The announcement of two new classes of lifeboat, the Trent and Severn on 10 December 1992, made the year something of a landmark for the RNLI. The introduction of even a single lifeboat class, let alone two, is an achievement far greater than meets the eye, for these are boats which combine the traditional attributes of lifeboats with performance obtained from technology close to the leading edge of design and construction. Such state-of-the-art lifeboats cannot be bought off-the-shelf, and the specification, in-house design, construction and trials which result in a new lifeboat class takes several years of intense effort by the RNLI's Technical and Operations departments.Fast Response The Trent and Severn classes arise from a proven and demonstrated need which can be traced back to the early 1960s, when the first high speed inflatable inshore lifeboats showed the value of rapid response.

The first fast all-weather lifeboat, the 44ft Waveney, followed in the mid-1960s, taking speeds up to 15 knots from the 8 or 9 knots of traditional lifeboats. The Arun (in 1971) and the Thames (in 1973) joined the fleet as larger fast lifeboats for stations where the lifeboat lay afloat, and the Tyne (in 1982) provided a fast lifeboat for slipway-launch stations. By 1983 the benefits of fast lifeboats was so proven that the Institution set itself the target of replacing all traditional lifeboats with new, fast designs by the end of 1993.

The last piece in that jigsaw was a carriage-launch fast lifeboat, and after considerable development the Mersey filled that need in 1988. By the end of 1993 the target will have been met and all 210 stations will have a boat capable of at least 15 knots, with the majority in the 17-18 knot region.

Moving Forward However 'afloat' lifeboats are now normally replaced after 20 years service, so the early Waveneys and Aruns are due for replacement in the early '90s. Although both classes have been highly successful more than two decades of progress led the RNLI to believe that an even better all-round design of afloat lifeboat could be produced to replace them when the time came.

As a result the machinery was set in motion to have two new classes ready for the early '90s. By 1989 the main requirements had been formalised - calling for a service speed of 25 knotf propeller protection and the ability to take the ground without damage, which neither existing class offered. Ease of maintenance, a range of 250 miles (plus 10% reserve), self-righting ability and numerous detail requirements were part of a brief which included safe operation in winds of 60 knots and seas up to 15m high. To complicate matters further there were length and beam constraints to maximise the use of existing mooring facilities.

Preliminary work included uprating the engines of an Arun, giving a maximum speed of around 23 knots. Although it was known that the increased fuel consumption and reduced range would not be acceptable, and that the Arun did not provide 'propeller protection or have the structural strength for these speeds the possibility of a 25-knot lifeboat was demonstrated.

The time needed to design and develop a lifeboat is considerable, so with two classes of differing size to be replaced, it was decided to use a geometrically similar hull (a GEOSIM) for the two classes of lifeboat, one 17m overall the other 14m but each developed from the same basic form.

The Concept The two principal objectives of extra speed and propeller protection are difficult to combine and required serious consideration during the early development. The relationship between speedand length is a critical aspect of hull design, and the new designs lay at the upper performance limit of a round bilge hull form and at the lower limit of a hard chine form.

Two methods of protecting the propellers were considered - putting the props in ducts, and recessing them within deep runnels with side keels. Ducted propellers were eventually rejected because of the amount of drag at the speeds being considered and concern that damage could result if the boat took the ground.

The Mersey already incorporated deep tunnels in the hull, and if this was combined with side keels similar to the Tyne there would be adequate propeller protection for the boat to take the ground safely. Weight and centre of gravity calculations indicated that boats of this type could be constructed in either aluminium or fibre-reinforced composite, but that to keep within the weight limits the choice of engine and transmission was limited.

Model testing The tunnels and side keels would add to the hull resistance, so model tests were carried out to establish the power required. Two models were tested for comparison - a hard chine hull developed by the RNLI and a round bilge hull based on the same parameters.

The resistance and power requirement for the two hull forms were comparable but, as expected, the resistance of the round bilge hull was less at lower speeds and the hard chine hull was better at higher speeds.

The water flow through the tunnels was of particular concern,but the tests showed it to be straight and strong with no 'cross-over' within the tunnel or around the side keels - either of which could have created a problem.

The bow-up attitude of the models was too large to begin with and although the optimum trim for forward vision was not the optimum for resistance a suitable compromise was achieved by the addition of 'wedges' at the aft end of the tunnels.

The difference between the performance of the two model hull forms was slight, so the only way to decide which hull to adopt would be to carry out sea-keeping tests in open water.

This was achieved in a novel way which avoided the expense of two prototypes. The models used for the tank tests were fitted with a small petrol engine, radio control equipment and instruments to measure roll, pitch, yaw, acceleration and helm response.

In March 1989 both models were tested in conditions equivalent to winds of up to Force 7, with significant results.

Although both models had impressive sea-keeping ability the hard chine hull proved to have better characteristics in the roughest conditions, notably in following sea conditions where the helm response was significantly better.

The sea-keeping trials confirmed the decision to continue development of the hard chine hull, and more tests were carried out on the self-righting capability of each model, the sea-keeping when at rest in regular and irregular waves and manoeuvrability in calm water.

