Review: Mercury FourStroke 5hp outboard motor

For 25 years the Mercury 5M outboard motor has been the best performing single-cylinder two-stroke 5 hp outboard 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. This portable engine also has a big displacement 102cc loopcharged powerhead which 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.

MERCURY 5HP PORTABLE OUTBOARD MOTOR

The Japanese-made Mercury F5hp outboard 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 portable Mercury FourStroke 5hp outboard 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 on the Mercury 5hp four stroke 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 the Mercury FourStroke 5 is straightforward. Intervals are 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 Mercury 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 Mercury F5 four stroke outboard 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 Mercury F5 outboard 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 Mercury FourStroke 5 hp outboard 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 TRADE-A-BOAT VERDICT

On the right hull the Mercury F5 outboard motor is a good alternative to the Mercury 5M. The F5 Mercury 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!

As of April 2014 the short-shaft F5 had a price of around $1800 and a spare propeller for about $120.

Thanks Tomos Marine, Marks Point, NSW, phone (02) 4945 3202, for supplying the loan engine.

MERCURY 5 HP OUTBOARD MOTOR SEA TRIALS

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

“Loop” of cruising with 10 per cent WOT and 40 per cent trolling operation, averaging 4kts

* With 20 per cent reserve.

MERCURY 5HP FOUR STROKE OUTBOARD SPECS

MERCURY FOUR STROKE 5HP PRICE: $1800 RRP

TYPE Single-cylinder four-stroke 5 hp outboard motor

REC. RPM RANGE 4500 to 5500

DISPLACEMENT 123cc

BORE x STROKE 59 x 45mm

WEIGHT 26.1kg

GEAR RATIO 2.15:1

WARRANTY 3 years

OEDA STARS 3

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Originally published in Trade-A-Boat #458, October / November 2014. Why not subscribe today?