Mighty light

Rick Huckstepp | VOLUME 25, ISSUE 3

At just 120kg, Yamaha’s new F70A four-stroke is no lightweight in the power stakes.

The make-up of the Australian outboard market is very interesting. Going on surveys conducted by Yamaha, two-stroke motors still account for 50 per cent of the market, but Yamaha says buyers are showing increased interest in four-stroke technology.

Sales figures generally are dominated by 40hp engines, which account for 40 per cent of Australia’s sales and in NSW, Victoria and Queensland, boaties have flocked to the 4.5m to 5.0m category of boats to the tune of 165,000 in total.

Little wonder, then, that Yamaha was keen to announce the arrival of its F70A four-stroke engine, which is aimed squarely at this end of the market.

Power-to-weight ratio was clearly a big focus for engineers, with Yamaha claiming the new F70A, at 120kg, is the lightest powerplant in its class; specifically, lighter than all other four-strokes and direct-injection two-strokes of similar output. According to Yamaha, the nearest four-stroke competitor weighs in at 155kg, and 145kg in the two-stroke category.

And while its 60hp F60A remains a big player in the market, this latest 70hp engine could be seen to be aimed squarely at the repower market and to those looking to maximise their power options on smaller, more economical boats.

The F70A boasts a 16-valve single overhead camshaft design and an inline four-cylinder configuration. By comparison with the F60A, pistons are 10 per cent lighter, while air intake volume has been increased by 17 per cent for more power. The intake tracts have been tuned and extended, while the intake box does a very effective job of silencing intake noise levels.

According to Yamaha, pre-release testing of the engine produced some revealing figures.

The first test boat was a Sea Jay 4.85 Haven Sports, which had a dry weight of around 500kg, with the hull rated to a maximum of 80hp. The second test rig was a CruiseCraft 4.85 Explorer, with a dry weight of 631kg and rated to a maximum of 90hp. Both boats were available for testing on the release day.

Yamaha says the Sea Jay, with two crew, achieved 60.95km/h at 6300rpm, with a fuel burn of 24lt per hour for a consumption rate of 2.54km/lt.

The CruiseCraft at the same rpm maxed out at 54.45km/h, with a total of 23.60lt, working out to 2.31km/lt.

The best distance per litre for the Sea Jay was 3.64km at 3500rpm/29.70km/h. This averaged out to a 8.15llt/hr fuel burn.

On the other hand, the CruiseCraft’s best figures were 3.20km/lt at 4500rpm, with an average speed of 36.95km/h. The fuel burn was 11.55lt/hr.

The test day saw three large adults aboard each of the boats and I have to say they performed admirably. Especially so the CruiseCraft, which would have weighed around 1100kg fully loaded. The F70A understandably did not have blinding hole shot on this hull, but climbed onto the plane easy enough and could maintain its speed during hard turns without bogging down.

The fact that these engines can swing the same props (‘K’-series) as the F80 through to the F115 engines is testimony to the amount of torque being generated and passed on to the gearcase, which has a ratio of 2.33:1.


Power: 51.5kW(70hp) at 5800rpm

Engine type: 16-valve SOHC inline 4-cylinder four-stroke

Displacement: 996cc

Compression ratio: 9.4:1

Weight(including prop): 119kg

Gear ratio: 2.33 (28/12)

Propeller: 13-5/8 x 14

Recommended retail price: from $10,412 (incl GST)