Engine Review - Mercury FourStroke 2.5 portable outboard motor

Ever since I acquired a 1981 3.8m Fairlite Gull dinghy in 2008 I’ve been wanting a low-emission portable outboard to match this hull’s efficiency. For example, it averages 4.1kts in calm water with a Mariner T2400 electric trolling motor that develops a mere 0.4hp.

Reviewing it shortly after acquisition with a Thai-built Suzuki DF2.5 portable outboard showed that while this engine performed as well as my 1993 twin-cylinder Johnson 4, the vibration levels were so high continued usage would have damaged my classic hull.

Portable Mercury 2.5 hp four-stroke outboard

So it was back to head scratching until late last year when Mercury offered me a FourStroke 2.5 portable outboard motor for evaluation. I had the choice of a 3.5hp version as well but figured the de-rated model would be better. It develops max power at 500 fewer rpm allowing for a coarser pitch prop and as with any displacement hull, engine load quickly falls off when the throttle is backed off, so slight over-propping is not a concern.

The other aspect of the 2.5hp FourStroke is that it’s designed for transom heights to 16in, which the Gull has as it was designed for US outboards that handle 16in transoms. The DF2.5 is designed only for 15in transoms, so on the Gull its prop was running in aerated water which also affected engine cooling.

Mercury 2.5 four-stroke portable outboard motor

Like the DF2.5, the Japanese-made FourStroke 2.5 portable outboard engine has a single-cylinder OHV engine with thermostatically-controlled water cooling and gear-driven pushrods, but splash lubrication is used instead of the DF2.5’s pressure system. Unfortunately the DF2.5 doesn’t have a low oil level warning light and if oil pressure falls off you won’t know until the engine seizes!

Whereas the DF2.5 has fixed ignition timing, the Mercury 2.5hp portable outboard motor has electronic advance enabling it to troll slower and perform better across the rpm range. Also a visible pilot water discharge is fitted, unlike the Suzy where you constantly have to reach over the transom underway to feel for the cooling water discharge. Cleverly the four-stroke Mercury 2.5 portable outboard motor can be flushed by attaching a garden hose without having to run the engine, though I always use a flushing drum with constantly overflowing water because it clears salt deposits from the water pump impeller and propshaft.

Powerhead access is better because the upper cowl can be completely removed from the lower cowl, though checking valve clearance adjustment won’t be easy as the rocker cover is tucked well down in the lower cowl. The one-litre fuel tank gravity feeds to the carbie which is alongside the powerhead to prevent vapour lock on hot days, assisted by hot air venting via water-trap air intakes in the upper cowl.

Changing oil in the 0.3lt sump is easy by removing an 8mm nut in the drain elbow – no dribbles down the leg. The recommended oil is Quicksilver FCW SAE 10W30 for all temperate climates, with servicing every 100 hours or annually after the first 20 hours. The warranty coverage is “3 + 2” years.

Performance

A quick trial with the standard I7 propeller showed the Mercury 2.5 FourStroke was way over-propped, so this was quickly swapped for a 5.7in pitch F6 plastic weedless prop. The loan engine normally started first pull hot or cold, with oil smoke appearing only after the engine had been transported on its side. After carefully running in the engine according to Mercury’s recommendations it was ready for performance, fuel flow and noise testing.

For these trials I took along fishing partner Tiana who declared the Gull/2.5 combo as perfect. The Gull reached 6.6kts compared to 6kts for the Suzuki pushing the same load of 310kg. Yet hull speed of 5kts was achieved at only one-third throttle opening.

Best of all were the low vibration levels, even at WOT where, due to cyclical slowing during exhaust and compression strokes, single-cylinder four-strokes are always at their roughest. At one-third to half throttle there was virtually no vibration, just that steady single four exhaust beat that’s music to my ears! And with good torque, punching into a 30cm chop and 10 to 15kt headwinds reduced cruising speed by only 0.5kt.

The Trade-a-Boat verdict

Tiana and I plan many more trips in the Gull which has been transformed into a low-emission fishing platform that’s oh so cheap to run.

Correctly propped the Mercury 2.5 hp suits displacement hulls to 4m and frankly is way too powerful for tenders such as the Walker Bay 10, for which the DF2.5 is a perfect match. The Mercury  2.5 FourStroke is beautifully engineered and with regular servicing should return years of frugal boating. For more info, visit mercurymarine.com.

Mercury FourStroke 2.5 portable outboard sea trials

Single Mercury FourStroke 2.5 outboard motor on 3.8m Fairlite Gull, F6 prop, total 310kg including two adults and fishing tackle, average of two-way runs on Lake Macquarie, NSW, waves to 30cm

* Loop of cruising including 10 per cent WOT operation, averaging 4kts equals 0.45lt/h. Sea-trial data supplied by the author.

Mercury FourStroke 2.5 portable outboard specs

Mercury FourStroke 2.5 price: 1230 RRP

TYPE Single-cylinder four-stroke petrol outboard motor

RATED BHP/MHP* 2.5/2.5 at 5000rpm

REC. WOT RANGE 4500 to 5500rpm

DISPLACEMENT 85.5cc

BORE X STROKE 55 x 36mm

GEAR RATIO 2.15:1

WEIGHT 18.4kg (dry, short shaft)

OEDA STARS 3

* Brake horsepower/metric horsepower

See the full version of this review in Trade-A-Boat #472, on sale November 26, 2015.