At the time this issue of THE LIFEBOAT is published the Mersey, the RNLI's latest class of lifeboat, will be on show to the public at the London Boat Show in Earls Court.

in this article Keith Thatcher, one of the RNLI's Naval Architects, looks at the work involved in designing and developing a completely new boat within the stringent parameters needed for a lifeboat to take the Institution into the next century. THE requirements of an organisation like the RNLI are constantly changing, and so the Institution periodically reviews its needs in the light of those changing circumstances.

One such review took into account the benefit of speed of response to a situation, and resulted in the move to the new generation of fast lifeboat typified by the Arun and the Tyne classes. However while new designs were available to replace boats moored afloat and those launched from slipways the missing link was a design of fast lifeboat which could be launched from a carriage across a beach or foreshore and replace the ageing Oakley and Rother class boats.

The urgent need for this new lifeboat became apparent early in the 1980s when it was discovered that the wood-hull Oakley and Rother classes were deteriorating faster than previously realised.

The Oakleys were introduced in 1958 and were, in any case, reaching the end of their useful life.

In the design of any new vessel the operators must decide the requirements upon which the design will be based, and in the case of the Fast Carriage Boat (FCB) these requirements were more restrictive than usual, since the new boat had to meet many of the principal characteristics of the Oakley and Rother classes.

In particular, the boat had to fit existing boathouses and be launched by the same method, placing severe limitations on the overall size and weight of the vessel.

It was also clear that the standard of crew and survivor accommodation in the new craft had to be improved, and the need for inherent self-righting and improved crew protection all pointed towards a watertight wheelhouse.

Although the design requirement specified a minimum speed of 15 knots, it was initially implied that 25 knots would be preferred as the maximum achieved in service conditions in calm water.

With an overall length of only 38ft the best waterline length that can be hoped for is about 34ft, and at 25 knots a craft of this length would be in the true 'planing' mode, where the boat is partially lifted by the effect of the water passing her hull and the length is not a limiting factor for speed. (The speed of a 'displacement' hull form, as used on earlier lifeboats, is strictly limited by the waterline length, and no amount of extra power will enable it to exceed a speed some 1.3 times the square root of the waterline length in feet, approximately 8 knots for a 34ft waterline - Ed) At this speed the most efficient hull is the deep-V, hard-chine form, used by most fast racing and pleasure powerboats which can give savings of up to 25 per cent in engine power for a given speed compared to a planing, round bilge boat of the same dimensions. The choice was complicated, however, by the need to provide a protected propulsion system for shallow water and on the beach. The alternatives available were to use a water-jet drive or recessed conventional propellers.

Traditional lifeboats have their propellers recessed in tunnels in the bottom of the boat, but at this stage in the design no one was sure whether a fully-planing tunnel hull could be made to work.

Water jets had been under evaluation by the RNLI for some time, but despite their obvious advantages - shallow draft and no projections to injure swimmers - their effectiveness was proving to be rather less than envisaged, mainly due to recurring problems when used on a beach. Particularly noticeable was the loss in performance resulting from damage to the impellers after rela-lively short periods of use, and a marked lack of performance astern, particularly in shallow water.

After extensive trials, water jets were eliminated in their present form and propellers running in tunnels became the only viable option.

Once the decision to adopt conventional propellers had been made, the design could proceed, and lines were drawn up for a deep-V hull with two bilge sponsons, with the propellers recessed between the sponsons and the centre keel.

Because of the quite radical shape of the proposed new boat it was decided to carry out model tank tests to determine the resistance and power requirements, rather than relying on computer predictions.

The Wolfson Unit at Southampton University tested a 1.28m, 1:9 scale model in March 1984, and with a few minor adjustments to reduce spray and improve running trim the results proved encouraging.

On completion of the tests it became obvious that the weight of the boat was critical to achieving the desired speed, and a study of material options was carried out - the preferred option being a lightweight composite. However for well established operational and maintenance reasons steel was the preferred hull material.

