I am sorry but, you are completely wrong. Mixed flow pumps are known as pressure pumps and axial flow are volume pumps. Also varying the discharge diameter on a mixed flow jet has little effect as compared to varying discharge diameter on an axial flow pump.
about effiency.
All impellers dont like to be efficient at all speeds. Here is a guide.
http://mywebpages.comcast.net/terijet/pumptype.gif
Maybe this is why a mixed flow pump is chosen over an axial or centrifugal.
Maybe there is another reason: Axial load.
A lot of good thought here. Haven't had much time to read lately...trying to catch up.
"Also it would interesting to know where the efficiency sweet spot for the pumps we use is."
Yep. A performance curve or two may answer a lot of these questions. How far off-design efficiency are we? If a typical centrifugal pump is 84-92%, how does that compare to mixed flow operating above bep rpm with a cut impeller and less than ideal npsh? Unfortunately, jet boat pump engineers don't seem to have these...
The other point is pump efficiency is typically quoted without intake loss and other appendages such as a nozzle and PD...right?
"I read somewhere that, on a 60mph jet the discharge speed at the nozzle is more like 75-80"
I have measured discharge speeds of 90 mph+ for your typical 65 mph lake jet. If I can just find them...
jer
I have measured discharge speeds of 90 mph+ for your typical 65 mph lake jet. If I can just find them...
(wondered where you been Jer)
Good brick wall 330hp goin to shit.
That puts the intake velocity at 28mph (relative to the impeller). Thats just about right for a 3.25" nozzle and a 5" impeller eye (ratio of 1:2.3. At this speed you are accellerating some 4000lbs or so of water a minute from 0 to 37mph or so. Bricks.
Reducing the nozzle reduces the amount of water you got to speed up, thats it. Trading M for V dont help a bit, Same force results. May even make the pump less efficient, depending on the RPM it is done at.
On the other hand reducing the inlet to the impeller seems like it would increase intake speed and reduce the speed the water would have to be accelerated to to get to the eye of the pump. Sure would mess up the hole shot though. Lack of NPSHr. Would have to be variable some way. I got an idea.
mikeT
Originally posted by miketsouth
I dont think so. I think it is the bricks.
Jets aint slow, they explode out of the hole
Jets get very inefficient at higher speeds
It is my feeling that the latter can be overcome. I have been working on it for a while. aint there yet though, but getting closer.
mikeT
Mike from all the testing I have seen and done It seems the highest pressure is in the vane not in the nozzle.
Maybe you can answer this question, I have asked several qualified and yet to get this answer.
I think 95% or better of the forward motion or propulsion comes from the impeller's forward pull.
As to Jets getting inefficient at higher speeds - Its the Get Loose factor from decreased wetted surface or Tail Walk. No Prop will pull with a jet while in the load cycle but as the jet must stay set to load, the prop can continue to climb out of the water. So its E.T. vs MPH story.
"Mike from all the testing I have seen and done It seems the highest pressure is in the vane not in the nozzle."
Of course the pressure in the vanes (and bowl) will be higher than in the nozzle. Don't need testing to figure that. I don't think Mike said nozzle pressure was higher...did he?
"I think 95% or better of the forward motion or propulsion comes from the impeller's forward pull."
What does impeller forward pull mean?
"As to Jets getting inefficient at higher speeds - Its the Get Loose factor from decreased wetted surface or Tail Walk."
Hull drag is a huge factor, but if we could eliminate hull drag from comparison, I think a pump's still less efficient than a prop. I'm thinking a well set v-drive has similar hull drag to a well set jet, yet the v-drive will have 10+ or more on the top end. Depending on speed, the higher the speed the more pump losses. But there's also intake loading you mention. At some point as the hull gets looser, a jet intake won't stay loaded. Is this the brick wall, or internal losses or some other mechanism?
jre
"Mike from all the testing I have seen and done It seems the highest pressure is in the vane not in the nozzle."
