Keith Thatcher, RNLI Naval Architect, continues a series of profiles of lifeboat classes Many people's image of a lifeboat launch is of a boat plummeting down a slipway into rough seas, disappearing into a cloud of spray at the foot of the slip to reemerge and forge her way purposefully seaward.

Although most lifeboats now make a much less glamorous departure, slipping their moorings and almost inconspicuously proceeding to sea, a few stations still remain where there is no suitable sheltered mooring and no alternative to the slipway launch.

These slipway lifeboat stations were almost the last bastion of the so-called 'traditional' lifeboat, the 8- or 9-knot 'double ender' with twin screws and tunnel hulls which served the RNLI so well for so long.

However, by the mid 1970s, new, faster lifeboats were well established at stations where boats lay afloat and the RNLI turned its thoughts to the possibility of a faster slipway-launched boat.

It had long been felt to be a contradiction in terms, but research into faster, round bilge hull forms by the National Maritime Institute showed it would be feasible to produce a boat of similar speed to the Arun which still provided the propeller protection needed for slipway operation.

Staff Target By 1977, the RNLI's Search and Rescue Committee had prepared an outline 'staff target' for the new boat, and a design steering group was_set up which included senior members of the RNLI's Operational and Technical staff and invited representatives of the NMI.

The chairman of the steering group was Mr P. Denham Christie, at that time chairman of the Boat Committee and a past coxswain of the Tynemouth lifeboat. The class name 'Tyne' reflects the close involvement of Mr Denham Christie in the development of the boat and his long association with the River Tyne and Tynemouth.

Because the new class of lifeboat was to take the place of existing slipway-launched classes such as the Watson and Barnett, the weight and size limitations at individual stations had to be taken into account in the design.

The parameters set at the first steering group meeting called for a maximum overall length of 47ft Sin and a maximum beam of 15ft Oin. Height from the underside of the keel to the top of the wheelhouse could be at most 13ft and the weight in service condition could not exceed 24 tons.

Although these parameters are quoted in Imperial units, the design was prepared in metric units.

The Tyne was the first RNLI lifeboat class to use this system of measurement, although it retains Imperial units in the Operational Number designation of '47-' .

Work proceeded smoothly but cautiously, and by 1979 models of the new design had been tank tested and then taken to sea in company with an Arun model of the same scale to test its seakeeping abilities.

Characteristics The principal characteristics of the hull design include soft, round bilges, a deep, fine bow and fairly flat, stern sections. The propellers are recessed into shallow tunnels and are further protected by deep bilge and centreline keels extending aft to the transom.

Corrosion-resistant steel was chosen for the hull plating and internal structure, with aluminium alloy for the deck and superstructures.

Glass-reinforced plastic (GRP) and timber were considered early in the design stages, but were rejected on the grounds of wear resistance and cost.

It had been decided early in the project to order two prototype boats so that modifications tried on one could be compared with the second, unmodified boat. In March 1980, Fairey Marine (East Cowes) was awarded the contract to build two boats, ON 1074 and ON 1075, (later to become 47- 001 and 47-002). The first boat, ON 1074, was delivered in March 1982 after successfully completing self-righting and other proving trials. ON 1075, followed at the end of the year.

Both boats were subjected to a period of evaluation, and their performance was compared with current station lifeboats in increasing sea conditions. Modifications were made to one boat at a time, the two boats then running together to evaluate the effect of the change.

The evaluation period proved outstandingly successful and the changes made to the prototypes were incorporated into the design for production boats.

It was decided to opt for at least two main construction yards.

Fairey Marine was a prime contender.

Otherwise, there were few small boatyards capable of.

or willing to, build complete boats at an acceptable price.

It was decided to adopt the 'hull part-assembly/fit-out yard' concept used for the Arun class.

Fairey Marine were contracted to build the first pan-assembly of hull, deck and superstructure for fitting out by William Osborne at Littlehampton. and a search was made for a second builder.

A steel fabrication firm. R.

Wright and Son at Foston, Derby, were contracted to build the remaining part-assemblies, and various fit-out yards were used.

