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wo-speed gearboxes have
been around for a long time.
Since approximately 1950,
many tugs and trawlers have had a
gearbox with two speeds on the output
shaf t to meet the conflicting
demands of high bollard pull and
good, economical free-running
speeds. The major drawback to these
earlier two-speed gearboxes was that
the gear ratio could be changed only
when the machinery was stopped.
That''s no longer the case with the
own two-speed gearboxes for his
raceboats back in 1986, and found
that they offered some benefits even
at lower speeds. They could transform
the performance of a boat using fixed
surface drives such as Buzzi''s own
Trimax Drives, and make a fast boat
much more flexible at both high and
low speeds. [For more on Buzzi and
his raceboat designs, see "The Tecno
40, " PBB No. 46, page 38—Ed.]
It was Buzzi who first understood
modern two-speed gearbox, which
had its origins in offshore raceboats,
rather than working vessels. In highspeed
racing craft, the upper gear
facilitates best top-end results in calm
conditions, while the lower gear
improves throttle response in waves.
The two-speed gearbox gives a
noticeable difference in performance,
particularly with diesel power, where
engine response is slower than that of
a gas engine. Multi-ratio gearboxes
can also benefit gas-engine raceboats,
though, which tend to have a very
narrow power band when tuned for
high power. In fact, Lamborghini
makes five- and six-speed gearboxes
for its raceboat engines so that the
maximum power band of the engine
and the response can be closely
matched to sea conditions.
Italian raceboat designer/driver
Fabio Buzzi, founder and owner of
FB Design (Annone Brianza, Italy),
began designing and producing his
that the advantages of the twospeed
gearbox could extend beyond
offshore raceboats to the highperformance
leisure and military
markets. After conducting extensive
trials in raceboats, Buzzi sold the concept
to ZF Marine, one of the world''s
major gearbox manufacturers. A newcompany
was formed—ZF Trimax—
as a joint venture between Buzzi
and ZF to market Buzzi''s Trimax
surface drives. The two-speed gearbox
development grew from this
association. ZF now holds the patent
on the two-speed gearbox, and is
currently the only manufacturer
supplying two-speed gearboxes to
the recreational and military markets.
One of the company''s motivations
for branching out beyond the raceboat
market was that it saw the
potential for two-speed gearboxes to
improve high-performance recreational
craft with surface drives. To a
certain extent, a trimmable surface
drive such as the Arneson or the b
can get some of the two-speed gearbox''s
benefits by trim adjustment.
Raising the drive causes increased
propeller slip; this can allow the
engine to rev higher to produce more
power, which can then be applied as
thrust when the drive is lowered
deeper into the water. That''s the way
to get a heavy boat on plane with
these trimmable drives. Fixed surface
drives tend to use engine exhaust
exiting in front of the propeller to get
a similar effect, but there isn''t the
same flexibility when the drive cannot
be trimmed.
Neither of these slip methods is
ideal, and some boatbuilders were
finding that customers do not fully
appreciate the finer points of adjusting
drives to suit the sea conditions.
With the drive too high, the engine
can over-rev; too low, and the engine
is overloaded and will overheat. The
two-speed gearbox can solve these
problems.
ZF''s gear unit bolts on to the front
of an existing ahead/astern gearbox
between the engine and the main
gears. This arrangement enabled ZF
to avoid having to redesign the entire
gearbox, and adds very little to the
overall length of the engine/gearbox
unit. The operation of the two-speed
section is electric/hydraulic, so that
the operator can select the appropriate
gear by flicking a simple up-anddown
toggle switch on the dashboard.
It''s possible to have a separate
JUNE/JULY 2001 123
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