Originally released in 1938, the 7L straight six supercharged two-stroke 6-71 stayed in production until 1997 until it was phased out due to non-compliance with exhaust emission requirements. In the AWB the engine was rated at a continuous operating output of 180 shaft horsepower (shp) at 1800rpm — shaft horsepower includes gear box losses and is the actual output at the prop, unlike most diesels that are rated crankshaft output. The original engines were fitted with a mechanical gearbox and when an Allison M hydraulic box was fitted years later it dropped to 174shp at the same rpm. The intermittent output was 210shp at 2100rpm and the 6-71 developed an impressive 813NM of torque at just 1600rpm.
Unusual for a displacement hull, the AWB has a straight bottom run up from just aft of amidships to the transom, with no hull rocker to make the hull squat aft when pushed past its designed hull speed of around 8.5kt. In fact, free-running, the AWB could reach around 11kt.
I know this because during my time in the Royal Australian Naval Reserve in the early 1970s, all my power boat seamanship and boat handling practice at Garden Island on Sydney Harbour was done in 6-71 powered AWBs. With its large diameter prop, coming alongside a jetty port side in required, just a quick spurt of astern gear (after coaxing the tramcar lever to engage) to walk the stern in. I've never since handled such a predictable single screw vessel.
Because the AWB has a relatively shallow hull depth the 6-71 sat in an unlined timber box just aft of the helm. When running the engine near maximum continuous rpm the noise from the supercharger was deafening. I'm sure that’s when my tinnitus began.
With the 6-71 long gone what’s a suitable replacement engine? Complete with gear box the engine was 1727mm long, 914mm wide and 1118mm high and weighed around 1100kg (1243 with Allison M) so a re-power engine would need to fit in the engine box and like all good repowers should be lighter than the original engine.
From extensive researching online of manufacturers' engine specs — most of which had woefully little technical information — I stumbled across a six-cylinder commercially-rated engine that would fit in the engine box, was lighter than the original 6-71, and developed slightly more torque, though at higher rpm. No other engine came close as a replacement.
IN THE DETAILS
Lurking away in Yanmar’s commercial range is the four stroke 6CH-HTE3-WUTE which, complete with heavy duty hydraulic gear box, is 1600mm long, a substantially narrower 738mm, and 1096mm high so it would fit the box nicely. Being turbocharged and inter-cooled, correct propping is essential, but the much lighter 940kg dry weight is the real bonus.
The 6-71 had two exhaust valves per cylinder with cylinder wall air intake porting and, like the 6-71, the Yanmar has push rod-operated valves and heat exchanger cooling, although it’s designed for a dry exhaust and a cooling water injected exhaust elbow would need to be fitted to suit the original 6-71 wet exhaust that exited through the transom, plus an enlarged exhaust system to prevent excessive back pressure. Also, the continuous and intermittent outputs are crankshaft-rated so would need to be reduced by around five percent to allow for gearbox losses to compare directly with the 6-71.
The commercial intent of the Yanmar is evident in the twin vertically-mounted oil filters and seawater-cooled lubricating oil cooler. The oil sump capacity is a massive 23L and Yanmar recommends using a mono grade SAE30 oil in temperate climates and SAE40 in tropical conditions. I recommend changing the engine oil and filters every 200 running hours or six months to ensure engine longevity. The wet cylinder liners also fitted provide way better longevity than dry liners because of more effective heat transfer from the combustion chambers to the cooling water passages.
The cooling system is also massive at 23L plus 0.8 for the coolant overflow tank, so this Yanmar should never suffer from overheating in tropical climates. Yanmar doesn't mention the alternator output, but the 24V starter motor is a beefy 4kW. The fuel injection system is an easily serviced in line design.
The standard instrument panel has an analogue tachometer with digital hour meter plus gauges for low engine oil pressure and high coolant temperature. Warning lights and alarms include lube oil filter blockage and low clutch oil pressure.
THE WRAP
It’s a shame the 6-71 has been discontinued, as it was such a reliable engine — but then it did have quirks such as having to always maintain a fast idle in neutral to prevent lube oil pressure getting too low. And of course being a two-stroke with supercharging the noise levels in close proximity to the engine were horrendous. The four-stroke Yanmar should be much quieter, even allowing for turbocharger whine approaching Wide Open Throttle.
Being a Yanmar it should enhance the re-sale value of the boat, if you can ever bear to part with it. After all, the AWB is a timeless classic and a throwback to days when oldies like me learned to handle single screw vessels without the aid of bow and stern thrusters.
For more on this engine email info@powerequipment.com.au or call (03) 9709 8500.
Find more about Yanmar here.
SEA TRIALS
Unfortunately, Yanmar doesn't supply details of continuous-rating prop power and fuel flow. Being a mechanically controlled engine it should not be operated continuously below 1250rpm or cylinder bore glazing from insufficient engine load and fuel over-supply relative to load may occur. I also suggest limiting maximum continuous rpm to 2250. The engine should be propped to reach 2700rpm under normal hull loading to allow for hull and prop fouling between haul-outs.
QUIC SPECS
ENGINE Yanmar 6CH-HTE3-WUTE
ENGINE TYPE Straight six turbocharged inter-cooled DI diesel
RATED OUTPUT (MAXIMUM) 276.1bhp/280.1mhp at 2600rpm*
RATED OUTPUT (CONTINUOUS) 252bhp/255.6mhp at 2550rpm
ESTIMATED SHAFT OUTPUT 239.4bhp/242.8mhp @2550rpm
MAXIMUM TORQUE 880NM at 1750rpm
DISPLACEMENT 6494 cc
BORE × STROKE 105mm × 120mm
DRY WEIGHT WITH BOX 940kg
*Brake horsepower/metric horsepower or PS