If you haven't shopped for an engine in a few years, you may be in for a revelation the next time you visit a showroom: the power plant just isn't what it used to be. The days of simple choices – inboard versus outboard or gas versus diesel – have been replaced by a bevy of options as both performance and price continue to grow.

VOLVO PENTAA quadruple installation of Volvo Penta's IPS pod drive system on a boat built by Lazzara Yachts.

Advances in engine technology, brought about by clean air regulations and the continuing trend toward larger and faster vessels, have resulted in a new generation of engines that deliver more power and fuel efficiency with less noise and pollution. Equally dramatic is that advances can be seen in all segments of the market, from lowly outboard engines to diesel-driven "pod-drive" systems with forward-facing props.

Even Grand Banks, long a standard bearer for traditional design, is debuting a new version of its 41 Heritage EU trawler fitted with the Zeus propulsion system made by Cummins Mercruiser Diesel. The boat will move using computer-controlled pods under the boat, rather than a shaft and propeller.

These modern engines do not come cheap – a four-stroke outboard can cost more than $20,000 – but they boast advanced, computer-controlled technology that is changing the face of marine propulsion. If you haven't been shopping in awhile, or if you are new to the sport, you may want to look at what is coming to market.

OUTBOARD ENGINES

The biggest sellers, and therefore the ones you're most likely to encounter if you are buying a new boat, are the four-stroke motors from Mercury, Yamaha and Honda. Outboards used to be two-stroke motors – like older lawn mowers, which required you to mix oil with gasoline – because they were simple, reliable, lightweight and powerful. But new emissions standards from the Environmental Protection Agency that went into effect in 2006 forced most engine-makers to turn to four-stroke technology, which more closely resemble the engine in your car and make it easier to comply with pollution laws.

MERCURYModern four-stroke outboards, now several years into production, have achieved major gains in performance areas too, in addition to traditional four-stroke characteristics like low vibration and noise levels, smooth running and good fuel mileage.

Theoretically, a four-stroke motor is a more complicated device, but engine makers have so much experience with them in other industries that they can safely be considered a mature technology. Any real problems you have with a four-stroke are more likely to result from something you did wrong – not changing oil, sucking sand into the cooling system in shallow water and the like – than they are from the complexity of the motor.

There is one exception to the four-stroke trend that is worthy of note: Evinrude's E-TEC motors. These are two-stroke engines that use high-pressure injection to finely tune the combustion process, meeting environmental standards while retaining some of the two-stroke's traditional advantages, which are low-end torque – good for acceleration – and top end speed. Evinrude offers the E-TEC in models from 75 HP to 225 HP. The company's other brand, Johnson, covers the lower end of the spectrum, from 9.9 HP to 25 HP.

The ownership differences between an Evinrude two-stroke and any four-stroke mainly involve maintenance. Because a two-stroke engine has its oil mixed in with its fuel, there's no oil and filter to change – though you do have to buy the special two-stroke oil and put that in the tank.

These bigger outboard motors, two stroke or four stroke, are expensive. You can easily spend more than $20,000 per engine for those in the upper 200 HP range. The current King of the Mountain in outboard power is the Yamaha F350, the first V-8 (5.4L) outboard, which produces 350 HP at 5,500 RPM. It weighs slightly more than 800 pounds and has retail price just north of $26,000.

CUMMINS MERCRUISER DIESELA Cummins Mercruiser Diesel Zeus pod drive installation.

So, what to choose? You will encounter die-hard fans of each brand in every marina, but the truth is that all of today's outboards are far more reliable than their ancestors. Most can be counted upon to start and run smoothly with no visible exhaust, and the fuel economy keeps getting better. That means selection comes down to the type of boat you own, the services in your area, how much you want to spend and your personal preferences. Do your homework.

INBOARD ENGINES

If your boat doesn't have an outboard, it has an inboard, meaning the engine is installed in a compartment under the deck. It used to be that these engines transmitted their power in one of two simple ways: via a shaft linked to a propeller or to a steerable propeller assembly known as a sterndrive.

But those options are multiplying. For example, an inboard engine today might be used to power a jet drive, which pumps water at high pressure through a steerable nozzle on the stern. Or, it might power a pod-drive, in which computer-controlled propeller mechanisms – the "pods" – are mounted underneath the boat rather than on the stern.

Whatever the propulsion system, inboard engines are likely to be gasoline-powered on smaller boats. Manufacturers including Volvo Penta, Mercruiser, Pleasurecraft Marine and one of its well-known divisions, Crusader, dominate the market. Most of these are marinized General Motors blocks and run all the way up to 8.2-liter V-8s, with 425 HP. Most manufacturers concentrate the bulk of their production nowadays on fuel-injected, computer-controlled and monitored engines, although at the lower end of the spectrum there are still some carbureted versions available.

CUMMINS MERCRUISER DIESELZeus drive mounting configuration.The advantages of a gas inboard are relatively lower cost – a top-of-the-line Crusader Captain's Choice 8.1-liter V-8 lists for about $13,000 – and the ease with which you can find a competent mechanic to help maintain it.

