RNLI lifeboats have a nominal working life of some 20 years, so when the Institution's current slipway-launched lifeboats reach the end of their twodecades of work in 2003 there will be an obvious need to replace them - but what should that replacement be?This question was posed nearly three years ago when the first steps were taken towards a new lifeboat, to enter service almost a decade later. Designing a boat is nowhere near as simple as it looks, and is certainly not just a case of sketching out a good-looking shape on a piece of paper. Even if you're designing the boat for yourself there will be several, often contradictory, 'musts' to incorporate, and when you're designing a new class of lifeboat the list of 'musts' is very long indeed.

All of the RNLI's new lifeboats are designed in-house, but the designers still have 'clients' - the crews on the coasts who will use the boats and the people who will operate and maintain them - and these clients are tough ones with specific features which must be included.

Slipway launching adds yet another dimension, for as well as having all of the other lifeboat attributes a slipway boat must also be compact enough to fit in a boathouse; light enough to recover by winch and her propellers must be fully protected while she is ashore.

In 1983 the Tyne was the best boat that could be produced for the task, but in the ensuing two decades much has changed - electronics and building materials have progressed and power units are lighter and more reliable for a given power. Simply to build more Tynes would be to miss an opportunity to develop a lifeboat which is as big a leap ahead of the Tyne as she had been ahead of the 9-knot double-enders.

In the very early stages thought was given to modifying the Tyne design - but with more speed and the need to carry a 'daughter boat' identified as major life saving aids considerable modification would be needed. So much, in fact, that it was simpler to start from scratch.

And what of the factors that had been traditionally taken as read? Lifeboats always had propellers and not water jets; lifeboats had traditionally been recovered stern first on to their slipways, and lifeboats had to be designed to fit their boathouses.

Or did they? First considerations One of the great opportunities provided by the Tyne replacement - known as the Fast Slipway Boat 2 - was to take a look at the wider picture, which is exactly what the first working party did in 1994, when the whole concept of the new design was first brought under one umbrella.

The Operations department first set out its stall with an outline set of requirements. Experience had told them that the 25 knots of the new Trents and Severns was a lifesaver, and that the ability to carry a readily launchable Y class for work close to the shore was invaluable. So these were built into the requirements - along with inherent self righting and other traditional lifeboat attributes.

A project team was then formed and tookup the challenge, involving the designers and operators and the surveyors, engineers and electronics experts who would be maintaining the boats through their working lives. The Shoreworks section was also part of the team, as it would have to build and maintain suitable boathouses.

Brainstorming sessions with coxswains and others involved in running the boats helped to refine the requirements, and the project team set to work.

First it went right back to square one, looking initially at whether there was a need for slipway-launched lifeboats at all. The position of slipway stations had been determined by 19th century criteria, and modern boats and harbour developments could have changed the situation.

In the event it was accepted that somewhere between 15 and 25 stations would remain slipway-launched in the foreseeable future, and the conclusion was that a replacement slipway boat was needed.

But did this boat need to fit existing boathouses? The Tyne had been designed around this constraint, but was it reasonable - and economic in the long run - to produce a design which would go into service in the 21 st century with limitations which could have been imposed by 19th century pulling and sailing boats? Slipways are very exposed structures and need constant repair and maintenance, so major rebuilds occur more frequently than at other types of station. This rolling programme of rebuilding, tied in with boat replacement, could provide boathouses for larger lifeboats if needed.

Several options were considered. For example, could operational needs be met by a boat which fitted all existing boathouses? Or by one which fitted half of them? Or even by a design which would fit only the largest of the recently refurbished stations? All of these possibilities were thoroughly examined, not just on the basis of initial expenditure on lifeboat and slipway, but also looking at 'through life costings' taking into account such things as the lower maintenance costs of the modern replacement buildings.

Still working on general principles the project team concluded that they needed a length of 15m (just over 49ft) to achieve the operational requirements, with an available height of 5m and a width of 5.5m.

This size of boat would fit approximately half of the existing boathouses, and the costings made sense when taken over the whole life of the boats and buildings.

Enlarging the smaller half of the RNLI's slipway stations also made sense when planning for the future - as the working party was aware that its successors in 2020 would find 19th century-size boathouses even more of a problem than they were today! Only when this concept was accepted and approved could work start on converting this agreed rectangular box shape into a workable lifeboat.

The design takes shape It may seem strange that the actual designing of the lifeboat started at what appears to be a fairly late stage - but this is in the nature of boat design which has so many variables that it's hard to find any constants! Experienced designers know, from established principles and accumulated experience, what basic dimensions are required and the many possibilities available.

Speed, for example, is affected by weight and engine power - the power-to-weight ratio.

The designers could use a simply-built, heavy hull and achieve the required ratio with lightweight, high-output engines, or they could use a sophisticated lightweight hull with lower-rated engines. Both options have an affect on reliability and maintenance.

