Jet Boat Pump-To-Engine Attachment: The How-To Guide

Jet boats use a propulsion system that combines an inboard engine with a jet pump. The jet pump draws water from an intake at the bottom of the boat and shoots it out through a nozzle at the back, propelling the boat forward. The boat is steered by a movable nozzle that aims the stream of water left or right. The jet unit is streamlined and does not sit below the bottom of the boat, allowing it to travel across shallow water. This is in contrast to a normal motorboat, which requires a deeper amount of water to operate due to its propeller being attached to the stern.

The jet boat pump's attachment to the engine

The jet boat pump attaches to the engine through a jet unit, which is a propulsion system that uses a jet of water to move the boat forward. This is achieved by drawing water from an intake at the bottom of the boat and shooting it out at high velocity through a nozzle at the back. The engine is connected to an impeller within the jet unit, which compresses and forces the water out of the nozzle. The jet unit also incorporates a steering mechanism, where the direction of the water jet can be changed to turn the boat.

The jet unit was first developed in the 1950s by Sir William Hamilton in New Zealand. His initial designs used a retractable marine propeller but later iterations employed a centrifugal pump driven by a bevel gear attached to a plywood boat powered by a Ford engine. This design was further refined with the help of engineer George Davison, and they made rapid advances in waterjet propulsion.

The jet unit offers several advantages over traditional propeller-driven boats. Firstly, it allows boats to operate in very shallow water as the jet unit is streamlined and does not extend below the bottom of the boat. This also eliminates the risk of damaging the propeller by hitting the bottom or underwater obstacles. Secondly, the jet drive system is contained within the hull, reducing the danger posed by propellers to swimmers. Additionally, the jet drive system is more compact and lightweight, making the boat easier to tow.

However, there are also some disadvantages to the jet drive system. The pump intake can become clogged with weeds, and ropes or lines can get tangled inside. The jet nozzle is not trimmable, limiting the boat's ability to adjust to changing passenger loads or sea conditions. The jet drive also lacks the rudder effect of a sterndrive system, which can make docking and no-wake speed navigation more challenging.

How the jet boat pump works

A jet boat pump works by generating a propulsive thrust. Water is sucked into the jet unit through an intake at the bottom of the boat and is then forced out through a nozzle at the back of the boat. This creates a high-velocity stream of water that propels the boat forward. The jet unit is streamlined and does not sit below the bottom of the boat, allowing it to travel across shallow water.

The jet unit contains an impeller or impellers (internal propellers) that compress and force the water out through the stator housing behind the impeller and then through the jet nozzle. The force of the water exiting the nozzle propels the boat forward. Steering is achieved by turning or pivoting the jet nozzle, which changes the direction of the water forced out of the back of the boat.

To reverse the direction of the boat or to bring it to a standstill, an astern deflector or a diverter (called a reverse bucket) is lowered into the jet stream after the water leaves the nozzle. This reverses the direction of the force generated by the jet stream, propelling the boat backward or bringing it to a stop.

The amount of thrust generated by the jet boat pump depends on the weight of the boat, the design of the hull, and the size of the impeller. The faster the impeller spins, the more horsepower is absorbed. The pump also needs to be spun at a faster rate to generate more thrust. Once the pump reaches a certain RPM, it starts moving more water, which increases the load on the engine and requires more torque and horsepower.

The jet boat pump works differently from a propeller boat, which uses a propeller to create forward thrust. The jet boat pump has no comparison with an outboard motor's principle of thrust, except that they are both sources of power. The jet boat pump works off of thrust and is an example of Newton's third law: "For every action, there is an equal and opposite reaction."

Differences between jet boats and prop boats

Jet boats and prop boats (also known as sterndrives) are both propelled by internal combustion engines but differ in their propulsion systems. Jet boats use a jet of water to propel the boat forward, while prop boats use a propeller.

Propulsion System

Jet boats use a jet pump to draw water from an intake at the bottom of the boat and shoot it out at high velocity from a nozzle at the back. The boat is steered by aiming this stream of water left or right with a movable nozzle. Reverse thrust is created when a diverter, or "reverse bucket", is dropped over the jet nozzle. The entire jet drive system is located within the hull, giving jet boats a minimal draft and the ability to operate in very shallow water. The compact four-cylinder engine that powers most jet drives takes up less space and is lighter than sterndrive engines, making the boat easier to tow. However, the pump intake can be clogged by weeds, and ropes can get tangled inside.

