Sterndrive

Key takeaways

Sterndrive belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sterndrive to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sterndrive from memory before moving on to harder problems.

Reference excerpt

A sterndrive or inboard/outboard drive (I/O) is a form of marine propulsion which combines inboard power with outboard drive. The engine sits just forward of the transom while the drive unit (outdrive or drive leg) lies outside the hull.

Operation The drive unit (outdrive) carries power from the inboard engine, typically mounted above the waterline, outboard through the transom and downward to the propeller below the waterline. The outdrive resembles the bottom half of an outboard motor and is composed of two sub-units: an upper containing a driveshaft connected through the transom to an engine which transmits power to a 90-degree-angle gearbox; and the lower containing a vertical driveshaft receiving power from the upper unit gearbox, transmitted through another 90-degree-angle gearbox to the propeller shaft. The boat is steered by pivoting the outdrive, as on an outboard motor; no rudder is needed. The outdrive is pivoted up for trailer travel and between uses to avoid fouling. The outdrive can be matched with a variety of engines in the appropriate power range; upper and lower units can often be purchased separately to customize gear ratios and propeller RPM, and lower units are also available with counter-rotating gearing to provide balanced torque in dual-drive installations. Sterndrive engines are similar to those used in true inboard systems. Historically the most popular in North America were "marinised" versions of Chevrolet and Ford V-8 automotive engines. In Europe diesel engines are more popular, ranging up to 400 hp in models such as the Volvo Penta D6A-400. Other brands of sterndrive include MerCruiser (produced by Brunswick Corporation's Mercury Marine, which also manufactures outboard motors). Mercruiser have a slightly larger market share in the US, while in Europe Volvo Penta has an approximate 80% market share.

History The history of stern drive power begins in the First World War with the production of the MAS of Isotta Fraschini, with two counter-rotating propellers. In 1948, Mercury Marine engineer Charlie Strang mated an aluminum car racing engine to the lower unit of an outboard motor, creating a marine propulsion system more powerful than the outboard motors available at the time. During the 1950s, he and fellow engineer Jim Wynne worked at Mercury under founder Carl Kiekhaefer, who was initially dismissive of and opposed to a technology that would later capture 80% of the market. In 1959 Wynne left Kiekhaefer and in less than 90 days developed his own stern drive, which he subsequently patented. The first commercial introduction of sterndrives was the Volvo Penta "Aquamatic" at the 1959 New York Motor Boat Show. Kiekhaefer introduced a MerCruiser outdrive in 1961 at the Chicago Boat Show. By 1962, 16 manufacturers were producing sterndrives. There was also the Outboard Marine Corporation (O.M.C.); however, due to several patent wars, O.M.C. ceased production.

Characteristics Advantages of the sterndrive system versus outboards include higher available horsepower per engine and a clean transom with no cutouts for the outboard installation and no protruding powerhead, which makes for easier ingress and egress for pleasure boat passengers, easier fishing and easier retrieval in rescue situations. Advantages of the sterndrive system versus inboards include simpler engineering for boatbuilders, eliminating the need for them to design propshaft and rudder systems; ease of trailering and shallow water maneuvering (with the drive trimmed up); also, a significant space savings with the engine mounted all the way aft, freeing up the boat's interior volume for occupancy space. This is of particular significance to consumers who are interested in "pocket cruisers," boats which have amenities like a head, a galley, and separate sleeping quarters in a boat less than 30 feet in length. The main disadvantages of sterndrives versus straight inboards is that they are more exposed. There are hoses, rubber bellows and oil lines in the water which can be damaged. There are also more components in the water which are exposed to corrosion. Maintenance on I/Os is more complicated than outboards in several ways, less in others. While sterndrive engines are not as readily removable as smaller outboards, larger outboards (which combine both large powerheads and outdrives) are bigger, bulkier, and heavier than separate engine and outdrive components on sterndrives. Sterndrive engine compartments may be cramped; however components are neither as tightly engineered nor physically as compressed and densely packed as outboards, which all must fit under a streamlined integrated cover. With both inboards and stern drives there is a fire and explosion hazard from gasoline fuel vapors within the engine compartment. Typically these boats must run a blower for several minutes prior to starting the engine, and when idling or moving below cruise speed. The blower exchanges the air within the engine compartment with fresh outside air. Because of their practical advantages and attractive cost, sterndrives have become extremely popular especially for use in pleasure boats, and there are many models of boats for which sterndrive power is the only available propulsion offered.

Illustrations

Worked examples

Example 1 — a first encounter with Sterndrive

Start with the simplest possible case. Write down what Sterndrive claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sterndrive before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sterndrive ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sterndrive

In research: Sterndrive appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sterndrive in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sterndrive is common in secondary-school and first-year university syllabi. It links to neighbouring topics Marine propulsion, Propellers, so understanding it makes those chapters shorter.
In everyday life: Look for Sterndrive outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Sterndrive in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Sterndrive means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Sterndrive out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Sterndrive in simple terms?

A sterndrive or inboard/outboard drive (I/O) is a form of marine propulsion which combines inboard power with outboard drive. The engine sits just forward of the transom while the drive unit (outdrive or drive leg) lies outside the hull.

Why does Sterndrive matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sterndrive?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sterndrive.