The Mercury 75 hp TLDI is a de-rated version of the Mercury TLDI 90 outboard, but because it’s so under-stressed for its output, it’s actually a better engine when mounted on the right hull.

The OEDA 3 Star Mercury 75 develops 74.0 hp at 5500 rpm with a wide open throttle (WOT) rpm range of 5150 to 5850. The three-cylinder powerhead displaces 1267cc and produces 120Nm of torque at 4000 rpm, whereas the Mercury TLDI 90’s maximum torque is 135Nm at 4250 rpm. So, on paper, the Mercury 75 hp is way down on output.

The Mercury 75 hp TLDI has the same orbital combustion process (OCP) developed by the Orbital Engine Corporation of Western Australia and utilises a mix of stratified and homogeneous combustion. This translates to incredibly low fuel consumption when trolling, yet plenty of torque and power to rapidly plane a hull. The variable-ratio oil injection has a large 4-litre powerhead-mounted tank, and oil is delivered by an electric pump. Lakeside Marine, the national Tohatsu distributor, recommends using Quicksilver OptiMax DFI oil in all its TLDI models.

The under-flywheel alternator produces a maximum of 40 amps, and the dry weight is 150kg. The gear reduction is a usefully low 2.33:1, enabling coarse-pitch and thrust-efficient props to be swung.

Servicing intervals are every 50 hours or annually after the first service at 10 hours, and the recreational-usage warranty is three years.

 

On The Water

Mounted on a Makocraft 4.85 MF Frenzy side-console tinnie, the review Mercury TLDI 75 outboard motor pushed exactly the same 900kg total displacement as the 90 on this hull, including two adults and test gear. However, Lakeside’s Dave Denny, one incredibly thorough guy, fitted a 15in pitch Power Tech stainless steel prop to allow the Mercury TLDI 75 outboard to rev right out. In contrast, the Mercury 90 engine was fitted with a 17-inch Power Tech.

Two inches can make a world of difference! The engine had less loading across its entire rpm range and, because it revved right out to 5800 rpm instead of 5500 rpm, it returned incredible fuel efficiency for a 75 hp outboard motor. The WOT fuel consumption was way down on Tohatsu’s stated maximum of 26lt/h because the engine could rev so freely. As with any fuel-injected petrol engine, loading it up simply uses more fuel because the EFI is trying hard to compensate.

As with the Mercury 90, using premium unleaded petrol and OptiMax oil, the Mercury TLDI 75 outboard motor started instantly hot or cold, with no oil smoke appearing at any time, just a slight oil smell when backing upwind. Across the entire rpm range, the vibration levels were slightly lower, and my gut feeling was that the engine was matched perfectly to the hull, which, after all, is rated to 75 hp.

 

Our Verdict

Forget OEDA ratings for this engine. Sure, 3 Stars translates to fewer exhaust emissions, but the real benefit for Aussie boaters is the lower fuel consumption. The TLDI system also has a diagnostic system that doesn’t require a laptop to analyse faults, resulting in less servicing technician time and potentially lower servicing costs than carbureted two-stroke engines in this power range.

It’s true that the Mercury 75 hp two-stroke, like its E-TEC and OptiMax competition, doesn’t have the smoothness of Honda’s four-cylinder BF75, but it’s substantially lighter and power delivery is quicker — important in situations like bar crossings.

As for reliability, it’s no secret in the Aussie boating industry that the OCP (also used in OptiMax engines) is the most reliable of all DFI systems. The inherent simplicity of the Mercury TLDI 75 is so refreshing compared to the complexity of double overhead camshafts and interference four-stroke engines, where, should the timing belt break, it’s goodbye boating pleasure.

 

Mercury TLDI 75 Performance

Pushing 900kg plus two adults.

1.9kts (3.5kmh)

700rpm (trolling) — 0.6lt/h

5.4kts (10.0kmh)

2000rpm (fast troll) — 3.0lt/h

5.9kts (11.0kmh)

2500rpm (offshore troll) — 4.2lt/h

11.3kts (21.0kmh)

3050rpm (clean plane) — 6.4lt/h

18.9kts (35.0kmh)

4000rpm (cruise) — 9.2lt/h

25.3kts (47.0kmh)

5000rpm (max cruise) — 16.8lt/h

29.1kts (54.0kmh)

5800rpm (WOT) — 22.5lt/h

Competing Outboards

Engine

Evinrude E-TEC 75

Mercury OptiMax 75

Weight

145kg

170kg*

HP / rpm

75.1 / 5000

73.9 / 5375

Displacement

1295cc

1526cc

Warranty

5yrs

5yrs

OEDA stars

3

3

* Mercury OptiMax 75 is de-rated from 115hp

Originally published in TrailerBoat #285, August / September, 2012.

 

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This article was written by Andrew Norton and previously appeared at TradeaBoat.com.au, now powered by Only Boats.


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