Marine Surveyors Lexicon


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Jet Drives

The Jet Drive is Simple. A jet drive is a water pump, which is driven by an inboard engine that has been specially prepared for marine use. The engine is mounted within the hull and connected directly to the pump. There is no clutch or gear box.

The jet receives water through an intake grate in the bottom of the boat, and ejects the flow beyond the transom and through a steerable nozzle. The pump impeller, in ejecting the water, creates an equal force in the opposite direction which propels the boat forward. Steering is done by swinging the nozzle and rudder right or left, as desired.

The boat's attitude can be controlled by pointing the nozzle up or down within a controlled range on models with underway trim.

Reverse is accomplished by lowering a reverse gate across the outflow, diverting the jet stream forward, beneath the hull. Lowering a reverse gate when the boat is going forward, can stop the boat virtually within its own length. (Stopping should be done carefully to prevent passengers or free objects from being tossed forward.) There is no extra strain on the engine or the Jet Drive when going from forward to reverse. The drive system can't detect the change; its rotation and load are not affected.

The jet drive does not know how heavy your boat is, or what speed the boat runs. Simply, the jet drive in your boat is matched to the engine. The jet drive should have been installed in your boat with a correct impeller to match the engine supplied with the jet drive. There is no requirement to change this impeller in an effort to increase out of the hole thrust or obtain better top speed.

A jet drive is easy to maintain. There are few parts in a jet drive. The direct-drive shaft has a thrust and tail bearing. The impeller is fixed to the shaft and completely enclosed. The intake grate screens out most trash...but if tough weeds, string or a small rope pass the grate, and remain in the impeller, these are easily removed through a hand hole which is normally covered.

With no gears and so few parts, overhaul of a jet drive may not be necessary for several years, or hundreds of hours of use. Parts are largely interchangeable and readily available. Access is simple and there's a minimum of downtime.

  Berkeley Race Jet

  Berkeley Standard Jet

Berkeley Jet Drive Specifics

The drives built by Berkeley will deliver about 1400-1800 pounds of thrust while pumping 3000 to 4000 gallons of water every minute at a pressure of 60 to 180 pounds per square inch (PSI).

The Berkeley jet drive is a mixed flow design (part axial flow, part centrifugal) with a 12" size designation. This 12" size designation is from water well sizing, meaning the bowl will fit in a 12" well casing. The original technology for Berkeley comes from the hydraulic principals used in the design of single through multi-stage line shaft turbine pumps which are used for submerged application in all types of water well or fluid transfer systems.

The balance of centrifugal and axial flow pump characteristics assures you of adequate thrust velocity for good performance in the popular boat speed ranges from 30 to over 60 MPH. The efficiency of the jet drive is readily seen once speed in the 40 plus miles per hour ranges are reached. Minor changes in the nominal 3¼" diameter nozzle are possible and may be effective in fine tuning. Usually a slightly larger diameter for boats with top speeds below 45 MPH and a small decrease in nozzle diameter where speeds above 65 MPH are possible.

Common Jet Drive Performance Upgrades

Nozzle Wedges: The wedges allow you to make incremental changes to the handling character of your boat. The wedge can be turned downward to possible fix a porpoising problem or upward to free a boat up that rides to wet.

Droop Snoot: Advantages of using a Droop Snoot: lowers the thrust point of the jet, allowing the jet pump to have better leverage to lift the bow up, which in most cases will increase your top end speed.

Rudder: For more low-speed maneuverability or improved high-speed tracking, a rudder is the answer. There are two models of bolt-on rudders. The first is an entry level economy plastic rudder, while the second is a sturdy cast aluminum version of the same rudder. Note: The plastic unit will not survive in aggressive boating situations. If you want durability then you need an aluminum rudder.

Race Prepped Bowl: The vanes on this bowl have been modified on the inlet to smooth and stabilize the water entry into the bowl. The outlet vanes are modified increasing exit velocity for less turbulence and straighter flow entering the nozzle. This is a must for the lake racer or any serious competitor.

Billet Bowl Stuffer: This billet aluminum stuffer mounts into the bowl directly behind the impeller. By closing up this clearance the hydro flow dynamics of the water exiting the impeller is substantially increased. The result is improved hole shot and acceleration. Note: This bowl stuffer is for use with a non-shoulder wear ring.

Intake Assembly: The High Performance Intake incorporates what's called a shoe and ride plate assembly. By using a shoe you can direct more water into the inlet of your jet pump thus increasing pump thrust. The ride plate gives your boat extra riding surface behind the shoe to give your boat more lift for increased speed.

Rock Grate and Loader Scoop: These are an entry level upgrade from a stock jet intake grate. They are designed to work best with stock or only mildly modified engine packages. By using an outer angle ramp design this unit can help to deliver more water into the jet intake for improved performance.

Place Diverters futuristic nozzle design with built-in ride plate will provide a quicker planning action and virtually eliminates drag caused by all reverse passages. The UP-DOWN nozzle brings another dimension in control. Now the driver can control the bow attitude of the boat from first thrust to high speed. This added control makes skiing easier for both the skier and driver. The driver keeps the bow low for faster planning and better visibility. The Place Diverter also allows mush slower planning speed, which is so essential when teaching a new skier, or for fuel economy. For the driver who has quickness and speed in mind, the Place Diverter is adjusted down for straight ahead, low bow takeoff and immediate planning with constant visibility. At high speed the driver adjust the diverter to the up positions and the bow rises, thus reducing the wetted area to improve top end performance. Also throws an exciting rooster tail.

Berkeley 12JG Replacement Parts Exploded View

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