To perform effectively, a deep-V hull must be kept light, and the use of steel imposed a severe weight penalty, to the extent that speed dropped and displacement increased above the level at which true planing could be achieved. This meant a complete reappraisal of the design, and the hull was re-drawn with a round bilge, semi-displacement form.

As part of the development work associated with any new boat, the RNLI has lately been commissioning large-scale models for sea-keeping trials. These models are 8ft to 10ft long, self-propelled, radio controlled, and are run in scale sea conditions with instruments on board to measure the boat's responses and motion. The results are then compared with those from a known lifeboat of similar size.

With the FCB, now the Mersey class, there were two problems first there was no comparable fast lifeboat of similar size, and second the models proposed would be almost one third full-size.

The cost of the trials was also a large proportion of the cost of a full-size boat and this, together with the need to demonstrate the boat's ability to launch and recover successfully off a standard carriage - impossible with a model - prompted the decision to build a full-size 'model'. It was estimated that a 38ft boat equipped to RNLI standards would weigh 14.25 tonnes. Since the Oakley and Rother classes average 12.5 and 13.25 tonnes respectively, the increase in weight was considered unacceptable.

To investigate this and other weight options the hull and superstructure were constructed from aluminium alloy, another 'first' for the RNLI.

The boat was built in late 1985 and, although initial speed trials proved disappointing, by August 1986 the boat had successfully self-righted and, by careful choice of propellers and some changes to the stern geometry, speed had been increased to 18 knots. Trials with the carriage also proved satisfactory.

The penalty for the development work to achieve satisfactory performance was a boat much modified forward and aft - and unusable as a service lifeboat. To incorporate these changes the design was again re-assessed and the lines amended. The hull was then tank tested by the Wolfson Unit in its final form, accurate data on power and propeller wake produced and two pre-production prototypes ordered, again in aluminium alloy.

To speed the production of the programme, the order for the two hulls was placed while the first boat, ON 1119, was still completing trials. Some construction work was carried out on one of the pre-production boats, ON 1124, before the re-assessment of the design and she therefore required modifying before completion.

The second boat, ON 1125, was built to the revised design from the start and as a result was completed slightly ahead of 1124.

Being the first boat afloat, ON 1125 was used as the trials craft and was subjected to an extensive programme of sea-keeping and performance trials, including selfrighting verification, speed, steering and carriage launch and recovery.

Finally, she was taken on a coast evaluation to the North East and North West coasts and to the Isle of Man. At each station she visited the crew were encouraged to use her as much as possible, and much useful data was fed back to the design team so that changes could be incorporated in ON 1124.

This development work enabled speedier completion of ON 1124, with the result that she effectively 'overtook' 1125 and was placed on station at Bridlington in November 1988.

After her extensive trials period ON 1125 needed a re-fit to bring her up to new lifeboat standard, but before doing so it was decided to gain further experience in a station environment. As a result ONI 125 was placed on temporary station duty at Dungeness during October 1988 for some four weeks, culminating in the French Lifeboat Sendee 'Manchex 88' exercise, during which ON 1125 'rescued' 15 survivors from a fictional Channel ferry disaster.

ON 1125 was at last re-fitted during the latter months of 1988 and placed on station at Hastings in January 1989.

At an early stage in the evaluation trials adverse reports had been received of the vessel's directional stability - constant rudder movements being needed to keep a straight course. In larger seas, and particularly when running before them, the boat could be made to hang on the face of a wave while the rudder was moved ineffectually from side to side.

On the plus side, the new hull was an exceptional sea boat, being far drier than expected, and having a very soft ride when pitching over large waves.

To solve the steering problem ballasting trials were carried out and it was found that with more weight moved to the extreme ends of the boat the steering was much improved.

Further improvements resulted from changes in rudder design.

A modified tank-test model, freerunning in waves in a tank, had confirmed that some adjustment of trim by ballasting was desirable, and also that handling was improved by reducing the length of the bilge keels. This change was tried full-size, but rejected as having little benefit and possibly creating problems when re-carriaging the boat.

