REVIEW: TOHATSU MFS5C LPG

Tohatsu Corporation is nothing if not innovative. It developed the world’s first battery-less EFI for four-stroke outboards and now has introduced its first small dedicated LPG outboard that runs on separate gas bottle.

But using LPG in an outboard does way more than just reducing these nasties. It makes a small outboard run smoother because there’s no transition from idle to main carbie jets. And of course per litre LPG is way cheaper than petrol, even though the engine consumes up to 15 per cent more fuel than a comparable output petrol engine.

The Japanese-made MFS5C LPG utilises the same base power head as its petrol MFS6CS counterpart, which develops 5.9 brake horsepower at 5500rpm. Switching to LPG means the engine loses 0.9 BHP at the same revs but testing the engine under identical test conditions shows the 5 develops the same bottom end torque as the 6 but loses out in the top end, needing a smaller prop to achieve similar Wide Open Throttle performance.

THE NITTY GRITTY

The power head has a gear-driven camshaft operating push rods and rockers to actuate the two overhead valves. Full pressure lubrication is provided with an oil level warning light on the engine pan. An effective thermostat helps warm the engine quickly from cold and maintain reasonably constant operating temperatures.

A five amp alternator is optional and Tohatsu recommends a 12 volt 40 amp/hour 350 CCA (cold cranking amps) battery for temperate climates and a 70 amp/hour 650 CCA unit for cold climates to prevent frying the battery on long runs to and from a favourite fishing spot.

Like the 6 there are six trim positions, a single shallow water drive setting and automatic full tilt and reverse locks. Again the engine must be in forward gear to tilt, a pain when setting shallow water drive.

In the 5 the fuel pump is replaced with a regulator which needs to be supplied with at least 1 bar or 14.7psi of gas from the bottle. A shut-off valve stops gas flow whenever the engine is stopped. The carbie is replaced with a mixer, which looks like a carbie except there’s no fuel bowl. And finally valve seats are hardened as unlike petrol, LPG is a dry and hotter burning fuel and provides absolutely no seat lubrication.

However there is an air/gas enricher just like a choke which must be pulled out when starting a cold engine and slowly pushed in as it warms. The 7.5kg capacity Sprint Gas fibreglass bottle supplied was well made with marine-grade brass fittings and the translucent cylinder showed the liquid gas level below half capacity.

Servicing intervals are every 50 hours or annually after the first 20 hours. Valve clearance adjustments aren’t needed for the first 200 running hours and removing the rocker cap to adjust clearances is easy.

The 0.45lt sump is a little light on capacity to absorb oil sludging between changes. But being a gas engine it’s unlikely there’ll be the same blow-by issues that afflict petrol engines during extended trolling periods. A large drain elbow under the sump makes changing oil easy. Tohatsu recommends using Quicksilver FCW SAE10W30 for all-round use.

The recreational usage warranty is three years.

ON THE WATER

The fully run-in loan 5 was an almost perfect match for my 2003 Sea Jay 3.4 Punt and outperformed a two stroke Tohatsu M5B under identical test conditions, though it needed a finer pitch prop to reach the top end of the recommended WOT rev range.

Cold starting isn’t as easy as the 6 as there’s no fuel primer bulb so it takes a while for gas to reach the regulator and about seven or eight pulls are needed compared to one or two for the 6. The engine has decompression starting where the exhaust valve is kept open until the engine fires so cranking it requires no more effort than the Tohatsu MFS3.5. Once the engine fires progressively pushing in the enricher knob allows the engine to settle down to a low idle with less vibration than the petrol 6.

Only on cold starting is there a slight gas smell but no oil smoke appears. Tohatsu recommends warming a cold engine in neutral for at least three minutes above five degrees ambient temperature and five minutes below. Before switching off the engine after each run or flushing I turn off the gas at the bottle and the engine runs for 40 seconds on remaining gas in the fuel line.

Using the third trim hole and swinging the same eight inch pitch prop the loan 5 accelerated as quickly onto the plane as the 6 but did “lock up” at the lower end of the recommended WOT rev range pushing my standard two adult and fishing tackle load. Swapping for a seven pitch prop allowed the engine to rev right out to just below where the rev limiter cuts in at 6300rpm.

Unlike petrol outboards where I can connect a fuel flow meter I could only work average fuel usage based on the weight of LPG consumed and not for specific revs. An electronic bathroom scales was used with increments of 0.1 KG so measuring was not as accurate as determining petrol usage in millilitres. However Tohatsu states the 5 uses 1.0 KG/hour at WOT but with the engine revving out to 6200 running the seven inch prop the loan engine was really using around 1.1 KG/hour. Elgas states 1.0 KG of LPG equals 1.96 litres so that’s a WOT consumption of 2.2 Lt/h compared to 2.1 for the petrol MFS6CS, which ran out to 15.3kts and 5700rpm swinging the eight inch prop.

After 2.5 hours of running with a total of 7.5 percent WOT operation, averaging 0.7 Lt/h the fuel/oil ratio was 1100:1, not bad for a small four stroke engine.

THE WRAP

Obviously the higher purchase price of the 5 will deter some boaters. But it is an excellent compromise between a four stroke petrol outboard and an electric outboard such as a Torqeedo. It’s significantly less expensive and the 7.5 KG gas bottle gives a much longer range than a comparable-weight lithium ion battery. The 5 performs well on flat bottomed punts to 3.5 metres without (in NSW) needing a driver’s licence or boat registration. It’s beautifully made and the crisp white paintwork looks so good compared to black or grey outboards. Also it’s safe for dentures and retaining hard-earned flab.

If other LPG outboards can be this good then I feel petrol for four stroke outboards will have a limited future. And the big winner will be the environment!

TOHATSU MFS5C LPG SPECS

TYPE Single cylinder four-stroke LPG outboard

RATED BHP/MHP* 5.0/5.0 at 5500rpm

REC. WOT range 5000 to 6000

DISPLACEMENT 123cc

BORE X STROKE 59 x 45mm

GEAR RATIO 2.15:1

WEIGHT 27.2kg (short shaft)

RRP $2084 (short shaft)

RRP gas bottle $198

OEDA stars 3 (plus Euro Recreational Craft Directive 2 star rating)
*Brake horsepower/metric horsepower

Check out the full review in issue #498 of Trade-a-Boat magazine. Subscribe today for all the latest boat news, reviews and travel inspiration.