Yanmar 6LPA-STP

Engine overhaul

A couple of years ago in Trade-A-Boat I discussed how a friend had settled on twin Yanmar 6LPA-STP turbo-intercooled diesels to replace the tired 7.2lt 330hp Chrysler V8 petrol engines in his 1980 Cresta 32 gameboat.

Since the initial report and after a year on the hard, Chris has re-launched the Cresta and by December 2005 had clocked up 25 hours on the engines.

While helping Chris to run in the Yanmars he discussed with me why he chose these engines over other makes. He said he really liked the electronic management system of Volvo Penta D6 engines and comparable Iveco diesels and carefully considered them, only to find them to be too long and high for the existing engine compartment. Their additional weight may have created hull trim problems, too.

Chris ruled out Steyr engines because he considered the output to be a bit low for the application with a “poor torque curve”.

The Chryslers developed the 330hp at 4400 to 4600rpm and between 600 and 700Nm around the 3000rpm mark. So Chris wanted engines that developed comparable power and torque, based on his need for a cruising speed of around 20kts for long distance offshore cruising and gamefishing. The engines had to weigh the same and, because of their length, height and torque curves Chris chose the Yanmars. Like the Chryslers, the Yanmars needed compact vee-drive boxes because, unusually, in the Cresta the engines are mounted just aft of amidships and the propshafts run at a 12-degree angle back to exposed props beneath the hull, which has a 20-degree deadrise at the transom.

Mounting the engines this way eliminates the need to use jackshafts from engine to vee-drive, or lifting the saloon floor for engine access. And when backing down on a gamefish, should any water find its way into the bilges through the aft cockpit floor hatches, it will only trickle onto the fuel and water tanks, rather than the engines, which are separated from the lazarette by a full bulkhead.

One aspect of Yanmar that Chris really liked was the comprehensive information supplied on the engine brochures, which includes detailed engine measurements and weight, plus power, torque and actual fuel usage curves.

In vee-drive configuration, the 6LPA-STP engines use the same single speed ZF63IV boxes as fitted to the MerCruiser 8.1S HO V8s (419.6hp at 4600rpm) that I recently tested in a SeaRay 40 Sundancer. But unlike the MerCruisers, which run 2.5:1 reduction ratios, Chris opted for 2:1 reductions, as the Yanmars are designed to reach 3800rpm.

As the Yanmars are flexibly-mounted, the installation mechanic fitted a flexible coupling between the gearboxes and propshaft. The couplings are over-engineered but should survive thousands of hours of usage.

Chris said the original props were 18 x 21-inch three-bladers, but too small for the Yanmars. Therefore, 19 x 21-inch cupped four-bladed props were fitted. The original exhausts were four inches in diameter but would have created excessive back pressure and a custom six-inch exhaust system with risers beneath the cockpit coamings was fitted. To simplify installation, a through-transom exhaust system was used. Unusually, though, the mufflers are right by the engines instead of near the transom, substantially reducing exhaust gas temperature where the hosing passes outboard of the 550lt fibreglass fuel tanks each side.

The fuel tanks were removed and the dirt and gunk was flushed from them before refitting. The fuel filters and lines were upgraded to cope with the increased flow and a return line for each engine installed, while the seawater inlets were increased from 1.5 to two inches and larger strainers were fitted.

Chris said the existing push-pull cable steering system was very heavy, so he fitted a new HyDrive hydraulic system, which was very easy to install. The existing single-lever throttle/gearshift control boxes were retained. Full instrument panels, which include key start/stop, a tacho, an hour meter and water temperature and turbo boost pressure gauges were supplied with the engines, and Chris said the electrics were all plug-in types and very easy to fit.

ON-WATER IMPRESSIONS
As Chris had invested so much time and effort into the re-power, he ran in the engines carefully for the first 50 hours. This involved mainly running them between 2000 and 3000rpm with no wide open throttle operation. The revs changed every 10 to 15 minutes to ensure the piston rings bedded in properly.

Only on cold-starting do these mechanically-injected engines blow any grey exhaust smoke and although there is some black smoke (through fuel over-supply) when lifting out of the hole, the engines run cleaner than expected.

When I tested the Cresta, I found it to be most cruise-efficient at 2500rpm, where we averaged 17.7kts using a total of 39lt/h.

For more details on the 6LPA-STP, contact Michael Blair at Power Equipment on (03) 9764 0711 or email power.equipment@yanmar.com.au