When the sailing ships of the 1800’s gave way to steam power new propulsion systems were invented to move cargo ships, ferries and tugboats to windward. Paddle wheels, both side and stern, and propellers offered shipbuilders and masters new methods of maneuvering their vessels. The descendants of these early power systems have been refined and updated, and today’s recreational boaters have a variety of choices for power and propulsion for small craft.
Gustave Trouvé, a French electrical engineer, invented the first outboard engine. The battery powered electric motor had its debut on the River Seine in 1881. Almost 30 years later, Ole Evinrude developed a very successful three horsepower gasoline outboard.
The outboard is a self-contained unit that is mounted on the transom of a boat. It consists of a powerhead, which contains an engine that can be fueled by gas, electricity and even propane. The lower unit contains the gear box and method of propulsion, usually a propeller, although water jets are available as well. Both methods of propulsion use directional vectored thrust rather than a rudder for turning.
Inboard engine with shaft drive
Outboard engines are popular for smaller, open watercraft usually up to the 35 foot LOA range. For traditional cabin cruisers and larger motoryachts an inboard engine with a propeller shaft extending through the hull is most common. A shaft seal, stuffing box or stern gland provides a watertight fitting where the prop shaft passes through the hull. The engine and transmission are secured inside an engine compartment with a propeller providing the thrust. Steering is controlled with a rudder connected to a steering wheel.
Inboard engine with stern drive
What looks like the marriage of an inboard engine with the lower unit of an outboard is an apt description of a stern drive. Also known as I/O (Inboard/Outboard) propulsion, it was first developed by Mercury Marine after World War II by mating a performance automobile engine with the lower unit of an outboard motor. Modern outdrives are a now two part affair, divided into an upper unit and a lower unit. The upper unit is connected to the drive shaft of the inboard engine and powers a transom-mounted gearbox angled at 90 degrees. The lower unit contains the propeller driven from the gearbox.
Compared to inboard engines with prop shafts, the I/O offers more interior space for the boat builder to use as a living area in smaller boats such as sport cruisers. It is also easy to trailer since the outdrive can be tilted up and a rudder is not needed for steering.
Inboard engine with water jet drive
Jet drives use water for propulsion. A high volume impeller pump, protected by a debris screen, sucks water from under the boat and then discharges it through a nozzle on the transom. Steering is accomplished by swiveling the nozzle port and starboard with a steering wheel. Since this is vectored thrust without a rudder, like an outboard engine or a stern drive, as soon as the jet stream ceases, so does the ability to steer. Reversing the flow of the water jetting out of the nozzle allows the drive to maneuver astern. Instead of using a transmission with a reverse gear, a deflector, known as a “clam shell” or “bucket”, is lowered to direct the discharge flow forward under the boat, and that causes the boat to move backwards.
Inboard engine with pod drive
Azimuthing podded drives, or azipods, were developed in the late 1980s for use on commercial vessels, such as oil tankers, cruise ships and tugs. These drives have a fixed propeller on a steerable pod mounted underneath the vessel. Rotating the pod through large angles allows ships to steer without the need of a conventional rudder.
The trickle down of this technology to recreational powerboats began in 2004, when Volvo Penta introduced its Inboard Performance System (IPS) pod drive. The IPS uses dual forward facing vertical drive legs protruding through the hull from an inboard engine. The forward facing propellers operate in a “pull” mode like a propeller driven aircraft engine. Zeus, a pod drive developed by Mercury Marine, uses aft facing propellers that operate in the traditional “push” mode.
Both manufacturers’ pod drives offer a greater efficiency over fixed shaft drives since much of the running gear is eliminated under the hull, which reduces drag dramatically. Combined with a joystick control, pod drives remove the need for bow and stern thrusters. Just tilt the joystick in the direction of desired travel and the boat responds immediately, whether it is forward, reverse or sideways.
Choosing the best engine and drive combination to purchase, factors in ease of boat handling, waters to be cruised, necessary maintenance and cost. The final selection, if done carefully, will give years of enjoyment on the water with the desired level of performance.