Tohatsu M18E2 and MFS18B2

Two or four-stroke?

Tohatsu's two-stroke M18E slots in neatly between 15 and 20hp outboards, weighing no more than the former but being a lot lighter than the latter. Since its Aussie release two decades ago, it has been one of the most popular outboards in Tohatsu's lineup, appealing to recreational and commercial fishermen alike.

But the trouble is, that due to its carburetted powerhead, it's rated only '1 Star' under the OEDA exhaust emission guidelines. Its 50:1 premix powerhead, while torquey and reasonably fuel efficient for output, is too dirty to be used on freshwater lakes and rivers.

So in 2001, Lakeside Marine, the national Tohatsu distributor, released the MFS18B which is both CARB 2008 and OEDA '3 Star' compliant. It has features over its two-stroke counterpart such as an upfront gearshift, automatic choke, an electronic-ignition timing advance and rev limiter, and it weighs 27 per cent more than the M18E.

GREEN MACHINE
Being green also comes at a price, with an increase in the recommended retail price of 35 per cent. But for freshwater anglers and boaters who care about the environment, the MFS18B is the only way to go. So how does it compare with the M18E?

Some time ago, Lakeside Marine provided a heavy duty 3.7m Stessl Edgetracker vee-nose punt to compare the engines. Both were manual-start longshaft versions wired with a tachometer to accurately check the revs. A handheld Lowrance GPS was used to measure speeds, and fuel-flow equipment used to compare fuel efficiency, or distance travelled for fuel used which is the real measure of fuel economy. Both engines were fitted with props best suited to their torque characteristics.

Spinning a 9.1-inch pitch Tohatsu alloy prop and pushing a total of 360kg including two adults and test equipment, the M18E trolled us at 3.3kmh and 700rpm using 0.8lt/h, whereas spinning an 11-inch prop and pushing 380kg the MFS18B averaged 4.4kmh and 950rpm using 0.4lt/h. This equates to a fuel efficiency improvement of 167 per cent or more than 2.6 times that of the M18E.

Cruising at 4000rpm and about two-thirds throttle, the M18E averaged 23.9kmh and 3.9lt/h, whereas, due to the additional transom weight, the MFS18B only just planed at these revs averaging 22.8kmh. It needed 4500rpm to cruise, where it averaged 29.1kmh and 3.6lt/h with an efficiency improvement of 32 per cent. Through a tight figure of eight turns at these revs, neither engine suffered prop ventilation, although the four-stroke maintained more constant revs than the two-stroke.

At Wide Open Throttle, the M18E averaged 42.3kmh and 5900rpm using 8.4lt/h compared to MFS18B's 42.8kmh and 5800rpm using 5.8lt/h. At WOT the MFS18B was a whopping 46 per cent more fuel efficient than the M18E and quieter, although vibration levels were higher despite the MFS18B being through-bolted to the transom. The efficiency increase is even more impressive considering the MFS18B shouldn't need an oil top-up between changes, whereas the oil cost must be factored into the total fuel bill for the M18E.

EASY STARTERS
Both engines were easy to start, although the MFS18B required two hands to cold-start compared to one for the M18E. No oil smoke appeared at any time with the four-stroke but, despite using a post break-in 50:1 mix, oil smoke appeared on cold starting and during extended trolling periods with the M18E. With its automatic choke, the four-stroke was also less finicky to start and when hot, both engines required only one hand. The upfront gearshift of the MFS18B made it easier to use than the side shift M18E.

Powerhead access in both engines is very good and the additional complexity of having a canister oil filter and a camshaft timing belt doesn't interfere much with access to the four-stroke's powerhead. Recommended servicing intervals are every 50 hours or six months after the first 10 hours and the waterpump impeller should be replaced every 100 hours or yearly. I also recommend checking the timing belt for damage at this interval.

The recreational-usage warranty for both engines is three years.

* As per manufacturer's recommended schedule excluding parts. Prices current as of January, 2008. Demo engines through Lakeside Marine, phone (02) 4392 6110, prop and servicing prices from Coast To Coast Boating, Morisset, NSW, phone (02) 4970 5541.