Outboard motors have seen considerable development in recent years. In fact, such has been the pace and scope of progress, that it could be argued that they bear little resemblance to their early predecessors.

The first outboard was developed by Ole Evinrude in 1909 in Milwaukee, United States. Almost a century later, approximately 10 million outboards are in use globally. The growth in sales and use goes hand-in-hand with the pace of development, resulting in engines becoming more compact, light, efficient, powerful and affordable.

With developments in computer chip technology and advancements in engineering, the modern outboard is a very sophisticated piece of electro-mechanical engineering. This article discusses some of these developments and how they have enhanced the experience of owning and operating a modern outboard engine.

Many of the more recent developments have been driven by the need to meet new environmental regulations that are coming into force around the world within the next year or two. The leading regulators are Europe and the United States, which are enforcing stringent new environmental regulations in 2007 and 2006, respectively.

One of the biggest developments in engine technology that has found its way into the outboard motor is electronic fuel injection, commonly referred to as EFI. EFI originated from the automotive industry and began to be incorporated into outboard motors in the mid-1980s. Early versions of EFI were sometimes troublesome, however later developments have seen dramatic improvements in engine reliability and fuel economy. In more recent years, EFI was further enhanced to incorporate multi-point fuel injection; this provides more fuel and air directly into the combustion chamber and results in more power through more complete combustion.

Digital engine management is now common, especially on the medium-to-larger capacity four-strokes. The incorporation of microcomputer-controlled sensors throughout the engine provides precise control of ignition timing, fuel injection and other adjustments, offering more performance gains. This level of control of engine functions also guards against engine damage, including knocking, over-revving, stalling, over-heating and insufficient lubrication.

Advances in intake and exhaust design have made the modern outboard significantly quieter in all modes of operation, in sharp contrast to its early cousins. Larger capacity air intake silencers have reduced high frequency inlet noise, while improved through-propeller exhaust systems have reduced the mid to low frequency noise levels.


New cowling materials and seals, along with improved fastening systems, have further reduced noise as well as resisting the ingress of unwanted water into the engine bay.

The inclusion of larger alternators within the modern outboard provides the opportunity for dual batteries, reducing the possibility of flat batteries and associated dramas out on the water. With more alternator output allowing dual batteries, one battery can be dedicated to engine starting and the other for all other on-board services such as navigation equipment, navigation lights, house lights, etc.

Outboard manufacturers are now including multi-function digital gauges as standard equipment to keep track of a multitude of engine functions and provide diagnostic information. These gauges provide sophisticated information such as running time, water contamination warning, engine check warnings, trim position, overheating warning and fuel consumption, along with the usual information such as rpm and voltage. At least one outboard manufacturer has also integrated its systems with a leading marine electronics company to provide a wide array of information about the status of the boat, fuel and the engine.

Problems associated with engine vibration have been addressed with some lateral thinking. The flywheel has been redesigned and repositioned under the engine, instead of the traditional location above the powerhead. This lowers the centre of gravity of the outboard and significantly reduces vibration.

One area that receives little thought from the average outboard owner is the layers of paint that protect their investment. The modern outboard utilises some of the most advanced paint and coatings technologies available today. Typically, the raw metal is etched with a special preparation then coated with two layers of epoxy-based undercoat, before receiving two further layers of two-pot finish. Finally, it is coated with a clear acrylic urethane to give a depth and shine to the final colour.

The end result, while pleasing to the eye, also helps protect engines from the ravages of salt water corrosion.


Until very recently, four-stroke outboards have always been considerably larger and heavier than their simpler two-stroke counterparts. This has been because of the relatively complex design of the cylinder heads and overhead valve actuation assemblies and drives used on four-stroke engines. But with clever use of advanced alloys and improved designs, there is now very little difference between the two. Casting and machining techniques have also contributed to the weight reduction, as has the in­troduction of high-tech, lightweight, non-metallic composites for some engine components. The difference is typically now in the order of around ten per cent between two- and four-strokes.

The vast amount of research and development in recent years has gone into four-stroke engines of all capacities. This has been driven by the increasingly strict exhaust emissions legislation in big markets, such as the USA and Europe. Two-strokes, by their very nature, tend to produce more hydro-carbons than four-strokes of equal capacity.

Modern four-strokes in typical medium-sized trailer boat applications tend to be three- four- or six-cylinder engines, either in vee or in-line layouts. Overhead camshafts are universal, while advanced counter-balancing provides smoother running and significantly reduces engine vibration, especially at idle or low rpm.

One of the most recent developments has been in the recycling of exhaust gasses to extract further energy from the partially burnt air/fuel mix. This has been achieved by some very detailed analysis and clever design changes to the combustion chamber. Significant developments in fuel injection technology have further improved efficiencies. These features not only make for cleaner running and more power; they also improve fuel efficiency. And these advances also reduce the impact on the environment.

The introduction of mechanical supercharging is a more recent development, resulting in much higher power and torque outputs, especially in the smaller capacity classes. Superchargers are basically air pumps that force-feed the engine with more air and fuel, thus resulting in a bigger ‘bang’ inside the combustion chamber. The end result is more power and throttle response for a given capacity of engine, with up to a 50 per cent boost over naturally-aspirated powerplants.

In just about every area, four-strokes have been refined to the point where the modern four-stroke outboard engine is a very sophisticated piece of engineering.



Most two-strokes now have separate tanks for oil and petrol, instead of the pre-mix arrangement of earlier times, in which the oil and fuel were mixed together prior to being fed into the engine. Most modern powerplants now employ oil injection, with precise amounts of oil being injected directly into the intake manifold to provide engine lubrication. One of the advantages of this arrangement is that the engine management system can vary the mixture in accordance with the requirements of the engine, which minimises oil consumption.

Some modern two-stroke engines use high pressure direct injection to drive the fuel into the cylinder. Extremely high pressure results in a finer atomisation of the fuel, making for more complete combustion. This is further enhanced by improved flow of the air/fuel mix through the cylinder, from intake to exhaust port.

The world’s first 100-horsepower outboard,
introduced in 1962 was cause for celebration.

Many of the latest outboards also have independently controlled air intakes to each piston for more precise control of the air/fuel mixture.

Two-strokes tend to be considerably cheaper than four-strokes of equivalent capacity because of much lower manufacturing costs and less moving parts. They are simpler engines to build, but cost benefits can be offset by higher fuel consumption, especially when run at higher revs. And despite improved fuel systems and more efficient combustion, it is becoming a real challenge for manufacturers to meet harsher emissions limits now coming into force.

It’s generally accepted that two-strokes are close to their development limits in today’s environmentally conscious marine industry. They still have their place as legitimate trailer boat powerplants, but the focus has well and truly shifted towards the more complex four-stroke engines.

Unless there are major breakthroughs in the near future, it seems certain that four-stroke engines will become the norm on most new boats. And it’s equally likely that we’ll see more power and torque being produced by smaller and lighter engines as engineers continue to find ways to give us a bigger bang for our outboard buck.

Next issue we’ll look at what owners need to do to keep their hardworking outboards purring reliably away to make the most of our time on the water.