Prop boats, on the other hand, use a propeller to create forward thrust. The propeller "chops" at the water, pushing the boat forward. Sterndrives mate a V6 or V8 engine with a steerable outdrive and propeller attached to the boat transom. The outdrive has a rudder effect, helping the boat to track true at all speeds, and the propeller offers efficient power transfer from the engine to the water. The drive can be trimmed to change the thrust angle of the propeller, helping to lift the boat and achieve a smooth ride. However, the outdrive and propeller are susceptible to damage if they hit the bottom or an obstruction in the water, and they pose a hazard to swimmers. Sterndrives also require more maintenance than jet drives and take up more space in the boat.

Steering and Handling

Jet boats are steered by angling the nozzles at the rear of the boat, and this can only be done under power. They offer superior speed and can make tighter turns, even in shallow waters. However, they have poor steering ability at lower speeds and when backing off the throttle, and they may wander about due to constant thrust from the engine. Jet boats are also purpose-built and cannot be converted from an existing boat. Mechanics for jet boats are also less common.

Prop boats are steered using a rudder behind the propellers or by turning off the entire outboard assembly on outboard motors. They are easier to control and steer than jet boats, as they offer more predictable handling. Prop boats can also be "held" in place, as they have a true neutral position where the propeller rotation can be stopped. They can power through weed beds and murky waters, as the boat's hull will push debris aside. However, the propellers can get fouled by long weeds, fishing nets, or rope tangles, requiring manual clearing of the propeller. There is also a risk of injury from the spinning propellers when entering or exiting the water.

Efficiency

Prop boats are more fuel-efficient than jet boats. Jet boats are also more susceptible to sucking up debris and fouling, especially in shallow waters with mud, sand, and stones.

Advantages and disadvantages of jet boats

A jet boat uses a jet of water for propulsion, with water entering the pump through an inlet at the bottom of the hull. The pump can be of a centrifugal design for high speeds or an axial flow pump for low to medium speeds. Jet boats have a number of advantages and disadvantages when compared to sterndrive (prop) boats.

Advantages of Jet Boats

Jet boats have exceptional shallow draft capabilities, with some able to operate in just 3 inches of water. This makes them ideal for shallow lakes or rivers. They also have good acceleration and turning ability, with a sportscar-like handling experience. They are safer than prop-driven boats as there is no exposed propeller, removing the risk of injury to swimmers or skiers. The compact engine of a jet boat takes up less space and is lighter than a sterndrive engine, making the boat easier to tow.

Disadvantages of Jet Boats

The pump intake can become clogged with weeds or debris, and ropes can become tangled inside. Jet boats are also generally louder and less fuel-efficient than sterndrive boats due to the higher RPMs required to turn the impeller. The unique handling characteristics of jet boats can also be a problem, with some models continuing to creep forward in neutral or behaving unpredictably in reverse. They may also struggle with low-speed handling and can be trickier to handle around docks. Finally, jet boats may have limited availability of specialized service, as some marine technicians will not work on jet drives.

History of the jet boat

The history of the jet boat dates back to the mid-1950s when New Zealand engineer Sir William Hamilton developed the first modern jet boat. Hamilton's goal was to create a boat capable of navigating the fast-flowing and shallow rivers of New Zealand, where conventional propellers were inadequate. This led to the creation of the waterjet unit, which draws water from under the boat through an intake and propels it out through a nozzle at the back, providing efficient navigation through rivers, rapids, and narrow canyons.

The early jet boats faced challenges with collisions and rocks due to their wooden construction. However, the introduction of fibreglass in the late 1950s paved the way for more resilient hulls, making them more durable and suitable for the rugged conditions. By 1957, jet boats had spread throughout New Zealand, and the formation of the New Zealand Jet Boat Association in 1962 further fuelled the popularity of this innovative watercraft.

The advantages of jet boats over conventional propellers became increasingly recognised. Jet boats offered superior manoeuvrability, safety for swimmers and marine life, and the ability to operate in shallow waters. They found applications in a wide range of fields, including passenger ferry services, rescue operations, patrol boats, and military use. The biggest jet-driven vessels are found in military use and the high-speed passenger and car ferry industry, with vessels capable of performing impressive "crash stops" and high-speed manoeuvres like the Hamilton turn or "jet spin".

Over time, jet boats have continued to evolve and improve, with companies like Hamilton Jet leading the way in commercial development. Today, jet boats are used globally, with applications in adventure tourism, exploration, pleasure boating, and racing. They have become an integral part of marine transportation and recreation, known for their thrilling rides and ability to navigate challenging waterways with ease.

Frequently asked questions