The most powerful of a trio of engines from 75-115hp, the Mercury/Mariner OptiMax 115 is the first direct fuel injection 115 to meet CARB 2008 three-star exhaust emission regulations. The only other DFI in this power range, the Evinrude 115 DI, currently meets CARB 2004 two-star and US EPA 2006 requirements.

The OptiMax 75, 90 and 115 are essentially OptiMax 200/225 V6 powerheads cut in half, which gives the OptiMax 115 the smallest piston displacement of any 115 on the market. And being a big three-cylinder engine, it has considerably higher vibration levels when trolling than the inline four-cylinder four-strokes and the V4 Evinrude.

But where the OptiMax 115 really scores is its incredible fuel efficiency; better than any other 115hp outboard currently available.

The demo Mariner was spinning an 18in-pitch stainless-steel Quicksilver Vengeance prop and was mounted on a Stacer 565 Coral Master Sports runabout, which displaced a total of one tonne including the two adults aboard.

The Mariner was tested using a handheld Lowrance GPS for speed and Mercury's user-friendly SmartCraft (standard with the OptiMax 115) for fuel flow, with an average of two runs recorded for accuracy.

Because of the Stacer's Mod Pod transom, designed to handle an outboard of up to 230kg, the much lighter weight of the Mariner allowed the hull to "lift" simply out of the hole. When the throttle lever was floored from fast idle, the hull planed in a couple of seconds and the engine reached 5000rpm so quickly that we had a hard time trimming out the leg fast enough to compensate. Through full-lock figure-eight turns up to 4000rpm, no prop ventilation occurred.

When trolling, the Mariner used the same amount of fuel as a Suzuki DF40 on a Clark Navigator runabout displacing 590kg and a Tohatsu TLDI 90 on a Signature 520C displacing 920kg - 30 per cent less fuel than an Evinrude 115 DI on a Quintrex 530 Freedom Sport displacing 1010kg, and 42 per cent less than a Yamaha F115A on the same hull but displacing 970kg. However, the vibration levels were higher than all but the Tohatsu.

At 2000 revs the Mariner used an incredible 36 per cent less fuel than the Evinrude at the same revs, which averaged 13.3kmh - and at 4000 revs, 16 per cent less than the Evinrude, which was averaging 44.7kmh. The Yamaha used 15.7lt/h but averaged only 39.0kmh, and in terms of fuel used for distance travelled, the Mariner still had the same fuel efficiency.

At Wide Open Throttle the Mariner averaged exactly the same speed as the Evinrude and Yamaha, but used 16 per cent less fuel than the Evinrude and one per cent less than the Yamaha.

The Achilles heel of the OptiMax 115 is its noise levels, which are slightly higher at WOT than the Evinrude and also higher than the wonderfully quiet Yamaha. The exhaust note tended to resonate through the Stacer's hull structure, although on a 'glass hull this should be less noticeable.

The demo Mariner started instantly hot or cold, although some oil smoke was emitted on initial cold starting, and when warming up the engine there was a slight oil smell - but none appeared after normal operating temperature had been reached.

Providing the anti-ventilation plate was kept at least three quarters immersed, power astern was good and the Mercury remote control box had smooth, progressive shifting into ahead or astern. Above 800rpm the vibration levels reduced to little more than the Evinrude or Yamaha.

Servicing the OptiMax appears straightforward, with the fuel injectors, belt-driven air compressor and alternator easily reached. The recommended servicing intervals are every 100 hours or yearly (also for waterpump impeller replacement) after the first 20 hours.

The warranty provides three years of coverage for recreational usage.

* Dead Slow Troll
** Offshore troll
*** Minimum plane
**** Clean plane
***** Most economical cruise
****** Maximum cruise
******* Wide Open Throttle