Four-strokes rule tender outboards

Traditionally, outboards for yacht tenders under 3m have been direct-drive two-strokes such as the Tohatsu 3.5 and the Mariner/Yamaha twos. But as the four-stroke Honda BF2D and Yamaha F2.5A are so fuel efficient and clean-running, why buy a two-stroke outboard?

Both engines have straightforward single-cylinder OHV engines with gear-driven camshafts and simple splash lubrication, which eliminates the need for an oil pump or low oil pressure warning system. Also, both engines can still be carried in one hand, although they're substantially heavier than the two-stroke competition.

While both motors comply with US EPA 2006 exhaust emission regulations and California Air Resources Board (CARB) 2004 requirements, Honda and Yamaha have taken completely different approaches to the engineering...

THE HONDA BF2D
Released on the Aussie market in mid-1998, the BF2D develops 2.0hp at 6000rpm from its 57cc powerhead and has a Wide Open Throttle (WOT) range of 5000-6000rpm. The claimed dry weight is 12.7kg.

Unlike its long-running BF2A predecessor, the BF2D has a centrifugal clutch and twistgrip throttle control, plus an entirely aircooled powerhead without the watercooled exhaust of the BF2A. At 2.42:1 the gear ratio is also substantially deeper, allowing the powerhead to rev more freely while still using the existing 4.7in pitch high-thrust plastic prop.

An automatic tilt lock is fitted and the four trim positions are easily adjusted, while a vertical sight glass viewed through the lower cowl enables instant oil-level checking.

Unfortunately, the large carry handle provided can't be used for carrying the motor, as the cylinder is then below the sump and sump oil can run into the cylinder. The motor must be carried using the small fingergrip in the aft end of the lower cowl, which becomes uncomfortable over long distances.

The aircooled powerhead eliminates the need to freshwater-flush the motor before stowing it away. When operating in weedy waterways, there's no risk of the motor seizing due to weeds blocking the cooling water intakes - which can happen frequently with small motors that don't have water intake screens. When negotiating snag-infested waters, the prop stops spinning by simply closing the throttle, unlike a conventional dog clutch, where the motor must be quickly shifted into neutral.

But because of the relatively low viscosity SAE10W30 Honda oil needed for effective lubrication of the rocker gear, the BF2D should not be operated in ambient temperatures higher than 32°C. This prevents it from being used in the tropics.

The loan motor starts easily and has only once blown oil smoke, after a three-month period of disuse. Mounted on my 2.4m roundbilge fibreglass yacht tender, pushing a total of 180kg and averaging 4kt with the throttle opening limited to half, the long term evaluation BF2D consumed 0.33lt/h. It also gave 2.7 hours of operation from the integral fuel tank, which has a useable capacity of 0.9lt. At WOT the Honda averaged 5.8kt consuming 0.85lt/h. At two-thirds throttle it towed a 6.7m Bluebird yacht displacing 1.7 tonnes at 3.5kt.

Undercowl access is good and recommended servicing intervals are every 50 hours or six months after the first 10 hours.

So far the loan motor has clocked-up 108 hours averaging 0.29lt/h with a total of 1% WOT operation and a fuel/oil ratio of 593:1. Over the last 36 hours of operation, with the throttle opening limited to half, the consumption was 0.25lt/h and the F/O ratio 893:1 - excellent for an aircooled four-stroke. But after 540 hours of saltwater leg/lower unit immersion, there was some rust on the overhead recoil starter pawl plate due to salty air being sucked past this by the cooling fan.

THE YAMAHA F2.5A
Released locally in mid-2002, the F2.5A has a forward-neutral gearshift and a thermostatically controlled, watercooled powerhead (plus a replaceable zinc anode in the cooling passages) developed specifically for marine usage.

The 72cc unit develops 2.3hp at 5500rpm and has a WOT rev range of 5250-5750. The lower unit straight from the two-stroke Yamaha 3A has a 2.08:1 gear ratio. The claimed dry weight is 17.0kg.

Although the F2.5A has a twistgrip throttle, an automatic tilt lock and a well-placed carry handle, the horizontal oil sight glass instead of a dipstick or vertical sight glass is just plain quirky. A 10mm socket wrench is needed to alter the four trim positions and carburettor access is poor. And although the demo motor started easily and warmed quickly from cold, it had higher vibration levels than the Honda.

Mounted on the 2.4m tender, spinning a 6in weedless alloy prop, pushing a total of 180kg and averaging 4kt, the F2.5A consumed 0.31lt/h, giving 2.9 hours from the 0.9lt integral fuel tank. At WOT the Yamaha averaged 6.0kt, consuming 0.80lt/h. At two-thirds throttle it towed the Bluebird at 3.6kt.

Recommended servicing intervals are every 50 hours or six months after the first 10 hours.

Due to the lack of a dipstick, I was unable to accurately check oil consumption on completion of testing. But after a total of 72 hours of saltwater leg/lower unit immersion, no corrosion was apparent anywhere on the motor.

PRICING AND SERVICING COSTS
As of October 2002, the BF2D retailed for about $1100. According to Jim Nicol from Honda of Blacksmiths, NSW (tel 02 4971 6077), excluding parts the servicing costs for the first year were $285, then $190 for the second year onwards. A spare plastic prop cost only $58.

The F2.5A retailed for around $1450 and according to Belmont Affordable Boats of Belmont, NSW (tel 02 4947 0055), excluding parts the yearly servicing costs are $255, then $185 onwards. A spare alloy prop cost $99.

AND THE WINNER IS...
The F2.5A is more powerful and more fuel efficient than the BF2D. If you operate in tropical waters, it's the only choice, because it can use SAE10W40 Yamaha oil, which provides adequate lubrication up to 40°C of ambient temperature. But for temperate climates, I would opt for the overall more user-friendly Honda.

Whichever motor you choose, the standard of engineering in both is years ahead of the two-stroke competition. From my extended testing of the Honda, there's no way I'd consider buying a small carburetted two-stroke ever again!