Mercury OptiMax 150 Outboard Motor Review

The Mercury Marine OptiMax 150 is the middle of three direct fuel-injection two-stroke outboards from 135 to 175hp, all using the same 60° V6 powerhead.

The 2.5lt loop-charged powerhead has been around for a long time and is one of the best Mercury engines. In carburetted and EFI form, it can develop up to 200hp. The combination of a relatively low WOT (wide open throttle) range — from 5250 to 5750rpm — plus a big cylinder bore, and short piston stroke ensure its long lifespan.

I’ve even tried a race version running on Avgas, and this pushed us to 84kts (155 km) in a 5.5m ski boat on the upper Hawkesbury River.

Durability and Efficiency

Unlike some of its four-stroke competition, the Mercury OptiMax 150 outboard motor is an actual marine engine with a diecast alloy cylinder block and head designed to handle the rigours of direct saltwater cooling. Spare parts are readily available, and Mercury even offers a rebuilding program for powerheads.

As with all Mercury OptiMax engines, the 150 utilises the Orbital Engine Corporation of WA dual-stage OCP (Orbital Combustion Process) direct injection with stratified and homogenous modes. This differs from the Evinrude E-TEC system in injecting atomised air and fuel rather than just straight fuel via unit injectors, as in a direct-injection car engine. At low revs (below 1500rpm or thereabouts), the air/fuel mix with fuel droplets down to only five microns is injected in a narrow conical pattern directly at the spark plug. This creates a normal air/fuel ratio of 14.7:1. However, this is as lean as 70:1 overall in the combustion chamber.

Above 1500rpm, the overall air/fuel ratio increases to generate the torque needed to plane a hull. Combined with fuel/oil ratios varying from 44:1 at WOT down to 400:1 when trolling, the 150 is very economical for its output. The standard in-boat oil tank holds 11.4lt.

Handling, Power and Fuel Consumption

At 201kg in an extra long shaft (25in), the Mercury OptiMax 150 outboard motor is much lighter than any direct four-stroke competition. I tested one on an Archer 58C plate-alloy cuddy cabin, pushing 1350kg and spinning an 18-inch pitch Quicksilver Mirage stainless steel prop. The demo 150 was slightly overpropped during the test and could have been done with a 17-inch prop. Even so, it performed flawlessly with low vibration levels across the entire rpm range and scarcely any harmonics through the hull structure.

The holeshot acceleration was matched only by other 150hp DFI two-stroke outboards, and only when I poured on the power did the typical Mercury V6 “howl” become noticeable. However, the Mercury OptiMax 150 outboard was noisier overall than the four-stroke competition.

There was absolutely no prop ventilation through full-lock figure-of-eight turns at 4000rpm — quite an achievement on a plate hull with an extended external keel.

Hot or cold, the 150 started instantly with no oil smoke appearing at any time, nor was there an oil smell when backing upwind. However, the Mercury control box shifting action was “notchy” and should be improved.

Providing the anti-ventilation plate was kept at least three-quarters immersed, power astern was good, and no cooling-water starvation occurred. When trolling, the fuel consumption was comparable to that of a 60hp four-stroke outboard and a carburetted 25hp two-stroke. Best of all, after one hour of performance testing, the 150 had used only 9.5lt—a superb figure for any 150hp outboard!

Powerhead access is good, with the belt-driven air compressor and 60amp/h voltage-regulated alternator easily reaching. The air and fuel injectors are also situated in neat blocks.

Price

Mercury Marine recommends servicing the 150 every 100 hours or annually after the first service at 20 hours. Providing this is done by an authorised service centre. The recreational usage warranty stands at five years.

As of April 2011, the recommended retail price was $19,00,0, with a spare Mirage prop that set you back $1080.

Mercury Optimax 150 Performance

Competitor Outboards

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This article was written by Andrew Norton and previously appeared at TradeaBoat.com.au, now powered by Only Boats.

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