Inboard-Outboard Engines: How Are They Made?

Inboard/outboard boat engines are a type of marine propulsion system for boats. They are a combination of an inboard engine and a lower outboard drive unit attached near the base of the transom. Inboard/outboard engines are heavier than outboards and are generally favoured for use on larger motorboats. The engine is mounted inside the boat, allowing for a full back deck for lounging, watersports or a swim platform. This also makes the engine quieter, resulting in a more enjoyable experience. However, one of the downsides of inboard/outboard engines is that the engine compartment takes up valuable space in the interior of the boat.

Inboard vs Outboard Engines: Pros and Cons

When it comes to choosing between an inboard and an outboard engine for your boat, there are several factors to consider. Both options have their advantages and disadvantages, and the best choice for you will depend on your specific needs and preferences.

Inboard Engines

Inboard engines are mounted inside the hull of the boat, usually connected to a propulsion screw by a driveshaft. They are more commonly found on larger vessels, such as yachts, where outboard engines would not provide sufficient power. Inboard engines tend to be quieter and allow for more space on the back of the boat, making them a popular choice for water sports enthusiasts. They also have broader power ranges and may be more energy-efficient, as they are modelled after car engines. However, inboard engines can be more challenging to service and maintain, as they are located in cramped quarters inside the boat. Repowering an inboard boat can also be a more complex and costly project.

Outboard Engines

Outboard engines, on the other hand, are mounted outside the hull, usually on the stern. They are typically found on smaller boats, such as fishing, recreational, and light commercial vessels, due to their simplicity and ease of use. Outboard engines are generally lighter, faster, and more efficient than inboard engines, and they are easier to service since they are easily accessible from outside the boat. Outboards are also self-draining and can be tilted out of the water, making them easier to store in winter and less prone to corrosion. Additionally, outboard engines often provide better fuel economy than inboard engines. However, one downside of outboard engines is that they take up space on the back deck, reducing the area available for seating or lounging.

Cost Comparison

When it comes to cost, outboard engines typically have a lower upfront cost and are less expensive overall compared to inboard engines. However, inboard engines may have a longer lifespan, which could result in lower long-term costs. It's important to consider not only the initial purchase price but also the ongoing maintenance and fuel costs when deciding between the two options.

User Experience

The type of engine you choose will also impact your user experience. Outboard engines are generally easier to operate and navigate, making them a good choice for beginners. They are also better suited for shallow water activities, as the motor can be tilted to minimize the draft. Inboard engines, on the other hand, offer a sleeker aesthetic and more space at the rear of the boat, which can be advantageous for water activities.

Ultimately, the decision between an inboard and outboard engine depends on your specific needs and preferences. Both options have their pros and cons, and it's important to carefully consider factors such as cost, performance, ease of use, and maintenance before making a decision.

How Inboard Engines Work

Inboard engines are a type of marine propulsion system for boats. Unlike outboard engines, which are mounted outside the hull of the boat, inboard engines are enclosed within the hull. They are usually connected to a propulsion screw by a driveshaft.

Inboard engines are contained within the structure of the boat and provide thrust and steering from the bottom of the hull. On most recreational boats, the style of inboard motor is an inboard/outboard motor, or I/O, which combines the engine mounted inside the boat with a steerable drive unit mounted on the stern – a sterndrive.

The engine is mounted inside the watercraft toward the centre to give good weight distribution. They are most popular on motorboats over 26 feet in length. The engine connects directly to a transmission, out of which comes a shaft that passes through the hull of the boat. The shaft is then attached to a propeller that turns to propel the boat. Because the shaft is fixed and does not swivel, a rudder is mounted behind it to provide steering direction.

Inboard engines may be of several types, suitable for the size of the craft they are fitted to. Boats can use anything from one cylinder to V12 engines, depending on whether they are used for racing or trolling. For pleasure craft, such as sailboats and speedboats, diesel, gasoline and electric engines are used. Many inboard motors are derivatives of automobile engines, known as marine automobile engines.

Some inboard motors are freshwater-cooled, while others have a raw water cooling system where water from the lake, river or sea is pumped by the engine to cool it. However, as seawater is corrosive, some seagoing craft use engines that are indirectly cooled via a heat exchanger in a keel cooler.

How Outboard Engines Work

Outboard motors are a common propulsion system for boats, typically mounted on the outside of the stern or transom. They are self-contained units, including the engine, transmission, and propeller. They are popular, especially for smaller boats, due to their ease of use, access, and maintenance.

The anatomy of an outboard motor can be divided into three main sections: the top (outboard powerhead), midsection, and outboard lower unit. The powerhead contains the internal combustion engine, which converts fuel into mechanical energy. This energy is then transferred to the midsection, which houses the driveshaft and provides a channel for the cooling system to the powerhead. The midsection also includes mechanisms for steering the boat and a tilt mechanism to lift the engine out of the water.

