MerCruiser 250 Sterndrive Review

Mercury Marine really has scored this time. Its silky smooth 4.5lt V6 MerCruiser 250 sterndrive engine mates perfectly with Axius joystick piloting and Skyhook dynamic positioning to make any twin-engined sportsboat handle on a dime.

Mercury Marine’s recently released 4.5lt V6 is a masterpiece of engineering and a specifically designed marine engine. Sure, pundits could say it’s just a six-cylinder version of the long-running 3lt MerCruiser sterndrive engine, but it’s way more. It also has Mercury’s magic cylinder bore and piston stroke dimensions where the stroke is 90 per cent of the bore, just like the 2.1lt powerhead in Mercury’s F75 to F115 outboards and, of course, the 3lt engine in the F135 and F150.

A nice touch is that when repowering a boat the footprint is the same as the superseded 4.3lt V6.

Sensibly, Mercury has stuck with the traditional V-block petrol sterndrive combination of chain-driven central camshaft and relatively short pushrods operating two valves per cylinder.

Of course being a 90-degree V6 there’s a single chain-driven harmonic balance shaft that rotates at twice engine rpm. Across the entire rpm range this virtually eliminates engine vibration. The exhaust risers have been designed to accommodate catalytic converters – thank God we don’t need this power-robbing nonsense in Australia. Yet!

Like the MerCruiser 4.3, the 4.5lt engine has EFI but the throttle body intake faces aft to reduce air induction noise. The engine can run on 91 RON standard unleaded but will perform better on 95 premium unleaded. The belt-driven voltage regulated alternator pumps out 70amps, up five from the 4.3.

Complete with Alpha drive leg, the dry weight for the raw watercooled MerCruiser 4.5 is only 33kg heavier than the similarly-equipped 4.3, so as the bulk of the engine weight is ahead of the transom it will hardly affect repowering a hull such as a Caribbean 26. Freshwater cooling is available and would be my choice for permanently-moored boats.

Maintenance

Maintaining the MerCruiser 4.5 is straightforward with regularly serviced items at the forward end. Mercury recommends servicing the 4.5 every 100 running hours or annually after the first 20 hours and using its own synthetic FCW (Four Cycle Watercooled) SAE 25W40 oil.

On the water

Enough of the boring tech bits. Now comes the joy of testing a boat that will spin on its own axis or hold position without having to lower the hook.

The Axius joystick works by splaying out the sterndrive legs when needed to spin or back a boat exactly where wanted, not normally possible with twin Bravo 3 legs that each have counter-rotating props. By simply twisting a dashboard-mounted knob the boat will move forward, aft, or spin, without needing a bowthruster. Two modes are available but I prefer the “docking mode” which still gives full control when needed (such as in strong cross currents) but has a gentler shifting motion, as the standard mode can get quite aggressive with
the shifts.

Skyhook is simply amazing. Choose a location, press a dashboard button to engage and, through GPS positioning, the boat will stay where wanted – as we found when testing our Sea Ray 310 Sundancer off waterfront houses on Miami’s Venetian Island chain – perfect for unnerving the residents!

Back to the engines. Both started instantly, hot or cold, with no oil smoke appearing at any time despite the hard life they’d already had as demo engines. When idling there was absolutely no vibration through the hull and, oddly, no characteristic V6 exhaust beat coming out of the hole where relative to engine rpm the engines are working their hardest. Now, with the engines so quiet, the main noise when planing is from the two sets of right-angle gears in the sterndrive legs. When idling along at 600rpm the engine noise is only 62dB at 3m – about the same as a Mercury 90 four-stroke outboard.

Developing 14 per cent more power than the 4.3 does comes at a price and the wide-open-throttle fuel consumption is around 25 per cent higher, but the real forte is mid-range fuel efficiency. It’s not bad considering the demo engines were pushing a total boat weight 26 per cent greater than the 4.3s I tested in a Sea Ray 315 Sundancer which, in terms of hull volume, was actually a smaller boat. And the top speed was six per cent faster!

Through tight turns at 4000rpm the EFI compensated for the additional engine loading and maintained constant rpm without touching the throttles.

The Trade-A-Boat verdict

I always loved the mechanical simplicity of the 4.3s and now Mercury has made repowering boats with slightly more grunt very easy. The engines suit a wide range of hulls in single and twin installations but twins are just perfect when combined with Axius and Skyhook.

Not only did my body flab remain intact but, after testing, my tinnitus was
no worse! For more on the MerCruiser 250 visit your local MerCruiser dealer.

MerCruiser 250 performance

Sea Ray 310 Sundancer with twin MErCruiser 250 sterndrives with Bravo 3 legs spinning 22.5in pitch props. Total displacement 5.8 tonnes; average of two-way runs over chop to 30cm.

* Note the maximum fuel efficiency zone from 3000 to 4000rpm. Sea-trial data supplied by the author.

MerCruiser 250 sterndrive specs

TYPE 90° V6, naturally-aspirated, EFI, marine engine

RATED HP 249.3 at 5000rpm

DISPLACEMENT 4500cc

BORE X STROKE 101.6 x 91.4mm

WEIGHT 426kg (dry w/Alpha leg)