MerCruiser TDI 3.0L review

The MerCruiser TDI 3.0L means you get diesel power in a bowrider. Now that’s smart thinking!

Traditionally, larger US-built bowriders have been powered by V6 or V8 petrol sterndrives. The idea of a thumping big displacement engine lurking under the aft sunpad just seems to make sense. But at the 2015 Miami International Boat Show, Sea Ray came up with a different concept and fitted a MerCruiser TDI 3.0L diesel in its 250 SLX, one helluva big bowrider.

Mated to a Bravo 3 sterndrive leg it returned trolling to midrange fuel efficiency you’d never get with a petrol engine. And in its upper rpm range it was actually quieter, so on long runs a normal conversation could be held. Alas no sweet V8 exhaust note!

MerCruiser TDI 3.0L

The turbocharged and intercooled base MerCruiser TDI 3lt engine is built by Volkswagen and is fitted to a range of VW and Audi vehicles. Though it develops a maximum torque output of 566Nm (about the same as a 6.2lt V8 petrol engine) at 2250rpm, 77 per cent or 436Nm are available at just 1400rpm and even at wide open throttle and 4200rpm 80 per cent or 453Nm. An electronically managed common rail system injects fuel at up to 26,000psi or 1770bar, right at the top end of injection pressures.

VW has used a unique vermicular graphite casting process for the cylinder block that it claims is lighter and stronger than conventional cast iron. Even with its standard freshwater cooling system and a Bravo 3 leg it weighs less than the recently released MerCruiser 4.5lt 250 V6 petrol sterndrive which has an estimated peak torque output of 390 to 400Nm at around 3200rpm.

Being an automotive base engine the MerCruiser TDI 3.0L has a plastic shroud atop the engine that needs to be removed to access the engine oil filter. The sump oil dipstick is forward, with the power steering pump reservoir and coolant overflow tank to port and water separating fuel filter to starboard. The injection system is normally self-bleeding but a hand pump is fitted just in case.

Unfortunately, despite its relatively small displacement the MerCruiser TDI 3.0L is substantially longer than even the MerCruiser 4.5, though the width is only marginally greater. The engine-only dimensions are 925mm long, 754mm wide and 762mm high.

Servicing intervals are every 100 hours or annually after the first 20 hours, but if using the engine infrequently I’d change the oil and filter every six months as turbocharged diesels don’t like dirty oil. The TDI 3.0L is rated to a maximum of 500 hours of running per year, with 12.5 per cent WOT operation or one hour in every eight.

MerCruiser TDI 3.0L on the water

As with the twins I tested three years ago in a Caribbean 26, the demo single started instantly with no exhaust smoke, though its idling and trolling noise levels were substantially higher than the twin 4.5s I tested the same day in a Sea Ray 310 Sundancer. The single Bravo 3 drive leg allowed for excellent hull control amongst the crowded Miamarina jetties off Bayshore Drive in Miami and the shift action was surprisingly gentle for a diesel.

Idling out to clear water the TDI 3.0L had a steady exhaust beat and as we opened the throttle the turbocharger quickly kicked in. Accelerating onto the plane with the engine under high load relative to rpm gave us the characteristic V6 exhaust note but once up this disappeared, replaced by drive leg gear whine.

Driving the 250 SLX/TDI 3.0L combo was way different to a petrol-powered bowrider. Because the engine only reaches 4200rpm, we planed at 2000rpm compared to 2500 to 3000rpm for a comparable output petrol engine and cruised at 3000rpm. And being a diesel, the maximum continuous cruising rpm is only 500 below WOT, whereas an automotive-derivative petrol V8 should be run continuously at least 1500rpm below WOT.

Through tight turns at 3000rpm the engine held its rpm without touching the throttle. However at WOT it was slightly overpropped, resulting in a higher fuel consumption than the twin 3.0Ls I tested which were slightly underpropped. To compensate for additional engine loading, electronically-managed injection systems will always pump more fuel in, so it’s better to underprop these diesels. At WOT the TDI 3.0L was only slightly noisier than the twin 4.5Ls at 4000rpm!

The Trade-a-Boat verdict

If you love pottering around waterways, the quietness of a petrol engine at low speeds is more appealing than a diesel that just wants to be worked. But if you do long runs to and from a favourite spot, the fuel efficiency of a diesel is hard to beat. At WOT the TDI 3.0L uses way less fuel than even the MerCruiser 4.5L but then servicing costs down the track will be higher than a comparable output V8, so you have to think hard about going diesel.

At least the TDI 3.0L won’t impact on tinnitus. And just like the 4.5L my flab didn’t get a workout either! See your MerCruiser dealer for more information.

MerCruiser TDI 3.0L performance

Single TDI 3.0L in Sea Ray 250 SLX bowrider, Bravo 3 drive with 23in pitch props, total displacement 3.5 tonnes. Average of two-way runs on Biscayne Bay, Miami over a low chop.

RPM	SPEED (KTS)	FUEL BURN (Lt/H)	NOISE (dB/3m)
800	4.9	1.5	67
1000	5.8	2.3	68
2000	14.9	16.7	79
2500	22.2	22	80
3000	28.9	32.6	81
3500	33.6	42	83
3950	37.8	51.5	85