Review: Tohatsu M3.5A outboard motor

By: ANDREW NORTON


Need a harbour kicker for your small keelboat? The 3.5hp Tohatsu M3.5A outboard motor has no peers.

Review: Tohatsu M3.5A outboard motor
The Tohatsu M3.5A outboard motor delivers plenty of power for an engine this size, yet it weighs only 13kg.

Way back in 1979 I acquired a 16-foot (4.9m) clinker launch powered by a 3.5hp Olds petrol inboard. Made in Maryborough-QLD, this OHV four-stroke engine had no neutral or reverse, just direct drive and cruised us at around 5kts sipping a mere 1lt/h.

This boat taught me true seamanship, such as learning how far the hull would travel coming up to a jetty with the engine switched off. The Olds was at the end of its lifespan and so many times we rowed back to the mooring in Berowra Waters, but back then I had an incredibly tolerant girlfriend who developed pretty good arm muscles when we shared the rowing.

Fast-forward several years to when I acquired my 22-foot (6.7m) plywood Bluebird cruising yacht. The main auxiliary outboard was always a bit heavy to lug about when I went for short sails, so I contacted Lakeside Marine, the national Tohatsu distributor, to borrow a long-shaft tohatsu M3.5AL. This direct-drive outboard was ideal for motoring between wind pockets on Lake Macquarie and as the Bluey was kept on a mooring off my house, why would I need neutral?

And at just 13kg you can repeatedly lift the Tohatsu outboard off the transom bracket without permanently damaging your back. Perfect for weaklings like me!

 

TOHATSU M3.5A

The M3.5A was introduced to the Aussie market back in 1975 and was developed from the long-running twin-cylinder Tohatsu B13A Sea Pigeon, which later became the M8A through to 1987.

The single-cylinder loopcharged M3.5A develops 3.5bhp at 4750rpm with a Wide Open Throttle range of 4300 to 5200rpm. The 74.6cc powerhead has only one piston ring but providing the engine is carefully run in on 25:1 for the first five hours (it used to be 10 hours on 20:1), it will act as both compression ring and oil scraper for countless years without wearing out. Once run-in the fuel/oil ratio is 50:1 and Valvoline two-stroke outboard oil or Nulon synthetic fortified outboard oil are recommended by Lakeside.

The CD ignition had no trouble coping with the damp bilge when rainwater found its way in. As the ignition system has fixed timing of 20 degrees BTDC, the M3.5A is most efficient running at or above 4000rpm, perfect for powering a yacht. The maximum torque of 5.6Nm is produced at 3900rpm.

The four trim positions handle a range of transom angles and a stopper knob holds the leg at full tilt. Three-hundred-and-sixty degrees steering also provides reverse if needed. The 1.4lt integral fuel tank is adequate for short runs and gravity feeds to a simple single-jet carbie.

The M3.5A has a clamshell cowling, so I recommend removing only one half at a time or re-assembly can be very finicky. A nice touch is that by simply removing the prop, two bolts and the gearcase end cap the waterpump impeller can easily be replaced without having to drop the lower unit.

Lakeside recommends servicing the engine every 50 hours or annually after the first 20 hours and the recreational warranty coverage is three years.

 

PERFORMANCE

The first job before introducing the M3.5A to its upcoming hard life was to swap the standard 5.7in pitch plastic weedless F6 prop for a 4.3in weedless prop to match the tall 1.85:1 gear ratio. This allowed the engine to rev well out and give the Bluey hull speed pushing a total of 1800kg, but also return incredible fuel efficiency when throttled back.

Even during the running-in period the Tohatsu always started first pull hot or cold and continued to do so for the four years I tested it. This engine never let me down.

Across the entire rpm range vibration levels were fairly high and over time this started separating the transom sheeting from its framing, but this would never be a problem in a ’glass hull.

After more than 100 hours of saltwater leg/lower unit immersion the Tohatsu was still in very good condition with hardly any corrosion or paint bubbling apparent.

 

THE VERDICT

Passengers aboard the Bluey always commented on just how well this tiny outboard pushed her (never an "it"), even into stiff headwinds where we could maintain 3 to 4kts at two-thirds throttle opening. I kept the Tohatsu for testing on other hulls after I sold the Bluey and only parted with it when I no longer had a hull with a long-shaft transom. That was a sad day as it was such a damned good engine.

The long-shaft model is only available on special order at just under 1000 (the standard short-shaft is generally ex-stick0. For your nearest Tohatsu dealer visit Tohatsu.com.au.

 

TOHATSU M3.5AL OUTBOARD MOTOR SEA TRIALS

Single 3.5hp Tohatsu M3.5A outboard motor. Average of two-way runs across a measured distance, calm water

RPM

SPEED (KTS)

FUEL BURN (LT/H)

4000 (2/3 throttle opening)

4.9kts

0.9

4900 (3/4 throttle)

5.6kts

1.4

5100 (WOT)

5.9kts

2

* Sea-trial data supplied by the author.

 

TOHATSU M3.5AL SPECIFICATIONS

TYPE Single-cylinder two-stroke petrol outboard motor

RATED HP 3.5

REC. RPM RANGE 4200 to 5300

DISPLACEMENT 74.6cc

BORE X STROKE 47 x 43mm

WEIGHT 13kg

GEAR RATIO 1.85:1

RRP Approx. $1000

WARRANTY 3 years

OEDA STARS 0

 

See the full version of this review in Trade-A-Boat #244, August / September 2014. Why not subscribe today?

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