Twin MerCruiser 3.0L sterndrives

Two years ago I reported on the idea of powering cruisers such as the Caribbean 26 with twin MerCruiser DTI 1.7L turbo-intercooled diesel sterndrives as an alternative to refitting a new pair of 190hp petrol sterndrives.

Since that article was written I have received reports about repowering older Bertram 25s with these diesels - a substantial drop in power from the twin 470 petrol sterndrives originally fitted, but apparently still returning good performance with the added benefit of dramatically-reduced fuel consumption when trolling offshore at displacement speeds.

But for many boatowners who don't regularly travel well offshore to reach fishing grounds, a substantially cheaper alternative is to fit twin raw-water (saltwater) cooled MerCruiser 3.0L petrol sterndrives, which develop 135hp at the prop compared to around 110hp for the DTI 1.7L diesel.

Mercury Marine doesn't provide torque curves for its petrol sterndrives, but having recently tested a 3.0L, its maximum torque output appears to be around that of the 264Nm produced by the diesel - though probably at slightly higher revs. A real sweetener is the recommended retail of $14,100 (September 2003) including an Alpha One leg with 2.0:1 gear ratio and power steering - about $8000 cheaper than the DTI 1.7L with the same leg, not to mention the lesser maintenance and repair costs down the track.

Originally released 26 years ago as a 120hp unit then later as a 140 before prop-rating became all the rage in the mid-'80s, the 3.0L utilises the same 101.6mm cylinder bore as the petrol MerCruiser V6 4.3 and V8 5.7lt sterndrives - but at 91.4mm, the stroke is 3.0mm longer. The 3.0L develops maximum power at only 4600 revs with a Wide Open Throttle rev range of 4400 to 4800, whereas the DTI 1.7L's range is 4200 to 4600. Complete with leg the 3.0L weighs 288kg - eight kilos less than the DTI 1.7L.

SENSIBLE ENGINEERING
Up until five years ago, few modifications were made to the 3.0L, which still remains Mercury Marine's biggest-selling sterndrive. Then came EST Analog electronic ignition with electronic timing advance and revised manifold design for reduced maintenance, easier starting in a damp marine environment and improved performance.

Thankfully Mercury retained the proven pushrod OHV actuation with hydraulic lifters and a cast-iron cylinder block and counterflow (non-crossflow) cylinder head, plus the simple twin-barrel carburettor with automatic choke.

Other features include a 65amp alternator with voltage regulation, mounted high on the block, away from bilgewater. The oil dipstick and canister oil filter are easily reached, as are the drive belts for the alternator and optional power-steering system, which is much more responsive than mechanical hydraulic steering systems. A sensible feature is the cooling water pump-out system - if the boat is drystacked in freezing conditions, any water left in the block and exhaust system can be quickly drained.

With their comparable torque output to the DTI 1.7L, twin 3.0L units should have no trouble pushing a Bertram 25 - which, depending on engine choice, displaces around two tonnes. The 3.0L provides excellent throttle response across the entire rev range; is relatively quiet for its output; and has reasonably low vibration levels, considering its piston displacement is very large for an in-line four-cylinder petrol engine, and that no vibration-reducing features like twin counter-rotating balance shafts have been fitted.

STUNNING PERFORMANCE
I have been unable to test a 3.0L against the DTI 1.7L in the same hull, so the following figures should be taken as a guide only.

Mounted in a 5.33m Sea Ray 176 SRX bowrider, the demo 3.0L from Hirecraft Marine of Toronto, NSW started instantly hot or cold by just turning the ignition switch (after the initial blower run to clear the bilges), warmed quickly from cold and never blew any oil smoke. Power astern was good and no cooling water starvation occurred. The ignition interrupter activated whenever ahead or astern gear was selected, ensuring virtually "clunk-free" shifting despite the Alpha One leg having a dog clutch.

Spinning a 21in pitch alloy Quicksilver prop and pushing a total of 1250kg including two adults, the demo 3.0L averaged 3.0kt on 900rpm compared to 2.8kt at the same revs for the DTI 1.7L in a Savage 5.5 Mako spinning an 18in alloy Quicksilver prop and pushing 1360kg. At 2000rpm the averages were 7.5 and 7.1kt, but the 3.0L achieved a clean plane at 12.9kt on 2500rpm compared to 15.0kt and 2750rpm for the diesel.

Cruising at 3000rpm the 3.0L had the edge on the diesel for quietness and vibration levels, and averaged 20.9kt compared to 18.6kt at the same revs, but consumed 22lt/h whereas the diesel used 9lt/h. Through tight figure-eight turns at these revs, neither leg suffered any prop ventilation.

At 4000rpm the 3.0L averaged 30.0kt using 32lt/h compared to 26.7kt and 17lt/h for the diesel, was noticeably quieter and accelerated much harder to WOT where, like the diesel, it was slightly underpropped. The WOT averages were 37.9kt on 4950rpm using 45lt/h for the petrol and 30.8kt and 4600rpm and 27lt/h for the diesel.

The petrol sterndrive also had better power trim and tilt feel - and when the trim gauge registered three quarters, the prop blew out but quickly regained its "bite" with a stab of the trim button, whereas the diesel's leg didn't recover quite as fast. But it could have just been the hull designs.

Obviously twin 3.0L units in a Bertie 25 won't be as quick, due to the lesser grip of the props outboard of the hull's centreline and the windage of the flybridge and possible clears - but they should still push the hull to better than 30kt.

COMPACT DIMENSIONS, LOW MAINTENANCE
Measuring 730mm long by 660mm wide and 533mm high, the 3lt engine is narrower and lower than the DTI 1.7L engine, which is 726 x 746 x 689mm - and this allows for better servicing access between the engines. Recommended servicing intervals are every 50 hours or six months after the initial 20-hour service, and Quicksilver SAE25W40 oil should be used in the sump. As the waterpump impeller is in the lower unit, it should be replaced every 100 hours or once a year or every six months for moored boats if using the boat infrequently.

Unlike the DTI 1.7L there's no camshaft timing belt to replace. But as with this engine, the diaphragm where the leg is bolted onto the transom should be checked for deterioration every three years for dry-stacked boats and preferably once a year for moored boats.