Jet Drive Propulsion

In my line of work, I am fortunate to learn about a wide variety of things from my students. I had occasion to hold a captain's license course in the vicinity of Fontana Lake in western North Carolina. A dam that backed up the waters of the Little Tennessee River and several creeks created Fontana Lake back in the mid 1940's.

Most of the students in the Fontana Lake captain's course were getting licensed in order to operate boats with "jet drive" propulsion. These boats use a gasoline powered inboard engine coupled to a "jet drive." The best way to describe the jet drive is to call it a "pump." It draws water through an opening in the bottom of the boat, "pumps" it through the drive and out a nozzle that is mounted on the stern of the boat.

The nozzle is connected to the helm by a linkage that allows the operator to direct the nozzle to the port or starboard. That's how the boat is steered. It's like having a fire hose on the transom and discharging it into the water to propel the boat and directing it from side to side to steer.

The company with the jet boats on Fontana Lake is in the business of taking up to eleven passengers for "joy rides." What makes the jet boat unique is that at fifty miles per hour they can "spin" the boat around (not a "U" turn but a spin). That is, in an instant you go from forward to backward.

The hull design on this type of jet boat is such that there is a lot of "side slip." The term "side slip" is used to describe the behavior of a boat that slides to port or starboard during a turn, similar to a car during a high-speed turn sliding (skidding) sideways. This type of jet boat is designed to take people in what I would call a "thrill ride."

Why do I bring this issue up? It has to do with steering a jet driven vessel. In addition to the Fontana Lake jet boats, there are many other vessels with jet drives. The most popular jet drive vessels are the "jet ski" and the "wave runner." They are also referred to as "personal water craft" (PWC). Also, there are a few manufacturers of small boats with jet drives.

Historically, all "jet drive" boats had one important thing in common. They all required a steady flow of water through the nozzle in order to steer. The nozzle is not a rudder. It is more like a short pipe through which water is discharged and directed port or starboard.

Turning a jet driven vessel requires a strong flow of water through the nozzle, which means you have trouble if you need to slow down and turn at the same time. In a prop driven vessel, you can release the throttle and still use your out-drive, outboard motor or rudder to turn the vessel. Not so with a jet drive.

A word to the wise; this type of vessel requires more anticipation. While there has been a movement to provide "off throttle" steering, before purchase or operation, do your homework! 

Until next time, I wish you clear skies, fair winds, and calm seas!