Looking at the wide variety of boat shapes it is hard to believe that each is related to the other. By starting with a dugout canoe and then lengthening, widening, changing the cross section and fiddling with the shape in other ways it is possible to arrive at racing yachts, speedboats or supertankers, merely by a process of evolution.

So, even when a naval architect sits down with a blank sheet of paper, his pen is influenced by other boats, his own and from other designers, and he evolves a new variation on a theme to suit the particular needs.

With so much data already available from the new 25-knot hulls for the Trent and the Severn it was inevitable that FSB2 would share many of their characteristics - so the starting point for the new hull was the Fast Afloat Boat shape (the Trent and Severn have virtually the same hull shape, even though they differ in size and almost every other respect).

The new hull would need to differ from the afloat boats in some ways, but with all of the full size and test tank model data available the effect of any changes could be predicted with accuracy.

While work went ahead on the basic shape, making use of extensive tank testing, studies were carried out on the other major variables - jet drive, fixed or controllable pitch props, and bow- or stern-first recovery.

Eventually the team concluded that the new lifeboat should have propellers. Although jets were not ruled out for future applications props were the answer for this particular job.

The jets had proved to be very manoeuvrable, but from the operational standpoint there was less in the way of documented reliability figures, particularly at launch and recoverywhere there was a possibility of picking up stones and other debris.

Meanwhile the tank tests were also showing that an extra metre of waterline length would be beneficial, and within the overall 'envelope' size the length taken up by the overhanging jet unit could better be used for this extra hull length.

The prototype Severn had already been fitted with controllable pitch propellers for extensive trials, and the project team and many lifeboat coxswains and crews were able to gain experience with this system. The final decision as to whether the propellers should have fixed or controllable pitch is still to be taken.

Bow- or stern-first recovery was also investigated at the same time, and it was decided that there was nothing to gain overall by changing from the traditional stern-first method. Agreed it was easier to drive the boat onto the slip, especially with one engine out of action, but an additional, level boat's length would be needed in front of the boathouse so that she could be turned for re-launching. The expense of this (and having to push the lifeboat over the level area before gravity took over) was enough to tip the balance. Stern-first recovery it would be.

Refining the concept While these options were being decided the tank tests had been steadily refining the hull. First of all with nine models, then whittling this down to three, to examine tunnel and keel shape, and then finally down to one preferred form.

The RNLI has become expert in using tank-test and full size data to predict the effect of changes, so the data from the discarded hulls was far from wasted. Armed with this valuable data, and that from the Trent and Severn tests, the final hull form could be adjusted and honed with predictable results until a final shape emerged.

Although even manoeuvrability trials can be carried out in the testing tank several of the shapes and configurations were also run on more open water (a nearby lake) using radiocontrol. Handling and directional stability are key factors in lifeboat design, and although heading into the weather may look spectacular the most difficult conditions are, in fact, when heading away from the seas - when a broach (rounding up across the waves) can be extremely dangerous.

Directional stability is therefore perhaps the most valued attribute, with up-sea behaviour and hull resistance as the next priorities.

Some aspects of handling can only show up under particular circumstances, and when one of the free-running models resolutely continued turning to port even after starboard rudder had been applied it became one of the discarded configurations! Care has to be taken when interpreting the free-running model tests though.

In the most spectacular free-running incident one of the model hull shapes went out of control and climbed the bank out of the lake - but the problem turned out to be nothing more serious than confused thumbs on the radio control sticks! The production boats With the hull shape more-or-less finalised you might be forgiven for thinking that the new lifeboat was almost ready to build. But no, that is really only the beginning, for now the details of the propulsion system (engines, gearboxes and props) have to be finalised, the hull construction agreed and the layout, controls and equipment incorporated.

At this point the RNLI is looking at another new route. The Institution probably knows more about lifeboats than anyone else, but there are specialist areas such as production engineering in which it doesn't have, or usually need, the very latest expertise.

In the development of the new boats from this point on the RNLI is considering pooling its lifeboat skills with those of the commercial world.

A number of contractors were approached and, if and when, a yard is chosen it will work with the RNLI's project team to produce the detailed drawings and specifications, by the end of 1997, before embarking on a four phase programme.

This will lead to the building of the first four production members of the new class - ready to take their place on lifeboat slipways by 2003.

Phase One will see the building of an experimental boat to prove the hull form, propulsion system and the launch and recovery capabilities.

This boat will have the bare minimum of fitting out (using cheap ballast to simulate expensive equipment) and might, for example, not even have a fully fitted-out wheelhouse.

With the hull form proved the experimental boat will be fitted out to normal lifeboat standards and assessed as a prototype by at least one operational lifeboat station. Crews will have the opportunity to get to know the new boat in depth and suggest changes for the production boats.

Once the final layout and equipment has been agreed a pre-production boat will be built to enable the boatyard to test its production methods.

This will be a full-blooded lifeboat for the relief fleet, the only thing on test being the builder's production methods.

With any building snags ironed out the fourth phase gets under way, and the new Fast Slipway Boat 2 begins its production run..