I have long preferred the concept of four-stroke and direct-injection two-stroke outboards to sterndrives. But after having tested the new 120hp (crankshaft rated) MerCruiser D1.7L DTI sterndrive in March, I've seriously had to rethink this preference!

The D1.7L is based on an Isuzu diesel and has an alloy cylinder head and DOHC actuation for the 16 valves. Unusually for an automotive engine of 1.7lt, it has direct fuel-injection (with glow plugs for cold-starting ease) but retains traditional mechanical injection timing. However, combined with the standard wastegate turbo and intercooling it complies with current IMO diesel exhaust emission requirements.

The engine itself measures only 726mm long, compared to 864mm for the 3.0lt 135hp petrol MerCruiser. Only in width (746mm) and height (689mm) does the D1.7L lose out to the 135, which is 584mm wide and 508mm high. Even so, when fitted in the Savage 5.5 Mako test hull there was still plenty of space alongside the engine box for comfortable quarter seats.

The dry weight is 296kg including closed circuit (heat exchanger) cooling and the Alpha leg, just 8kg more than the 135 which has open circuit (seawater) cooling.

The closed circuit cooling allows the D1.7L to operate in the temperature range for which it was designed, reducing gas blow-by past the piston rings and cylinder wall corrosion through condensation. Open circuit automotive-based outboards, such as the Honda BF115, run much cooler than they were originally designed to handle.

While the D1.7L weighs 62kg more than Honda's extralong BF115, the bulk of the weight is half-a-metre forward of the transom and not slightly aft of it, as with the Honda. As a result, the D1.7L-equipped Mako actually trimmed better than the BF115-equipped Mako I tested three years ago.

The demo D1.7L started instantly each time, although there was some black smoke on start-up. There was also the traditional diesel 'rattle' at the trolling speed of 900rpm in gear, however this all but disappeared at 1000rpm.

Spinning a four-bladed 18-inch pitch alloy prop and pushing a total of 1320kg (including two adults), the demo engine trolled the Mako at 5.2kmh. At 1000rpm the average was 7.2kmh, while 2000 rpm averaged 13.2kmh and was ideal for offshore trolling with the fuel consumption a low 4.0lt/hr.

The D1.7L planed us at 2750rpm and 27.8kmh, which also proved ideal for maintaining a clean plane in the blustery sou'easterly-driven 1.5m swells off Newcastle Harbour, while consuming only 7.5lt/hr. Back in the Harbour, and over a 15cm chop, at 3000rpm the D1.7L cruised quietly averaging 34.5kmh using 9.0lt/hr. Through tight figures of eight at 3300rpm there was no prop ventilation. The standard power steering was direct, yet reasonably light.

'Flooring' the throttle at 3000rpm created no black smoke and the engine accelerated better than any four-stroke outboard in this power range. At 4000 rpm it averaged 49.5kmh using 17lt/hr, while the Wide Open Throttle (WOT) was 57.2kmh on 4600rpm consuming 27.0lt/hr. At these revs we could still hold a normal conversation at the helm.

In comparison, spinning a 17-inch stainless steel prop and pushing a total of 1200kg (including two adults), the BF115-equipped Mako averaged 48.2kmh on 4000rpm, consuming 15.1lt/hr, and 64.4kmh on 5200rpm at WOT, using 36.0lt/hr.

Servicing the D1.7L appears straightforward and the engine oil dipstick and filler cap, oil filter, leg oil level tank, sedimenter, water-separating fuel filter and engine coolant overflow bottle are all easily reached.

Recommended servicing intervals are every 100 operating hours or once a year after the first 20 hours or three months, but the engine oil and filter should be changed every 50 hours or six months, whichever comes first. Hamilton's Marine, which supplied the demo rig, checks cylinder head bolt tension, valve clearances and the two waterpump impellers every service.

The D1.7L has changed my views on small sterndrives and, mated with the Mako, I would have no hesitation in choosing this option over a four-stroke or DFI two-stroke outboard!