Recently a friend contacted me about repowering his Cresta 32 gameboat. He said the existing 7.2lt 330hp Chrysler V8s were getting a little tired, and he wanted to replace them rather than sell the boat and buy a newer one.

He was concerned that he would lose too much money on selling the Cresta with petrol inboards, and there would be too much of a price gap if he decided to buy another boat.

Initially he considered fitting twin 8.1lt MerCruiser Horizon V8 petrols but no matter how long he tried surfing the internet he couldn't get torque figures on the engines - and on pricing them, he found that they were almost as expensive as comparable diesel engines.

Unusually, the Cresta runs vee-drives with the engines just aft of the cabin and the stern glands beneath the engines. Combined with a fixed bulkhead aft of the engine compartment and the location of 1100lt fuel and 300lt water tanks under the cockpit, the owner was restricted by both space and engine-weight limitations.

This ruled out fitting the Volvo Penta D6 diesels he had been interested in.

Yanmar: the only choice
After exhausting all potential engine choices including the relatively lightweight MerCruiser ES300, the owner settled on the twin 4.2lt straight-six Yanmar 6LPA-STP units and set about negotiating a price for the two engines.

He told me there was a huge discrepancy between the various agents, and he ended up securing the two engines for $18,000 less than the highest price quoted.

Based on the Toyota LandCruiser diesel, the 6LPA-STP has 24 valves actuated by a toothed, belt-driven overhead camshaft and develops a maximum crankshaft output of 311hp at 3800 revs based on 1.0hp = 746W and running on Japanese-grade distillate, which has a specific gravity of 860 grams per litre. On poorer Aussie grade distillate, which is 839 grams per litre, the output drops to 296hp at the same revs.

A lesser-spec version of this engine is also available and would be more suitable than the STP model for light commercial applications. Known as the 6LPA-DTP is has 12 valves, develops 246hp at 3800 revs on Aussie-grade fuel and produces a maximum torque of 650Nm at 2500 revs.

In comparison, in the Landcruiser 100 the electronically managed turbo-intercooled 1HD-FTE engine develops 202hp at 3400 revs and 430Nm of torque at 1800 revs, compared to 700Nm at 2600–3000 revs with a peak of 710 at 2800 revs for the mechanically injected 6LPA-STP.

Even at 1800 revs the Yanmar produces 480Nm, so this engine requires very careful propping to avoid overloading the engine when planing a hefty gameboat.

According to Alan Steber of Stebercraft at Taree NSW, who my friend used to install the engines, the Cresta displaces seven to eight tonnes, and Alan recommends that with normal operating loads including full tankage, the Yanmars should be propped to reach 3900 revs.

The slightly undersquare engine with a 94mm cylinder bore and 100mm piston stroke has a wastegated, watercooled turbocharger and boost-compensated distributor-type fuel pump to reduce black exhaust smoke when the engine is under heavy loads such as coming on to the plane.

The fuel injectors have two-stage springs to reduce injector "creak" noise, and the ladder-frame, full block-width crankshaft bearing cap structure reduces lower block-area distortion and bearing slap.

Yanmar says the toothed rubber camshaft timing belt reduces timing gear noise, while the 12 balance-weight crankshaft cut harmonic engine vibration - but although the belt has a lifespan of about 800 hours, for a marine engine I prefer the inherent reliability of a camshaft driven by a chain.

The STP version has a bigger-capacity engine-oil cooler and gallery cooled pistons to reduce heatsink in this area.

Fuel consumption
Yanmar's current engine brochures help buyers to make an informed decision when choosing a marine diesel, and show actual fuel consumption based on a standard prop power curve, crankshaft torque output and prop and full load kilowatts - the latter indicating power at both the crankshaft and prop.

It's just a matter of converting the kilowatts to horsepower. But bear in mind that the fuel-consumption figures shown are for displacement hulls where there is not an increase in engine load when the pushing a hull over the planing "hump".

On water, tests I've conducted indicate that fuel efficiency is worse just before a hull planes than when it's out of the hole.

A quick glance at Yanmar's figures shows the engine uses 9lt/h at 1800 revs, 14 at 2200, 20.5 at 2600, 30 at 3000, 41 at 3400 and 63lt/h at 3800 revs, which is economical for a mechanically injected diesel of this output. However, in the rev range at which the hull is coming onto the plane, the consumption will be higher - perhaps as much as 10 per cent.

Once the hull is planing, the fuel efficiency - or distance travelled for fuel used - will increase significantly because the engine load has diminished for the throttle opening in that rev range.

Compact dimensions
In bobtail form or minus gearbox, the 6LPA-STP is 1065mm long, 574 wide and 739mm high, and has a dry weight of only 408kg.

Fitting a single-speed ZF 63A hydraulic gearbox with eight-degree down-angle at the output flange (Yanmar doesn't supply its own boxes for this engine) adds 140mm to the overall length at the flange, and about 60kg to the weight.

The reduction ratios available are from 1.22:1 to 2.52:1, with the 1.56:1 and 2.04:1 ratios suitable for most planing hulls. ZF also makes parallel and vee-drive boxes for this engine available in a range of reduction ratios from 1.51:1 to 2.78:1 for the parallel drive and 1.29:1 to 2.48:1 for the vee drive.

As the Cresta has only 1.5in diameter shafts, the 1.56:1 ratio would be the most suitable as although it's more thrust-efficient, the 2.00:1 ratio would probably transmit excessive engine torque through these relatively small diameter shafts.

Small diameter shafts in planing hulls is one of the reasons ZF's two-speed boxes have not been as popular in Australia as in Europe and the US, because most owners here don't want to upgrade the shafts to handle the additional torque when the gearbox is in "low" gear.

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