The question of whether gas or diesel is a better power choice, some other factors that come into play such as hull efficiency and windage in superstructures.
Diesel becomes the better choice in direct proportion to the amount of weight being propelled. 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 and 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. 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.
Diesel's great advantage is carrying more load with less strain on the engine due to higher torque generated. 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 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 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.
Internal displacement is the best measure of an engine's ability to deliver power efficiently. The rule for service life is that the more power is squeezed from an engine block, the shorter it's life span. 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 / Weight: Other factors come into play, engine speed and weight. There is no escape from the fact that fast turning diesels have shorter life spans. Slow speed diesels can be longer lived precisely because they do turn much slower.
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.
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 a year. will never see any advantage from lower fuel costs or consumption.
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, 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.
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 small thing that usually adds up to big dollars. Small boats with big diesels are usually the worst offenders.
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 60's, it didn't work then and won't work now, never mind all the smoke they'll put out about "technological advances
