SternDrive

A sterndrive, or inboard/outboard drive (I/O), is a type of marine propulsion. The engine (or upper unit) is located inboard just in front of the transom, delivering power via a shaft running through the transom to the drive unit (usually called the outdrive or lower unit) on the outside of the hull, which resembles the bottom half of an outboard. The sterndrive includes the gearing for the system and carries the propeller. Steering is accomplished by pivoting this unit, just like with an outboard motor -- no rudder is needed. The engine on its own is typically the same as those used in true inboard systems, with the most popular in North America being reworked versions of Chevrolet and Ford V-8 automotive engines. In Europe, diesel engines are quite a bit more popular, with up to 370hp available with Volvo Pentas D6A-370. The two most well known brands of sterndrives are Volvo Penta (part of the Volvo Group) and MerCruiser (produced by Brunswick Corporation's Mercury Marine. Mercruisers have a slightly larger stake in the United States' market; however, in Europe, Volvo Penta has a huge market share -- around the 80% mark.

Sterndrives offer several advantages over outboards, including higher available horsepower per engine and a clean transom with no cutouts for the outboard installation and no powerhead sticking out, making it easier to get in and out for pleasure boat passengers and for easier fishing. Sterndrives offer several advantages over inboards, including less complex engineering for boat builders, preventing the need for the design of propshaft and rudder systems; also, a large amount of space savings with the engine mounted completely aft, freeing up the boat's interior for occupancy space. This is ideal for consumers who are interested in "pocket cruisers" -- boats that feature amenities like a head, a galley, and separate sleeping quarters -- all in a boat less than 30 feet in length.

The main disadvantage of a sterndrives in lieu of straight inboards is that they are quite a bit more exposed, with hoses, rubber bellows, oil lines, and other components in the water that can be damaged or corroded.

Inboard/Outboard maintenance tends to be more complicated than outboard maintenance for numerous reasons. For example, oil changes can require complex pumping mechanisms, and engine repairs are often quite a bit more complex than outboards because of super tight spaces in the engine compartment. In certain boats the entire engine has to be removed to perform even simple repairs where all that is required of an outboard is the removal of the cover.

One danger with both inboards and sterndrives is that of fire/explosion due to fuel vapors in the engine compartment. These boats usually require a blower to run for several minutes prior to starting, and when idling, exchanging the compartment air with fresh, outside air.

Due in large part to their useful advantages and attractive price tag, sterndrives have become very popular for use in pleasure boats. In addition, there are numerous boat models for which sterndrive power is the only option available.

Inboard Engine/Crankcase

An inboard motor is a marine propulsion used in boats. An inboard motor differs from an outboard motor in that the engine is enclosed within the craft, rather than completely outside the hull, and is typically connected to a propulsion screw by a driveshaft.

For pleasure craft, both diesel and gasoline engines are used. Many inboard motors are modified automobile engines, also known as marine automobile engines. The invention of the sterndrive propulsion leg improved design so that auto engines could easily power boats.

For larger craft, including ships (where outboard propulsion would in any case not be suitable) the propulsion system may include many types, such as diesel, gas turbine, or even fossil-fuel or nuclear-generated steam.

Some inboard motors are cooled via freshwater, 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, due to the corrosive nature of seawater, some seagoing craft have engines which are indirectly cooled using a heat exchanger. Other engines, usually small single- and twin- cylinder diesels specifically designed for marine use, use raw seawater for cooling and zinc sacrificial anodes are used to protect the internal metal castings.

Outboard Engine/Crankcase

An outboard motor is a propulsion system for boats, composed of a self-contained unit, including an engine, gearbox, and propeller or jet drive. Outboards are designed to be attached to the outer side of the transom and are the most common way of propelling small watercraft. In addition to providing propulsion, outboards also provide steering control, due to their design allowing them to pivot over their mountings, thereby controlling the direction of thrust. The skeg can also serve as a rudder when the engine is not running. In comparison to inboard motors, outboard motors are very easy to remove for storage or repairs. When out of service, or being drawn through shallow waters, outboards can be tilted up to protect the prop and lower unit, to avoid seaweed accumulation and underwater hazards, and to clear road hazards while trailering.

Portable: Small outboard motors up to 15 horsepower are very portable, due to the fact that they are affixed to the boat via clamps, thus easily moved from boat to boat. These portable motors usually have a manual pull start system, with throttle and gearshift controls affixed on the body of the motor and a tiller for steering. Small, portable outboard motors typically have integral fuel tanks and provide sufficient power to move a small dinghy at around 8 knots (9.2 mph). These smaller outboard motors are most often used to power small craft such as jon boats, dinghies, and canoes; to provide secondary power for sailboats; and for trolling aboard larger craft, due to their ability to more effectively run at trolling speeds. When used for trolling purposes, the motor is often placed on the transom alongside, and connected to, the primary outboard, enabling helm steering.

Large Outboards: Large outboards are typically attached to the transom (either directly or via a bracket), and are linked to helm controls. These larger outboards range from 2-, 3-, and 4-cylinder models capable of generating anywhere from 15 to 115 horsepower suitable for hulls up to 21 feet long, to powerful V-6 and V-8 cylinder blocks that are rated for up to 350 hp suitable for hulls up to 38 feet or longer.

Electric-Powered: Often called "trolling motors", electric outboards are typically used on very small craft or on small bodies of water where gasoline motors are prohibited; as an auxiliary means of propulsion on larger craft; as repositioning devices while fishing for bass and other freshwater fish; and many other applications where silent, easy operation and zero emissions take precedence over speed and range.

Diesel: Diesel outboards are available as well; however, their relative weight and cost make them impractical for most purposes. What's more, diesel engines tend to give off toxic exhaust emissions, which is a highly undesirable feature for most boaters.

Pump-Jet: Outboard pump-jet propulsion is also an available option. Though pump jets are generally less efficient than an open prop, they are actually quite useful in situations where shallow water operation is important.

It is imperative that you select a motor that is matched to the hull, both in terms of power and shaft length. Overpowering can be very dangerous and underpowering typically results in a boat that is unable to perform in the role for which it was acquired. Boats that are built in the United States have a Coast Guard Rating Plate, which identifies the maximum horsepower recommended for that hull. A motor that has less than 75% of the maximum usually results in less-than-desirable performance. Outboard motor shaft lengths are standardized to fit 15-inch, 20-inch and 25-inch transoms. Should the shaft be too long, it will extend farther into the water than necessary, in turn creating drag that impairs performance and fuel economy. If the shaft is too short, the motor will be prone to ventilation. What's more, if the water intake ports on the lower unit are not submerged at the ideal level, the engine will most likely overheat, resulting in damage beyond repair.

Motor height on the transom is also a very important facet in achieving ideal performance from your outboard. The motor should be as high as is possible without ventilating or loss of water pressure. This height will minimize the effects of hydrodynamic drag while underway, giving you far greater speed. Ideally, the anti-ventilation plate should be around the same height as, or up to two inches higher than, the keel, with the motor being in neutral trim.