For modern boats there is a choice. Wood, glass reinforced plastic, steel and aluminium are all very good, well tried materials. The first three have all been used for RNLI lifeboat hulls while aluminium is used for superstructures. All have given good and reliable service. Each material, however, has its own particular advantages and disadvantages which, being known, can be taken into account when the choice is being made for any specific purpose. Whichever material is chosen for a lifeboat, her building will be under the constant surveillance of the Institution's own overseers and throughout her life she will be in the care of the Institution's surveyors of lifeboats.

by James Paffett A HUNDRED YEARS AGO, or even 50, there would have been no argument about it. Lifeboats, indeed all boats, were made of wood. The material was cheap and plentiful; everyone was familiar with it, working it demanded only the simplest of hand tools, and the shipwright's art was built upon unbroken experience stretching back to the Bronze Age. A well-built wooden craft is still a joy to look at. Why bother with anything different? When it is gone into closely, there are in fact some very good reasons for looking for alternative materials. Wood is awkward to join; it burns; it rots; it gets eaten by marine worms; it will not hold oil so that fuel must be carried in separate metal tanks; it is variable in properties and is not manufactured to standard specifications. But above all wood is becoming prohibitively expensive, both in first cost (because there are fewer trees) and in fabrication (because of the cost of hand labour). Not surprisingly, we now see on all sides small craft made of steel, aluminium and above all glass reinforced plastic (GRP). If GRP had been invented first, no one would seriously suggest chopping down trees, sawing up the logs and nailing together the pieces to make boats: quite unpractical, the experts would say, look at the fire danger, rot, worm . . . and expense.

Glass reinforced plastic Let us consider the alternatives to timber in more detail. In many ways GRP is the answer to the boatbuilder's prayer.

Weight for weight the material is far stronger than any wood.

It does not need to be joined at all—the hull is laid up as a single piece of continuous material from stem to stern. GRP is self-coloured and non-corrodible, so that it does not need to be painted, except under water to prevent fouling. It presents a very smooth surface to the water flow, and it is not attacked by worm or rot. It can be formed with ease into complex shapes, including in particular 'double curvature surfaces' which are difficult to form in metal. Above all, repeat hulls can be laid up in a standard mould, enabling a prescribed hull shape to be turned out time and again as required: a great advantage in quantity production.

There are, of course, some snags. Although it is possible to produce GRP boats with fairly elementary plant and inexperienced labour, the quality of the final product depends very much on close, continuous and competent supervision and control of the whole process. Unfortunately GRP is not easy to inspect after it has set; there are no obvious signs or convenient tests which will point to faults in chemical mixing or lay-up procedures. The steel shipbuilder has his raw material rigorously inspected and tested for him at the steelworks, but the GRP builder makes his own material as he goes along. A builder who skimps his inspection and production control may turn out a hull which looks sound, but which is permeable and gains in weight by soaking up water, or which blisters on the surface or develops other defects in service. Bad workmanship may, of course, occur in any trade; it just happens to be more difficult to spot in GRP than in metal or timber. All reputable boatyards in the UK are aware of these hazards and have quality control developed to a fine art, with Lloyd's Register of Shipping at hand with advice and inspection services. The pinnacle of GRP technology is reached in the yards building mine-hunting warships for the Ministry of Defence (MOD); rigorous control over all stages of assembly produces hulls which can withstand not only the normal rigours of the sea but also a battering from exploding mines, surely the ulimate test of any hull structural material.

There are other points to note about GRP. It does not catch fire easily, but it does burn. Although very strong, it does not resist abrasion well and craft which take the ground or are hauled up beaches are liable to get scratched and scored on the bottom. If the damage extends through the surface plastic layer and reaches the glass fibre there may be some absorption of water and loss of strength. Fastening items of machinery and heavy equipment to a GRP hull needs special care, and difficulties have been met in keeping powerful diesel engines anchored securely in place on their seatings.

Finally, and most important in the lifeboat service, there is the question of ease of repair. A GRP vessel begins life as a monolith: the material is continuous from end to end, just as a tree is continuous timber from root to twig. But when a GRP hull is damaged, by having a hole knocked into the bottom, say, we need to let in a new piece and join it to the old structure. This can undoubtedly be done, but a satisfactory repair demands very careful preparation of the cut edges to give a long overlap or 'scarph', and the laying up and jointing of the new structure needs quality control every bit as tight as in new construction. No matter how good, the joints can never quite approach the strength of the unbroken material; and there is as yet no really satisfactory way of testing the new joints for strength.

