Mercury’s 115 hp FourStroke and 125 hp OptiMax Outboard Comparison

Mercury puts its new lightweight 115hp FourStroke up against the punchier 125hp OptiMax two-stroke—both tested on identical Streaker hulls. It's a head-to-head showdown between modern efficiency and proven performance. Which outboard comes out on top? Let’s find out.

TwoStroke vs FourStroke Outboards

Boat engines are getting lighter and lighter as outboard manufacturers find more ways to pull maximum performance from more efficient blocks, mountings and drive mechanisms. 

With the release of its magnificent new Mercury 75 to 115hp 2.1lt four-stroke outboards last year, industry giant Mercury said: “Unthink everything you know about four-strokes.” This new range followed the release of the industry-leading Mercury 150 hp FourStroke back in 2013 and now its 135hp sibling.

The Mercury OptiMax range of direct-injected two-stroke outboard motors has been a winner. They provide drastically improved fuel efficiency combined with gutsy performance, reasonable noise levels, and similar service costs. The mid-range 125hp OptiMax has been a very popular engine in a high-demand section of the outboard market, while many (but not all) four-stroke outboard competitors in the 90 to 130hp range have traditionally been quite disappointing.

Clash of the Mercurys: The Ultimate Face-off

So how will a new Mercury 115hp FourStroke compare against the 125hp OptiMax?

Our good friends at Streaker provided us with two identical hulls, one a Streaker 5700 Navigator and the other a Streaker 5700 Bluewater. The only variance was small changes in deck construction. The weights and wind resistance would be virtually identical, as would the fuel and passenger load.

The Streaker Navigator had the Mercury 125hp OptiMax, and the Streaker Bluewater, the all-new Mercury 115hp FourStroke, fitted with the optional CommandThrust gearbox.

The Mercury four-stroke was mounted on the third hole and fitted with an Enertia 19P three-blade stainless steel propeller, allowing for the larger gearcase and higher ratio of 2.38:1 from the new CommandThrust leg. The Mercury OptiMax two-stroke engine was fitted slightly higher, on the second hole, and had a Mercury Vengeance three-blade 18P stainless steel propeller combined with its 2.07:1 gearcase ratio.

Our experience from past testing is that the new CommandThrust gearcase doesn’t necessarily give you greater top-end speed but instead delivers a very noticeable increase in positive drive throughout the entire rev range with no discernible slippage and a more solid and controlled drive at the wheel. The difference in handling characteristics is amazing, especially when you compare the two gearcase options on identical rigs, as we have.

The Results Are In

The results speak for themselves. The all-new Mercury FourStroke achieved a top-end speed of 37.8kts at wide open throttle at 6000rpm, where it used 44lt/h. It just pipped the larger Mercury 125hp OptiMax at the post, the OptiMax delivering 37.3kts at 5700rpm WOT but using less fuel at 40.5lt/h. So, with 10 less horsepower, the all-new Mercury four-stroke ever so slightly outperformed the larger OptiMax 125 on paper and in the water, but it used a trickle more fuel to do it.

Realistically, the comparison was as close as it gets, allowing for windage, etc. However, what the figures don’t describe is the four-stroke's more pleasurable acoustic characteristics and that indescribable feeling of driving a much more satisfying and efficient combination.

The figures revealed a similar story throughout the rev range. For example, at a more realistic common travelling speed, at 4000rpm, the two outboard motors again returned comparative figures. The new Mercury 115hp four-stroke did 22.4kts using a much more satisfying 20.5lt/h; the Mercury OptiMax was ever so slightly less, at 21.65kts at the same 20.5lt/h. The difference is negligible, and such minimal differences may be influenced by wind gusts at the time—but remember that the four-stroke is rated at 10hp less than the Mercury OptiMax.

Figures are good, but what I need to describe is the positive holeshot from the Mercury 115 FourStroke. It certainly delivered greater acceleration throughout the vital low to mid rev range where you need it. The slightly larger prop and gear ratio delivered smooth, positive, and gutsy power right through the rev range, whereas the OptiMax felt a little peakier and certainly lost some traction through both acceleration and turns.

The Final Verdict

Weight is vital for outboards, and the all-new Mercury 115 four-stroke weighs in at 165kg, 5kg under that of its 125hp OptiMax stablemate, even with its larger and heavier Command Thrust gearcase. This is an enormous achievement in outboard technology and should be applauded.

Four-stroke engines are famous for their longevity and the Mercury FourStroke range is backed by its enormous worldwide service network, the comfort of its lifetime maintenance-free valve train design, a three-year corrosion warranty and a non-declining 3+2 = 5-year factory-backed warranty.

Wrap all this up with Mercury’s huge propeller selection to provide a perfectly matched package, and the pleasure of boating gets better and better.

115 HP Mercury FourStroke Performance

5.7m Streaker 5700 Bluewater with single 115hp Mercury FourStroke, two passengers and more than half fuel.

* Sea-trial data supplied by the author.

125 HP Mercury Optimax Two-Stroke Performance

5.7m Streaker 5700 Navigator with single 125hp Mercury OptiMax two-stroke, two passengers, and more than half fuel

*Sea-trial data supplied by the author.

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This article was written by and photos provided by John 'Bear' Willis and previously appeared at TradeaBoat.com.au, now powered by Only Boats.

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