ensure that they retain the capacity to take punishment so necessary in a lifeboat.

This experience with GRP has reinforced one of the points made in the 1983 article - the supreme importance of inspection and quality control during the building operations.

Quality control The quality and strength of steel is ensured by careful analysis and testing at the steelmaker's works, but the boatbuilder makes his own GRP laminate as he goes along, and quite minor errors in mixing and application can result in structure which is weaker or heavier or less watertight than it ought to be. These are faults which are difficult to detect after completion short of cutting out samples for laboratory tests. This procedure is tedious and expensive, and as it leaves the hull with sampled areas it is understandably unpopular.

The glass reinforcement in GRP can take the form of 'woven rovings' (WR) or 'chopped strand mat' (CSM), which is a mass of randomly-oriented short fibres. CSM can be laid up by hand or spray and the lay-up is difficult to inspect effectively, particularly with spray application.

Weight is more easily controlled where the fibre takes the form of WR, a woven cloth of known weight per square metre. The recent experience with the GRP boats by no means rules out the use of GRP in the future, but it does mean that CSM is unlikely to be used in important work.

The development of the new fast carriage-launched boat, now known as the Mersey class, has brought new materials to the RNLI.

The weight of a carriage boat needs to be held down to something like 12 tonnes to enable it to be handled across the beach, and this,combined with the need for speed, imposes a severe limitation on the hull weight.

Early on in the design stage it was found that this ruled out the use of steel, so that it was not possible to adapt a scaled-down version of the Tyne for use across the beach. GRP was considered, but its poor resistance to impact and to abrasion by sand and shingle argued against its use in a boat which was certain to get thoroughly abraded in service. This left aluminium as the only suitable hull material.

A prototype boat was built in welded aluminium alloy - not an operational lifeboat, but a kind of sea-going full scale model for use as a guinea pig. Several modifications were tried out in this vessel, including changes in bow and stern shape. The welded construction of the hull, combined with the relative absence of internal fittings, made such changes easy to carry out.

When the speed, performance and layout satisfied the RNLI operational staff the design was finalised and the building programme proper commenced.

The first operational aluminium alloy RNLI lifeboat, ON 1125 Sealink Endeavour, went on service at Hastings in February 1989, and there are now eight other aluminium craft in various stages of construction.

When afloat an aluminium lifeboat looks much like any other lifeboat to the casual observer, showing the usual colour scheme out of the water. However, the lower part of the hull shows up as bright bare metal - with no paint at all.

As a carriage boat spends relatively short periods afloat there is no need for any anti-fouling composition to prevent weed growth, and aluminium protects itself when both in and out of the water with a thin skin of self-generated oxide corrosion. The upperworks are painted in the normal RNLI style for recognition purposes.

Fibre reinforced composite During the time taken to design and commission the aluminium Mersey class there have been important developments in boatbuilding techniques, in particular the emergence o'f FRC - fibre reinforced composite.

At first sight this looks much like GRP, but the fibre is not necessarily glass, and the plastic is not necessarily the polyester we are familiar with in conventional GRP.

The new fibres are Kevlar (the trade name for a synthetic aramid fibre of great strength manufactured by Dupont), carbon, boron, and high-performance glasses referred to as R-glass and S-glass. Hulls of spectacular strength and lightness have been built for racing sail and power craft by using combinations of these fibres in sophisticated ways, usually in combination with epoxy or modified polyester resins.

FRC technology has been proved in the racing and defence fields to such an extent that the Institution was led to investigate the possibil-ity of building Mersey class hulls in FRC, as an alternative to aluminium.

With advice from SPS of Cowes, specialists in the use of FRC, a prototype boat was designed within the RNLI and built by Greens Marine of Lymington. This vessel has behaved very well in preliminary sea trials, and when fully fitted out will still be slightly lighter than the aluminium boats now in service.

One of the snags with GRP hulls based on polyester resigns is their indifferent resistance to abrasion; scuffing on sand and shingle can grind away the surface layer of plastic to expose the raw glass fibres and allow moisture to enter. This was one of the reasons for choosing aluminium in preference to GRP for the new carriage boat.

