Two- versus Four-Strokeseatrials
Single M5B and MFS 5C LPG on flat-bottomed 2003 Sea Jay 3.4 punt, swinging 8-inch and
7-inch props and pushing 285kg and 290kg respectively. Average of two-way runs on
Lake Macquarie, NSW, in calm water. Range is in nautical miles for the 2.5/12lt tanks and
7.5kg gas bottle respectively with a 10% reserve. In reality the 7.5kg bottle shouldn’t be
filled to more than 80% capacity and this is also listed in the following figures.
ENGINE M5B MFS 5C LPG
TROLLING 1100rpm 1300rpm
KTS 1.9 2
LT/H 0.3 —-
RANGE 14/68 —-
PLANE/CRUISE 4900rpm 5800rpm
KTS 11.9 12.6
LT/H 2 —-
RANGE 13/64 —-
WOT RPM 5200 6200
KTS 13.8 14.7
LT/H 2.5 —-
RANGE 13/59 —-
“Loop” of cruising with 10% WOT and 40% trolling operation averaging 4kts
LT/H 0.87 0.6
RANGE 10/49 88/70Tohatsu M5B
versus MFS 5C LPG
FROM EIGHTIES TO BREAKTHROUGH TECHNOLOGY...
T
he M5B is the
longest-running
two-stroke 5
on the Aussie
market. Originally
released in 1985 to replace
the air-cooled M5A, the
water-cooled M5B featured
electronic ignition timing
advance and a through
exhaust gas hub system
to extract the maximum
amount of power and torque
from the single-cylinder
loop-charged engine. A fullgear shift, six trim positions,
a single shallow-water drive
setting and automatic reverse
and full tilt locks plus twist-
grip throttle control were
provided.
Initially, two separate
models were available, one
with a 2.5lt integral fuel tank
and the other with a remote
12lt tank, but more recently
these were combined to
offer anglers the choice of
integral and remote tanks.
Five different prop pitcheswere available to suit a wide
variety of hulls.
Apart from changes to the
cowl shape the current M5B
remains the same as in 1985,
including using a 50:1 premix
after the first 10 hours on
25:1.
Released last year and
based on the MFS 6C
introduced in 2000, the 5C
LPG runs on propane or LPG
and is Australia’s cleanest-
running 5hp engine. Some
top-end power has beenlost compared to the 6C
but bottom-end torque is
the same and because the
LPG burns so thoroughly
the 5C LPG has way lower
vibration levels making it the
smoothest-running single-
cylinder five I’ve ever tested.
A rev limiter cuts in at 6300.
The leg, transom bracket and
lower unit are the same as the
M5B but only a remote tank
model is available, normally
connected to a 7.5kg (14.7lt)
fibreglass SprintGas bottle.
The overhead valve engine
has a gear-driven camshaft
operating push rods for
the rockers and pressure
lubrication with low-oil
warning light on the engine
pan. And the upfront gear
shift makes using the 5C LPG
so much nicer.
During testing I measured
gas usage by weight so fuel
usage at a given set of revs
wasn’t possible, but overall
the loan 5C LPG used slightly
more fuel than the 6C under
identical test conditions.
Power head access in
both engines is excellent
and servicing intervals are
every 50 hours or annually
after the first 20 hours. The
recreational usage warranty is
three years for both engines.tradeboats.com.au 33