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Judging
by the number of letters I get and the questions I see on Internet
forums, many people are still quite uncertain about the costs and
benefits of gas and diesel engines. "Which is best?" they
ask, as if one form of power were universally better than the other.
Like most things in life, each type of engine is better suited for
some applications than others. It all depends on your needs, the
type of boat you own, and how you use it.
Diesel engines are not something you should take
lightly. There's a good reason why the rush to put them into cars
back in the 1970's flopped. They are not the ideal power source
for all applications. The all encompassing, ultimate, operative
descriptive adverb that covers diesel engines is the word COST.
More about that in a moment.
There is something about diesel engines -- they
seem to have a certain mystique about them. I never really felt
that way, perhaps because I used to build race car engines, but
apparently a lot of people do. My view is that they are dirty, smelly,
noisey and cause a lot of vibration. They're also expensive and
there is very little about them that you can repair yourself. Given
the choice in most instances, I'd choose gas. Whether its
the deep, throaty growl of a Detroit Diesel, or just a status
thing, I really have no idea. But I do know that there's an awful
lot of mythology about diesels, much of which simply isn't true.
In order to clarify the issues somewhat, I'll break them down into
separate categories.
Basic Principle #1: In my view,
one of the worst things that can happen is to underpower a planing
power boat. That's because the engines in a boat that is underpowered
are going to be constantly operated at the peak of their power output,
meaning that the engines are going to under their maximum load most
of the time. That's a prescription for both short engine life and
poor fuel economy.
Basic Principle #2: The
second worst thing that can happen is to buy engines where the manufacturer
has squeezed the maximum available horse power from the engine.
The reason is that you end up with the same result as #1 above if
that maximum amout of power is needed to run the boat at normal
speeds. Running an engine at maximum power greatly reduces service
life (often by a factor of four).
Basic Principle #3: Diesel
engines are a lousy choice for people who don't use their boats
much, or who frequently make only short jaunts. High speed diesel
engines should not be run at low speeds for either long or short
periods, and then shut down. The problem here is that diesels build
up carbon in the cylinders at low speeds; they need to be run
long and hard to burn out that carbon after slow speed running.
If not, then carbon builds up in the ring grooves of the pistons
and will eventually cause the rings to stick. When that happens
severe, even catestrophic engine damage will result. High speed
marine diesels (2000 RPM and over) are not made for idling around.
Basic Principle #4: There
is a concerted effort going on by manufacturers to make diesels
lighter and less expensive. They are doing this by changing a lot
of formerly cast iron parts to aluminum. They tried that 25 years
ago, it didn't work then, and I don't believe that it will work
now. Oh, they say they're got all kinds of great new alloys now.
Really? Aluminum is still anodic to just about every other metal
on a boat, and I see absolutely no reason to believe that these
newer engines will fare any better. Those that tried it and experienced
disastrous failures include: Detroit Diesel, Perkins, U.S. Marine,
OMC, Volvo, Cummins, Chris Craft and a host of others I can't remember.
Caterpillar has wisely stayed away from aluminum, and I would too.
If you buy an aluminum diesel, color yourself guinea pig.
The first two principles apply to both gas and
diesel engines. The ideal situation is where the boat only requires
80% or less of the engine's maximum power to push the boat at the
anticipated cruising speed. To further understand this, let's take
an example. Say we have a 454 cubic inch gas engine which is typically
rated at 350 HP, but in this case the engine is in a larger boat
and has been bumped up by an engine marinizer to 400 HP. Now we
have a situation in which the engine is not only putting out more
power, but is pushing a heavier load. Just as you human body is
going to wear out faster if you spend your life doing hard labor,
so are those engines. Not only are they under more strain, but the
higher horsepower translates to generating a lot more heat and friction.
Each additional increase in horsepower over what the basic engine
block was designed to achieve, reduces engine life drastically.
Engine Speed Diesel engines
gained the reputation for long service life early on in the history
of the engines, and mainly from engines that were used in commercial
operations. These were big, very slow turning engines, usually in
the 600 to 1,000 RPM range. Obviously, not only is an engine
running at that speed turning four times slower than an engine at
2400 RPM, but the heat generated and the strain is a great deal
less.
Then we hear about how long truck engines last.
Well, there's a whole lot of difference between a load that is rolling
on wheels and a hull that is pushing tons and tons of water aside.
Even though truck diesels turn faster, they have a lot less load
on them, so they also last longer, mainly due to continuous operation.
Then there are companies like Stuart-Stephenson who take stock diesel
engines and micro tune them by essentially remanufacturing them
to extraordinary tolerances, for use in such applications as power
generators. These engines can have service lives as long as 60 -
80,000 hours. But that again is because they are operating continuously
under constant loads.
The long service life of the diesel is not a myth
when used in its proper application; its only a myth in pleasure
craft, where engines are operated intermittently, infrequently,
at high and low speeds, and usually under very heavy loads and adverse
conditions.