To quantify the results of the ballast experiments, and relate them to other classes of lifeboat an examination of several geometric parameters was carried out. The most meaningful was the evaluation of 'Radius of Gyration', which is ameasure of the boat's resistance to change of direction, both horizontally or vertically.

Sailing yachts, which need to respond quickly to movements of the rudder, have a small Radius of Gyration and go to great lengths to keep weight centralised and away from the ends of the boat, but lifeboats have to be directionally stable and therefore need larger values, with the weight more evenly distributed.

Calculations for Radius of Gyration showed that the re-ballasted Mersey had the highest value, the Tyne class next lowest and the Arun lower still - results which are born out in practice. As confirmation the one-off steel Arun, which is often said to be a better sea boat, was found to have a higher value than the majority of the class, falling between the Tyne and the Mersey. Once the correlation was seen, the theory became obvious. A small boat such as the Mersey is influenced strongly by sea conditions because of her lighter weight. Since in relative terms she is also travelling faster than the Arun or Tyne, the speed of response to these outside forces will be quicker, and hence require faster reactions by the coxswain.

To slow the speed of response weight is moved into the ends of the boat, creating greater momentum and increasing resistance to directional changes. As modified, ON 1125 became almost docile and could be run in quite large following seas with impunity.

The Mersey project calls for up to 40 boats to be in service within four years. The most attractive way to achieve this, and one adopted by all quantity boat builders, is to use a moulded hull. The resulting standardisation of shape allows the use of pre-constructed fit-out modules and pre-assembled components to give a shorter building period. Since time is money the costs also reduce.

The Institution has experience of a GRP production run in the Arun and Brede classes, but it was felt that the Mersey's arduous service requirements called for a more sophisticated material. A survey of the market resulted in the decision to build a trial boat in FRC - fibre reinforced composite. The term can cover many different materials, and is the generic name for all composite materials, but in the Mersey it represents an epoxy resin matrix reinforced with glass and Kevlar fibres - see 'The Material Revolution' (The Lifeboat, Spring 1989).

Samples were tested for impact strength by dropping a 12kg steel projectile from a height of 8.5m on panels of wood, steel, aluminium alloy and a number of different FRC laminates, each designed to the requirements of the Mersey hull. Wood offered no resistance, steel and aluminium alloy dented, but the FRC intended for the Mersey was undamaged, even by repeated impact.

The brief for the first FRC boat, ON 1148, was to build a boat similar to 1124 and 1125, but utilising the unique properties of FRC. The fit-out was kept to a minimum, since the purpose was to test the material, not the fit-out. However full electronics were fitted, since one problem to be overcome would be electronic interference.

The basic hull laminate is a sandwich consisting of a main skin of 7-8mm, a 70mm core of medium density PVC foam and a thin inner skin of about 4mm. In each skin the properties of the various reinforcements are used to produce the strongest hull for a given weight. Deck, bulkheads and superstructure and all internals are also sandwich construction for strength, stiffness and light weight.

To minimise electronic interference, all wiring is screened and in steel conduit. The inside faces of the wheelhouse are sprayed with zinc, and all electrical items earthed to this coating, which is in turn earthed to the steel keel shoe.

The boat was delivered in March 1988 and subjected to extensive evaluation trials. She was run on and off Dungeness beach a total of 243 times, a test estimated to represent 20 years' launchings, and dragged for a mile over a surface of shingle, sand and mud.

After these trials there was little more damage than scuffing of the bottom paint. Impact strength was further tested by dropping the boat from a height of 12ft on to water, resulting in drenched spectators but no damage. ON 1148 has now been fully fitted out as a lifeboat and is undergoing coastal evaluation and trials, she is also being used to test items of equipment being considered for future boats and will eventually serve in the relief fleet.

It was always intended that the first production boats should be of aluminium alloy construction in order to get the class building under way. Lessons from the pre-production-prototypes had shown some small changes to be desirable and orders for eight boats with these incorporated were placed in June 1988.

In the meantime, the experiment with ON 1148's construction method having proved successful, a production run of FRC boats has been ordered, taking full advantage of the modular construction method made possible by the material..