The outboard lower unit, submerged underwater, contains the shift mechanisms, driveshaft, prop-shaft, gearset, and other components crucial for propelling and steering the boat. It translates the engine's power to move the boat forward or in reverse. The shift mechanisms control the rotation of the propeller, and the gears transmit rotational force by applying force against the teeth of another gear.

Outboard motors come in a variety of power sources, including electric, gasoline two-stroke, gasoline four-stroke, and diesel engines. They offer benefits such as high horsepower-to-weight ratio, ease of installation and maintenance, and extended maintenance intervals. Modern advancements have made two-stroke and four-stroke engines comparable in reliability, weight, speed, and fuel economy.

History of Inboard Engines

The history of inboard engines began with the first marine craft to utilize inboard motors being steam engines, dating back to 1805 with the Clermont and the Charlotte Dundas. Harbour tugs and small steam launches were also among the first to adopt inboard steam engines. It wasn't until the 1880s that the naphtha engine emerged, but these early boat engines had low power and high fuel consumption.

In 1887, Gottlieb Daimler and Maybach built a four-cycle boat engine and tested it on the Neckar River. This was a significant development, as it paved the way for the first commercially available engines, which were built by Sintz in America from 1893.

The late 19th and early 20th centuries witnessed a surge in technological advancements, and motorised pleasure boats became a luxury for the wealthy. However, the average fisherman or weekend boater typically relied on rowboats. This disparity led to experiments with portable electric and gas engines that could be adapted for rowboats.

In 1896, the American Motor Company of Long Island, New York, built 25 portable boat motors, believed to be the first gasoline-powered outboards. These early outboards featured a horizontal cylinder, vertical crankshaft and driveshaft, a tiller arm for steering, and a 90-degree gearbox below the water ahead of the propellers.

In 1903, Cameron B. Waterman created an outboard motor with an air-cooled motorcycle engine connected to a propeller. Production began in 1906, and by 1907, the engine was redesigned to be water-cooled, significantly increasing sales. The success of Waterman's outboard motor showcases the evolution of inboard engine technology during this period.

In the early 20th century, inboard engines continued to gain popularity, and companies like Evinrude and Johnson Motor Co. made significant advancements, such as lightweight designs and die-cast aluminium castings. During World War II, companies like OMC focused on producing engines for military purposes.

Post-war industrial boom led to a surge in outboard motor production, and companies like Evinrude, Johnson, Mercury, Mariner, Yamaha, Suzuki, Tohatsu, and Honda played pivotal roles in the evolution of outboard technology. This era witnessed the introduction of features such as recoil starters, removable motor cowls, forward-neutral-reverse gear shifts, and remote fuel tanks.

The Future of Inboard-Outboard Engines

Electric Propulsion:

The marine industry is witnessing a cautious approach to the adoption of electric power plants, with companies like Mercury Marine leading the way. Electric propulsion is expected to become mainstream, but this transition hinges on advancements in battery technology. Higher energy density, improved power output, and reduced costs of batteries will be crucial in accelerating the growth of electric power plants in boats.

Hybrid Systems:

While fully electric propulsion faces challenges due to the high power demands of marine environments, hybrid systems show promise. Mercury Marine's Fathom energy management system, for example, combines lithium-ion batteries with sensors, switches, and controllers to meet electrical demands on boats without relying solely on the engine. This approach allows for silent and vibration-free power while docked, with the engine powering the boat when needed.

Increasing Horsepower:

Engine manufacturers are continuously pushing the boundaries of horsepower. Ilmor Marine, for instance, has introduced a 630-horsepower engine for towboats, doubling the horsepower of previous models. This trend is driven by consumer demand for bigger boats and more powerful engines, creating a cycle of innovation.

Emission Limits and Technology:

Stricter emission limits have played a significant role in the evolution of inboard-outboard engines, sparking a revolution in 4-stroke technology. This trend is expected to continue, with engineers optimising combustion processes and minimising emissions. Additionally, the integration of technologies such as autopilots and joysticks will drive a more holistic design approach.

Engine Cooling:

Engine cooling remains a critical aspect of inboard-outboard engine development. Engineers are focusing on developing effective cooling systems that can regulate engine temperature without compromising power output. This is particularly important for high-horsepower engines in demanding conditions.

Design and Functionality:

Inboard-outboard engines will continue to evolve in terms of design and functionality. Improvements in weight distribution, steering control, and torque will enhance the overall performance and handling of boats. Additionally, the layout of the engine compartment will be a key consideration to maximise space utilisation and provide a more enjoyable boating experience.

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