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. 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."[1]

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.

Power Options:
Gas -vs- Diesel Part II
by David Pascoe

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.

torque.gif (106227 bytes)

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.