Horsepower Vs Torque Another
consideration, which many people find difficult to understand, is
the relationship of torque to horsepower. Horsepower alone is not
the sole criteria of how powerful an engine is. Torque is the amount
of kinetic energy the engine builds up as a result of the rotating
components -- crankshaft, flywheel and pistons. Torque is measured
in the number of foot/pounds of power that is required to stop a
running engine at differing speeds (In brochures this is often stated
as the "power curve"). Most engine manufacturers provide
torque/horsepower curves to illustrate the power range. Diesels,
being heavier engines, as well as having more power with each stroke
of the pistons, develops more torque lower on the torque curve.
Gas engines develop their maximum horsepower toward the top of the
curve, meaning the greatest power develops at maximum engine speeds,
where it is least useable.
Torque is not a constant, but varies over the power
curve, as shown in the graphs for a diesel engine below.
Notice how there is a steep rise in the curve, and then how it flattens
out near the top of the RPM range. Gas engines have curves that
are the opposite of diesels, rising gently along the bottom, and
then the curve shoots up sharply.
For heavier boats, the best engine is the one that
develops its maximum torque lowest on the curve, meaning that the
engine is developing more power at lower speeds. Since lower speeds
mean less strain, heat and friction, these factors will translate
into longer engine life, just as you will probably live longer with
a regime of moderate exercise over a life of hard labor.
There is a point in boat size and weight at which
gas engines can no longer push the boat efficiently. This is usually
around 35-38 feet and 16,000 lbs., depending on hull shape and the
amount of hull and wind resistance. Boats with flatter bottoms require
less power, as do low profile boats such as express cruisers as
compared to a flybridge fisherman with a tower. For anyone contemplating
a purchase in this size range, the ideal choice becomes rather clear.
Remember what I said about operating engines on the edge of their
power range.
Where Torque is Most Needed The
torque/horsepower curve is most critical for those boats that operate
at speeds only slightly above planing speed, e.g. the ones that
are operating at the top end of the torque curve. But wait! Notice
that the diesel engine torque curve begins to flatten near the top.
That means that for slower boats, at the very point you need the
most power, is the point where it begins to fall off.
This is precisely the reason that you do not want to buy a boat
that is underpowered: you will always be wanting to operate it at
the worst possible speed.
The Dilemma Most questions of
choice arise for boats in the 28 to 34 foot range where either type
of engine is available with adequate horsepower. Gas engines have
the advantage that they are cheap to buy and cheap to
repair. Diesels are just the opposite; for the price of one
Caterpillar 3208 of comparable power, you could buy three gas engines.
For the price of a smaller, in-line 6 cylinder diesel, you could
buy two gas engines. Thus, cost-wise, unless you really need diesel
power, they are not very practical. The advantage of the diesel
comes only at the point where the extra torque is needed because
a gas engine would simply be under too much strain to have an adequate
service life.
Fuel Consumption If you
are planning on some serious long range travel, particularly if
fuel stops are not available, then fuel consumption becomes
and issue. Diesels will typically burn 1/3 to 1/2 the amount of
fuel as their gas counterparts. But considering the cost of the
engines versus the amount of fuel you will burn during your period
of ownership, fuel savings in monetary terms is a non issue.
Cost of Ownership Initially,
the cost of maintaining the new diesel engines is low. The
problem arises in that diesels will not tolerate neglect and a lack
of maintenance like gas engines will. Whether you are buying new
or used, if you don't follow the maintenance schedule "by the
book," then the cost of owning the engines can become very
high indeed. Just so you know where I'm coming from, let me say
that I have been performing diesel engine failure analysis for insurance
companies for over 25 years now. Having performed countless hundreds
of these analysis's, I have a pretty good idea of both why these
engines break down, and the cost to repair them. And its not EVER
cheap.
First understand that diesel engines develop a
tremendous amount of heat and power at lower speeds. Failure to
maintain the engine's basic systems results in catastrophic engine
damages far more rapidly than gas engines. For example, gas engines
will usually survive common overheating situations, whereas because
the higher internal temperatures developed by diesels, overheating
can occur very rapidly and more often than not is fatal to the engine.
The older an engine gets, the more prone it is
to overheating. That's true of gas engines, but even more true of
diesels because the cooling systems are more elaborate, often involving
turbo chargers and intercoolers. Turbochargers utilize the hot exhaust
gasses to operate a turbine that forces more air into the engine.
This is air that is heated by the exhaust system, and before it
enters the engine has to be cooled first, which is the reason for
the intercooler. All of this puts more strain on the cooling system
and makes it all the more critical that the cooling system be maintained
in top condition.
Unfortunately, most boat owners -- I'd venture
to say at least 90% of them -- do not understand their engines very
well. Moreover, the cost of maintaining the engines, especially
when they get 4-5 years old, begins to escalate. Cooling system
overhauls (heat exchanger, oil coolers, fuel coolers and intercoolers)
are typically measured in thousands, not hundreds of dollars. That's
because you may have as much as a total of 8 coolers to maintain.
This provides the motivation for owners to skimp on maintenance.
While performing surveys, I see that most owners won't even
change their air intake filters (they are nearly always dirty),
so trust me when I say that most boat owners simply run their engines
until they quit. Most boat owners go to great lengths to try to
convince me what a wonderful job they do of maintaining their engines.