Yes the highest pressure should be at the vanes. The pressure available for thrust is at the nozzle. Just before the vanes i suppose it is all velocity. Dont know what the pressure would be just after the impeller.
"I think 95% or better of the forward motion or propulsion comes from the impeller's forward pull"
There is a good argument against this. If the impeller is pulling the boat then it is pulling it down. There is an elbow there.
There is another good argument against this. The impeller would pull the same way regardless of nozzle size or direction. Take the nozzle off and what happens. Throttle up with the nozzle turned and what happens.
There is another good argument against this. If the impeller was pulling, the inlet would be in a vacuum. It is under pressure most of the time.
The thought that an impeller or propeller pulls water doesnt make sense. The propeller pushes the boat and the water. The impeller pushes the water, and then the water pushes the boat. Even when you seem to suck water up a straw, you aint pullin it. Atmospheric pressure is pushing it.
It is hard to pull water.
I dont think the impeller pulls.
At some point as the hull gets looser, a jet intake won't stay loaded. Is this the brick wall, or internal losses or some other mechanism?
i suppose that:
The bricks come in much before boat speed causes the pump to become unloaded. Look at pics of a regular, well trimmed jetboat running thru the water at 65mph...aint much in the water but the pump. Still 330hp and 65mph. Put a 330hp OB or I/O on that same boat and you could drag two anchors at 65mph.
I dont think speed makes the pump lose efficiency. It makes the pump/boat SYSTEM loose efficiency. The pump dont know anything but pressures presented it. No pump curve i have seen even mentions suction pressure except that it has to be above a minimum. From then on it is just differential pressure that dictates the flow rates.
That dont mean that the way the water is entering the pump dont affect it. It affects it very much. Non linear flow, turbulence messes up the axial loading and can induce cavitation in just parts of the the impeller, reducing efficiency and causing all kinds of problems.
The faster you go in a jetboat, the harder it is to get the water into the pump just right, or at all. I like the idea of the bubble as a director device for the intake at speed. I have never seen one but understand the principles.
mikeT
Sounds like the bottom line is that the biggest theft of efficiency and a resulting lower hp to speed ratio is either the drag caused from water entering the pump or from the turbulence and the resulting cavitation. I know that in applications where precision control of the flow is needed we use flow straighteners inline up stream from the pump to keep the turbulence to a minimum. If the drag is the main culprit then other means of loading the pump may improve the performance. The flow straighteners may not be an option as they require a certain number of pipe diameters upstream of the pump to be effective. Maybe changes in the intake grate designs could help to perform this task. Since the wetted surface is limited at speed changes in how the pump is loaded would seem to be limited as well.
The real bottom line...
http://members.cox.net/lvjetboy/FasterIsBetter.jpg
jer
This is some good reading guy`s!!! Too bad everything is against everything...lol
Mike you are so right about the "the intake being under most of the time"
That tell`s us that it may be able to suck but sucking is not the key factor to moving the boat. If you tied the boat to a dock and hit the gas it will suck water but I think that if you hooked up a gage to the bowl you would see a huge drop in bowl pressure because the intake would have to suck the water and not being force fed.
From what I have found on my test`s, it look`s like the key is to load the pump intake (intake pressure) with out overloading it causing to much tail lift. We all know what happens when we get to much tail lift (No more intake pressure) there for no more forward push from the pump`s outlet.
I`m designing a new bottom for my Hydro right now with the idea of keeping my intake pressure at about 14lbs. I`m resizing the bottom part of my pod that my intake bolts on to lessen the amount of tail lift to help keep the boat wetter. One problem I have found is that when I put to much shoe in I get a wild ride on shut down so I`m trying to keep the shoe to a minimum. I think that I can narrow the pod to about 2" wider than the intake( 1" on each side of intake), install a bubble in front of the intake to aide in loading, I might be able to keep my intake pressure up at higher speeds and have a safer shutdown because the bubble will do the loading instead of the shoe and the narrowing of the pod will lessen the lift.
What do you all think?
Hope I didn't wonder off to far?
I`m just always looking for new ideas!
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