Modifications The present Tyne is essentially the same as the first prototypes, although a keen-eyed observer will detect some changes.

The hull shape of all the boats K essentially the same, although the production boats differ from the prototypes in deck line. Close examination reveals a step in the sheet-line of ON 1074 and 1075 (47-001 and 47-002). On these two boats, the deck aft of the step has a very heavy camber which is covered by a lightweight false side deck. Although entirely satisfactory, this was felt to be an unnecessary complication and the production boats have a normal continuous sheer and camber.

To design a boat weighing no more than 24 tons in operational condition required fine control over the weight of all components.

The engine room space is severely limited and the need for engines of high power-to-size and powerto- weight ratios was apparent from the outset. The RNLI had some experience of the General Motors 8V71 engines in the 50ft Thames class and these were chosen for the prototypes as they offered the best power-to-weight ratio.

By the time the production boats were being considered, the GM 8V71 engine had reached the end of its production 1 i fe and the newer GM 6V92 was adopted.

The first ten boats, including the prototypes, are fitted with a standard GM gearbox, the Allison Type M20. As the class building programme progressed it was seen that there was some benefit in changing to a lighter gearbox with a slightly improved reduction ratio. The ZF 160 BW gearbox was chosen as the most suitable alternative and all boats from and including ON 1114(47- 01 Dare so fitted.

The change involved a re-alignment of the engine beds, which could not be done to earl ier boats. A new gearbox from Twin Disc has recently become available and steps are in hand to replace the ageing Allison boxes with Twin Discs during survey.

Deck Material At the same lime as the change of gearboxes was being made, it was decided to change the main deck material (excluding the wheelhouse structure) from aluminium alloy to steel.

Fitting the aluminium alloy deck to the steel hull was always difficult and involved a 'riveted connection' at the deck-to-bulkheads and deck-to-hull joints.

This was a potential source of corrosion and had caused a few alignment problems. An all-steel deck offered simpler hull construction and, although increasing the weight, would simplify surveys, especially as the boats grew older.

To facilitate fitting of the superstructures, a new steel-to-aluminium alloy transition joint was used, known as 'Kelocouple' — a strip of material composed of layers of steel, aluminium and aluminium alloy, explosively bonded together. The steel deck is welded to the steel side, the aluminium alloy superstructures are welded to the aluminium alloy side, forming a positive mechanical connection without the use of rivets or bolts.

These modifications resulted in extra weight so the self-righting ability of the boat was reviewed.

It was found that an increase in superstructure volume would be beneficial. So, from boat number 30 (ON 1039). the aft cabin height was increased by 125mm. All Tynes have now had this modification carried out while undergo- ing survey.

Production continued smoothly until, in 1989, all the scheduled slipway stations had been allocated Tynes. Production ceased with boat number 40 (ON 1158).

Although originally conceived as a slipway-launched lifeboat, the protected propellers of the Tyne also make her an ideal lifeboat for shallow waters.

As a result, some stations where the lifeboat lies afloat but has to operate over sand bars or rocks (or has access problems at low tide) have also been allocated a Tyne. These stations include Lowestoft and Whitby.

Arranmore and Salcombe.

HISTORY OF THE TVNK CLASS 1977 Stall' target approved by SAR Committee. Steering group set up.

1979 Design finalised, production drawings prepared.

1980 Prototype boats 47-001 (ON 1074) and 47-002 (ON 1075) ordered at Fairey Marine.

1982 Prototypes delivered 1983 Production boat re-design - Mkll revised sheer and engine change.

1984 First production boat (Mkll) delivered - 47-003 (ON 1094)Padstow. Prototype boat. 47- 001 (ON 1074). to station at Selsey.

Dual construction programme set up. Complete boats from Fairey Marine. Hull, deck and superstructure for fit-out at other yards from R. Wright & Son.

1986 Mklll boat introduced - 47-011 (ON II14) Angle. Steel main deck., superstructure attached by Kelocouple. Gearbox change from Allison to ZF.

1987 Aft cabin height raised to give increased reserve of self-righting ability for the greater displacement of Mk III boats.

1990 Last boat delivered - 47- 040 (ON 1158)Shoreham..