Yanmar 420 diesel

In a recent issue of Trade-A-Boat we reviewed Yanmar's straight six turbo-intercooled 6LY2(M)-STE diesel, a 5.8lt engine that suits a wide variety of applications.

As with all Yanmar turbo-intercooled diesels, it has fixed mechanical injection timing. Via the use of a relatively high compression ratio and high pressure injection timing, it complies with current IMO exhaust emission regulations.

Unusually for a 'big' diesel, the 420's stroke of 110mm is only slightly greater than the 105.9mm bore measurement. The 420's bore/stroke measurements effectively reduce piston speed at high revs.

To ensure efficient gasflow, four valves per cylinder are used, but reliable pushrod valve actuation has been retained from earlier models.

The 420 develops 308.9kW (414hp at 1.0hp = 746W) at the crankshaft at 3300rpm and a maximum of 1110NM of torque at 2500rpm. The maximum continuous rating is 257.4kW (345hp) at 3100rpm.

Given its output, the 6LY2(M)-STE is a compact engine and in bobtail form (minus gearbox) measures 1274mm long, 708 wide and 709 high, while the weight is only 515kg. Even with Yanmar's standard single speed gearbox, the length is increased by only 200mm.

As we've come to expect from Yanmar, weight is also very competitive - even compared to petrol powerplants. For example, the 420's dry weight is 617kg, compared to MerCruiser's 370hp 8.1lt Horizon inboard at 513kg (with a single speed box).

I first tested the Yanmar 420 engine in July 2000 in a rather exotic hull -- a Fabio Buzzi 38.

By way of background, the FB38 in military form is used by the Hellenic Coastguard of Greece. The highspeed craft has a stepped planing surface to reduce hull drag when planing, measured 11.85 x 2.32m and displaces about five tonnes with a full fuel load and four adults aboard.

To maximise performance, two speed ZF gearboxes were fitted and had a 1.305:1 ratio in first and 1.012:1 ratio in second. The drive was through Trimax surface drives, similar to the Arneson concept but fixed in one position and relying on a separate rudder for steering. Five-bladed Swiss Rolla props were used.

To get the boat out of the hole, the two-speed box and prop aeration were used in combination.

The relatively low first gear of the box allowed the motor to quickly reach its torque band, while the exhaust gases exited directly onto the leading edge of the props and allowed them to ventilate until the hull was fully planing.

Once planing, the exhaust exits above the waterline and doesn't interfere with the lower half of the prop biting into clean water.

The only real drawback to this system is that when reversing the exhaust gases dramatically reduce the props' bite. Still, the FB38 is all about going forwards. An exciting ride, it points to the performance potential of modern diesels.

In the FB38 installation, the 420s started instantly from cold, with slightly more diesel rattle when idling than the new Yanmar 500, but were still very smooth and quiet for straight six engines.

In first gear the FB 38 planed at 15-20kt on 2000rpm. At 2500rpm, a quick flick of each dash-mounted toggle switch engaged the electronically controlled second gear with a slight jolt and the speed jumped from 30 to 40kt. There was absolutely no exhaust smoke as the load increased when second gear was engaged.

At WOT and 3300rpm the FB 38 averaged 63.5kt!

My second chance to sample the 420 was in September 2001, when the engines were fitted to a boat quite a bit closer to Yanmar's bread and butter, a Black Watch 36 flybridge sportsfisher.

The 36 measures 11.00 x 3.75m and has a conventional Raymond C Hunt style vee hull with a deadrise of 18? at the transom. With six adults aboard and a full fuel load, the displacement was around 10t.

This time the Yanmars drove through single-speed ZF gearboxes, having a 1.96:1 reduction, and were spinning 21 x 28in four-bladed props. Both engines started instantly and at low speeds emitted black smoke only when throttled aggressively by the helmsperson using the one engine ahead/one astern technique, to spin the boat in its own length.

The only other time any smoke appeared was as the hull approached the planing hump with the throttle well open.

At 700rpm the Yanmars averaged 3.7kt and again were very quiet and smooth-running. However, with 1500lt of fuel (1290kg) under the cockpit and the relatively short planing surface, down-angled trim tabs were needed at all times. On some hulls this can affect planing speeds, however, according to TAB's boat testing team this is not a significant factor with Black Watch's 36 or 40 foot models The 36 passed the planing hump at 19.3kt on 2800rpm, where the props were absorbing 210kW and the engines consuming 50.0lt/h each. The best cruising revs were 2900rpm, where the 36 averaged 22.9kt and used 58.9lt/h each motor.

At these revs the Yanmars remained unobtrusive, with a low turbo whine.

Even though the motors were revving well above their maximum torque range, they accelerated strongly out to WOT. This is where the ability of a diesel to run continuously at a couple of hundred revs below maximum scores every time over, say, carburettored petrol V-eights, particular when used in 'performance' orientated applications such as sportscruisers and sportsfishers.

The WOT average of the 420 in the BW36 was 28.2kt on 3400rpm, consuming 90lt/h per motor.

As with the installation in the Fabio Buzzi 38, at WOT the vibration levels were very low and the effective engineroom insulation further reduced engine noise.

For more on the Yanmar 6LY2(M)-STE contact Power Equipment, tel (03) 9764 0711 or fax (03) 9764 0678.