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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. Patently untrue. The
average life expectancy of a marine diesel engine in a pleasurecraft
is somewhere around 1500 hours between major overhauls. The average
boat reaches this in about 8-10 years, meaning that the average
annual operating time averages around 150 hours. If that seems unrealistically
low, consider that that translates into 2-1/2 weeks of eight hour
days. Most boats have years when its even less than that. If this
surprises you, it may surprise you even more when I tell you that
gas engines average around 900 hours before overhauls.
The Problem With Light Weight Engines There
is a direct relationship between service life and the weight of
engine blocks and cylinder heads. The heavier, or thicker the castings,
the longer they will last. That's one of the reasons why older engines
just seem to go and go, while we often refer to the engines of recent
years as "throwaways."
The problem with light castings is a problem of
both strength and heat distribution. Diesel engines, with their
350-550 lb. internal cylinder compression, develop tremendous heat
within the cylinders and heads. When castings heat up they expand,
and when casting thickness are unequal, this can lead to cracking.
It follows then, that the thinner the casting, the weaker it is,
and therefore more prone to heat distortion and cracking.
This has been one of the major problems of trying
to adapt light weight automotive engines to marine use. Because
the loads are much greater, more heat is generated, and therefore
more distortion of the castings occur. And when distortion occurs,
the close tolerances of the moving internal parts such as crankshaft,
bearings and journals, rods, pistons and cylinder walls goes out
of whack. The end result is an early demise of the engine. Therefore
the move to adapt high speed, light weight small truck engines to
marine use results in an engine with a decidedly shorter service
life. One of the most common problems that we see with light engines
is the frequent cracking of cylinder heads, which is the first place
that designers seek to reduce weight.
Over-fueling Another reason why high performance,
light weight diesels don't last long is related to over-fueling.
When you take an automotive engine that doesn't require as much
power to push its load, and increase its power output, you do so
by increasing the amount of fuel and air. This not only creates
much more heat, but it has yet another side-effect: the increased
fuel injected into the cylinder washes away the lubricating oil
on the cylinder walls. This is true of both gas and diesel engines.
The primary cause of all high performance engine failure is related
to the pistons. This is closely followed by failures in the valve
train, which is greatly stressed by increased heat and stress. To
overcome these problems, these automotive engine systems must be
completely redesigned. Unfortunately, they often are not.
Why not? You have to understand that the marine
engine market is a rather limited market that doesn't generate the
kind of revenues that the automotive market does. Over the years,
this has been a universal problem for marine engines of all kinds,
namely that the marine conversions simply don't go far enough to
account for the differing service loads. And with the push to produce
more efficient and clean-running diesels, the problem of marine
conversions promises only to get worse, not better.
Small Boats and Diesel Engines: The question
of whether gas or diesel is a better power choice dissolves for
boats of a certain size or weight. I draw this line somewhat arbitrarily
at around 16,000 lbs or 35 feet. I say "arbitrarily" because a lot
of other factors come into play such as hull efficiency and windage
in superstructures, but generally speaking you can use these numbers
as a general guide line.
Diesel becomes the better choice in direct proportion
to the amount of weight being propelled. In a word, the reason is
"torque." Horse power and torque are two different measures of power.
Torque is a measure of the kinetic energy that builds up in a rotating
engine. The higher the torque, the more power it takes to slow the
engine down or, in other words, it takes more power to make it work
harder or, the engine will carry a heavier load with less strain.
Diesel engines develop more torque for several reasons. One is because
of their greater mass: heavier parts develop more kinetic energy.
But they also have compression ratios three times that of a gas
engine, which also develops more torque. Gas engines develop most
of their horse power at the top end of their RPM curve; diesels
develop more power lower on the speed curve because of their greater
torque, which can be thought of as the reserve power behind the
rotating shaft.
Thus the diesel's great advantage is carrying more
load with less strain on the engine due to higher torque generated.
(This is only true for heavy marine diesels; small, lightweight
diesels, such as those made for small trucks, have a much lower
advantage simply because the torque is lower). When dealing with
lighter loads, that advantage disappears. There is also an issue
of kinetic energy, which is energy that builds up in rotating parts
such as flywheels, which helps sustain the load. Another advantage
is that the diesel will develop that power with significantly less
fuel. But that advantage is nullified by the much higher initial
cost of the machine itself. The only real advantage is in the amount
of fuel tank space savings since you can have smaller tanks with
a diesel. Otherwise, few boaters run enough fuel through diesel
engines for fuel savings to make up for the high initial cost.
By the time a boat reaches 16, 000 lbs. or
around 36 feet, it is approaching the limit where a gas engine can
power it efficiently. Not only is there the issue of weight, but
the water resistance on a larger hull. Gas engines begin
to build up too much internal heat and the strain begins to result
in lower service life in larger boats. We're talking here about
the big block, 454 CID V-8's that are the largest gas engines available.
These engines at 300-340 hp usually do very well so long as they're
not pushing a too heavy load.
