Originally Posted by YachtForums
Let me begin with some jet-pump fundamentals...
To better understand how the intake gullet works, we need to take a closer look at the jet-pump and the impeller. The impeller itself will only scatter water, and is highly inefficient. But, when placed within a "shrouded" environment, it becomes a ducted propeller. This shrouded configuration produces greater efficiency than its open, non-shrouded counterpart. The reason is simple. The duct controls water and forces it backwards as opposed to a propeller which allows water to slip outwards.
Impellers (and jet-pumps) work on the principal of positive and negative pressure, or a push/pull concept. As a blade rotates, it pushes water back (and outwards due to centrifugal and accelerated force). At the same time, water must rush in to fill the space left behind the blade. This results in a pressure differential between the two sides of the blade: a positive pressure, or pushing effect on the blade face and a negative pressure, or pulling effect, on the backside of the blade. This action occurs on all the blades around the full circle of rotation.
Thrust is created by water being drawn into the impeller and accelerated out the back. However, due to the spiraling effect (vortex) of water leaving the trailing edge of the blade it must pass through stators (straightening vanes) to "true" its trajectory. Stators also increase velocity by "catapulting" water, similar to the way a "kick" works on the trailing edge of a propeller blade. To further enhance velocity, water passes through the venturi before finally exiting the pump as thrust. As we discussed earlier, the venturi works on the principle that a restriction or reduction in line size will cause water to accelerate if the same volume is to be realized at the other end of the restriction. This is where you get the "jet" in pumps. Finally, a steering nozzle is used to vector or deflect thrust for yaw direction.
Impeller design and efficiency is strongly linked to the other components that make up the jet-pump, i.e., the intake gullet, its volumetric area, the laminar transition of the intake housing, stator blade area (including angle of trajectory), venturi rate of compression, venturi "bowl" area, exiting orifice dimension, mass and weight of the hull, and pump placement or depth within the same.
The intake gullet is the recessed area within the hull leading up to the entrance of the jet pump. This area plays a vital role in jet pump efficiency. There are a multitude of factors that determine its length, size, shape and depth. Much of this has to do with the operational parameters the vessel was designed for, such as hull speed and displacement.
For instance, a larger vessel with greater displacement may choose an intake gullet design with a more gradual rake leading up to the jet pump entrance. This maximizes the amount of water available for acceleration. In this scenario, intake gullet vacuum is not as critical because the weight of the hull (and the depth of the pump) will keep the intake cavity primed. In contrast, a light, high speed hull that rides closer to the water's surface, may use an intake gullet with a more aggressive rake and a reduced intake gullet area. This decrease in cavity size, increases the vacuum (or negative pressure zone) at the intake, which helps reduce ventilation brought on by a higher speed planing hulls that operate near the water's surface.
Several variables effect water flow to the pump, including the speed and density of the impacting water due to the forward motion of the hull when underway, and the amount of negative pressure created by the intake housing under operation, which is directly relevant impeller pitch, rpm, and blade area.
Ultimately, the best intake gullet design would be variable in size. In other words... larger for acceleration and smaller for high speed operation, to maximize intake vacuum when aeration is present.
On the subject of intake gullets, which are only one aspect of jet-pump integration and configuration, I should expand on the venturi...
Of all the components that make up a jet pump, the venturi is by far the most critical component in dimension, shape and size. It is the final stage of acceleration that water will receive prior to expulsion. The venturi, for those of you not familiar, is the shroud located just after the stator blades (directing vanes) and the part of the jet pump that the steering nozzle or thrust deflectors are most commonly connected to.
The exiting size of the venturi's orifice is generally half the size of the dimensional area of the intake gullet footprint, or a 2-to-1 reduction. Quite simply, the venturi is a reducer or compressor, and in the case of water, which can not be compressed, it is an accelerator. As I've said, venturi's function on the principal that a reduction in size of a flow path will cause water to accelerate if the same volume is to be realized at the other end of the reduction. The venturi is one of the most important links or stages in jet pump design. Without it, the jet pump as we know it... would be rendered benign.
By increasing or decreasing the size of the venturi's exiting orifice, where the water is expelled, you can effectively control backpressure, velocity and the intensity of the exiting thrust. Each are inter-connected and controlled via the inner bowl camber (rate of compression), entrance and orifice dimension, and time of reduction (travel).