The results confirmed the earlier findings, and later in 1989 the decision was taken to proceed with the hard chine hull for both new classes.

Construction Early weight estimates indicated that the new boats could be built in either aluminium or fibre reinforced composite (FRC). However, later more definitive estimates showed that to obtain the required performance the weight constraints were extremely tight. As construction in FRC enables substantial weight to be saved without loss of strength this material was chosen.

The design of the laminate for the composite is complex, taking into account not only the overall strength but also the direction of the loads. Pressures exerted on lifeboat hulls are high, and experience of boats in service was used to derive the design pressures required.

The self-righting ability of a lifeboat relies largely on a watertight deckhouse, which must remain intact, so as the boats were to be designed to withstand a 'pitchpole' (stern-over-bow capsize) in addition to a normal capsize the deck and deckhouse structure had to be designed with this in mind.

It was decided in the first instance that the larger boat would have a hand-laminated single-skin bottom panel (supported by continuous longitudinals) with vacuum-bagged sandwich side panels supported by transverse bulkheads and web frames. Sandwich laminates would be used for deck, superstructure and bulkheads.

Polyester or modified poly-ester and urethane acrylate type laminating resins were used.

For the smaller boat the thick sandwich monocoque-type structure using epoxy pre-impregnated reinforcement, similar to that of the Mersey, was best suited to the limited space and subdivision required for the boat.

Throughout the development various combinations of resins and reinforcements were examined to reduce weight and maintain strength.

Meeting the weight and centre of gravity limits for the boats meant very close monitoring during construction andthiswasextendedto the fitting out stages, where the boats were positioned on load cells so that they could be weighed at regular intervals.

For a lifeboat to survive and retain her self-righting capability any water which enters a damaged hull must be contained by subdivision of the hull, and in the larger Severn this is achieved by using a double bottom in all spaces except the engineroom, two longitudinal bulkheads and transverse bulkheads. The smaller Trent relies on the form of construction (FRC sandwich) for double-bottom protection and subdivision.

Machinery and layout The layout of both classes takes account of the need for easy maintenance of the machinery and also the provision of adequate space for seated survivors.

The engine room is aft, with the two Caterpillar diesel engines of the Severn (1,050bhp each) driving the propellers through a 'U' drive gearbox. In the Trent, however, space limitations led to a novel approach in which one of the twin MAN diesel engines (808bhp each) is turned round, driving the propeller in a conventional manner, while the other works through a 'U' drive.

As a result the engines can be removed through large hatches in the aft deck, whereas previous RNLI lifeboats - with engine rooms further forward and in-line shafts - have sometimes needed partial dismantling of the wheelhouse to remove an engine.

A full-scale mock-up of the wheelhouse and accommodation allowed interested parties to assess the arrangements, particularly the medical aspects and survivor handling.

Survivors are best placed in a position of least motion, and the Severn achieves this with seating forward of the engine and fuel tank space, and in the after part of the deckhouse. Space limitations in the Trent prevent this ideal location but a suitable compromise has been found.

The deckhouse also has six crew seats and direct access to the below deck spaces, which contain the galley and toilet facilities.

Stretchers can be supported by block and tackle from a deckhead runway, allowing them to be moved horizontally and to be positioned at any angle - a facility requested by the Institution's Medical and Survival Committee.

The deck sheer line of both boats was dropped in a similar way to the Arun to reduce the freeboard amidships so that survivors can be more easily brought aboard. A 'well deck' is provided in the Severn in which a crew member can stand to lean over the side without the fear of falling overboard.

As with the Arun the Severn is provided with a Y class inflatable, launched and recovered by a lightweight aluminiumProof of the pudding Full-scale trials are carried out on all RNLI prototypes, normally in four stages: first the builder's trials, then technical trials followed by operational trials and, finally, evaluation at various stations around the coast.

Builder's trials are carried out on all RNLI lifeboats, and particularly prototype craft. They check the functioning of all the equipment and systems, last about ten weeks and include weighing, inclining trials, fuel calibration, self-righting trials, four- and six-hour machinery trials, equipment trial, speed and fuel consumption trial.

After a prototype boat has been handed over by the builder, technical trials begin to confirm that the design satisfies the requirements. Typically these include speed and trim, turning circles, manoeuvrability, crash stops, instrumented sea-keeping, noise and vibration levels, bollard pull, shaft revolution per minute and torque measurement.

Most are carried out in calm water, but the sea-keeping trials are held in varying sea conditions and are of particular interest.

The boat is fully instrumented to measure vertical acceleration forward, vertical acceleration amidships, roll angle, pitch angle, yaw rate, rudder angle and wave elevation. The responses are recorded for various sea conditions, speeds and different headings and, as these trials have already been carried out on other classes of lifeboat it is possible to compare the sea-keeping capability and seakindliness of the boat. The data is also used to help predict the cumulative effect of the boat's motion on the operating efficiency of the crew.After completion of the technical trials, and any modifications, the operational staff start to evaluate the boat as a working lifeboat. In addition to normal day-to-day seakeeping and passage experience these trials include such things as mooring, anchoring, Y boat operation, towing, helicopter operation, man overboard recovery, night use, fire-fighting and damage control, pilotage and casualty handling.

