V8 petrol engines and fuel economy aren’t phrases typically found in the same sentence, but the repowering of a 7.3m Naiad RIB proves that Yamaha’s new 300hp four-stroke V8 can offer fuel-saving opportunities.
Naiad RIBS are designed at the company’s offices in Picton, but built in various locations around the world. That arrangement arguably makes for one of the world’s most ideal boat-manufacturing operations – as work environments go, it doesn’t get much better than the Marlborough Sounds. Company owner Steve Schmidt lives in an idyllic spot in the deepest recesses of the Sounds (water access only) and uses a 7.3m RIB for his daily, 18km commute to and from the Picton office (9km each way).
The boat’s a 2004 design, and since the first model rolled off the production floor, it’s become widely accepted as a sea-kindly workhorse – with refined aesthetics. Today, scores of 7.3s are in use all over the world by rescue organisations, Coastguard teams, fisheries authorities, tour operators, Antarctic scientists and even the US military.
Naiad offers plenty of interior customisation with its boats, but the 7.3’s hull design is fairly sacrosanct. And its most unusual feature is the double-stepped configuration – something more typically found on high-performance race boats. Stepped hulls, says Schmidt, reduce friction – and improve fuel economy. Until recently, Schmidt’s boat had been powered by a 300hp, two-stroke Yamaha HPDI, and while Schmidt had no issues with its performance, its reliability was problematic, and over the four years it required three new powerheads.
Rising fuel prices were also beginning to hurt. And living in the Sounds, he was also keen to reduce engine emissions and eliminate the inevitable two-stroke smell. Intrigued by the performance data from Yamaha’s new 300hp V8, he opted to swap the two-stroke for a four-stroke.
That was four months ago. The consensus? “I just love it – it’s a beautiful engine, and definitely more fuel-efficient than the HPDI. The combination of GPS and Yamaha’s fuel-monitoring technology made it easy to monitor the differences. With the HPDI we were getting 1.4km per litre – the V8 returns 1.6km per litre.”
He admits to initial scepticism: “What are the odds a 5.3-litre V8 will be more fuel efficient that a 2.7-litre V6? Especially considering that the V8 is some 100kg heavier?”
The boat’s overall weight is now 1500kg. At WOT (5900rpm) the V8 propels the Naiad to 100km/h. Schmidt is particularly impressed by the greater smoothness of the V8 and its quietness – especially at idle. “It obviously changes its note through the rev range, but it’s a pleasant, muted growl, never intrusive, and above 4000rpm it’s silky smooth.”
He also believes the extra weight actually suits the boat. “We’ve provided many of Australia’s Coastguard rescue units with 7.3m Naiads, and they all fit twin rigs – it’s standard redundancy policy – and the boat’s seem to thrive on the configuration. For my part, I think our boat was a little ‘corky’ at rest. The extra weight has slightly altered the trim and seems to have settled it down.”
Any negatives? “Perhaps the only criticism is the steering – it’s incredibly light, and I would prefer to have more feedback. Because of the stepped hull, the boat is very light at speed, and if you inadvertently give the wheel a twist, the boat takes off fairly smartly.”
The changeover, he adds, was fairly seamless, with the new engine’s gauges fitting precisely into the holes from the HPDI’s gauges. He did have to remove the boat’s aft stainless steel tower (for protecting the cowling) to accommodate the larger engine, but will now reconfigure and reinstall it.
A 10m Naiad fitted with Yamaha’s twin 350hp V8s won Boat of the Year at Australia’s Marine Industry Association Awards earlier this year.
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