Amazingly, with the engine throttled back to 23-25 knots, this is good for over 1000 miles. Graham can cruise from Darwin to Broome if he feels like it.
Issue: September 2003
Until they started building hulls for Naiad RIBs, WA's Kirby Marine had probably never built two identical boats. They are the epitome of WA's niche aluminium boat building industry. And they have built an impressive range of boats, from cray boats to abalone dive boats, from trailable leisure boats to a 53 knot, 54 ft luxury day boat. The kind of quality they routinely produce is shown by the specialised work other boat builders get them to do.
If, for instance, you want a highly polished stainless steel anchor for your 50m mega yacht, then Kirby's is the phone number you need. Because of the consistent quality, Kirby Marine was the builder chosen by naval architect Gavin Mair to be the exclusive builder of his Air Rider designs.
They are complex boats and need a master's touch. And they are worth the effort. Put boldly they are cathedrals, or tri-hulls. You always hesitate to use those words, because so many mediocre and bad examples were built in the past. Most of them had tremendous drag, which is absolutely the opposite of the Air Riders. The biggest reason for the twin tunnels is to direct air to generate lift and improve the ride. It's a good theory that a few others have tried with a remarkable lack of success. Gavin Mair's solution was endless research and trials, head achingly complex calculations, and sheer persistence.
Kirby's biggest customers for Air Riders have been commercial abalone divers. Some near incredible boats under 8m have come out of the factory. They have to be built bullet-proof to withstand being dumped onto trailers on surf beaches, and yet reach 50-knot speeds flat and hold speeds not much less in appalling seas. Specialised stuff and perhaps not entirely what every leisure customer wants. Yet a growing number of discerning leisure boaties talk seriously to professional fishermen and they have been steered to the benefits Air Riders can give to their boating.
A fine example of the efficiency gains on offer is given by the relatively modest performer Kirby built for a WA diving enthusiast. Graham Gunness wanted a boat to take him home in comfort and speed in beam, or following seas created by 30 knot winds. He also wanted lots of range. And carrying capacity for lots of diving gear and lots of friends. Kirby's Air Rider answer was 9.2m by 3.2m and a trailing weight (yes, it gets towed) of four tonnes. A sizeable chunk of boat, yet without the excessive beam a lot of people associate with 'tris'. The main hull has the kind of deadrise usually seen on offshore racers and that was only possible because of the stabilising effect of the wing hulls at rest.
Instead of what would have been a very twitchy mono, Graham virtually got the stability of a barge. But not the drag. Powered by an extremely modest 230hp four-cylinder Yanmar diesel, she puts out 30 knots with full fuel and 10 divers with two bottles each and all their gear aboard. And that gear includes a sizable compressor. The fuel load is noticeable too, 1000lt. Amazingly, with the engine throttled back to 23-25 knots, this is good for over 1000 miles. Graham can cruise from Darwin to Broome if he feels like it. Although he probably won't, as crocodiles make most of that trip extremely diver unfriendly.
That's cheap boating and achievable by no other boat of similar size and abilities. All the wanted sea keeping abilities are there as well. The owner is still delighted with the behaviour after working the boat hard over two years and our more limited experience of it suggested something very special. Super steady at rest Hal (named after the computer in 2001 a Space Odyssey) literally got up and ran when given the throttle. There was a definite sensation of lift. Handling was completely unlike a catamaran and with none of the cat's nervous behaviour in quartering seas. As much as anything it resembled a 6m monohull in the responsive way it let us flick her around.
Trailing a boat this big with heavy loads on board for long distances can be stressing on a hull, but less on this one than virtually any other design. Kirby builds these Air Riders to USL standards ' the code for Australian commercial vessels ' which puts in quantities of frames and stringers far beyond what most other countries call for. On top of the USL, larger numbers of stringers go in to form the complex shape. The skin that follows duplicates that shape, and on its own would nearly be a self-supporting monocoque. Skin and framework together are as nearly indestructible as you could get. More of an investment than a consumer commodity, but still very cost effective and not slow to build.
All the metal for the Air Rider designs is cut from the electronic drawings on a computer driven plasma cutter: absolute accuracy and minimum wastage. Kirby Marine has a bank of standard designs, which can be almost infinitely customised, or they can create one-offs, but inevitably with some cost premium.
Words by Mike Brown