REVIEW: TOHATSU M9.8B VS YAMAHA F9.9S MHB

By: Andrew Norton

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REVIEW: TOHATSU M9.8B VS YAMAHA F9.9S MHB
TOHATSU M9.8B AND YAMAHA F9.9S MHB

Released on the Aussie market in 1997 and uprated from its M8B counterpart introduced a decade earlier, the M9.8B is one of Tohatsu’s most popular outboards. It’s light for output but has tonnes of grunt with top-end performance that leaves the M8B for dead. It still operates on a 50:1 premix after the first 10 hours on 25:1.

The power head has thermostatically-controlled water cooling and mechanical ignition timing advance. The transom brackets are virtually identical to the M5B, but the much larger leg design for efficient exhaust gas scavenging meant it could only be steered 130 degrees instead of 360. Also the cooling water intakes were relocated above the gear case torpedo for easy attachment of flushing muffs, while the zinc anode doubled as a trim tab to reduce steering load.

Power head access is excellent and servicing intervals are every 50 hours or annually after the first 10 hours. The recreational usage warranty is three years.

The F9.9 is a "stroked" version of its F8C counterpart (now both engines use the same power head) and leaves its four-stroke competition for dead. The counterflow engine has so much grunt that it actually outperforms the M9.8B despite weighing a whopping 54 per cent more. Though having a belt-driven camshaft, the four-valve engine is a non-interference type where the valves won’t contact the piston crowns if the belt breaks. Electronic ignition timing advance is fitted and the ignition module includes a rev limiter set at 6200. Across its entire rpm range the F9.9 vibrates less than the M9.8B and is way quieter at or near WOT. And the upfront gear shift makes operating the F9.9 so much easier than the M9.8B.

However, the transom bracket from the two-stroke Yamaha F15 is a bit archaic with only four trim positions and two shallow-water drive settings. But at least the intakes for the thermostatically-controlled water cooling are above the gear case torpedo so flushing muffs can be quickly connected.

Power head access is excellent and servicing intervals are every 100 hours or annually after the first 20 hours. The camshaft belt has a designed lifespan of 800 to 1000 running hours.

The recreational usage warranty is four years.

SEA TRIALS

Single M9.8B and F9.9 on 3.6m 1988 Sea Al Super Skua tinny, swinging 9.5-inch and 8.5-inch props and pushing 285kg and 300kg respectively. Average of two-way runs on Lake Macquarie, NSW, in calm water. Range is in nautical miles from 12lt plastic remote fuel tanks with a 10% reserve.

ENGINE M9.8B F9.9
TROLLING 940rpm 900rpm
KTS 2 1.9
LT/H 0.7 0.3
RANGE 30 68
PLANE RPM 4400 4500
KTS 13.4 12.8
LT/H 3.5 2.9
RANGE 41 47
CRUISE RPM 4700 5000
KTS 14.4 15
LT/H 3.6 3.1
RANGE 43 52
WOT RPM 5700 6100
KTS 19.2 19.5
LT/H 5.3 4.2
RANGE 39 50

"Loop" of cruising with 10% WOT and 40% trolling operation averaging 8kts

LT/H 2.5 1.8
RANGE 34 48

SPECIFICATIONS: TOHATSU M9.8B

ENGINE

TYPE Loop-charged 2-stroke

RATED BHP/MHP* 9.7/9.8 at 5500rpm

WOT RANGE 5000 to 6000rpm

DISPLACEMENT 169cc

BORE X STROKE 50mm x 43mm

GEAR RATIO 2.08:1

DRY WEIGHT 26kg

RRP $1902

OEDA stars $1

*Brake horsepower/metric horsepower. Note the M9.8B is $380 less than the MFS 5C LPG when the gas bottle is included in the price.

SPECIFICATIONS: YAMAHA F9.9

ENGINE

TYPE OHC four-stroke

RATED BHP/MHP* 9.8/9.9 at 5500rpm

WOT REV RANGE 5000 to 6000rpm

DISPLACEMENT 212cc

BORE X STROKE 56mm x 43mm

GEAR RATIO 2.08:1

DRY WEIGHT 40kg

RRP $3199

OEDA stars 3

*Brake horsepower/metric horsepower.

Check out the full review in issue #502 of Trade-a-Boat magazine. Subscribe today for all the latest boat news, reviews and travel inspiration.

 


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