Four-stroke tender outboards

When it comes to powering tenders from 3-3.6 metres, many people opt for single cylinder two-strokes such as the Mariner/Mercury/Tohatsu and Yamaha fives. But as the single cylinder four-stroke Suzuki DF6 and Tohatsu MFS 6A are so fuel efficient and clean-running, there's really no excuse for buying a two-stroke outboard.

Unlike their de-rated 5hp counterparts, these motors are capable of maintaining a planing cruising speed on a 3m dinghy and subsequently are more fuel efficient overall. Both sixes have watercooled OHV engines with gear-driven camshafts and pressure lubrication and feature full gearshifts, shallow water drive settings and optional 12V alternators. They comply with US EPA 2006 and California Air Resources Board (CARB) 2004 exhaust emission regulations.

The DF6
Released late last year, the DF6 is the most powerful version of a trio of engines. It develops 5.9hp at 5250rpm from its 138cc crossflow powerhead and has a Wide Open Throttle (WOT) range of 4750-5750rpm. The gear ratio is 1.92:1 and the shortshaft model weighs 25kg.

Features include a large rear-mounted handle for easy carrying without fear of sump oil flooding the cylinder, solid-colour upper and lower cowls, five trim and two shallow water drive positions, 180° steering, a long tiller arm, 1.5lt integral fuel tank with an optional 12lt plastic remote tank and six amp alternator. The exhaust relief is directly under the lower cowl to reduce back pressure, the waterpump impeller housing is stainless steel for a long lifespan in saltwater and the oil sight glass enables a quick check of oil level without using the dipstick.

An anode is fitted to the leg in addition to one atop the anti-ventilation plate, giving the DF6 very good protection when alongside a yacht or cruiser with copper antifouling. There's also provision for through-bolting the motor to a tender's transom should the boat be slung on davits.

The loan motor started easily hot or cold and warmed quickly from cold, refused to suffer carburettor vapour lock in 35° heat and provided the motor was stored upright before using, no oil smoke appeared at any time. Due to the above-prop, exhaust power astern was excellent and the motor provided great manoeuvrability in tight confines.

Mounted on a 3m deHavilland John 10 punt, pushing a total of 235kg and spinning a 7in pitch prop, it planed us at 4800rpm and cruised effortlessly at 14.5kt on 5100 revs using 2.0lt/hr. The WOT average was 15.8 on 5830 consuming 2.4lt/hr.

Spinning the same prop, mounted on a 3.6m Sea Al Super Skua dinghy and pushing a total of 285kg, the DF6 achieved a clean plane and averaged 12.6kt at WOT on 5350 revs, consuming 2.6lt/hr. Across a 4kt 'loop' including 10 per cent WOT operation, it averaged 0.68lph and using Quicksilver SAE 10W30 oil the fuel/oil ratio was 850:1.

After 50 hours of saltwater leg/lower unit immersion, no corrosion was apparent anywhere on the loan motor.

Servicing access is very good and maintenance intervals are every 50 hours or six months after the first 20 hours. The waterpump impeller should be replaced every 100 hours or once a year.

Suzuki recommends using SAE10W30 or 10W40 oil.

The warranty coverage is two years for recreational applications.

The MFS 6A
Released in 2000, again this motor is the most powerful of a trio of engines. It develops 5.9hp at 5500 revs from its 123cc crossflow powerhead and has a WOT range of 5000-6000rpm. The gear ratio is 2.15:1 and shortshaft weight is 25.5kg.

Features include six trim and one shallow water drive positions, 150° steering and an optional five amp alternator. A 12lt plastic remote fuel tank is supplied, but there's no option of an integral tank and the upper cowl needs removing to access the dipstick. Also, the incorrectly-placed carry handle necessitates the motor be carried upright with two hands.

Again on the John 10 but spinning an 8in alloy prop, the loan Tohatsu started easily hot or cold with slightly less effort required than the DF6, but blew some oil smoke on cold starting, suffered vapour lock on hot days and had higher vibration levels across the entire rev range.

It planed the punt at 5000rpm, which was also the best for cruising, where it averaged 11.0kt using 1.8lt/hr. The WOT average was 15.3kt on 5740 revs, consuming 2.1lt/hr.

On the Sea Al and pushing 285kg, a 7in pitch prop was needed to let the motor reach its correct rev range. At WOT it averaged a semi-planing 10.5kt on 5850 revs, consuming 2.1lt/hr and across the 4kt loop used 0.72lt/hr. The fuel/oil ratio was 820:1.

After 50 hours of saltwater leg/lower immersion there was slight rusting of the propshaft.

Servicing access is good, with intervals recommended at every 50 hours after the first 10 hours, and water pump impeller replacement every 100 hours. Tohatsu recommends SAE10W40 oil year-round.

The MFS 6A has a two year warranty for recreational applications.

Pricing and servicing costs
As of February, the DF6 retailed for $1989. According to Hamiltons Marine of Mayfield (NSW), a spare prop was approximately $200 and servicing costs (excluding parts) were $335 for the first year then $229 for each year thereafter.

The MFS 6A retailed for $1943 and according to Neptune Marine of Slacks Creek (Qld), a spare prop was $140. Excluding parts, servicing costs for the first year were $295, then $230 for each year thereafter.

And the winner is...
Both these sixes are beautifully engineered and as I stated in this column back in January Trade-A-Boat, there's no way I'd even consider buying a small two-stroke outboard when I could have technology as good as these four-strokes.

But the DF6 is the clear winner. Its well-positioned carry handle makes it much more user-friendly, the integral fuel tank is a real advantage when there's no space aboard a crowded tender for a remote tank and the oil sight glass simplifies day-to-day operation.

And while the Tohatsu is more fuel efficient at WOT, I just love the Suzy's mid-range torque, which is exactly what a tender motor needs.