Mercury 5F outboard motor review

Going green means compromises. See how that applies this 5hp Mercury outboard motor.

For 25 years the Mercury 5M outboard motor has been the best performing single-cylinder two-stroke five engine on the Aussie market. Its 20kg dry weight plus impressive torque and reasonable fuel efficiency make it unbeatable in its power range for powering cartop dinghies and punts from 3 to 3.5m. And its big displacement 102cc loopcharged powerhead creates enough vibration to reduce flab when trolling, without numbing a tiller arm while planing. Early models were made in Belgium but the current Japanese models have a way better finish. Best of all, in NSW you don’t need a boat driver’s licence to operate it – one way you can prevent being ripped off by a government!

Trouble is it has an OEDA 1-star exhaust emissions rating and frankly is too dirty to be used on freshwater lakes and rivers.

This is where the Mercury’ four-stroke F5 outboard motor comes in. It has an OEDA 3-star rating plus an upfront gearshift that makes holding a rod and controlling the engine way easier. Its overall fuel efficiency, or distance travelled for fuel used, is also way lower than the 5M.

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MERCURY F5 OUTBOARD MOTOR

The Japanese-made F5 has come a long way since its original local release 15 years ago. Back then it had a side gearshift but the current model, released four years ago, has the upfront shift and a well-rounded upper cowl that won’t snag fishing lines.

The F5 develops five brake horsepower (1bhp equals 746W) at 5000rpm with a Wide Open Throttle range of 4500 to 5500rpm. The 123cc crossflow engine has a gear driven camshaft with pushrods to operate the OHV arrangement and unlike the 5M, there’s a thermostat to maintain even running temperatures. The pressure lubrication has a low-oil-level warning light and the electronic ignition timing advance means no linkages to adjust. A 5amp unregulated alternator is optional.

Standard are a 1.15lt integral fuel tank and a 12lt remote for longer runs, while the six trim positions allow for fine tuning of leg angle. One shallow-water drive position is fitted plus a strong tilt lock for fully tilting the engine. The dry weight is a reasonable 26.1kg but unlike the 5M, a large carry handle isn’t fitted so two hands are needed.

Servicing is straightforward with intervals at 100 hours or annually after the first 20 hours, though the 0.45lt sump is a bit small for a five. I recommend using Quicksilver FCW SAE10W30 oil for all temperate climates. The recreational usage warranty is five years.

PERFORMANCE

This is where the compromises appear. Mounted on my 1978 flat-bottomed de Havilland John 10 punt cartopper, swinging the standard 8in pitch alloy prop to match the 2.15:1 gear ratio, a loan 5M easily planed our 255kg total displacement, including two adults and fishing tackle. Once up, it could also be throttled back and maintain a clean plane with good fuel efficiency.

Not so the F5 which has the same prop and gear ratio. While it delivered way better trolling efficiency with hardly any vibration through the tiller arm, the additional 6.1kg on the transom was enough to increase hull loading and bottom sheeting friction to the point where only at WOT did the F5 plane the John 10.

Throttling back dropped the hull off the plane to an inefficient semi-planing mode. Also, at or near WOT vibration levels were way higher than the 5M due to the cyclical reduction of rpm during the exhaust and compression cycles. This happens with all single-cylinder four-strokes.

The through-prop hub exhaust limits power astern and unlike the 5M which can be swung 360 degrees, the F5 has only a 160-degree steering angle to prevent sump oil from flooding the cylinder when the engine is tilted. So it’s a matter of coming into ramps slowly!

Carbie vapour lock sometimes occurred after the engine had been stopped for 15 to 20 minutes making restarting a bit of a chore, but longer periods created no issues. After a total of 50 hours of saltwater leg/lower unit immersion no corrosion was apparent anywhere on the loan engine.

THE VERDICT

On the right hull the Mercury F5 is a good alternative to the 5M. The F5 is well engineered and with regular maintenance should last years, while being very economical to run. It doesn’t reduce flab while trolling but at or near WOT, driving the F5 is like having a gym and outboard rolled into one, making it better value than an outboard alone!

MERCURY 5 OUTBOARD MOTOR SEA TRIALS

Two-way runs across a measured distance using portable tachometer and in-line fuel-flow gear

Engine	Mercury F5	Mercury 5M
TROLLING
SPEED (KTS)	2 (trolling)	2
RPM	1100	1100
FUEL BURN (LT/H)	0.16	0.27
CRUISING
SPEED (KTS)	8.9	11.5
RPM	4500	5000
FUEL BURN (LT/H)	1.3	1.7
WIDE OPEN THROTTLE
SPEED (KTS)	11.6	16.7
RPM	5200	6000
FUEL BURN (LT/H)	1.7	2.5