Review: Tohatsu M5B outboard motor

By: Andrew Norton

Presented by
  • Trade-A-Boat

On the right hull, the Tohatsu M5B outboard is a great little engine for saltwater anglers.

Review: Tohatsu M5B outboard motor
The Tohatsu M5B outboard is also sold as the Mariner / Mercury 5. Spare parts should never be a problem.


Originally published in TrailerBoat #286, September / October, 2012.

Originally released in 1985 to replace the air-cooled M5A, the Tohatsu M5B outboard was developed to compete directly with the Yamaha 5C engine, introduced in 1982.

The Tohatsu M5B outboard differed from the Tohatsu 5C in that it had a loop-charged powerhead instead of a cross-flow unit and no thermostat. It also ran on a 50:1 premix, instead of Yamaha’s 100:1 ratio. These aspects aside, the pair were very similar.

Since 1988 I’ve conducted long-term reviewing on three Tohatsu M5B outboarda, the latest being a well-used 1996 model owned by my fishing partner, Di. Apart from slight differences in the upper cowl shape and that my friend’s engine features a remote 12lt plastic fuel tank in addition to the integral 2.5lt tank, there are no significant mechanical differences between them.



The Tohatsu M5B outboard has a 102cc single-cylinder powerhead developing 5hp at 5000rpm, with a wide open throttle (WOT) rev range of 4500rpm to 5500rpm. The ignition and timing advance are electronic and a full gearshift is fitted in addition to the 360-degree steering. There are six trim positions along with a shallow-water drive setting. The reverse lock automatically engages when reverse is selected and the easily-set full tilt lock makes beaching a cinch. The engine has a dry weight (no fuel) of 20kg and the 2.15:1 gear ratio enables relatively coarse-pitch props to be fitted for better planing efficiency.

The Tohatsu M5B outboard has an OEDA "1 star" rating. By way of comparison, the Yamaha 5C engine hasn’t been tested, but with its cross-flow powerhead a "0 star" rating is likely.



Like any single-cylinder outboard, the Tohatsu M5B outboard needs to be mounted on the right hull to achieve its full performance potential. Di’s engine has been tested on three different hulls over the past five years and it really only performs well on one of them.

For example, on my early ’70s 3.4m Savage Gull dinghy, spinning the 8in-pitch alloy prop and pushing a total of 280kg (including two adults and fishing tackle), all the Tohatsu M5B outboard could manage at WOT was 12.3kts (22.8kmh). This just got it on the plane, but as soon as the revs were backed off the hull fell back into an inefficient semi-planing mode. This is because the engine simply didn’t have the torque to overcome the high hull loading relative to the small planing surface.

On my 2003 flat-bottomed Sea Jay 3.4 Punt the Tohatsu M5B outboard motor performed much better but it was still sensitive to weight.  When the total weight was increased a mere 20kg, from 265kg to 285kg, a clean plane wasn’t achieved until 4900rpm and 11.9kts (22.0kmh). At this point WOT on the Tohatsu M5B engine was only 5200rpm and the average speed of 13.8kts (25.7kmh) indicated the engine was working too hard. All Japanese outboards perform best at or near the top of the manufacturer’s recommended WOT rev range.

Also, over my standard small outboard "test loop", which saw the boat averaging 4.0kts (7.5kmh) and included 10 per cent WOT and 40 per cent trolling operation, the fuel consumption rose from 0.69lt/h to 0.87lt/h — an increase of 26 per cent. So my advice is this: choose your hull design and occupant weight very carefully!

Despite its unknown previous life, Di’s engine normally starts first pull hot or cold, although not having a thermostat, it runs cold while trolling. However, switching to premium unleaded petrol has made a big difference to oil smoke emissions and trolling quality, even using the same distributor-recommended semi-synthetic Valvoline Outboard 2-Stroke Oil.

The remote fuel tank is handy for longer runs, especially as it’s small enough to fit between the transom and aft thwart of older three-thwart tinnies. Over the five years, the fuel consumption with a total of 7.5 per cent WOT operation has been a reasonable 0.76lt/h, so the M5B is definitely not a fuel guzzler.


Boat engines

More boat engine reviews

Find marine engines for sale.



Providing servicing is performed by an authorised Tohatsu dealer, the recreational-usage warranty is three years. As of January 2012 the Tohatsu M5B outboard motor had a price of $1536 retail, with a spare alloy prop costing $120. The Tohatsu M5B engine is also sold as the Mariner and Mercury 5, so spare parts availability shouldn’t ever be an issue, even in remote areas.



Tohatsu has done a great job with this simple yet affordable and relatively fuel efficient outboard that suits tinnies to 3.4m. The Tohatsu M5B is easy to operate and straightforward to maintain and service, and with its once-a-year servicing (or every 50 hours) after the first 10 hours or two months, even allowing for higher fuel and oil costs it’s far cheaper to own than a four-stroke five.



Fitted to a Sea Jay 3.4 Punt, total weight 265kg)




1.9kts (3.5kmh)

1100rpm (trolling)


10.3kts (19.1kmh)

4300rpm (minimum plane)


12.7kts (23.5kmh)

5000rpm (max. cruise)


15.3kts (28.3kmh)

5700rpm (WOT)





Mariner/Mercury 5

Yamaha 5C










Dry weight



Warranty (years)



OEDA stars




Originally published in TrailerBoat #286, September / October, 2012. Why not subscribe today?


Want the latest stories delivered straight to your inbox? Sign up for the free TradeBoats e-newsletter.