Steel The second alternative material is steel. Steel has for a century been accepted as the natural, indeed the only conceivable, material for big ships. In recent years steel has been working its way down the size scale, so that now we find quite small vessels being built in steel where timber would have been the choice a few years back. Steel has many advantages. It is relatively cheap and widely available.

Steelworking skills are widespread. Steel sheets and bars are manufactured to tight specifications, reliable and consistent in their properties. Steel is fire, worm and rot proof. It does rust, but methods of preventing corrosion are well understood and effective. There is no joining problem, as a weld in steel develops the full strength of the parent material; an all-welded steel structure is effectively monolithic. With modern gas-shielded arc welding plant steel sheet can be reliably welded in quite small thicknesses so that steel can now compete for lightness and strength with GRP—it can more than compete with timber—in the larger kind of boat hull.

Welded joints can be inspected easily for quality; the grosser faults are visible to the eye, while internal flaws and cracks can be spotted with X-ray or ultrasonic probe equipment. A properly built steel hull can withstand a remarkable amount of forceful distortion without leaking, far more than an equivalent GRP or timber hull. Repairing a damaged steel hull is a relatively straightforward business; the damaged structure is cut away bodily with oxy-acetylene torches, the cut edges are cleaned up, the new structure is built and welded into place. On completion the whole is literally as good as new since the welds develop the full strength of the steel.

Aluminium If we can build a structure in steel light enough to compete with timber, should we not be able to build one even lighter still in aluminium? Indeed we can. Aluminium, volume for volume, has only one-third the weight of steel. While pure aluminium is soft and weak, the addition of a small percentage of other metals yields a range of alloys some of which are fully as strong as structural steels, so that if lightness is our sole criterion aluminium will win every time.

One cannot visualise a steel air liner. At sea, however, aluminium has made less progress. While some very successful aluminium alloy marine craft have been built, particularly in the USA, the material has so far met limited success in the UK commercial market. What are the snags? Aluminium costs more than steel, but there is more to it than this. The stronger light alloys are now weldable, and need to be joined together by riveting; aeroplanes are held together by thousands of rivets, leading to structural complications which disappeared from ships a generation ago, much to the relief of naval architects and builders. Riveted structures are awkward to build, and even more awkward to repair after damage. They easily leak in the face of distortion or damage. Weldable aluminium alloys are available, but they are less strong than the aircraft materials, and if we use them to gain the advantage of simplicity and watertightness then we have to use thicker material to maintain strength and so lose some of the weight advantage over steel.

Welding of aluminium demands rather special plant and special skills, and so far only a few boat yards in the UK can tackle aluminium hull construction or repair. Even so, the all-aluminium-alloy small craft is well worth considering, particularly if the operator is prepared to pay for high performance. The light-alloy assault boats built by Fairey Allday for the MOD show what can be done in this medium.

Steel and aluminium The two metals can, of course, be used in combination, and it is not uncommon now to see small vessels with steel hulls and aluminium deckhouses. This combination makes for stability. A fundamental requirement in designing a ship or boat is to get the centre of gravity (CG) of the vessel in the right place. If the CG rises too high above the keel then stability is reduced and capsizing in waves becomes more of a danger.

Designers exploit the weight difference between the two materials to keep the CG down; they use the robust, familiar but heavier steel for the main hull and the lighter but more expensive aluminium for the deck, deckhouses and upperworks.

By these means a given hull can carry a rather larger and more elaborate top structure than would be possible if steel were used throughout, without detriment to stability. If the enlarged deckhouses are fitted with watertight doors, so as to retain their buoyancy even with the craft upside down, it may even be possible to retain positive righting moments at all heel angles up to 180 degrees. This means that the craft will be self-righting and will bring herself back on to an even keel even if she is turned on to her back by the seas. The achievement of self-righting, a very desirable feature in a lifeboat, is greatly eased if we can use aluminium for the higher parts of the boat's structure.

However, mixing steel and aluminium brings its own difficulties. The two metals cannot be welded to one another, so that the junction between them has to be riveted or bolted.

There is also some danger of electrolytic corrosion in the metal around the junction if it is not well designed, painted and maintained. These problems, though, are well understood and there are now many composite craft giving excellent service at sea.

The choice The boat designer who turns away from wood thus has a complicated choice to make. There are three main alternative materials, which can be used alone or in various combinations.

