Impala 4.5 S

Paul Smith tests the Impala 4.5 S - the subject of jet boat racing folklore in the deep South.

Impala 4.5 S
Impala 4.5 S

The Impala range of jet boat hulls have become ensconced into jet boating folklore - especially in the South Island and in particular, Canterbury - since their inception in the early 1980s. Originally produced by Bruce Binnie's Jet Services Ltd as the successor to the enormously popular and infamous Jet 44 Invader, the Impala hull was developed to cater for the growing "high performance" sector of the jet boat market.

In 1980, the vast majority of jet boats were constructed of Glass Reinforced Plastic with only a smattering of metal hulls being produced, and these tended to be rather more agricultural in design and execution. This was also the time when New Zealand jet boat racers were the undisputed champions of the world and jet boat racing fever was filtering through the ranks of recreational boaters, especially in Canterbury.Drawing on influences from American manufacturers, the Impala at 5.2 metres in length was longer and sleeker than most of the existing designs. It compared with similar designs from Andrews Fibreglass (A500 and A540) and Hamilton Marine (Jet 73). At the time of its introduction, many of the diehard Jet 44 owners were captivated by the Impala's racing lines and impeccable handling characteristics and quickly made the change, ensuring Jet Services Ltd was kept busy with a constant stream of orders.

The first examples of the Impala utilised a wide delta keel section in conjunction with six planing strakes and a 17-degree deadrise. This endowed the boat with very good shallow water performance, although the ride tended to be a little harsh in rough water at speed. A radiussed keel section was introduced later, which was formed in Bisalloy steel. This had the twin benefits of improving high-speed handling and of increasing the strength and durability of the hull along the critical keel area and thereby dramatically reducing hull maintenance.

By the mid-1980s, the phenomenon of jetsprinting was gaining in popularity and this brought with it a change in the market requirements toward shorter and lighter boats. Jet Services Ltd responded by trimming nearly three quarters of a metre from the overall length of the Impala mainly from the long bow area, and releasing the Impala 4.5 S. This boat soon became popularly known as the "Short Impala". In later years, the design would be shortened and narrowed considerably more to produce a dedicated Jetsprint version, but only six or so of these boats were produced.

The Short Impala became very popular as a capable, versatile jet boat. In the mid-1990s I owned two of these boats - one powered with a 4.1 litre Ford Falcon engine and Hamilton 773 waterjet, the other with a V12 Jaguar and Berkeley waterjet, which I raced with moderate success. In the late 90s I navigated in two full seasons of racing for Christchurch racer Dan Lysaght in his 350ci Chev-powered Impala 4.5 S, and even today I boat with a couple of good jet boating mates, each of whom have "Long Impalas" with potent Chevrolet power. I guess it is fair to say that I am quite fond of the marque.

A few years ago, Bruce Binnie retired from boat building and put the Impala moulds into storage. Recently Binnie was approached by Andrew Brady of Timaru, an experienced boat builder. After negotiation, Brady purchased the moulds to the Impala 4.5 S and the rights to the Impala brand. He commenced business under the Impala Jets Ltd name and has produced a number of new hulls from the mould. Plans are also afoot to produce the Long Impala again.

Brady has just completed assembly of his 4.5 S demonstrator, which he kindly made available for review.

Construction and design
The Impala 4.5 S is, at 4.5m in length, exactly consistent with the average mainstream recreational jet boat. However, its beam of 1.95m means that it has a slightly higher beam to length ratio than many of its competitors. This, to my mind, is a good thing as it allows the boat to carry weight well and provides better shallow water performance at slightly slower hull speeds. The 4.5 S continues the 17-degree deadrise of the longer version, and is fitted with a radiussed bisalloy steel keel plate. Again, three planing strakes either side of the keel extend forward to the stem to provide bow lift. The flat section of the keel behind the jet intake and just forward of the transom is subject to high levels of wear and tear. On the Impala 4.5 S this is protected by a stainless steel wear plate.

The hulls are carefully hand laid in polyester resin and chopped strand mat. In a departure from previous Impalas, the new boats are fitted with two full-length timber engine bearers that are securely glassed in. Sealed marine plywood is used to form the floors between the bearers out to the chines, which is glassed in. A removable plywood sole lies across the bearers to give access to the bolts holding the keel plate in place and to conduct repairs to the centre section if required.

The GRP deck is reinforced in critical areas with marine plywood and is bonded to the deck using rivets and high strength glue to form a strong monocoque. The Perspex windscreen and black powder coated alloy grab rail are then affixed to the deck along with quality powder coated deck fittings.

Two front bucket seats are mounted on GRP bases, which are glassed to the floor, while the extended double bucket rear seat is mounted on an alloy base, which is screwed to the floor allowing removal as required for repairs and maintenance.

The flat dashboard extends quite a way back from the windscreen and a moulded step on the passenger's side assists entry to the cockpit. This in itself is not arduous as the screen is raked back and very low. However, the dash seems to encroach on usable cockpit space of which there is not an abundance. The instruments that monitor engine function are mounted centrally on the flat dashboard and comprise tachometer, oil pressure, engine coolant temperature and fuel.