Somewhere in the mid-range of the gas inboards you start to see competition from diesel engines – engines that dominate the power ratings all the way up into the 3,000 HP range (and technically higher, given that virtually all large cargo ships are diesel-powered). In smaller boats, you will see diesels made by Volvo Penta, Yanmar and the Cummins-Mercruiser Diesel, a joint venture between the two companies. In larger boats which require more power, manufacturers such as Caterpillar, MAN, MTU and Detroit Diesel come into play. These are the big boys of the diesel world, with horsepower ratings that run to the thousands. This is also the last of the big iron, a segment of the market where old-style mechanical injectors and engine controls are still in evidence.

DIGITAL DIVIDE

But that is growing increasingly rare. If your idea of a diesel is that hideously noisy, blue-smoke-belching Mercedes down the street, you're in for a pleasant surprise. The latest diesel engines, introduced over the last five years, are quieter and release less exhaust because noisy mechanical injectors have been replaced by computer-controlled fuel injection. These engines meet the latest emissions standards and have added to the appeal – safety and fuel economy – that diesels have traditionally enjoyed.

Indeed, computer microprocessor-controlled marine power is becoming common throughout the market. While many traditionalists still balk, charging that the additional complexity makes these engines difficult for average boaters to repair, there's no denying the trend. Computers have controlled our car engines for years now, helping us to meet ever-more stringent emissions requirements, and the migration aboard our boats is inevitable.

Along with these new electronically-controlled engines have come new information systems that allow the helmsman to see enhanced diagnostics and monitoring. The newest engines use a standard information highway called a CAN bus to transmit information to and from the engines. The result is electronic displays at the helm that can read out everything there is to know about the engines, from the fuel burn rate to the temperature of the exhaust.

One side effect – hailed in some circles and reviled in others – is that these same systems can be used to control those engines and transmissions without mechanical cables and pulleys. This technology, first developed in the aviation world, is called fly-by-wire, meaning that electrical signals transmit the helmsman's wishes to the machinery. Thus, clean new helm designs with only a single control lever per engine – rather than the standard black shift and red throttle for each – are becoming more common. These electronic controls are available on everything from outboards to big diesels. Older hands make require some time to adjust to the simplicity, but most experts say it's a better system.

CUMMINS MERCRUISER DIESELZeus pod with aft-facing, contra-rotating propellers, integrated trim tab above and protective skeg below.

POD-DRIVES

Some of the products appearing on the market in recent years are downright futuristic. By now you've probably heard of Volvo Penta's IPS system, and Cummins Mercruiser Diesel's Zeus system. While there are some significant differences, they both have one radical element in common: two or more steerable drive pods under the boat, connected to gas or diesel engines right above them in the engine compartment.

A computer uses sophisticated software, tuned for each boat model, to interpret the movements of a joystick at the helm and determine in which direction the pods turn and how much thrust is applied. The pods are not physically linked together. The reason they can maneuver the boat so effectively is that they can be turned in different directions, such as one in forward and the other in reverse. While still relatively new, many people who have driven pod-equipped boats say that they completely eliminate the need for a bow thruster. They many also reduce the learning curve for new boaters when it comes to docking.

When sophisticated new precision autopilots are connected to this computerized system, the boats become capable of precise station-keeping, holding an exact heading and position while standing still, as you might while waiting for a bridge to open. They can compensate well for wind and current combinations by using high-precision GPS receivers.

Pod drives have other attributes. They eliminate the long shaft and strut assemblies hanging down underneath the boat, which create drag. The companies therefore claim that the propellers bite into "clean" water that is not disturbed by turbulence, and are therefore more efficient. Volvo Penta claims efficiency gains of up to 30 percent over a shafted inboard installation on the same model boat.

Both pod drives are installed so that the pods will break off cleanly in the case of a major underwater collision or very hard grounding, and are designed so that a break-off does not hole the boat. Volvo Penta says there have been several accidents with IPS boats – hard groundings – and that the drives sheared off as designed, leaving the boats floating and intact. There are an estimated 2,000 IPS-equipped boats already in the water, which itself is a testimonial to the design.

Another advantage of pod drive designs is that they save space inside the boat. The engines are installed in a compact package right above the drives, typically right near the stern of the boat. This gives interior designers more room to work with. Grand Banks says that the Zeus installation saves so much room they were able to add a second stateroom to the design.

THE FUTURE

Pod-drive technology is developing rapidly, with new models appearing in both higher and lower horsepower configurations. Volvo Penta, which was first on the scene with its diesel-powered IPS, has just rolled out new versions compatible with its larger gas engines and with some much higher powered diesels for bigger yachts.

Mercury Marine has taken the pod-drive concept and applied it to sterndrives – a natural evolution – by removing the tie-rod that connects the two sterndrive legs and using a Zeus-like computer to move the two independently. Early demonstrations of this configuration, called Axius, show that it can provide maneuverability similar to the original systems. Mercury plans to have it in production in 2008, and Volvo Penta is expected to bring something similar to market for its sterndrives.

Another innovation to look for is a configuration in which pods and engines are separated. Already in use aboard large cruise ships, this configuration uses large electric motors inside the waterproof pods to drive the propellers directly, powered by large diesels inside the hull that serve as generators, providing electricity to the pods via heavy cables. One major advantage to this arrangement is that you can now separate the engine and pod unit, giving the interior designer even more flexibility. Most marine architects must design around the weight of the diesel engine, but this configuration would allow the designer to put that heavy diesel virtually anywhere.

Systems like this are expected to break into the production market, though it may take several more years – another thing to look for as engine technology marches forward.