Internal displacement is the best measure of an
engine's ability to deliver power efficiently. And the ratio of
CID to horse power (divide CID by engine HP is the simplest measure
of how much service life can be expected. The inviolable rule for
service life is that the more power is squeezed from an engine block,
the shorter it's life span. A 350 CID block generating 260 HP is
going to last a whole lot longer than the same block putting out
350 HP, whether its gas or diesel. That's why the old 6-71 Detroit
Diesel will run darn near forever at 265 HP from 465 CID, but self-destruct
in 6-800 hours at 450 HP. A ratio of 1:2 is about ideal for a marine
engine, but at 80% to 90% at least yield reasonable service life.
At 1:1 and above it should be considered a high performance engine
with a very short service life indeed.
Speed -vs- Weight Yet other factors come
into play, engine speed and weight. There is no escape from the
fact that fast turning diesels have substantially shorter life spans.
Slow speed diesels can be longer lived precisely because they do
turn much slower. But when you soup them up, that advantage is lost,
for a variety of reasons. Diesel engines running at 3200 to 3600
RPM are lightweight automotive engines for which good service life
should not be expected in marine applications. A vessel in water
and a vehicle on wheels are two entirely different load situations.
The light weight diesel was not designed to push heavy vessel loads
any more than the gas engine was.
Whether its gas or diesel, its a universal axiom
that the faster you want to go, the more it will cost you, not only
in terms of fuel costs, but in terms of engine life. We've already
discussed why high performance diesels have a very short service
life, but I've not yet mentioned that high power gas engines suffer
the same fate.
A pair of medium weight diesels can easily weigh
2,000 lbs. more than a pair of gas engines. In a 30' boat, an extra
ton is going to result in a considerable speed loss because of that
extra weight. In terms of speed, this gives a considerable edge
to the gas engine. While everyone knows that gas power is faster,
few people consider this point. The light weight diesel at least
gains the advantage over the heavier counterpart in terms of speed
potential, but looses out in the long run on longevity.
The One Big Diesel Advantage If one is willing
to travel at slower speeds, the one great advantage that diesel
holds over its gas counterpart is lower fuel consumption, lower
fuel cost and greater range. If fuel range is a consideration, then
diesel wins hands down. Of course this is entirely dependent on
how fast you want to travel; if you want to run at the same speeds
as gas power is capable of, then even that advantage fades.
Yet many people make the mistake of thinking that
because fuel costs are less, the overall operating cost is less.
This is simply not true when you figure how much lower cost diesel
fuel you have to burn to make up for the added cost of the engines
themselves. The "average" boater running his boat at 150 hrs/yr.
will never see any advantage from lower fuel costs or consumption.
Not when the option for diesels costs something like $20,000 or
more. At even a very optimistic savings rate of $0.50 gallon, we're
looking at 40,000 gallons just to make the break-even point
By now you should begin to understand why small,
light weight diesels are not necessarily a better choice for small
boats. That is, of course, unless you just "want" diesels, which
a lot of people do, but not necessarily for rational reasons. The
argument for gas engines is that they're cheap, efficient, and far
less costly to maintain. And they are certainly just as reliable
as diesels, all things considered.
If you still want diesels in that 28 or 32 footer,
just remember that you're paying a very substantial premium for
them without much in the way of benefits.
And since we're talking about small boats (well,
small by some people's standards), if maintenance costs are a concern
to you, think twice about buying a boat with large engines crammed
into small spaces. If its going to cause you pain to write a check
for $1500 or $2000 for what should seem to be normal maintenance
work, you had better consider whether a repairman has to dismantle
part of the boat in order to change a water pump or whatnot. A customer
of mine recently complained of just that. In order to change a water
pump, the mechanic had to dismantle part of the galley, remove permanently
installed carpeting, and finally cut a hole in the deck which had
no hatch. And when he was done, he had to put it all back just like
it was. My customer blamed the mechanic for overcharging, but really
it was his own fault for not paying attention to what he was buying.
Beware that there's always a trade off for the
boat that seemingly has everything, because the extra space was
obtained at the expense of engine room or compartment space. When
the engines are put in with a shoe horn, rest assured that every
aspect of maintenance is going to cost you more, and sometimes a
lot more. This is particularly true when considering a used boat.
If the front and outboard sides of the engines can't be seen, yet
alone reached, problems develop that aren't observed, and therefore
not maintained or repaired. There's not much chance of discovering
a serious problem and correcting it before serious damage is done.
When surveyed, boats with tight engine compartments almost invariably
are found to have more engine problems than boats with engines that
can be reached on all sides. Its a seemingly small thing that usually
adds up to big dollars. Small boats with big diesels are usually
the worst offenders.
Trends: New EPA rules are going to have
a major impact on diesel engines. The mandate to make them lighter,
more fuel efficient and cleaner is going to translate into engines
that are vastly less reliable. Why? Because they're going to start
cutting out all that necessary extra cast iron, and in many areas
start replacing it with cast aluminum. The marine engine industry
tried cast aluminum once before back in the late 1960's; it didn't
work then and won't work now, never mind all the smoke they'll put
out about "technological advances." The German transmission manufacturer
Z-F has tried making cast aluminum gear boxes, a foray into the
future that has blown up in their faces along with their gearboxes.
Aluminum is simply too weak to handle these kinds of loads, too
heat sensitive, and unresisting to corrosion to be an acceptable
substitute. Once these new rules go into effect, I predict a healthy
future for remanufacturing old engines.
Related Articles: Gas -vs- Diesel Part II
Posted in 1997. Revised
January 20, 2001
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