But all I have to do is take one look at the engines to see that
its all a Bill Clinton story. If they won't change a hundred dollars
worth of air filters, they're sure as heck not going to spend two
grand for a cooling system overhaul.
Therein lies the basis for my reticence about recommending
diesel engines for small boats. Diesel engines are costly to maintain
and my experience shows that most small boat owners are not willing
to pay the cost. There are over 100 Marine diesel repair shops in
Broward County Florida (Fort Lauderdale) and they're not all there
providing routine maintenance. Mostly they fix engines that go bang.
Just check out many of the forums on the web and you will see many
owners posing questions about engine problems. They do that because
they're trying to find a cheap way out by avoiding having to pay
a $70/hr. expert.
Most engines go bang as the result of overheating
due to a failure to maintain the cooling system. Cooling systems
do not get maintained because (1) owners don't understand how critical
it is, and (2) they can't afford to. And that's all there is to
it.
Bottom Line If you have
a choice of gas versus diesel, your first criteria should be to
determine whether you can really afford to own diesels. The initial
price is only part of the cost. If you can't afford to write big
checks for routine maintenance, then you're going to be a whole
lot better off with gas. Yes, I'm engaging in some tough talk here
but, you know, after hearing these endless excuses for 30 years,
one comes to realize that people like to fool themselves when it
comes to buying something they can't afford.
Let me tell you about how many V-8 diesel engines
that I see with ONE new cylinder head. I see that a lot. Now, why
would an engine only need one head? Since they come in pairs, doesn't
it make sense that if one is bad, the other won't be long to follow?
Of course it does, but if one head is found to have a problem --
and since the other one doesn't at the moment -- and since its VERY
expensive to fix the problem, the owner crosses his fingers and
HOPES the other one won't go bad anytime soon. Why would an owner
do this? Well, I'll let you answer that question.
Another thing I see are are single replacements
of exhaust manifolds and risers. Manifolds and risers don't last
forever, and they, too, are very expensive, and the engines will
have either two or four of them. Yet its common to see engines with
only ONE replaced. Of course the owner doesn't understand that riser
or manifold failure can totally wreck the engine by causing water
leaks into the cylinders. Though I'd have to bet that the
mechanic who replaced it told him that he could expect the others
to fail soon, and that he should replace ALL of them now. Of course,
that meant facing up to Big Bill, but the owner took a look at his
checking account and decided he couldn't afford it. These are usually
some of the guys who say they owned their boats for 10 years and
never spent a penny or had a single problem. If you believe that,
I've got some great Asian stocks I'd like to sell you.
Performance Diesels Harken
back to what I said about squeezing the maximum amount of power
out of an engine block, along with what I said about cooling systems.
I'm still stunned that I hear guys talking about diesel engines
running 3, 4, even 6 thousand hours. I can count the number of engines
I've seen with 3000 hours on them that have never been rebuilt on
the fingers of one hand. And virtually every one of them were in
slow speed, long range cruisers with superb maintenance. I raised
a lot of flack from my previous article in which I stated that the
AVERAGE diesel runs about 1000 -1100 hours before overhaul. I stick
by that number, but you have to understand that the AVERAGE includes
high performance diesels, as well as those folks who do not maintain
their engines at all; the ones who run them until they stop.
High performance diesels cause a tremendous drop
the average since the go fast diesels often go bang at 600 hours
or less. If anyone would like to challenge that, take a tour
of the Fort Lauderdale diesel shops and check out the hour meters
on some of these boats. You'll find many of them are less than two
years old.
Diesel engines are capable of having a long life
when the power to displacement ratio is low. But when they start
jacking up the power, beyond what the manufacturer originally intended,
that benefit disappears. There is a very simple formula you can
apply to estimate service life: simply multiply the cubic inch displacement
of the engine times one. The result is the maximum amount of horse
power you can have and still expect a reasonable service life. A
6V92 engine is 552 CID; at powers greater than 550, these engines
don't last. At 450, they'll go 10 - 15 years easily.
We recently surveyed a yacht with a pair of 8V71N
(naturally aspirated) Detroit Diesels rated at 325 hp that hadn't
been overhauled since new 1981. Now, an 8V71 has a 568 cubic inch
displacement; the fact that these engines have a 0.56:1 power/displacement
ratio explains why they could run so long.
Conversely, divide the CID by the horse power,
and the greater the result UNDER the factor of one (1), the longer
engine life you can expect. If you have an engine with an 0.70 CID/HP
ratio, then you can expect 3000 hours engine life. Otherwise, you
are a victim of the myth.
Finally, if you are a novice boater, and even worse,
you don't know anything about engines, then you might as well throw
darts at brochures as far as making engine choices are concerned.
There's lots of folks out there plunking two hundred or five hundred
thou for a nice diesel boat because they figured anyone marketing
a product that big or expensive must know what they're doing. This
entire web site is testimony to the fact that many don't. Sure,
you probably get a good engine warranty, but it won't warrant you
for the builder for putting the wrong engine in a boat.
Take some good advice and get some professional
advice before you buy. It can save you from a world of hurt.
Related Article: Power
Options : Gas Versas Diesel
Posted July 14, 1998
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