The new FRC materials based on epoxy resins were claimed to be superior to GRP in their resistance to abrasion, so to put this to the test the prototype FRC Mersey, ON 1148, was dragged along the sand and shingle beach at Dungeness without skids to see how the material would respond.

After a to w of more than one mile the bottom was found to be hardly affected, the fibre reinforcement being marginally exposed at one point only. This brutal treatment, representing a full life-time's beach exposure for a carriage boat, would have been utterly destructive if applied to a timber hull, and would have severely damaged the surface protective coatings of a metal hull.

Construction in FRC is attractive to the Institution for reasons other than strength. Fast boats are needed urgently to replace the ageing fleet of eight-knot carriage lifeboats, but the construction of aluminium boats is limited by the number of boatyards willing and able to build in this material to RNLI standards. If a parallel programme could be set in hand to build in the new material the overall rate of production could be speeded up.

Production line building One possibility being studied is the setting up of an organisation in which a central builder undertakes the production of a considerable number of boats, subcontracting parts of the work to other builders - deckhouses to one, bulkheads to another and so on - for final assembly in the master yard. Such an arrangement would represent a radical innovation in the Institution's building methods.

A necessary concomitant of building in FRC on this scale will be a tight system of quality control. As already indicated, the strength and weight of a GRP structure depends closely upon the supervision and control exercised during the laying-up process, and this is true to an enhanced degree with the newer materials.

To exercise adequate quality control in the building of FRC hulls the Institution may have to call upon the aid of specialist concerns with the requisite expertise in polymer chemistry.

The building process used for the prototype boat, and contemplated for the follow-on FRC Merseys, involved a sandwich-type structure.

The main hull consists of inner and outer skins of Kevlar- and glass-reinforced epoxy, separated by a layer of PVC foam.

This produces a very strong, stiff structure, which does not need support from intermediate frames between bulkheads. The smooth, unobstructed interior greatly simplifies fitting- out, and the built-in buoyancy of the foam ensures that the boat will not sink even when every compartment is open to the sea.

The foam serves two roles, as a structural member and as the equivalent of the old'barricoes', or buoyancy boxes, used in the early lifeboats.

In building an FRC hull the Kevlar and Rglass are used in the form of woven cloth, the warp and woof being oriented in a controlled manner to handle the stresses predicted by the designers. No chopped strand mat is used.

The outer skin is first laid up in a female mould and the cloth and resin consolidated by using vacuum bags - large bags which surround the entire laminate and from which the air is partially evacuated so that the bag is sucked tight against the material. Heat is applied by electric elements tocure, or harden, the resin.

When the outer skin is complete the foam core is fitted, followed by the inner skin of fibre and resin. This generally resembles the outer skin, but the resin is one which cures at room temperature to avoid exposing the foam core to heat.

The process is one of some cornplexity, and building in FRC demands standards of cleanliness and temperature control far removed from the sometimes draughty and dusty conditions of the average boatyard.

This argues for concentration of the manufacturing process into a small number of specialist establishments, with the rate of building kept up by a rapid throughput of hulls.

The Institution has in the past tried to spread the lifeboat building programme over a number of yards around the coast, to maintain competition and to keep the industry familiar with RNLI methods and standards, and will only abandon this for the 'production line' approach after the closest consideration.

Once the boats are in service and become due for survey it should be possible to place the repair and maintenance work in the usual way.

The Future What of the future? Aluminium is now well established, and we shall have a number of aluminium Merseys. Whether we are to have FRC Merseys as well will depend upon investigations now in progress, but the signs are promising. Meanwhile, studies have begun in the RNLI design office into a new Fast Afloat Boat to succeed the Arun class in a few years' time.

If FRC fulfils its early promise the new FAB may well be built in FRC. If not, we have aluminium to fal I back on, or - at some cost in speed - steel.

But of one thing we can be sure, the hull will not be of wood..