Marine propulsion – Jet drive – Having means to increase fluid drive means efficiency
Reexamination Certificate
1998-07-17
2001-03-13
Swinehart, Ed (Department: 3617)
Marine propulsion
Jet drive
Having means to increase fluid drive means efficiency
Reexamination Certificate
active
06200176
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to marine jet drive systems and, more particularly to marine jet drive systems which include or are retrofitted with a preloader for reducing cavitation. The invention also relates to such a preloader which may be retrofitted to a standard marine jet drive system. The invention further relates to a method for reducing cavitation during operation of a marine jet drive system.
BACKGROUND OF THE INVENTION
Marine jet drive systems for watercraft generally comprise a water conduit disposed toward the rear of the watercraft which conduit comprises an intake portion, a pump portion and a discharge portion. A pump impeller within the pump portion is generally driven by a shaft which extends from a prime mover through the intake portion to the pump portion. Water from under the watercraft enters the intake portion and passes to the pump portion. The water pump impeller in the pump portion increases the energy of the incoming water which then flows from the pump into a discharge chamber in the discharge portion. Water exits from the discharge portion as an organized jet providing propulsion for the watercraft.
In conventional jet drive systems, atmospheric pressure is the only force pushing water into the intake portion when the craft is at rest and the eye, or front face of the pump impeller, is above the water level or only partially submerged, thus the net inlet head at the pump inlet is less than atmospheric. Accordingly, at start-up, the pump impeller must first generate a sufficient suction head to draw water from the intake into the pump. This results in a delay or slippage until full water flow through the pump is achieved. In addition, because of initial uneven water flow through the pump, cavitation, which is caused by the formation and collapse of partial vacuums in the flowing water, occurs around the impeller with the consequences of a reduction in water flow thrust as well as damage to the impeller surface.
Prior systems have sought to alleviate these problems by providing marine jet pumps with multiple stages in which a first stage impeller is driven at a slower speed or in counter-rotation to a second and/or subsequent stage impellers. In general, the first stage impeller is larger than the subsequent stage impellers so as to build up water flow through the pump in successive stages of low pressure rise. In addition, such systems house all of the impellers within the pump portion in a manner whereby the first stage impellers are enclosed at their outer perimeter such that water flow is restricted to axial flow through the impeller.
Examples of such prior systems include U.S. Pat. No. 3,328,961, which employs a pump assembly having a larger, slower rotating first stage impeller and a smaller, faster rotating second stage impeller. The impellers are mounted on and driven by separately rotating concentric shafts and are peripherally closely adjacent to the internal surface of the pump housing.
U.S. Pat. No. 3,405,526 describes a multiple stage hydraulic jet propulsion apparatus which combines a slower rotating first stage axial flow impeller and a faster rotating second stage mixed flow impeller. As with the '961 patent, the impellers are mounted on separately rotating concentric shafts within the pump housing so that the peripheral edges of the impeller vanes are closely adjacent to the walls of the housing.
In U.S. Pat. No. 3,531,214 a multistage jet pump is disclosed in which a plurality of successively smaller impellers are radially driven from a common shaft by means of gears on the shaft which mesh with gear teeth on peripheral rings of the impellers. The effect is that the impellers are fully enclosed peripherally by their respective ring gears and, due to the differing gear ratios, are driven at successively higher speeds.
U.S. Pat. No. 5,634,831 discloses a water jet propulsion unit which employs two counter-rotating impellers within the pump. Mounted on concentric, counter-rotating shafts, the impellers are calibrated so that any radial flow created by the upstream impeller is converted into axial flow by the downstream impeller. However, like the other prior art, the impellers of this unit are within the pump housing with their peripheral edges closely adjacent to the housing wall.
Thus, the prior art seeks to solve the problems of delay and cavitation in marine jet drive systems by complex mechanisms. Although these prior art systems may be somewhat effective at improving the efficiency and performance of jet drives once the watercraft powered by them are in motion, the first stage impellers of these systems remain at a position where the eye of the impeller is above the water level or, at best, only partially submerged and the net inlet head is less than atmospheric. Thus, the first stage impeller of the prior art multistage pumps suffers from the same problems that its presence seeks to cure in the subsequent stages.
SUMMARY OF THE INVENTION
The present invention provides a preloader device for marine jet drive pumps which improves start-up and acceleration performance and reduces cavitation using a simple assembly which may be readily incorporated into the production of jet drives or retrofitted to existing units whether of the axial or mixed flow type. The inventor has found that it is not necessary to provide pumps with multiple stage impellers of decreasing size rotating at increasing speeds, or counter-rotating impellers. Rather, improved acceleration and reduced cavitation are obtained by the simple expedient of mounting a preloader device, such as a smaller propeller, impeller or screw type device, on the pump impeller drive shaft ahead of the pump impeller and, preferably, within the intake portion of the marine jet drive, without requiring modification of the pump assembly. This puts the preloader device within the intake portion and in axial alignment directly in front of the eye of the pump impeller. The preloader device increases the net positive suction head available (NPSHA) to the pump impeller in any type of marine jet drive unit thereby raising water within the intake portion and, consequently, pushes water into the pump across the eye of the pump impeller thereby improving initial start-up and acceleration while simultaneously reducing cavitation in the pump impeller under acceleration of the marine jet drive unit. The size and pitch of the preloader device, together with the space between the preloader device and the pump impeller may vary according to the application and needs of the particular marine jet drive unit. Unlike the pump impeller in the marine jet drive unit, the preloader device, since it is of a smaller diameter, is not enclosed at its outer perimeter, but is open; this permits water to not only flow directly into the preloader device, but also around it, thereby providing a more even loading or priming of the pump impeller which improves the performance and acceleration of the drive as well as reducing cavitation in the pump impeller.
The present invention, in one aspect thereof, provides an improvement for a marine jet drive comprising a housing having a forward intake portion, a rearward discharge portion and a pump portion disposed therebetween. The pump portion comprises a housing having an impeller rotationally driven within the housing by a shaft extending through the intake portion. The improvement comprises a preloader device mounted on the shaft forward of the impeller and within the intake portion of the jet drive housing, whereby the preloader is rotationally driven concurrently with and at the same speed as the impeller.
The present invention further provides a device for preloading a marine jet drive pump to reduce cavitation in a jet drive pump impeller; the device including a water propeller mounted on the shaft of the impeller and sized to be rotatable within an intake housing of a marine jet drive unit ahead of and in axial alignment with the eye of the jet drive pump impeller. In operation, the water propeller causes water to be drawn from the intake portio
Sherman & Shalloway
Swinehart Ed
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