Yanmar 500hp diesel

Released at this year's boat show circuit, Yanmar's 6CXM-GTE2 is a fine example of how to extract maximum performance from a diesel without resorting to electronic engine management.

The power and torque that Yanmar has been able to extract from this 7.4lt straight six is brilliant, especially when compared to the now-discontinued 7.4lt V-eight petrol MerCruiser. In standard EFI form the inboard MerCruiser developed 310 propshaft horsepower at around 4500rpm, yet the Yanmar develops 493 crankshaft horsepower (or 368kW at 1.0hp = 746 watts) at only 2900rpm. And whereas the Merc developed its maximum torque above 3000rpm, the Yanmar develops it at only 2100rpm and the crankshaft output is a massive 1500 NM.

To achieve this engine performance without incurring a weight penalty, Yanmar took the existing 7.1lt 6CX(M)-ETE, which developed 414hp (308.9kW) at 2700rpm and 1200 NM at 2200rpm, and increased the stroke from 125 to 130mm. At 110mm the bore remained the same. The 7.1lt already had 24 valves with pushrod valve actuation, but Yanmar increased the fuel injection pressure and utilised two-stage injector springs.

The result is a motor that, at 825kg in bobtail form (minus gearbox), weighs only 5kg more than its predecessor and has the same 1504mm length. At 876mm the new motor is 6mm wider, but at 905mm it is a significant 30mm lower, despite the longer piston stroke.

As with its predecessor, the new Yanmar has a six-point mounting system (four for the engine, two for the gearbox) that dramatically reduces torsional vibration from the crankshaft compared to four-point mountings. The crankshaft has 12 balance weights and runs massive bearings to handle the increased torque.

So how well does Yanmar's new engine perform, especially considering it's one of the few recreationally-orientated diesels available with mechanical injection timing?

In September Power Equipment, the Australian distributor for this motor, provided twin units for evaluation in a 12m Black Watch 40 gameboat. The engines were mated with single speed ZF handed gearboxes having 1.769:1 reduction ratio and matched to 22 x 29in four blade props.

Carrying about 10 adults (there were people everywhere) and, according to Power Equipment, displacing 12.5 tonnes, the 40 was the biggest hull I'd fit these engines in. Unfortunately the hull was a little dirty due to sitting in the water for a few weeks without antifouling but, though unsightly, this had only a slight effect on the overall performance.

On initial start-up, after the motors had been left unused for a few days, there was some grey exhaust smoke, a combination of unburnt engine oil and condensation in the cylinders. But the engines started within a few seconds of cranking over and after a few minutes the smoke all but disappeared. Surprisingly, for a mechanically-timed diesel, there was no black smoke when manoeuvring gently and the single lever electronic Morse controls gave fingertip manoeuvring; although there was a noticeable clunk each time 'ahead' or 'astern' were selected. Black smoke only appeared when the helmsperson became aggressive with the one engine ahead/one astern technique.

Running down the Tweed River (Northern NSW) on a slack tide from the Black Watch factory, the Yanmars averaged 6.0kt on 750rpm, consuming 3.2lt/h each motor. At 1000rpm my Lowrance GPS showed 8.1kt and a useful offshore trolling speed, where the consumption was 4.8lt/h each, an excellent result for a diesel this powerful.

As the revs were steadily increased, there was no sign of black smoke until around 2000rpm and 17.3kt, when the hull was starting to plane. We achieved a 'clean' plane (there was still a rooster tail) at 19.5kt on 2200rpm consuming 41lt/h each and by 2500rpm were cruising quietly at 24.4kt using 51.0lt/h. At Wide Open Throttle we averaged 32.8kt on 3000rpm, with the motors slightly underpropped and consuming 105lt/h per motor and absolutely no exhaust smoke apparent.

On the run from Tweed Heads to Southport (about 11nm) we found that over the 1-2m sou'easterly swell we could average an effortless 27kt on 2650rpm (50 below the maximum continuous revs) consuming 70lt/h for each motor. When I stuck my head down the engine room hatch at these revs there was hardly any engine vibration and very little whine from the turbos. However, in the cockpit directly atop the props there was either some propshaft or prop vibration, but certainly not enough to be of any concern.

Having tried twin 414hp (308.9kW) Yanmars fitted with two speed ZF boxes in a Fabio Buzzi 38 last year, I found that the Yanmar 500s worked very hard in the Black Watch 40. Of course the 18? transom deadrise, tuna tower and 1900lt of fuel (1630kg) under the cockpit concentrated weight aft and significantly affected our running angle, but frankly once planing there was not a lot of throttle left.

I prefer diesels to have an easier life, and an example of a really good installation was the MerCruiser 1.7L DTI I tested in a Savage 5.5 Mako earlier this year. The motor planed us at 2750rpm, cruised at 3000 and wound out to 4600, giving us a lot of throttle to play with once planing. But when your top speed and revs are only 12-13kt and 800rpm more than minimum planing speed, there's a lot to be said for a two speed gearbox to lower engine revs once planing.

With a ZF two speed and a 30% reduction in revs in the second gear there would be much less engine load at low speeds, allowing the engine to reach its torque band faster. And in first gear the freer-revving engine would reach turbo-boost revs faster and be less likely to suffer from cylinder bore glazing when trolling offshore. In the Black Watch 40 the engines would have to be revved to about 2600 before engaging second gear, which would reduce the revs to about 2000, the minimum needed to keep the hull planing.

But having said this, the Yanmars did an excellent job in the 40. They were throttle-responsive at all revs, very quiet and smooth-running and some of the cleanest-running mechanically-timed diesels I've yet tested.

For more information on these Yanmars contact Michael Blair at Power Equipment, tel (03) 9764 0711.