The prototypes also undertake a circumnavigation of the British Isles to enable as many people as possible from stations around the coast to evaluate and comment on them. Feedback is obtained from crews and operational staff and used to help with modifications and improvements for later production boats.

Modular Fit-out Lifeboats are built by independent boatyards around the country and, until recently, were constructed in the conventional way, with the machinery, operational and accommodation spaces fitted out after the basic structure had been completed.

The increasing complexity of modern lifeboats means that the fitting out is a very complicated and labour-intensive operation, which is particularly acute in the smaller boats where access to the various compartments is limited. When the Mersey was being planned the structure was designed to be built in modules, so that large sections of the structure could be fitted out before being installed in the main structure. This system has proved so successful that four months was saved in the production of a Mersey. The prototypes of the Severn and Trent classes were, of necessity, fitted out in the traditionalmanner, but production boats will take full ad vantage of the modular system.Construction Materials... Wood construction is often thought to be synonymous with theRNLI, but in fact the last wooden lifeboat entered service in 1982 and will be replaced by her FRC successor later this year.

Steel has been used successfully for hull construction for almost thirty years but where the weight of steel cannot be tolerated aluminium, introduced in 1988 for ten Merseys, has been used successfully.

The RNLI ordered its first glass reinforced plastic (GRP) lifeboat, a Nelson 40, in 1968 principally for material evaluation. The construction was very conventional for the time, using orthothphalic resins reinforced with woven ravings and chopped strand mat.

In 1971 the Arun class lifeboat was developed, specifically for construction in composite materials, and from wooden prototypes the world's first purpose-built GRP offshore lifeboat evolved. The first boats used a similar construction to the Nelson 40, but improved resins and reinforcements were incorporated as they became available.

The need to improve speed, durability and survivability has led to the increasing choice of FRC as a construction material, and in the mid '80s the increasing weight of lifeboats, as a result of the need for increased power and additional equipment, led to a re-evaluation of construction materials.

Panels of timber, steel, aluminium alloy, GRP and FRC of equivalent strength and representative of actual boat structures were made up and tested by dropping a 12kg projectile on them from a height of 9m. The wood panels failed totally and the conventional GRP panels showed some delamination and some failures. However, the FRC, steel and aluminium panels performed satisfactorily.

With evidence that an FRC boat could be built to withstand the same pressures but at a lower weight than steel or aluminium a prototype Mersey was built from FRC in 1987. She satisfied all requirements for impact resistance, abrasion resistance and shock loading in extensive trials designed to simulate some 20 years of life for an average beach-launched lifeboat. In just over one week the prototype was driven on and off a beach nearly 250 times and dragged almost a mile over a shingle, sand and pebble surface.

After these demanding trials she was tested for shock loading by being dropped 3.5m into the water from a crane - with no adverse effects.

As a result the majority of the production Merseys, 27 boats, was ordered in FRC and further development has enabled the material to meet the needs for even more performance in the replacement classes of 'afloat' lifeboats.Principal Dimensions Severn Length overall 17.0m (55ft 9in) Beam 5.5m (18ft Oin) Draught 1.68m (5ft 6in) Displacement 37.5 tonnes (36.9 tons) Speed 25 knots Engines Two Caterpillar 3412 TA diesels 1,050bhp each at 2,150rpm Fuel capacity 5,500 litres (1,200gal) approx Trent Length overall 14.0m (45ft 11 in) Beam 4.53m (14ft 10in) Draught 1.295m (4ft Sin) Displacement 25.5 tonnes (25.1 tons) Speed 25 knots Engines Two MAN D2840LXE diesels 808bhp each at 2,300rpm Fuel capacity 4,100 litres (900 gal) approxThe Cn/Stdl Bull... Although a design of modern, fast lifeboat is now available for all situations the RNLI is always looking to the future and anticipating future trends and developments. At the moment it is felt unlikely that the speed of all-weather lifeboats will exceed the 25 knots of the latest designs, although the smaller inshore lifeboats will probably be capable of around 40 knots within the next few years.

The trend towards lighter diesel engine installations for a given power and reliability will undoubtedly continue, and the overall reliability and suitability of this type of power source is unlikely to be challenged in the foreseeable future.

Propulsion systems, however, could well change, with water-jet drives likely to be a serious consideration by the year 2000. Considerable evaluation of the operational suitability of jet-drive would have to be undertaken, but the advantages for a lifeboat would be considerable. The elimination of conventional propellers, rudders, their protective appendages and compromise design features would reduce resistance and therefore power requirements as well as improving performance in following seas.

Clearly the next few years will see many further changes in requirements and advances in technology, not least in the refinement of the composite materials which have already contributed to the success of the latest generation of lifeboats.'Fast Work' is based on the paper The design and development of modem lifeboats' presented to the Institution of Mechanical Engineers on 27 January 1993. The authors of the paper were F. D. Hudson CE.O FRINA, MIMARE (Chief Technical Officer, RNLI), I. A. HickS BSc, CE«G, MIM.c-E (Technical Manager, RNLI) and R. M. Cripps BSC, CENG, MRiNA (Design Manager, RNLI).