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Marine Engines : Power Options

Time:2017-11-09 17:19Turbochargers information Click:

power options gas or diesel

Power Options: Gas Versus Diesel

by David Pascoe

One of the most frequently asked questions I get concerns the power choice of gas versus diesel. Lately I've received more e-mail on this subject than any other. My difficulty in answering that question has a lot to do with common misunderstandings about the nature of these engines. Most people make choices based on popular beliefs, without any real understanding of the nature of this rather complex subject. In this essay I will attempt to dispel some of the myths, and give a brief discussion of the basis by which one should consider the pros and cons of each choice. Myth #1: Diesel is safer than gas.

For some reason or other, the fear of gasoline explosions, which are very rare, but had caused some rather spectacular accidents thirty years ago, just won't die. The facts are that gas engines are very safe and you probably stand a better chance of dying or being injured in an airline crash that you do in a gasoline fire or explosion. Yes, gas engines do pose a carbon monoxide hazard, but most of this hazard comes from gas generators.

Diesels are safer from the standpoint of explosions as diesel oil vapors are not explosive. Prior to the advent of water cooled turbochargers, statistics show that fires caused by diesel engines ran nearly 5 times the rate over gas engines. With the introduction of water cooled turbos, the rate of fires has come way down. 

Of far more concern is the issue of carbon monoxide poisoning. Diesel exhaust produces far less CO than gas exhaust, though diesel exhaust produces suphur dioxide that can quickly cause nausea, but is not life threatening. Gas generators are responsible for most instances of CO poisoning, with leaking exhuast systems and station wagon effect a very distant second place. If you plan to do much overighting at anchor with a generator running, diesel is definitely the way to go.

Myth #2:
Diesel engines run for thousands of hours before requiring major maintenance.

Diesel engines gained the reputation for longevity based on their use in continuous operation such as trucking, generators and commercial vessels. Diesel in trucks and commercial vessels can run for thousands of hours because they often are run continuously without ever being shut down, or shut down only infrequently. Without going into a technical explanation, this is what accounts for long life in commercial applications. That does not mean, however, that they last longer in terms of the calendar: commercial engines run for vastly more hours, but have to be rebuilt just as frequently by the rising and setting of the sun. 

In pleasure craft use, diesels not only don't run continuously, but they are often rarely run. And in this case, it is the disuse that leads to their early demise. The reason for this is due to corrosion. An engine that is not running, especially for extended period of time like weeks, yet alone months, develops internal corrosion in all parts of the systems so that wear is greatly accelerated. An engine that is running all the time precludes most of this corrosion from occurring. Diesel engines in pleasure craft almost never wear out; they break down due to corrosion damage and other maintenance deficiencies. 

Myth #3: Diesels are more economical.

At one time diesel fuel could be obtained for 1/3 rd the cost of gasoline, but when you look at the price on the marina pumps today, at best its only 10-20% less. Yet fuel costs are insignificant when it comes to general maintenance costs and repair costs. I'll use two engines of comparable power to illustrate, a Crusader 350 HP gas and a Caterpillar 3208, 340 HP diesel engine. I have here on my desk a major overhaul bill for each. The Crusader engine was removed from the boat and rebuilt on the bench in a shop; the Caterpillar rebuilt in place in the boat. Costs: The Crusader bill was $3,211.48 and the Caterpillar $8,945.04, nearly triple the cost. Did the Caterpillar run substantially longer to justify the additional cost? No, it didn't. In fact, the Cat engine was only one year older than its gas cousin, and both engines had 800+ hours on the hour meter. Diesel parts are much more expensive and mechanics charge on average about 50% higher labor rates over gas mechanics. 

Myth #4:
An engine with low engine hours as registered on an hour meter is better than one with high hours.

Remember that hour meters turn on and off with the ignition key while the cosmic time clock never stops ticking. Why is this important? Because corrosion and internal degradation continues at a more accelerated rate when the engine is not running than when it is. A six year old boat with only a few hundred hours on the meter is telling you that it hasn't been used much. That means that it is much more likely to have wear and corrosion related internal damage than one that has had much more use. A recent example is a 5 year old 36 boat with 195 hours on the meter that required major cylinder head and turbo charger repairs, about $6,000 worth. 

Myth #5: A diesel engine can have an expected life expectancy of several thousand hours.

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