MOTC Manual - edited

Outboard motors consist of a power head, midsection (exhaust housing), and lower
unit (gear box).
Outboards can be tilted up and down (some manually, and some hydraulically) to
trim the boat, and for increased clearance when on a trailer.
Inboard/Outboard - These are also referred to as “I/Os”
or “stern-drives”. I/Os are generally heavier than
outboards. They consist of a motor (typically four-stroke)
mounted inboard and an outdrive attached low on the
transom and has a lower unit like an outboard motor. The
outdrive swivels from side-to-side to provide for the
steering of the boat. It can also be tilted up and down to
provide boat trim while underway, and for clearance when
on a trailer.
Inboards (propeller & jet) – Inboards with a propeller drive and rudder are
mounted forward compared to I/Os, and the motor connects directly to a
transmission out of which comes a shaft that goes through the hull of the boat as it
passes through the "stuffing box". The shaft is then attached to a propeller that turns
to propel the boat. (The stuffing box is a cylinder through which the shaft passes.
The shaft is surrounded by a stuffing material which when compressed between the
cylinder wall and the shaft
prevents excessive water
from entering the boat.)
Since the shaft is fixed and
does not swivel from side to
side, a rudder is mounted
behind the shaft and
propeller to deflect the flow
of water to provide steering
direction.
Inboards with a jet drive are mounted near the stern like I/Os. The motor links
directly to the jet drive, although some inboard jet drives have a rudimentary
transmission.
Motor Kill Switches
A basic kill switch is activated when a key, connected by lanyard
to the operator, is pulled. This shuts off the motor, protecting
the operator if accidently thrown from the helm. Kill switches
are preinstalled on outboards with tiller steering, but not always
on boats with a helm. Aftermarket kill switches can be installed
if not present but required or prudent.
Kill switches come in many designs. Those designed using a
toggle can be deactivated without a key. Some that need a key
to be deactivated have a spare key mounted nearby the switch.
DOI – Motorboat Operator Certification Course Chapter 2 – 2