ZF's two-speed gearbox range

Up two speed!

ZF's range of two-speed gearboxes represents a potential leap forward for owners, designers and builders of fast planing craft. Manually-selectable, the units are just the thing for a wide range of recreational and light commercial usage. Andrew Norton.

There's nothing new about two-speed gearboxes in boats. European trawlers have had them for decades, giving vessels the choice of a lower ratio for trawling and higher for normal cruising.

Apart from the obvious benefits of two forward ratios, the deeper reduction of the reverse gear enables an engine to more quickly reach its torque band for faster stopping and improved manoeuvrability astern (including backing off a sandbar). For this reason, many single-speed boxes have deeper reverse ratios than forward gear important in a yacht where engine power is very limited in relation to hull displacement.

But until late last year, no company had made public a manually-selectable two-ratio box for pleasure craft.

ZF Marine has changed that with a range of electronically-shifted, though manually-selectable, boxes and ratios suitable for pleasure duty and light commercial usage.

FAB FOUR
Four distinct types of boxes are available. The first is a parallel offset series (comprising six separate models) where drive from the crankshaft output flange drops in the same plane to the propshaft input flange, as do most traditional type of single-speed forward-neutral-reverse gearboxes.

The smallest of these, capable of handling pleasure duty-rated diesels from 303 hp at 2300rpm to 422 at 3200, is the HSW 90 TS which is available in ratios from 1.25:1/0.97:1 to 1.970:1/1.526:1 and weighs a very reasonable 66kg. According to tests conducted by ZF, the latter selection of ratios is about the most popular for recreational planing hull applications.

Largest of the pleasure duty-rated parallel offset series is the BW195 P1 TS, able to handle from 1475hp at 2100rpm to 1615 at 2300. The ratios are much closer together, examples being 1.086/1.015-0.821 to 2.030/1.898-1.534, while the all-up weight is 495kg.

The second series (six separate models) is the Offset 'A' Down Angle, where the crankshaft remains parallel to the waterline but the propshaft angles down several degrees. This type of box, in single-speed form, was made popular by Volvo almost two decades ago and has been adopted by many diesel manufacturers. It enables the engine installation angle to be reduced while still providing sufficient down-angle for the propshaft and also allows an engine to be located further aft away from accommodation, hence its popularity with yachties.

Smallest of these is the HSW 90 ATS with 8° down-angle at the propshaft connection flange. It is pleasure-rated from 303 hp at 2300rpm to 422 at 3200 and available with ratios from 1.220/0.942 to 2.530/1.960. It weighs 63kg.

Largest is the BW 195 AP1 TS with 10° down-angle, rated from 1475 at 2100rpm to 1615 at 2300. The ratios are 1.273/1.190-0.962 to 2.030/1.898-1.534 and weighs 495kg.

The third series (three separate models) is the V-Drive Integral, popular when designers locate engines aft under the cockpit floor to balance the hull under varying load conditions. Providing a hull has the buoyancy aft to take the weight of twin diesels, locating them aft enables variable load components such as fuel tanks to be positioned amidships, so that whether full of near empty, hull trim doesn't significantly alter.

Smallest is the HSW 90 VTS, rated the same as the 90 TS and ATS models but incorporating a 12° vee angle and available in ratios from 1.290/0.999 to 2.480/1.922. This box weighs 80kg.

Largest is the IRM 300 VTS with a 14° angle and rated from 452 hp at 2300rpm to 590 at 3200. Ratios are 1.252/0.981 to 3.094/2.423 and weighs in at 149kg.

The fourth series is the V-Drive Remote, where the box is positioned well ahead of the engine and a jackshaft is used to transfer power from the crankshaft to box. This enables an engine to be located right aft, but reduces propshaft angle by positioning the V-drive further forward.

The one model available, known as the BW 190 VTS, is available in a range of variations from 190 VTS to 195 VP1 TS, all with a 10° angle.

The ratings are from 1,234 hp at 2100rpm to 1615 at 2300, while ratios for the 190 are 1.273/1.180-0.962 to 2.920/2.730-2.207 through to 1.273/1.190-0.952 and 2.030/1.898-1.534 for the 195. All variations weigh 520kg.

REAL PERFORMER
Exhaustive testing conducted by ZF in the US shows real performance increases and fuel savings compared to single-speed boxes.

One example quoted by ZF was a 12.5 metre Sea Ray, displacing 12,400kg and powered by twin Volvo 73P diesels fitted with HSW 110 ATS boxes having 1.95/1.52 ratios.

According to ZF, at 1000rpm in first gear, the Sea Ray averaged 17.6kmh consuming 10lt/hr per engine, but in second gear at the same revs it averaged 17.3 and a mere 2lt/hr!

At 2000rpm the first gear average was 20.6kmh and 39lt/hr compared to 34.1 and 28 in second. And at WOT first gear average was 51.8kmh and 70lt/hr compared to 63.8 and 65 in second!

The only drawback was slower acceleration out of the hole in second gear, 32.1secs from 0-2600rpm compared to 13.8 in first. But with a two-speed box, why would you accelerate onto the plane in second? That is the beauty of two-speed gearboxes especially in planing hulls.

Further tests by ZF showed that shifting up at the right time is crucial for good planing hull performance. For example, a 9.5m gameboat powered by twin 420hp Yanmar 6LY2-STE diesels performed best when the ZF boxes were upshifted at around 2500rpm. Shifting at 2000rpm the 8300kg hull planed in eight seconds, while at 3000 it took five and a half. But at 2500rpm it took only five seconds.

Mating a diesel such as the 6LY2-STE to a ZF two-speed also lowers the all-up weight. For example, with a single-speed gearbox this motor weighs 617kg, but fitted with the HSW 110 ATS with 8° down-angle the weight drops to 599kg!

For more information on ZF two-speed gearboxes, contact Gary Bain at ZF Australia, tel (02) 9674 6222.