Mercury 6hp portable four-stroke outboard

 

The Japanese-made Mercury 6hp FourStroke has a smaller displacement than its direct four-stroke outboard motor competition, but this translates to overall better fuel efficiency.

The Mercury has gear-driven pushrod actuation of the rocker gear, a thermostatically controlled cooling system, electronic ignition timing advance and pressure lubrication. A low oil level light is behind the handle and a 12V 5amp alternator is optional. There are six trim positions with a single shallow-water drive setting and automatic full tilt and reverse locks. There’s also an upfront gear shift.

Cleverly Mercury offers a 1.15lt integral fuel tank and a 12lt plastic tote tank as standard, yet these don’t affect powerhead access. Unfortunately the tote tank fuel line doesn’t detach from the tank making cleaning the tank filter screen a pain.

 

Maintenance and warranty

The engine oil dipstick, spark plug and inline fuel filter are easily reached and removing the rocker cover to check valve clearances is easier than the Mercury 2.5 FourStroke I’ve been testing. The well-rounded upper cowl has effective water-trap air intakes that also vent engine heat on hot days.

The 0.45lt engine sump is a bit small for a six to absorb oil dilution during extended trolling periods, so I’d change the oil every six months or 50 hours. A sump oil drain elbow under the engine pan makes changing the oil easy. I recommend using Quicksilver FCW SAE 10W30 oil for all temperate climates. The warranty coverage is “3+2” years.

 

Engine performance

Swinging the standard 8in alloy prop, the loaner six was well-matched to the flat-bottomed Sea Jay 3.4 Punt which measures 3.4 by 1.42m and has six transverse frames, 1.6mm sheeting and all-welded construction. The perfect small skinny-water casting platform! Carefully run-in before my long-time co-tester Richard Ardizzone and I commenced performance, fuel efficiency and noise testing; overall the six was easier to operate than the competition. Its decompression arrangement made starting effort no greater than the 2.5 and oil smoke appeared only after the engine had been transported on its side.

However having to leave the six in forward gear to set shallow-water drive or fully tilt it, is a pain and strange considering the two-stroke five can be tilted in neutral. Way easier than having to overcome prop thrust to tilt the six enough to engage shallow-water drive.

Trolling is the forte of any four-stroke outboard and the six delivered incredible fuel efficiency. Not only were vibration levels low but across the entire rpm range it was quieter than the two-stroke five. Midrange vibration levels were reasonable for a four-stroke single but increased at WOT.

 

The Trade-a-Boat verdict

Mounted on a flat-bottomed punt up to 3.5m and pushing no more than a total of 300kg the Mercury 6hp FourStroke is the way to go compared to a two-stroke 5. Sure it costs more to buy and service, but the engineering and fuel efficiency are so good the extra investment is worth it.

Sea trials

Mercury 5M and Mercury 6hp FourStroke on a Sea Jay 3.4 Punt, standard 8in pitch prop, two adults and fishing tackle totalling 285 and 290kg respectively, average of two-way runs on Lake Macquarie, NSW, calm water.

	Mercury 5M	Mercury FourStroke 6
Trolling
RPM	1100	1400
SPEED (KTS)	1.9	2.5
FUEL BURN (LT/H)	0.3	0.2
Fast troll
RPM	2000	2000
SPEED (KTS)	4	4.1
FUEL BURN (LT/H)	0.7	0.4
PLANE AND CRUISE
RPM	4900	5100
SPeED (KTS)	11.9	12.3
FUEL BURN (Lt/h)	2	1.7
WOT
RPM	5200	5700
SPEED (KTS)	13.8	15.3
FUEL BURN (LT/H)	2.5	2.1
Cruise inc. 10% WOT and averaging 4kts
FUEL BURN (LT/H)	0.87	0.52