Yamaha 15F

When Yamaha's 246cc loopcharged powerhead first appeared locally in the Mariner 15 in the late 1970s, it offered boaters fuel efficiency and performance unheard of in this power range.

At the time, all the popular 15s had crossflow powerheads, which not only returned lacklustre performance, but were also fairly thirsty to operate.

So when Yamaha entered the Aussie market under its own name with this motor in late 1982, and also offered the pollution (and fuel cost)-reducing benefits of a 100:1 petrol/oil mix, the company was clearly on a winner! The only trouble with this engine was its antiquated shallow water drive/reverse lock system and side-mounted gearshift lever nowhere near as user-friendly as its US-built competition.

So in 1995 Yamaha completely revised its 15, creating the 'F' model. This motor had well-rounded upper and lower cowls for a low, easy to cast over profile, an upfront gearshift, automatic setting of the two shallow water drive settings, full tilt and a slick new lower unit with removable cooling water intake screens. Best of all was the cold-start system, which combined the choke with slight throttle opening and ignition timing advance. Yes, Yamaha's 15 had come of age, and combined with the long-proven powerhead was the motor with the answer for the vast majority of boaters.

Though for freshwater fishos who do a lot of Dead Slow Troll (DST) operation, the smoother-running crossflow Johnson 15 remains the better choice. I commenced long-term evaluation of a 15F in 1996 and found that for user-friendliness, it was a full generation ahead of the Yamaha 15 I had lived with for some time in 1991. However, unlike that engine, the 15F has always used far more fuel at DST than it should, even on my relatively light 3.6m Sea Al Super Skua tinnie.  For example, spinning the standard 10.5-inch pitch alloy prop and pushing a total of 295kg (including two adults and fishing tackle), at a DST average of 750 revs and 3.7kmh, it consumes 0.9lt/hr or as much as a 430cc Yamaha CV20!

Subsequently, now when I want to troll any distance, I mount an electric outboard.

However, on the main carbie jet, the 15F is a completely different engine. It planes the two-adult load cleanly at 19kmh on one-third throttle, while at Wide Open Throttle (WOT) it averages 46.2kmh on 6060 revs consuming 7.2lt/hr. Under identical test conditions, the 15F returns 23% better fuel efficiency than the Johnson 15 at WOT, and over a mix of 10% WOT operation and normal cruising averaging 15kmh it consumes only 2.7lt/hr.

Cold-starting is always achieved first pull regardless of how long the engine has been unused, but the engine is somewhat smokier than a Johnson 15 for about the first five minutes. The engine warms very quickly and with its effective thermostat, stays hot while trolling.

At all speeds, the pilot water discharge is clearly visible and even on one of the shallow water drive settings, with the anti-ventilation plate partially exposed, there's no cooling water starvation.

Servicing the 15F is straightforward and during the first two years intervals are recommended every 50 and 100 operating hours after the first 10. After the first two years, the engine requires servicing once a year (also for the waterpump impeller) or every 100 hours. For owners who do their own maintenance, Yamaha's own Gearcase Lube Pump makes quick work of changing gearcase oil.

The only concern during the past three years has been some water finding its way into the gearcase past the lower unit oil fill/drain plugs. But I certainly have no complaints about the engine's saltwater corrosion resistance, and apart from a few paint chips on the prop and lower unit the engine is still in showroom condition.