Yamaha Z150P

De-rated from its Z200N counterpart, Yamaha's understressed High Pressure Direct Injection (HPDI) Z150P suits commercial as well as recreational applications.

The Z150P utilises the same direct fuel injection system as the Z200N, where fuel is injected directly into the combustion chambers at 750psi or 50kg/cm2 using three pumps to achieve this pressure.

The first pump is driven by alternating crankcase pressure and the second (3kg/cm2) electrically incorporates a vapour separator. The third and high-pressure pump is driven by a cogged belt directly from the crankshaft and injects fuel particles at only 20-30 microns.

Instead of the Optimax DFI system, which uses a combination of stratified and homogenous combustion depending on engine revs, the HPDI uses homogenous only with an air/fuel ratio of around 20-25:1 at all revs.

Each of the six cylinders have an independent throttle body air intake with butterfly valve connected to an oxygen sensor in the exhaust system and the Electronic Control Module (ECM). This senses air temperature, barometric pressure, throttle and crankshaft position, fuel pressure, engine temperature and of course oxygen levels to maintain the correct air/fuel ratio regardless of load and barometric pressure. The ECM also advances the ignition timing for cold starts and reduces revs should the engine overheat or oil pressure drop off.

If the sensors, high-pressure fuel pump or ECM fail the motor can still function on the electric fuel pump and limp back to the launching ramp, whereas if the air compressor in Optimax models fail the system shuts down completely. A laptop computer is plugged-in to diagnose any faults.

The fuel/oil ratios are no leaner than in the carburetted Yamaha 150F and vary from 50:1 at Wide Open Throttle (WOT) down to 200:1 at Dead Slow Troll (DST). But even in following winds the two motors I've tested refuse to emit any oil smoke or even an oil smell.

Both demo motors started instantly hot or cold and at low speeds (under 2000 revs) had none of the air compressor noise of their Optimax competition, although there was a slight whine from the high-pressure fuel pumps.

Mounted on a Southwind SF550WR Walkaround, spinning a 17-inch pitch stainless steel prop and pushing a total of 1500kg including four adults, the first Z150P demo motor provided as much power as you'd ever want on this hull. It achieved a clean plane at 28.9kmh on 2900 revs and cruised quietly at 59.0kmh on 4000 revs consuming 25.6lt/hr. Through tight figures of eight at these revs there was no prop ventilation.

The WOT average was 78.7kmh on 5600 revs using 56.6lt/hr and a normal conversation could still be held at the helm.

On Southwind's recently-released SF580 Cuddy Cabin, spinning a 17-inch alloy prop and pushing a total of 1500kg including two adults, the second motor handled this hefty hull almost as well. It trolled at 5.5kmh at 700 revs with hardly any vibration being transmitted through the hull and achieved a clean plane at only 27.5kmh at 2800 revs. It also held a clean plane effortlessly at 3000 revs and 28-30kmh into 1-1.5m ocean swells off Barrenjoey Head on Sydney's Pittwater and these revs and this speed would suit extended coastal operation while returning excellent fuel economy.

At 4000 revs it cruised quietly at 54.0kmh and again no prop ventilation occurred through tight figures of eight. The WOT average was 72.2kmh at 5250 revs.

Servicing the Z150P appears straightforward and recommended intervals are every 50 operating hours or six months for the first two years after the initial 10-hour service, then every 100 hours or once a year thereafter, which also applies to water-pump impeller replacement.

In my opinion the Z150P, particularly when mated with the Southwind SF550WR or SF580 Cuddy Cabin, is the logical alternative to the direct Optimax competition. It's well engineered, beautifully finished and should provide years of reliable low-pollution power on fibreglass hulls in the 5-6m range.