Yamaha F25A

By incorporating a counter-balancing piston into what is a relatively simple twin-cylinder powerhead, Yamaha has produced an engine that performs as well as its three-cylinder Honda BF25 competition but suffers none of its carbie-balancing problems for just like Yamaha's torquey F15A there's only one carbie with an accelerator pump.

Another advantage of the F25A is the standard option of power trim and tilt, which makes it ideal for retiree-oriented aluminium runabouts such as the 4.2 metre Quintrex Getaway.

The demo F25A provided by Hunter Valley dealer Terrace Boating was well suited to this hull and though somewhat underpropped, provided a reasonable blend of planing performance with all-day trolling capability. Our total displacement was 560kg including two adults and basic gear.

Spinning a nine-inch pitch prop that would have been better suited to a semi-displacement LFB hull around 5.5 metres, the demo F25A wound out to only 38.2kmh at 6280rpm, consuming 9.8lph. Fitted with an 11.25inch prop it could have reached almost 43.5kmh at 5600 revs, using around 9.5lph.

Noise levels at WOT (Wide Open Throttle) were comparable to the Honda BF25 with slightly higher vibration levels across the entire rev range. Overall, the F25A was much smoother than comparable twin-cylinder two-strokes.

The F25A's anti-ventilation plate was located midway between the hull bottom and full length external keel, and prop ventilation only occurred in hard-over WOT figure of eights.

By using the quick-acting power trim to full advantage, the F25A planed the Getaway almost as fast as a two-stroke 25 not bad considering the relatively heavy weight for its output. But because of the underpropping, 5180rpm were needed to achieve a clean plane at 29.9kmh, where the F25A consumed 5.9 lph.

Fortunately, the second F25A tested was fitted with a 10.5inch prop. An electric start, manual trim and tiller steer model, it was well suited to the Quintrex 420 Hornet Trophy, which displaced a total of 500kg. We planed at 28.3kmh on 4210rpm consuming 5.0lph and at WOT we averaged 42.7kmh (6010rpm) using 9.6 lph. This consumption figure is about 20% less than a comparable two-stroke 25.

Unlike the Getaway there was no prop ventilation. Prop-steer torque was minimal with the tiller arm a comfortable reach from the Trophy's helm seat. At DST (Dead Slow Troll) this motor averaged 3.6kmh on 830 revs, consuming only 0.5lph 40-50% less than a two-stroke 25. Of course, there was no oil smoke.

Fitted with cold-start fuel primers, both motors started instantly cold or hot and both stayed hot while trolling. No cooling water starvation occurred, providing the anti-ventilation plates were kept immersed and at all speeds the pilot water discharges were clearly visible.

Undercowl space is a little tight but the F25A still appears straight forward to service. It's made easier with features such as a spin-on oil filter. Recommended servicing intervals are every 50hrs for the first two years after the initial 10hr break-in period when the valves must be adjusted. This schedule is followed by every 100 hours from then on, including replacing the waterpump impeller.

The oil in the large 1.7 litre-capacity sump should be changed every six months, regardless of how many hours are clocked up.

Four strokes don't suit all applications and you should really work out whether you can afford the higher purchase price and servicing costs of one. If, like most saltwater boaters, you clock up only 40 hours per year, my advice is to stick with a two-stroke such as Yamaha's CV25.

If you fish a mix of fresh and saltwater, do a lot of trolling and care about the environment, Yamaha's F25A is the way to go.