The helm is mounted high and to port as is traditional in jet boats. Toggle switches operate bilge and fuel pumps. The forward/reverse is also electrically operated via a toggle switch mounted on the port side. The floor mounted foot throttle was mounted a little too close to the seat for my long legs, but placement of this and other controls is variable at time of construction.

The cockpit sole and sides are fully lined, which makes for a very quiet environment. Twin alloy fuel tanks of 50 litre capacity each are mounted under the rear deck either side of the jet unit, while the engine is protected by full GRP cover, which is upholstered to match the seats.

Performance and handling
This example has been fitted with a 4.0 litre Toyota/Lexus V8 engine. Such engines are becoming increasingly popular in jet boat and other marine applications. They offer modern design and construction features and can be sourced second hand from Japan at moderate prices. The engines are of all alloy construction, feature quad overhead camshafts actuating four valves per cylinder and are ideally suited to locally built engine management systems.

Although the alloy construction suggests a light overall weight, in practice the rugged construction and high number of moving components means there is little difference in the overall weight of this engine in comparison with an alloy head smallblock Chevrolet or Ford. However, the Lexus motor is extremely smooth in its operation and is capable of producing excellent fuel economy, especially at cruising speeds.

Unfortunately, its four-litre displacement means that the Lexus engines give away almost 30 per cent to a six-litre Chevrolet. In a jet boat application, the Lexus struggles to deliver its best performance on the still popular Hamilton 773 waterjet and the current Hamilton 212 waterjet. I was curious to determine just how well the Lexus engine would perform when coupled to the Scott SJ110 waterjet fitted to the demonstrator.

Like the Hamilton 212, the Scott SJ110 is a large capacity, single stage mixed flow waterjet that utilises a large diameter nozzle outlet when compared with older waterjets. Generally speaking, such waterjets require more grunt to drive them, and it has been difficult to match the input required with the engine power outputs at the appropriate engine speeds. In the case of the Scott SJ110 however, the impeller fitted is a perfect match to the power curve of the Lexus V8.

We found the Impala would respond very quickly when the throttle was mashed open with no cavitation at all off the mark. By the same token, the tachometer needle swung around to 5400rpm, very close to the engine speed at which maximum power is developed. A comfortable cruise could be maintained at 3000rpm and at 4000rpm the boat seemed to be getting along quickly indeed. However, when the engine was given its head from 4000rpm, the power delivery was most impressive, and this despite the engine being fitted with log style water-cooled exhaust manifolds that do little to encourage healthy engine breathing. This is the first time I have experienced a Lexus engine in a jet boat that delivers on the promise of its technological pedigree and this is due entirely to the perfect match in power requirement of the SJ110 waterjet.

Even though the design of the Impala hull is quite old now, there is very little that can be done to improve what is already a superb handling boat in most river conditions, especially at the speeds the Lexus/Scott Jet combination is capable of delivering.

The topdeck design does show its age, particularly in respect of the windscreen, which is low, raked back and virtually useless as a means of deflecting wind from the driver or passengers.

We took the opportunity to test the Impala 4.5 S's ability in white water during the course of our expedition in the Waimakariri Gorge. The river was flowing quite high after a recent fresh, and the standing waves in Hamilton Rapid were larger than usual. We found the Scott jet provided good grip in the aerated water, while the moderately flat bow sections provided adequate lift and the turned down chines did a fine job deflecting water down and away from the hull.

The hull responds well to inputs from both the helm and the foot throttle. Although on this occasion we didn't trial this boat in the braided section of the river, experience with the marque confirms Impalas have a formidable reputation when negotiating tight, twisting braided water especially at speed. The impeccable hull handling is enhanced by the light and precise weighting of the Scott steering nozzle and the instant throttle response of the fuel-injected engine.

As one of only two notable brands of jet boat now offered in GRP construction, the Impala will still appeal to the sports-oriented, performance jet boater who is not interested in extreme adventure boating. Performance of GRP hulls on the shallow braided river systems of the South Island has much to recommend it - especially when it comes to pushing the boats back into navigable water. As GRP construction technology continues to improve we can expect to see the differences in weight in comparison with alloy hulls to decrease. Furthermore, the GRP hulls are less expensive than similar sized alloy hulls, which require painting, floorboards and seat bases to bring them up to the same specification.

The Impala 4.5 S was presented on a solid galvanised steel, single axle trailer and had a full road cover. You can expect a new boat built to the same specification as that tested to set you back around $38,000 including GST, which represents pretty good value for money.

For those looking for more performance, Impala Jets Ltd is looking to re-establish the 5.2m Long Impala which, although it has an impressive racing pedigree, is still a very respectable performer in the recreational/pleasure application.

Impala 4.5 S
Length overall: 4.5m
Hull length: 1.95m
Deadrise: 17 degrees at transom
Keel: Radiussed, 6mm Bisalloy keelplate
Construction: Polyester resin, chopped strand mat, timber bearers, plywood floors glassed in.
Engine: Toyota / Lexus 4.0 litre alloy V8, quad cam, 32 valve. Est 260hp @ 5600rpm. Link Electrosystems ECU.
Waterjet: Scott Jet SJ 110, single stage high volume mixed flow, 118mm nozzle.
Price: $38,000.00 inc GST (as tested).
Manufacturer: Impala Jets Ltd, tel 0800 467 252.

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