Family runabout boats up to about 26 feet in length may be powered by a sterndrive or jet drive propulsion system. Both are great systems, and each offers advantages and disadvantages.
Jet Drive Boats
Jet-powered boats use the same propulsion system used in personal watercrafts (PWCs) that mates an inboard engine to a jet pump. The pump draws water from an intake in the bottom of the boat and shoots a high-velocity stream of water from a nozzle at the back of the boat to propel it forward.
Common Features & Advantages of Jet Boats
- Jet boats may have one or two engines and jet pumps.
- The boat is steered by a movable nozzle that aims the stream of water left or right. A diverter (called the reverse bucket) drops over the jet nozzle to create reverse thrust.
- Because the entire jet drive system is located within the hull, a jet boat has minimal draft and can operate in very shallow water—there’s no propeller or outdrive that could be damaged by hitting bottom.
- Also, because the jet drive impeller is contained within the pump, there’s no propeller to pose a danger when swimming around the transom of the boat.
The compact four-cylinder engine that powers most jet drive systems takes up less space in the boat and is also lighter than a sterndrive engine, which can make the boat easier to tow on a trailer.
Jet Boat Drawbacks
A jet drive system does have some disadvantages.
- The pump intake can be clogged by weeds, and a stray ski rope or dock line can be drawn into the pump and get tangled inside.
- Unlike a sterndrive the jet nozzle is not trimmable, so the boat’s running angle cannot be adjusted to accommodate changing passenger load or sea conditions, or to gain speed and efficiency.
- Because a jet drive does not have the rudder effect offered by a sterndrive it may not track as well at no-wake speeds and can be trickier to handle around a dock.
A sterndrive system mates a V6 or V8 engine located inside the boat with a steerable outdrive and propeller attached to the boat transom (a sterndrive is also often called an inboard/outboard, or I/ O).
Common Features & Advantages of Sterndrives
- The outdrive has a rudder effect to help the boat track true at all speeds and propeller offers very efficient power transfer from the engine to the water.
- The drive can also be trimmed—which changes the thrust angle of the prop—to help lift the boat on plane from a start and then to raise the bow at speed to achieve a smooth ride and reduce drag, improving top speed and efficiency.
- Because it extends below the keel of the boat, the outdrive and its propeller are susceptible to damage if they impact the bottom or an obstruction in the water, like a rock or tree stump lurking just below the surface.
- The propeller can also be a hazard to swimmers behind the boat.
- A sterndrive also requires more maintenance than a jet drive. The steering and trim use hydraulic systems that may require lubrication and maintenance, and in cold climates the engine should be “winterized” with an antifreeze solution to prevent the block from cracking in freezing weather. A jet drive is self-draining and does not require this step.
- The sterndrive engine is heavier than a jet engine, and takes up more space in the boat.