Petrol vs Diesel

Fifteen years ago, while working part time for Harvey Halvorsen, I had the opportunity to skipper two of his boats. It involved taking his friends out on Sydney Harbour for lunch and dinner cruises.

One of the boats was Kirribilli, the last Halvorsen 40 built and the tender to America's Cup contender Gretel in '67. Powered by twin 454ci Chrysler Golden Lion 300hp V-eights, she was capable of 25kt, but could just as happily burble around the Harbour at seven or eight knots.

The other was Gerralang, one of the last timber Island Gypsy 36s. A pure displacement trawler yacht, she was powered by twin 120hp straight-six Ford Lehman diesels and was capable of about 11kt.

While Gerralang was more suitable for entertaining guests, with her 3m tender slung on davits over the transom, Kirribilli was the one that guests favoured.

One reason was that the freshwater-cooled V-eights were silky smooth and created virtually no vibration. Another was that in following seas, there was no diesel exhaust smell that could turn a less than sober guest positively green!

Things have changed a lot since those days. Diesel powerplants are cleaner and greener (in the modern sense of the word) but petrol powerplants have also advanced to the point that many casual boaties could be better off with a spark-ignited powerplant under their hood.

SAFE AND SOUND
There are a number of commonly-held misconceptions that the latest petrol engines have gone a long way to address.

Above all, a lot of boat owners worry about the 'dangers' of inboard petrol engines. The volatile nature of petrol vapours are the clear concern of many 'experts'.

It's fair to say that this is a hangover from the past to a large extent. Back in the good ol' days, the engines didn't have the sealed carbies, fuel-injection systems and electronic ignition set-ups of modern petrol engines.

It should be noted, however, even then with safe practices and due diligence I never had a problem. Before switching on the bilge blower for several minutes prior to starting the engines for the first time each day, I would lift the engine compartment hatches and check for any leaks around the fuel pumps and carbies. The hatches would be left open for about an hour while I readied the boat for guests.

Once I was sure there were no leaks or petrol smells and the oil and water levels were okay, I'd close the hatches, run the blower and start the engines.

This was done by disconnecting the shift component of the Teleflex controls, fully advancing and closing the throttles once to prime the carbies, then pushing the starter buttons. The engines always started first kick and after five to 10 minutes of warming at fast idle could be used normally.

With modern multipoint injection petrol engines, such as MerCruiser's 350 MAG MPI (5.7lt) and 454 MPI V-eight (7.4lt), you just operate the blower for a few minutes, then turn the key as you would an injected car engine. The management system automatically richens the air/fuel mix until the engine has warmed.

Another concern of boat owners is the cost of running petrol engines. This is something private boat owners should note as it is largely only a factor if the motors are operated at, or near, Wide Open Throttle (WOT) all day.

Indeed, in many applications when used at low throttle, modern injected petrol engines consume little more than some diesels. And with carburetted motors such as MerCruiser's standard 260hp (at the propshaft) 5.7L, more economical running is just a matter of keeping the revs below where the accelerator pump takes over and engages the second carburettor barrel.

Even at WOT, a 5.7L Mercruiser consumes around 80lt/hr compared to 60 for the excellent 230hp (at the crankshaft) 3.5lt turbo-intercooled Yanmar diesel a powerplant which provides comparable torque. The consumption figures drop to about 25 and 20lt/hr respectively while just on the plane.

Another consideration with MPI V-eights is that the management system actually operates the powerplant more efficiently at any given revs than a diesel engine. For unless your diesel has electronic engine management, the injection timing is essentially fixed and provides the right amount of fuel over a relatively small rev range.

OUT FOR SERVICE
Some years ago Mariner Cruisers conducted research among owners of its vessels and found that most clocked up about 150 operating hours in the first year, followed by 50 hours each year thereafter. And though all the cruisers had planing hulls, many owners were content to potter around at displacement speeds, knowing they had planing capability to get them home at day's end.

Running a petrol engine at low speeds for extended periods, particularly a freshwater-cooled one, creates no servicing problems. But do this to some diesels (especially older turbo-charged units) and you run the risk of glazing the cylinder bores, leading to excessive oil consumption and blowby gases in the oil sump down the track. Indeed, most engine manufacturers say the biggest problem (apart from incorrect propping) with turbo diesels is that people don't work them hard enough!

And if servicing your petrol engine is a concern, bear in mind that with OEself-tuning' engine management systems the only real maintenance is changing the oil and oil filter every six months (regardless of how few hours are clocked up), and cleaning or replacing the fuel filter and spark plugs every other season.

Indeed, these service intervals will increase as more engines like MerCruiser's 454 MAG MPI Horizon become available.

A WEIGHTY ISSUE
The aspect of weight with many marine diesels is no longer a problem. Yanmar has led the diesel field here with its powerplants offering excellent hp/kg ratios. Indeed, the above quoted example of the carburetted 5.7L MerCruiser V-eight versus Yanmar 230hp turbo diesel sees the petrol engine around the 400kg mark complete with gearbox compared to 375kg for the 3.5lt Yanmar.

However, in many other cases the petrol powerplants are significantly lighter. Compare, for example, MerCruiser's own D-Tronic units. Fitted with variable injection timing, MerCruiser's 4.2lt D-Tronic diesel which pumps out 240hp and has comparable torque to the 300hp 350 MAG MPI V-eight suffers a weight penalty. The D-Tronic weighs 551kg compared to 390 for the V-eight. Its turbo-intercooler system requires oil and filter changing twice as frequently as the V-eight.

Even the lazy, understressed 310hp 7.4lt MPI V-eight weighs only 503kg. In its uprated 454 MAG MPI Horizon version it pumps out 380hp for another 30kg in weight!

IN SUMMARY
It is in midrange craft that the modern petrol powerplants are making a 'comeback'.

Our dayboat feature this issue is a prime example all three craft are petrol powered and proud of it.

It's interesting to note that according to Pathfinder Marine's Richard Pym, about 50% of people buying his Carver cruisers up to about 9.0m in length stick with petrol engines, while owners of larger boats stick with diesels.

Neither of us can work out why this is so, especially when the much higher initial purchase price of diesel powerplants is taken into account. Perhaps the fault lies with insurance companies who continue to offer lower premiums for diesel-powered boats!

What's obvious is the end decision must be based on how frequently your boat is used, how hard you work the motor(s) and how much you're prepared to pay for servicing.

If you have a canyon runner or operate a charter fishing business, the lower fuel and insurance costs make diesels very attractive.

If, however, you use your boat infrequently and don't plan to do any cruising offshore, then I believe petrol powerplants deserve a good hard look.