- Location
- SW Michigan
I posted this over on PWC today in the Kawi section, but I thought I would post it here too. Just thought I would share this information. Take it for what it is worth.
The age question of whether one should taper bore or straight bore the exit of the reduction nozzle has been around for a while. In an attempt answer this question, and understand better the optimum nozzle configuration for a particular setup, I put together a simulation model. At this time the model does not take into account the added pressure on the pump intake due to the watercraft moving, however for an A to B comparison, the values should still be valid. I should be able to also look at pump cone angles too, however it was held constant for this investigation.
The simulation I put together includes an engine (with an estimated torque vs. rpm curve of an kawi 1100), pump, and reduction nozzle geometry. The model I created adjusts itself based on pump pressure (and pump drive torque) and engine torque until an equilibrium is met. So you if you incorporate a smaller nozzle, you can really see peak engine rpm decrease. Then looking at the outputs of the simulation, one could compare engine rpm, mass flow, exit velocity, and resultant thrust. I still need to validate the model with some actual test data, but it seems to make sense with a 1mm change in diameter making about a 100-200 rpm change depending on where you are at and whether you are using a straight vs. taper nozzle.
Jet pumps work by the means of change in momentum, so (mass flow)x(velocity out -velocity in) = Thrust force. The greater the peak thrust force, the faster you will ultimately be able to go since you will have more force to overcome drag. You are also able to look at how quickly the thrust force increases, which links to acceleration.
Conclusion
The conclusion I have come to, is the taper at the end of the nozzle decreases jet pump thrust. Holding all other things constant, having a straight bore for the final exit of the provides greater thrust. The simulation suggest the net thrust improvement will be in the range of 4-5% between the optimum 1.5deg taper bore and the optimum straight bore.
Some other points:
So hopefully some of what I said here makes sense. I could, of course, be missing some things in this simulation, but I have been working in the hydraulics field for quite some time and my expertise is in hydraulic, pneumatic, and mechanical simulation. Anyways, I thought I would share this work since it ultimately means nozzles are easier to bore, and of course you get more out of your machine.
The age question of whether one should taper bore or straight bore the exit of the reduction nozzle has been around for a while. In an attempt answer this question, and understand better the optimum nozzle configuration for a particular setup, I put together a simulation model. At this time the model does not take into account the added pressure on the pump intake due to the watercraft moving, however for an A to B comparison, the values should still be valid. I should be able to also look at pump cone angles too, however it was held constant for this investigation.
The simulation I put together includes an engine (with an estimated torque vs. rpm curve of an kawi 1100), pump, and reduction nozzle geometry. The model I created adjusts itself based on pump pressure (and pump drive torque) and engine torque until an equilibrium is met. So you if you incorporate a smaller nozzle, you can really see peak engine rpm decrease. Then looking at the outputs of the simulation, one could compare engine rpm, mass flow, exit velocity, and resultant thrust. I still need to validate the model with some actual test data, but it seems to make sense with a 1mm change in diameter making about a 100-200 rpm change depending on where you are at and whether you are using a straight vs. taper nozzle.
Jet pumps work by the means of change in momentum, so (mass flow)x(velocity out -velocity in) = Thrust force. The greater the peak thrust force, the faster you will ultimately be able to go since you will have more force to overcome drag. You are also able to look at how quickly the thrust force increases, which links to acceleration.
Conclusion
The conclusion I have come to, is the taper at the end of the nozzle decreases jet pump thrust. Holding all other things constant, having a straight bore for the final exit of the provides greater thrust. The simulation suggest the net thrust improvement will be in the range of 4-5% between the optimum 1.5deg taper bore and the optimum straight bore.
Some other points:
- Kawasaki and Yamaha high pressure die cast these parts. They need to have a certain amount of draft in the casting in order to pull the part from the mold. Coincidence or not, most of the high pressure die cast aluminum parts I have worked with over the last several years have all called out a nominal 1.5 deg of draft. I have not measure it, but I have read 1.5deg is what the end diameter is at on the Kawi nozzles.
- The optimal nozzle exit diameter will be different based on the nozzle taper angle. For example: To achieve the greatest thrust at max rpm(that the engine can spin at that pump power) - the nozzle configuration for a straight nozzle may be 82mm, however for a 1.5 degree taper nozzle, it may be 83mm. The net thrust at with the taper nozzle at 83mm will be still less than the thrust with straight at 82mm though. The simulation suggests you will always need a larger taper bore than an equivalent straight bore to reach the optimum point for each design. The nozzle size will still play a role in RPM and hookup, but the taper has shown to not be beneficial in all cases I have evaluated.
- Like I said above the nozzle works from change in momentum. However for a higher efficiency pump system, you desire to have the greatest mass flow/velocity combo going parallel to the axis of desired motion. This is why the pump has stator veins. It is an attempt to “straighten” the water and direct it so more of it is going straight out the back. Without the stator, a great of the waters velocity will be spinning and not helping provide thrust in the forward direction. So saying this, having a taper at the end of the nozzle is directing water in a direction which is not parallel to the axis of desired motion. The outer most water in the nozzle is also crashing into the inner most water actually going more parallel.
- As the aeration level increases in the pump, the taper nozzle performance becomes closer to the straight nozzle performance.
So hopefully some of what I said here makes sense. I could, of course, be missing some things in this simulation, but I have been working in the hydraulics field for quite some time and my expertise is in hydraulic, pneumatic, and mechanical simulation. Anyways, I thought I would share this work since it ultimately means nozzles are easier to bore, and of course you get more out of your machine.
