FDS Outdrive

The FDS (Flexible Drive Shaft) Outdrive is unique because it can be used as a hard water, semi-surfacing, or surface drive, whereas other drive systems such as the Arneson or Trimax only function efficiently in surface mode.

The FDS is protected by a full Australian patent for the innovative coupling that functions as a constant velocity joint. Yet the drive has only one moving part - the prop shaft. Prop thrust is absorbed by the engine's gearbox and the flexible coupling, which allows for 15 degrees of trim and 40 degrees of steering, enables engine torque to be transmitted to the prop via a combination of ball bearings and counter-wound springs encased in a strong neoprene housing. Gary has tested the drive unit for hundreds of hours under sometimes severe operating conditions Gary claims the FDS is 99 per cent efficient.

At only 26kg with prop, the FDS Outdrive is about half the weight of a comparable sterndrive leg such as the MerCruiser Alpha unit, except that instead of aluminium being used for the leg housing, antiventilation plate and gearcase/skeg, the whole unit is fabricated from 316 stainless steel, with paintwork to suit customers' requirements. The prop shaft is water-lubricated and inexpensive inboard shaftdrive props can be used.

Installing the FDS is a DIY job once a recess has been made in the bottom of the hull's transom. The FDS prop shaft passes through the transom via a small hole with seals to prevent water entering, and a fairing plate flush with the hull bottom is installed beneath the neoprene coupling housing to ensure uninterrupted water flow to the prop.

To steer the FDS a compact mechanical hydraulic system such as a HyDrive or SeaStar unit is fitted and an electro-hydraulic ram is attached to the antiventilation plate to provide up and down trim. The prop shaft is then connected via a flexible coupling to the gearbox which, along with the engine (unless a down-angle box is fitted) is mounted at 12 degrees.

To allow for access to the inboard side of the FDS and gearbox coupling, the engine is mounted 30cm to 50cm ahead of the transom. Compared to a sterndrive the FDS Outdrive has minimal ongoing maintenance costs, but is more expensive to set up. Complete with a 1:1 Borg Warner gearbox, a carbie 5.7lt MerCruiser Ski inboard retails for about $15,000, whereas with leg the carbie 5.7lt 250 sterndrive is about $23,000.

Add $4000 for an FDS Outdrive and about $800 for hydraulic steering and, with the 5.7lt inboard the cost is up to about $20,000. However, a spare Lundberg prop is only $220 compared to $700 for a stainless steel Quicksilver Vengeance stern drive prop.

Gary supplied a standard production FDS Outdrive for evaluation in his fibreglass 6.3m side console hull powered by a freshwater-cooled 350 Chev running electronic ignition and a two barrel 500 CFM carburetor, driving the FDS via a 1:1 ZF hydraulic gearbox. The prop was an 11.5 x 16-inch raked Lundberg.

With the FDS trimmed up the boat drew only 30cm, allowing us to bring it right into shore. Trolling at 800rpm, we were able to turn the boat in just over one hull length. In deeper water we opened the throttle and using a few degrees of down-trim planed our 1400kg total cleanly at 25.5km/h on 2200rpm and achieved a slow cruise of 29km/h at 2500rpm.

As the revs increased we trimmed up the FDS to slightly above the normal 12 degrees prop shaft angle and, at 2600rpm, averaged 40.3km/h and at 2800rpm, 45.1km/h. At 3000rpm the average was 48.3km/h and fuel flow 34lt/h. Through full lock figure of eight turns at these revs with the trim angle left untouched, there was no prop ventilation.

Leaving some throttle opening, at 4000rpm we averaged 64.4km/h using 49lt/h with only a small rooster tail from the FDS. As the temperature gauge was nudging 100 degrees there seemed little point in pushing the Chev any harder but, with the drive trimmed right up, we would have gained another 300 to 400rpm and reached almost 70 km/h. Imagine the performance in a 5.5m ski boat!

The FDS Outdrive warranty is one-year unlimited hours for recreational usage. For more details, phone Gary Rigby on 02 4994 5154.