When reviewing diesel inboards, I've alluded to ZF's 2 Speed gearboxes with monotonous regularity. In my opinion the concept is brilliant in its logic, but until you actually test a product you never really know how good it is!
Back in July, ZF Australia gave me the opportunity to test the 2 Speed box in a long, narrow offshore raceboat powered by twin 420hp Yanmar sixes. These drove through ZF 110 GTS boxes (with 1.305:1/1.012:1 ratios) and Trimax Surface Drives - similar to the Arneson concept, but fixed in one position and relying on a separate rudder for steering.
The boat itself was the biggest centre console I've driven and was built by Fabio Buzzi (or FB Design), an Italian boatbuilder that specialises in high-speed military and offshore raceboats from 10.8 to 23.5m.
The model I tested was the FB 38, which in military form is used by the Hellenic (Greek) Coastguard. It's available with twin Yanmars from 300-420hp each and according to FB Design these motors were chosen for their low weight and compact dimensions for output.
The FB 38 measures 11.85 by only 2.32m and is designed to fit inside a standard 40ft shipping container for transportation anywhere in the world. It displaces 4.1 tonnes with the twin 420s.
The standard version I tested had four seats arranged in pairs behind a curved windscreen framed by strong stainless steel rails and a dash accommodating full instrumentation and a GPS/plotter. Ahead of this was an open cockpit with moulded seating flanking the cockpit (and storage space under this). There was a small cabin forward with single berths for limited overnighting and ahead of this a full anti-dive (or what FB Design calls 'anti-stuff') stemhead section to prevent burying of the fine forward sections in following seas.
The four seats are known as 'Tecnos' and use an electrically-controlled hydraulic piston to provide variable impact cushioning. They were snug-fitting but once in them provided complete isolation from wave shock.
The FB 38 has a multi-stepped bottom with double chines and the two main planing strakes either side ending about amidships. Then there's a stepped section with one strake per side and finally another step-up to a strakeless section ahead of the twin propshafts. Running aft to the transom from this last section is a box-section (between the propshafts, props and rudders) called a 'central flap' by FB Design.
Hinged at the aft end of the strakeless section is a long 'Seaplane' trim tab either side set in a small step-up to create virtually a flush surface from the strakeless section to each tab. Towards the hull centreline the step increases and incorporates the through-hull thrust bearings for the ZF 2 Speed boxes just inside the hull, which in turn are directly coupled to the engines. This arrangement places the engines as far aft as possible for maximum cockpit space.
The propshafts run only a short distance from the through-hull thrust bearings to the semi-surfacing five-blade Swiss Rolla props. Immediately behind the props are semi-balanced spade rudders positioned so that when the boat is fully planing only the lower half of the props and rudders are below the hull bottom, reducing draft underway to about 60cm!
All underway fittings such as the trim tabs, thrust bearing flange, propshafts, prop supports and props and rudders are of stainless steel.
To get the boat out of the hole a combination of the 2 Speed box and prop aeration is used. The relatively low first gear of the box allows the motor to quickly reach its torque band, while the exhaust gases exit directly onto the leading edge of the props and allow them to ventilate until the hull is fully planing. Once planing the exhaust exits above the waterline and doesn't interfere with the lower half of the prop biting into clean water.
According to FB Design, offshore racing has proven the fixed surface drive to be much more reliable and to create less drag than the sterndrives favoured by Mercury Racing. Also the relatively low revs of the Yanmars means they won't wear out after two race seasons as apparently happens with the much higher-revving MerCruiser petrol engines.
The only real drawbacks I found were that when reversing, the exhaust gases dramatically reduced the props' power and because of the props' massive pitch and drag of the long trim tabs the Yanmars were heavily loaded in reverse, resulting in some black exhaust smoke.
However there was little in the way of a 'clunk' when forward or reverse gear were engaged.
Standard are twin lever controls, a bit disconcerting after single levers, but according to FB Design this allows for fine tuning of engine revs in rough seas.
In first gear and carrying four adults the FB 38 planed at 28-37kmh on 2000 revs, using the trim tabs to get our stern-heavy hull out of the hole. At 2500 revs and at the peak of the engines' torque curves, a quick flick of two dash-mounted toggle switches engaged the electronically-controlled second gear with a slight jolt and the speed jumped from 55-75kmh.
At Wide Open Throttle (WOT) and 3300 revs the FB 38 averaged 118kmh over a 20cm chop on Sydney Harbour. Although the boat leapt around a lot at this speed and was quite noisy with the above-water exhausts, it held a straight course well with surprisingly little wave shock being transmitted through the Baltek balsa core sandwich bottom skin. However there's no denying the FB 38 is a wet boat!
To reduce the space needed to turn the FB 38 at WOT, which tended to skip across the waves (mainly because of the semi-exposed rudder blades and hull's directional stability), we had to drop the speed to about 90kmh.
Back at 30kmh the hull turned in its own length with the boat banking steeply into the turn. We then waited until the revs dropped to 1600 (the maximum downshift revs) and shifted to first gear. There was no jolt as it engaged and then the throttles could be slammed open. Once 2500 revs were reached, a quick flick into second saw the FB 38 accelerating like an outboard-powered ski raceboat with the turbos howling on full boost. Who says diesel-powered boats can't be fun!
For more on the ZF 2 Speed gearbox, contact Gary Bain, tel (02) 9674 6222 and for the FB 38 and Trimax Surface Drive, contact FB Design, tel +39 0341 260105; or email: firstname.lastname@example.org.