Tohatsu M9.8B Outboard Motor Review

Originally published in TrailerBoat #281, April / May 2012.

The Tohatsu M9.8B and Tohatsu M18E2 are two of the most popular outboards in this long-running Japanese manufacturer’s line-up, and with good reason.

TOHATSU M9.8B

Based on the Tohatsu M8B (which, incidentally, turns 25 this year) the portable Tohatsu M9.8B was released on the Australian market in 1997 and quickly established a following amongst anglers who wanted more power than an 8hp outboard but not the weight of a 15hp. All of the competitions’ 9.9hp engines are de-rated from 15hp, hence their much higher weights.

Sensibly, Tohatsu retained the basic marine engine concept over the past 15 years but improved the Tohatsu M9.8B through subtle changes, such as incorporating a zinc anode / trim tab beneath the antiventilation plate, fitting removable screens over the cooling water intakes and switching the drab grey colour to the current indigo.

The Tohatsu M9.8B outboard motor develops 9.8hp at 5500rpm, with wide open throttle (WOT) ranging from 5000rpm to 6000rpm. The 169cc twin-cylinder loop-charged powerhead has thermostatically-controlled cooling and of course CD ignition, but mechanical timing advance. The gear ratio is 2.08:1 and the shortshaft weight is only 26kg. Six trim positions and a shallow-water drive setting are provided and the automatic reverse lock eliminates the need to flip a lock-down lever.

Manual and electric start models are available in both tiller-steer and remote control versions with the electric start engines retaining the manual overhead recoil starter. An unregulated 12V / 7A alternator is optional in manual start models.

ON THE WATER

The Tohatsu M9.8B outboard motor I borrowed from Lakeside Marine, the national Tohatsu distributor, was a fully run-in loan unit. Swinging a 9.5in-pitch alloy prop it was mounted on my 3.6m Sea-Al Super Skua tinnie, on which I’ve tested countless outboards up to 20hp since 1988.

The review engine required no more cold-starting effort than my Tohatsu M8B. Operating on a 50:1 mix of standard non-E10 unleaded and semi-synthetic Valvoline Outboard 2-Stroke oil, it blew oil smoke only on cold starting and after 15 minutes or more of trolling. Providing the antiventilation plate was kept immersed power astern was good and at all speeds the cooling water telltale was clearly visible.

The loan Tohatsu M9.8B outboard engine performed much better on the Super Skua than my M8B swinging an 8.3in prop and pushing the same 285kg total, including two adults and fishing tackle. It easily maintained a clean plane at cruising rpm, whereas the M8B was working hard across its entire rev range on this hull and is really better suited to tinnies to 3.4m. No prop ventilation occurred through tight figure-of-eight turns at WOT and across the entire rpm range the engine had low vibration levels and was surprisingly quiet for a carbie two-stroke engine.

On completion of each group of tests the spark plugs were spotlessly clean and after a total of 50 hours of saltwater leg / lower unit immersion, corrosion appeared limited to slight surface rusting on the end of the propshaft.

PRICE

The recreational-usage warranty is three years and as of March 2012 the manual shortshaft model had a price of $2188 RRP, with a spare prop costing $140.

THE TRADE-A-BOAT VERDICT

Several readers have asked me why I run a Tohatsu M8B instead of the M9.8B. The answer is simply that I have two frequently used tinnies rated to 8hp and I never fit more power than listed on the hull compliance plates. Were these hulls rated to 10hp then the Tohatsu M9.8B outboard motor would be my choice, for the performance gain at no extra weight.

Tohatsu has done an excellent job of packaging a lightweight, compact, relatively fuel efficient and low-maintenance engine that suits tinnies to 3.6m. And since Lakeside changed the servicing requirements to every 50 running hours or annually after the 10-hour check-up, owning a Tohatsu M9.8B outboard engine has become considerably cheaper.

PERFORMANCE

On a 3.6m Sea-Al Super Skua tinnie.

COMPETING OUTBOARDS