Yamaha F15

Despite still being most popular horsepower class in Australia, two-stroke 15hp engines have largely been overshadowed by the four-strokes in recent years. For anglers who troll extensively or mainly fish freshwater, the Honda BF15, Johnson/Suzuki DF15, Mariner/Mercury F15, Tohatsu MFS 15B and Yamaha F15A are a better choice than any comparable two-strokes.

But for anglers who fish mostly saltwater, don't use their engines frequently enough to justify the additional initial and servicing costs of a four-stroke and who want a reasonably portable 15hp, the fishing-orientated Johnson 15, Mariner/Mercury 15 and Yamaha 15F are still worth buying. And of all the 15s I've tested, based on my evaluation of one since April 1996, Yamaha's 15F provides the best blend of performance, user-friendliness, fuel efficiency and with its 100:1 petrol/oil mix, relatively low pollution.

The 15F's loopcharged powerhead is nowhere near as smooth-running as the crossflow Johnno and Merc and shakes around a bit when trolling. But if I want to troll any distance I mount my MotorGuide T30 electric outboard to starboard of the Yammie. This provides speeds of 2.3kmh to 6.4kmh and will run for more than two hours on a 50amp/hour deep-cycle battery.

Carefully run-in according to Yamaha's recommendations using a 25:1 petrol/oil mix, the test motor was then put through a series of trials under identical conditions to its direct Johnson and Mercury/Mariner competition. Test boat was a 3.6m Sea Al Super Skua aluminium dinghy, which displaced 285kg including two adults, safety gear, ground tackle and fishing tackle. The 15F ran the standard 10.5in pitch alloy prop, the loan Johnson a 12in alloy prop and the loan Mercury a 10.5in alloy prop.

Initially the idle circuit air/fuel ratio was set too rich, and when trolling the 15F consumed 0.9lt/hr, whereas the Johnson 15 averaged 3.7kmh and 0.7lt/hr on 750rpm and the Merc 15 3.9kmh and 0.8lt/hr. No revs for the Mercury were picked up on my Shoptach portable tacho. But by turning the idle circuit pilot screw half a turn to lean the air/fuel ratio, the 15F averaged 4.5kmh on 850rpm using 0.6lt/hr, returning 42 percent better fuel efficiency than the Johnno and 53 percent over the Merc.

All three motors planed the Sea Al on half throttle and provided more than adequate power on this relatively lightweight hull. At Wide Open Throttle the 15F averaged 46.2kmh on 6060rpm using 7.2lt/hr, the Johnno 42.4kmh on 5650rpm consuming 8.1lt/hr and the Merc 41.3kmh using 8.0lt/hr. The 15F was 23 percent more fuel efficient than both the Johnno and Merc.

Over a "loop" of normal cruising and including 10 percent WOT operation averaging 15kmh, the 15F consumed 2.7lt/hr compared to 3.2 for the Johnno and 3.3 for the Mercury, resulting in 19 percent better efficiency than the Johnno and 24 percent over the Merc.

Cold starting has always been first pull regardless of how long the motor has been left unused and since the leaning of the idle circuit, very little oil smoke appears. The motor warms very quickly, stays hot while trolling and the dry exhaust relief makes for a very quiet motor. At all speeds the pilot water discharge is clearly visible and even with the anti-ventilation plate partially exposed, power astern is good and no cooling water starvation occurs.

Servicing the 15F is straightforward and during the first two years, servicing intervals are recommended every 50 and 100 operating hours after the first 10 hours, then every 100 hours or once a year, which also applies to waterpump impeller replacement.

Since testing commenced, the 15F has needed three new waterpump impellers and one new thermostat, as although the motor has been freshwater flushed after every saltwater use aluminium chloride crystal (commonly mistaken as salt) build-up in the cooling water passages has been a problem. Also there are signs of the prop bushing slipping on the prop splines, despite the prop not having hit an underwater obstruction. But no corrosion has appeared anywhere on the motor. The warranty provides two years of coverage for recreational applications.