Tohatsu 15hp

The recently released MFS 15B is the first of Tohatsu's twin-cylinder four-strokes and is also available in a 9.9hp version. The bore is the same as the single-cylinder MFS 5A.

Standard features include an up-front gearshift, automatic choke (similar to that in Yamaha's discontinued 232cc four-stroke 9.9) and engine protection features such as a rev-limiter and automatic rev reduction should the oil pressure drop. The rev-limiter cuts in at 6250rpm and should the oil pressure fail the revs drop to 2000rpm, requiring the engine to be switched off before it will restart.

Sensibly Tohatsu Corporation has fitted a spin-on oil filter as with the direct Mariner/Mercury and Yamaha competition. Honda's BF15 is now the only four-stroke 15 not to have this feature.

The MFS 15B is a bulky and heavy motor that's better suited to bolt-on applications. In longshaft form it weighs a hefty 52.5kg dry, but part of the reason is that it has a deep 2.15:1 gear ratio. This puts the Tohatsu midway between the 323cc Mercury F15 with a 2:1 ratio (51kg) and Bigfoot model with its 2.42:1 ratio (58kg).

I recently tried an early-production MFS 15B on a 3.7m Stessl Edgetracker supplied by Lakeside Marine.

Because the motor had been laid on its side before being mounted on the Edgetracker, cold starting required several two-hand pulls and some throttle before the motor would start. However no oil smoke appeared at any time and once started the auto choke kept the revs at about 1100rpm for a couple of minutes before dropping back to the normal 1000rpm idle. And when the motor was restarted after a 15-20 minute break the choke stayed off and only one hand was required to fire it up with none of the hot-start problems that afflict other auto choke-equipped motors such as Mercury's F40.

Spinning a 9.8-inch pitch alloy prop and pushing a total of 380kg (including two adults and fishing tackle) the demo MFS 15B trolled at 4.2kmh at 900rpm consuming 0.4lt/hr.

At trolling speed the motor was so quiet you could hardly hear it and vibration levels were comparable to the competition and much lower than the two-stroke 15hp loop-charged models from Tohatsu and Yamaha. Providing the antiventilation plate was kept immersed no cooling water starvation occurred and at all times the pilot water discharge was clearly visible.

Because of the extra transom weight over a two-stroke 15 a clean plane was not reached until 4100rpm and 19.8kmh when set on the third trim hole (there are six positions) for best all-round performance. We found that 4500rpm gave the best cruise where the demo motor averaged 26.4kmh consuming 3.6lt/hr and through tight figures of eight at these revs there was very little prop ventilation and the Edgetracker stayed on the plane.

The carbie accelerator pump ensured rapid acceleration over 4500rpm and the Wide Open Throttle (WOT) average was 39.9kmh at 5900rpm consuming a reasonable 5.2lt/hr. However, although noise levels were relatively low for a four-stroke 15, the vibration was so intense that my throttle hand went numb after about two minutes at WOT. Vibration levels were acceptable to about 5000rpm but over these revs they were much higher than the competition. Clearly the MFS 15B was not designed to be used continuously over 5000rpm.

Servicing the MFS 15B appears straightforward with the oil filter to port and the single carbie to starboard. The camshaft drive belt is covered by a plastic shroud and all wiring around the powerhead is logically laid out. Recommended servicing intervals are every 50 hours or six months after the first 10 hours and the waterpump impeller should be replaced every 100 hours or once a year.

Apart from the WOT vibration levels the MFS 15B is a very user-friendly motor and with regular maintenance should prove as reliable as its popular two-stroke 15/18hp counterparts!