Acquainted to Outboard Motors

The first engines most boaters use are outboard motors. Outboards are engines featuring an integral drive system including a drive shaft and propeller, all in a single unit that can be moved from boat to boat. All of them still look more or less like the motor designed by Wisconsin inventor Ole Evinrude and patented in the United States on September 16, 1910. Evinrude went on to found Outboard Marine Corporation (OMC), one of the world’s largest manufacturers of outboards. An outboard motor includes a gasoline engine mounted at the top of a long shaft and a prop mounted at the bottom. The motor clamps on the stern of a boat, and the whole thing—motor, shaft, and prop—pivots to steer the boat. If the boat already has a steering mechanism in place, such as a tiller and rudder, the motor stays stationary and the boat steers like normal. The outboard concept was particularly useful in the early years when mostly smaller boats were used for recreational boating, and a single motor could easily be moved from one boat to another. Individuals who owned these light, portable motors could carry it in the trunk of their cars to boat rental operations anywhere, and in the booming economy before 1929, lots of folks had spare cash to spend. Thus they made Ole Evinrude and his offspring quite wealthy.

Outboards today offer the advantages of high performance, low weight, and push-button trim and tilt of the lower unit, which makes it easier to control the running attitude of a boat—the angle of the boat bottom to the surface of the water—and makes it easier to operate in shallow water. Outboards are also easy to steer due to the “vectorable” thrust—that is, the propeller itself can be angled to push the stern to one side or the other. The picture below shows the lower unit of an outboard.

Let’s run through the parts of the lower unit of an outboard:

• Tab. The tab offsets the tendency of a prop to “walk” sideways slightly at some trim positions and balances the steering force. (Not to be confused with “trim tabs” that control running trim, usually found on larger boats with inboard power.)
• Bullet or nosecone. The bullet or nosecone houses the forward and reverse gears and prop shaft.
• Propeller. The propeller grips the water and propels the boat by pushing water backward. Three-bladed props are most common. The propeller on a boat is commonly referred to as the prop. The shaft that connects it to the drive shaft from the powerhead is called the prop shaft.
• Skeg. The lowest extension of the lower unit, the skeg protects the prop from striking obstructions and also helps the boat track straight.
• Lubricant fill hole. The lower unit is full of oil to provide lubrication. This hole allows draining and refilling.
• Anti-ventilation plate. This flat plate helps prevent the prop from drawing in surface air and losing its grip on the water. It is often called the “cavitation plate,” a usage that is technically incorrect but common enough to be widely understood.
• Cooling water pick-up. This small intake allows the water pump to pull water through the lower unit and up into the cooling passages of the powerhead. It must be kept free of debris at all times.

In nautical terms, an engine is the power unit itself without the drive train and propeller. A motor is a complete power unit including a drive shaft and prop. Thus, inboard and stern-drive power plants are usually referred to as engines, while outboard power plants are usually called motors. (And just to make things more confusing, a motor is also an energy-conversion device such as an electric or hydraulic motor that draws power from an engine located elsewhere on the boat.) The lower unit of an outboard and a stern drive are nearly identical. This section couples the power put out by the engine to the prop, which causes the boat to move through the water.

Tabs on outboards and stern drives are usually made of zinc, which acts as a “sacrificial anode” in salt water. The “sacrifice” it makes is to literally be eaten up by galvanic corrosion in order to protect the more important aluminum and steel parts of the motor. When a tab becomes severely depleted, it can be replaced by loosening one bolt. Inboards usually have a similar zinc bolted in place on their shaft, and it, too, must be replaced periodically.