Increasing the venturi's expulsion size will decrease backpressure, and allow water to be processed more rapidly, thus moving the hull (mass) forward at a faster rate due to more available thrust, but sacrifices top speed because of reduced compression. Decreasing the venturi's expulsion size will create more backpressure, which results in less water being processed, but increases the velocity at which it exits. This results in higher speeds, but does not give the mass of water necessary for greater acceleration. Venturi designs are usually a compromise to give maximum acceleration and top speed.
The real reason that an adjustable venturi is necessary and holds so much value is that because pumps do not run fully loaded at higher speeds. They ventilate, thus inducing air into the equation. Because the amount of water available for compression at higher speeds is reduced, due to the introduction of air, there is less water density available for thrust. By reducing orifice size, density is enhanced, thus speed is increased. This technology is really not new. The original inception, whose origins date back to the Messerschmitt 262 and the "movable onion", were the forerunners to afterburning turbo-jets. Because flow-is-flow, whether it be water or air, some theories cross-platform. The only difference is air can be compressed and water can not. Oh yeah, one is quite a bit denser than the other too.
In early 1984, our research team began conceptualizing and theorizing the potential of an adjustable venturi and later developed the V.G.V. (variable geometry venturi) This unit operated on the principles mentioned above but utilized hydraulics to control orifice diameter, which was necessary given the huge amounts of thrust created on the research vehicles we developed. In 1987, a very unique material was made available, current regulated (electrical stimuli), that lined the inner walls of a venturi (or bowl) and controlled exact camber and orifice dimension. This material has future applications i.e., artificial limbs, robotics, etc. Unfortunately, it is under regulation for now and there is no access to it.
Controlling rate of compression realized significant performance and efficiency gains. This is one of the most important aspects of venturi design. By shortening the length of the venturi and increasing the rate of compression (along with a larger exiting orifice) it would generate increases in mass velocity. And sub-sequently, increasing the length of the venturi and reducing the rate of compression (along with a smaller orifice) would yield increases in speed, primarily due to reduced drag and increasing the density of flow available. Inner wall flex and fluctuation is critical as well. The reason that I mention flex is because it is conceivable to utilize a material with built in flex to accomplish some desirable characteristics.
Other aspects of venturi design are critical as well. Orifice length is an example. By elongating the orifice, you can effectively "true" the trajectory of water, similar to the difference between the accuracy of a pistol and a rifle. By "truing", I'm referring to the explosion that water experiences during rapid collision within the bowl. This results in a very diffused spray pattern exiting the venturi. Elongating the exit will give water a chance to "compose" itself and thus travel in a true path resulting in tighter expulsed trajectory.
One of our first VGV’s was an adjustable venturi that utilized inner bowl “feathers” actuated by an aperture that closes concentrically. While it was mechanically a very cool-looking contraption, much like the afterburning tail-feathers on a fighter jet, it was hydrodynamically incorrect. The reason is simple, while it reduced orifice size it also increased the rate of compression while failing to control trajectory. Properly configured and controlled, the device had great merit.
Original design's of Bernoulli's work (remember your physics) and the principals behind a venturi are still applicable today. If you are familiar with his work and you happen to be familiar with the development of the SR-71 BlackBird (Lockheed), you have witnessed the future of venturi optimization. God Bless Kelly Johnson, he was so far ahead of his time.
A properly designed venturi can yield significant acceleration gains and top speed gains. A really good design will become exponentially more efficient with speed. In other words, the faster you go.. the more efficient it becomes! Venturis work on thrust and pressure. Wherever you have thrust you have the potential to create vacuum. Wherever you have pressure, you have energy. And in the case of venturis, that pressure can control a multitude of variables… and this entire process can be executed with NO MOVING PARTS!
Hull lift can effect the performance (efficiency) of a jet pump. Large yachts generally don't have to contend with this, as they run with so much wetted surface and have so much weight, they are not prone to exiting the water at higher speeds. Smaller and lighter craft are much more susceptable to the problem of ventilation. This is when a boat achieves sufficient speed that the hull exits the water in rough, varying water conditions. At this point, the intake gullet is prone to ventilation, or "breaking" vacuum. Without vacuum, the pump can not draw water into the intake gullet and thus ventilation occurs. The result is a loss of efficiency and a loss of speed.
One of the benefits of jet pumps and the vacuum they create is "artificial weight". Because the intake vacuum (remember; negative pressure) is pulling down on the hull, it not only helps to keep the hull (PWC) planted in the water, it can provide a better ride in varying conditions because this simulates increased weight.