Tohatsu MFS 18B

Tohatsu Corporation has added yet another model to its low-pollution outboard line-up, bringing the total of these motors to eight, with six four-strokes from four to 18hp and the direct-injected two-stroke TLDI 50 and 90hp models.

Tohatsu's MFS 18B is an uprated version of the MFS 15B (released in December 2000), which provides a useful performance increase over the 15 without a weight penalty. Although it won't match a four-stroke 25 for performance, it's 18% lighter than the equivalent manual start longshaft Yamaha F25A.

The MFS 18B has the same features as its less powerful counterpart, including an upfront gearshift and an automatic choke. Engine protection features include a rev limiter and automatic rev reduction should the oil pressure fall below a pre-set level.

In June, Lakeside Marine, the national Tohatsu distributor, supplied an MFS 18B on a 3.7m Stessl Edgetracker, enabling me to directly compare this motor to the MFS 15B.

Because the 18 had been stored on the Edgetracker before testing commenced, it started first pull cold with only a firm two-handed grip needed. Within two minutes the auto choke had brought the warm-up revs down to the normal 1100 in neutral. No oil smoke appeared at any time, and when the motor was restarted after a 15-20 minute break, the choke stayed off and only one hand was required to start it.

Providing the antiventilation plate was kept immersed, no cooling water starvation occurred and power astern was good. At all times, the pilot water discharge was clearly visible, but Tohatsu should reverse the operation of the reverse lock/shallow water drive setting lever, which currently needs to be up for 'run' and down for 'tilt'.

This time the motor was through-bolted to the transom, reducing vibration levels across the entire rev range to about the same as the Mariner/Mercury F15 and Yamaha F15A. There was no finger-numbing vibration above 5000rpm, which had occurred with the MFS 15B attached to the transom using only standard thumb screws.

Initial trials of this motor showed the standard 9.8in pitch alloy prop to be way too small, so the prop was re-pitched to 11in. Pushing a total of 380kg (including two adults and fishing tackle), the demo MFS 18B provided a real performance edge over the 15, particularly when we carried an additional 60kg aboard.

With the two-person load aboard, the Dead Slow Troll average was 4.4kmh on 950rpm consuming 0.4lt/hr, exactly the same consumption as the MFS 15B spinning the same prop and pushing the same load. A clean plane was achieved at 22.8kmh on 4000rpm, whereas the 15 did this at 19.8kmh on 4100rpm.

The best cruising revs were 4500 for both motors, but the 18 averaged 29.1kmh whereas the 15 averaged 26.4kmh and both consumed 3.6lt/hr. Through tight figure-of-eights at these revs there was no prop ventilation.

At Wide Open Throttle (WOT) the 18 averaged 42.8kmh on 5800 using 5.8lt/hr compared to 39.9kmh, 5900rpm and 5.2lt/hr for the 15.

When the additional load was carried aboard the Edgetracker, the 18 had the same DST average and fuel consumption as for the lighter load. Surprisingly, at 22.5kmh the clean plane speed was slightly lower, but an additional 200 revs were needed to get us up. At the cruise revs of 4500 the 18 averaged 29.0kmh, consuming 3.8lt/hr with no prop ventilation occurring through tight figure-of-eights, while at WOT the average was 40.3kmh on 5600rpm consuming 6.3lt/hr, still an excellent result for an 18.

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

Tohatsu will have to reduce the relatively high DST revs before the 18 can compete with its direct four-stroke competition. But for saltwater anglers who want a low-pollution outboard that provides significantly better performance than a 15 without the weight penalty of a 25, the MFS 18B fills the gap perfectly!