Shortshaft Mercury 15

Mercury rising

Released locally in 1988, the Mercury 15 has been unique in its power range because it’s the only two-stroke outboard to have a combined throttle and gearshift on the tiller handle. For serious amateur anglers, this one-handed control is simply unbeatable because it enables the other hand to be left completely free for holding a rod while trolling.

But there’s more to the 15 than just one-handed steering and throttle/gearshift control. Simply by pushing down on the tiller handle when planing with the throttle backed off momentarily, the gearcase drag allows the leg to be trimmed out slightly for reduced hull skin friction and subsequent drag when running downwind.

Of course, upwind it’s not possible to trim the leg in, but forward motion can always be reduced to trolling speed and the leg trimmed back in to keep the bow down. In addition to the three trim positions there are three shallow water drive positions - ideal for sneaking up shallow waterways to a favourite ‘hole’.

Developed from the US-built 8 and 9.9, released locally in 1986, the 15 differs from its Tohatsu-built Super 15 counterpart in that it has a crossflow cylinder head, enabling the US-built 15 to troll smoothly for hours. In comparison, the loopcharged Super 15 is best left for drift fishing and quick runs from and back to the launching ramp. Vibration levels when trolling are much higher.

Cold starting is also easier with the US-built engine, as pulling the knob out a couple of times ensures the engine has the right amount of fuel for starting. Yamaha’s 15F comes close with its combined choke and throttle and ignition timing advance but, although potentially less troublesome than a primer system, cold starting the Yammie is not as positive as the Merc.

Some time ago I had a Mercury 15 for six months and at no time did any ‘slop’ develop in the throttle/gearshift linkages and the overlap between shifting and throttle advance always remained positive. The loan engine always started first pull from cold and emitted oil smoke only below about 2000rpm.

The loan engine warmed quickly from cold and always started easily when hot, without suffering any fuel vapour lock.

Mounted on my 3.6m Sea Al Super Skua tinnie, spinning a 10.5-inch pitch alloy prop and pushing 285kg, the Merc trolled at 3.9km/h using 0.8lt/h, and averaged 41.3km/h at wide open throttle using 8lt/h. Over a ‘loop’ of cruising, trolling and including 10 per cent of WOT operation averaging 15km/h, the Merc used 3.3lt/h. This gave more than seven hours of running from the standard fuel tank.

A lot more gutsy than a US-built 9.9 I had tested a couple of years earlier on a similar hull, the 15 had no trouble towing a child on a boogie board at planing speeds with two adults and one child aboard. It was reasonably quiet at WOT, with very low vibration levels at the tiller arm due to the crossflow powerhead.

No corrosion was apparent anywhere on the loan engine, even after 100 hours of saltwater leg/lower unit immersion. Owner reports indicate that the Merc 15 has consistently good saltwater corrosion resistance.

Recommended servicing intervals are every 100 hours or once a year (including waterpump impeller replacement) after the initial 20-hour checkup. Powerhead access is excellent with the spark plugs, carbie and throttle/gearshift linkages easily reached. The bowl-type fuel filter, located where any contaminants in the fuel can be quickly seen, is a nice touch.

Prop-swapping is very easy compared to the direct  competition.

For recreational usage the warranty coverage is three years.