Tohatsu M5B

Mighty Five

Released on the Aussie market in 1985, the Tohatsu M5B and its re-badged Mariner and Mercury counterparts are the most powerful single-cylinder two-stroke fives available.

The M5B has a full gearshift in addition to 360-degree steering, six trim positions and one shallow-water drive setting, and a full-tilt position. There are also effective steering and throttle friction adjusters. But unlike its Mercury Marine counterparts, the M5B is available in either integral or remote fuel tank versions, whereas the former are available with both integral and remote tanks, just like Suzuki's DF6.

Because of its relatively small piston displacement, the M5B needs to be mounted on the right hull to achieve full potential. Tests I conducted in October, 2007, on a fully run-in M5B supplied by my fishing partner Diann, showed that performance was at best adequate on my 3.4m Savage Gull dinghy but much better on my 3.4m Sea Jay punt. This was because the punt has a flat bottom and a relatively large planing surface compared to hull loading.

Spinning the standard eight-inch Tohatsu alloy prop and pushing 285kg including two adults and fishing tackle, the M5B averaged 3.9kmh and 1100rpm using 0.3lt/h. Although Wide Open Throttle was needed to plane us; once up, the throttle could be backed off slightly for a clean plane at 22kmh and 4900rpm. WOT averages were 25.7kmh and 5200rpm using 2.5lt/h, and across a "loop" of 10 per cent WOT and 40 per cent trolling operation averaging 7.5kmh, fuel consumption was 0.87lt/h.

In comparison, on the Sea Jay (pushing 290kg) my long term evaluation DF6 (see TrailerBoat 223) performed much better. Spinning the standard seven-inch Suzuki alloy prop to allow for the taller gear ratio, it trolled us at 4.3kmh and 1200rpm using 0.2lt/h, and planed us cleanly at 22.4kmh and 5000rpm. The WOT averages were 28.0kmh and 5690rpm using 2.6lt/h, and across the 7.5kmh loop it used 0.58lt/h.

TrailerBoat readers may wonder why I've compared the DF6 with the M5B instead of the DF5. The reason is simple: as the DF6 weighs the same as the DF5, why have the latter when for the same weight you can have an additional 0.9hp. However, Tohatsu's M6B is a de-rated version of the M8B and weighs 6kg more than the M5B.

As expected of a single-cylinder loopcharged outboard, the M5B had higher vibration levels when trolling than the single-cylinder DF6. But as revs increased the tables turned and above 3000rpm, the M5B was significantly smoother than the DF6, although at WOT it was much noisier. I've also found this quirk when testing two and four-stroke portable generators, where on light load the four-stroke is smoother but on full load significantly rougher than a two-stroke.

Cold starting was the same for both outboards, normally first pull hot or cold, though being a premix two-stroke the M5B always blew oil smoke for the first five minutes until warm. With its effective thermostat the DF6 warmed quickly from cold and stayed hot when trolling, whereas not having a thermostat the M5B ran very cold after about five minutes of trolling, but surprisingly blew little more smoke than its Tohatsu M8B counterpart with a thermostat. Perhaps the use of semi-synthetic TC-W3 Valvoline Outboard Oil (recommended by Lakeside Marine, the national Tohatsu distributor) had a lot to do with this, as using it also virtually eliminates spark plug fouling.

Carrying the M5B and DF6 is easy with their large handles but the 5kg lesser weight of the Tohatsu is noticeable over longer distances.

Powerhead access for the M5B is very good, particularly in the remote tank version. Recommended servicing intervals are every 50 hours or six months after the ten-hour initial checkup and the waterpump impeller replaced every 100 hours or yearly.

The M5B is rated "1 Star" under the OEDA exhaust emissions scheme and has a three-year warranty for recreational usage.