Choosing an engine

Tender Loving Care

Tender outboards live a hard life. They often run for only short periods and are rarely warmed-up fully before ferrying passengers to the shore and back. Then they are stowed away for long periods, usually without a freshwater flush.

Unless your boat is equipped with transom davits or has sufficient deck space to lift the tender aboard using the boom, the outboard must be manhandled aboard from the tender. This means that unless you are very strong and/or don't mind permanently injuring your back, outboards under 40kg are the way to go.

This weight limit effectively rules out four-strokes over 8hp, as 9.9's and 15's start at around 43kg. So you're limited to two-strokes to 20hp or four-strokes to 8hp.

Whether you buy a two or four-stroke outboard depends entirely on your personal lifestyle. If the motor will be used infrequently, a two-stroke is far more tolerant of long periods of disuse, whereas a four-stroke must be run at least once a month to maintain protection from internal corrosion around the bearings and piston rings.

Because of their shape, inflatables require a lot more power to push them than comparable tinnies (particularly in rough weather), and this is where the much better power-to-weight ratio of a two-stroke scores over four-strokes. In the event that your outboard gets a mouthful of water (not uncommon in tender moments) it's much easier (and cheaper) to revive a submerged two-stroke.

My tender motor recommendations have been broken up into two and four-stroke categories and there are a total of six motors. This is not to say that other brands won't do a good job, just that these motors are better suited to the task of powering a tender than the others. And some new models were not available to test for this guide; for example, there's a new Johnson 3.5 with a clutch, but to date OMC Australia has been unable to supply one.

TWO-STROKES
In this category I recommend the Mariner/Mercury 3.3, the Tohatsu M9.8B and Tohatsu M18E2.

All operate on a 50:1 petrol/oil mix which will protect the bearings and piston rings far better than a 100:1 mix. Sure there's more oil used, but for the short periods these motors will generally be used this is of little consequence.

The Mariner/Mercury 3.3 (left) has a single-cylinder 74.6cc loopcharged powerhead, a clutch for ease of handling by kids and a lift-off upper cowl for servicing ease. It weighs 14kg and suits tinnies to three metres or displacement rowing dinghies to 3.8m.

Mounted on a three-metre punt, spinning the F6 prop and pushing a total of 225kg (including two adults and fishing tackle) it averaged 12.8kmh at Wide Open Throttle (WOT), consuming 2.0lt/hr.

Tohatsu's M9.8B is an up-rated version of the 13-year-old M8B and has a twin-cylinder 169cc loopcharged powerhead. It weighs only 27kg and provides performance closer to a 10hp than an eight. The motor is compact and easy to use and suits tinnies to 3.7m or inflatables to 3.2m.

Mounted on a 3.6m tinnie, spinning the optional 9.5-inch pitch prop and pushing a total of 285kg (including two adults and fishing tackle), at three-quarters throttle it cruised at 26.7kmh, using 3.6lt/hr, and averaged 35.6kmh at WOT, using 5.3lt/hr.

Tohatsu's M18E2 provides almost as much power as a 20 without the weight penalty. In shortshaft form this twin-cylinder 294cc looper weighs 37kg and suits tinnies to four metres and inflatables to 3.7m.

Mounted on a 3.7m Stessl Edgetracker V-nose punt, spinning the standard 8.5-inch prop and pushing a total of 360kg (including two adults and fishing tackle) the test motor cruised at 20.6kmh on 4000 revs, using 4.0lt/hr, and averaged 43.1kmh on 6000 revs at WOT, consuming 9.8lt/hr.

Like the M9.8B, the strong Tohatsu ignition system and effective thermostat kept oil smoke emissions to a minimum while trolling.

FOUR-STROKES
In this category I recommend the Honda BF2D, Tohatsu MFS5A and Yamaha F8C.

Two years of intensive evaluation have proven Honda's BF2D to be the best all-round motor for tinnies to three metres and rowing dinghies to 3.7m. After a total of 180 hours of saltwater leg/lower unit immersion no corrosion was apparent anywhere on the loan motor!

The BF2D has a single-cylinder OHV 57cc powerhead driving through a centrifugal clutch (just like a brush-cutter). It's the easiest motor on the Aussie market for kids to use and because it's air-cooled doesn't require regular freshwater flushing, only an occasional dunk in a bucket of freshwater for the lower unit to wash salt deposits from the driveshaft/gearcase seal. Complete with oil it weighs 14kg.

Mounted on a three-metre punt and pushing a total of 225kg (including two adults and fishing tackle) it cruised on two-thirds throttle at 10.0kmh, consuming 0.6lt/hr, and averaged 10.5kmh at WOT, using 0.9lt/hr. And on a 3.1m clinker fibreglass rowing dinghy pushing a total of 190kg, at one-third throttle it averaged 9.4kmh, consuming a mere 0.4lt/hr!

Just remember that this motor uses relatively light viscosity SAE 10W30 oil for its splash/mist lubrication system. Care is required when using this engine in high ambient temperature conditions.

Tohatsu's MFS5A has no trouble handling tinnies to 3.6m and inflatables to three metres. It has a single-cylinder 123cc OHV powerhead with thermostatically-controlled cooling and weighs 25.5kg.

Mounted on a three-metre punt, spinning an eight-inch prop and pushing a total of 235kg (including two adults and fishing tackle) it averaged 21.5kmh at WOT, consuming 1.7lt/hr. On the 3.6m tinnie, spinning a seven-inch prop and pushing the two-adult load (total 255kg), at WOT it averaged 19.7kmh, using 1.9lt/hr.

Yamaha's recently released F8C suits tinnies to 3.6m and inflatables to 3.2m. It has a twin-cylinder 197cc OHC powerhead with thermostatically-controlled cooling and weighs 38kg.

Mounted on the 3.6m tinnie, spinning an 8.5-inch prop and pushing the two-adult load (total 285kg), at three-quarters throttle it cruised at 28.4kmh, consuming 2.8lt/hr, and at WOT averaged 32.6kmh, using 3.6lt/hr.

CONCLUSION
All the motors mentioned in this guide are easy to maintain and with reg ular servicing should provide years of reliable power. Always use the manufacturer's recommended lubricating oil and with the four-strokes change the oil every six months regardless of how few hours are clocked up in this period.