The advantages and disadvantages are finely balanced, and in ordinary commercial production the final choice will probably be dictated by money: by the relative costs of the material itself and of the labour needed to fashion it into a boat and to maintain and repair the craft throughout its life.

In non-commercial services such as defence and lifesaving other aspects, such as resistance to damage and ease of repair after damage, may need to be considered. A material which fully satisfies the needs of the weekend yachtsman may not show up so well in a military assault boat or other craft subjected to harsh usage. And this includes lifeboats.

The RNLI built exclusively in wood for almost the first century and a half of its existence. However, the Institution was well aware of the growth and success of GRP in the recreational boat industry in the years following the war, and in order to obtain first-hand experience of this material the RNLI purchased a lifeboat embodying a Keith Nelson GRP hull in 1968. This boat has been closely monitored in service and the GRP hull has proved entirely satisfactory.

The rising cost of good quality timber, and of the labour necessary to work it to the Institution's standards, provided ample incentive for seeking alternative materials for lifeboat hulls. In the light of experience with the Keith Nelson boat stationed at Calshot, and of information received from many quarters, notably the MOD and Lloyd's Register of Shipping, the Institution took the plunge and began building the Arun class boats in GRP, the first three having been completed in wood. Twenty-three GRP Aruns are now in service and performing excellently.

The latest type of fast lifeboat being brought into service, however, the 47ft Tyne class, has a steel hull and aluminium deckhouses. Does this mean, the Institution has been asked, that the RNLI has tried GRP and found it wanting? Should the ordinary yachtsman think again before ordering a GRP hull? In fact, no serious new snag has been found in the GRP lifeboats. Such drawbacks as exist with GRP, and which are outlined earlier in this article, are all well known and can be coped with by competent builders and operators. The yachtsman has no call to depart from his allegiance to what has become his traditional building material. But it may be of interest to explain the Tyne choice in more detail.

The Tyne was designed in the RNLI's own design office.

All possible hull materials were carefully and impartially considered, and steel was finally chosen because of the ease with which it can be repaired. The Tyne is a slipway boat. She is intended to be launched direct into the sea, sometimes a very rough sea, from coastal slipways. The launching process, and even more the subsequent recovery and hauling back up the slipway, constitute very rough treatment for any boat. No matter how good the crew's seamanship, the occasional bump must be expected, and if repairs are needed it should be possible to complete them quickly without calling on specialised plant or labour which may not be available at slipway sites, some of which are very remote. For speed, efficiency and convenience of repairs there is no doubt that steel has the edge over all competing materials. The main hull, then, of the Tyne is in steel; the superstructure is in aluminium, which helps to make the boat self-righting.

Closely related to ease of repair is ease of alteration: the ability to change a structure from one shape or layout to another. In more leisurely days a lifeboat could serve usefully for 30 years without any structural alterations at all. But now we have fast, high-performance boats, tightly filled with equipment. Technical developments come thick and fast, and we must be prepared to re-fashion each craft several times during her life to keep abreast of changes: to fit new types of engines, perhaps, or steering systems, or to modify hull shape in the light of new theories of seakeeping. Steel lends itself to being cut and re-joined indefinitely; a steel hull can even be cut in half amidships and a new piece let in to lengthen it without too much difficulty, certainly far more easily than in timber or GRP. The Institution, while being very pleased with the present performance of the Tyne, takes some comfort from the fact that her robust steel structure will lend itself to bold changes and extensions if the need arises some time in the future.

What of other fast lifeboats? The Arun, Thames and Waveney classes are berthed afloat, normally in sheltered waters, and so are not exposed to the hazards of the slipway launch and recovery process. The possibility of grounding or other damage in the normal course of rescue operations still remains, however, so that steel still has some appeal on the grounds of ease of repair. The Waveney and Thames classes are already in steel; the Aruns, having begun life as a wooden design, are now being built in GRP. It can be argued that it would make for uniformity of practice and equipment if all these lifeboats, which after all do not differ so very much in size or function, were built of a common material. And in fact a study has shown that an Arun could be built in steel with no loss in performance compared with the two materials already tried.

If steel is adopted as a standard main hull material in all the large boats, however, it by no means follows that the RNLI is abandoning GRP, which continues to give excellent service in the 33ft Brede lifeboat and smaller types. There are horses for courses, and materials for boats; choice depends on service conditions, now and in the future. Making the right choice is a complex and technical business; it is not easy, but the RNLI chooses with great care and is convinced it has the best answers for 1983..