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Thread: Jet pumps and volumetric flow data

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    Member Yukoner's Avatar
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    Default Jet pumps and volumetric flow data

    Was pondering engine options, as i find myself doing a lot these days, and was wondering why none of the manufacturers, Outboard Jet Inc included, publish or offer volumetric flow data (CFM or L/S) vs. Hp curves for their various pumps or impeller sizes?
    Say for instance if OBJ Inc had the data for their 7 3/8" V-4 impeller, and the Hp needed to obtain that CFM, then we, as customers, could then compare the RPM/Hp curve for our present motor to a different motor, and get a pretty good picture of performance characterstics as they relate to CFM of water out the jet.
    You could then look at your 115 ETEC say, and see that you are putting out 100 CFM at 100 HP at 5000 RPM, as compared to a 115 Yamaha 4s moving 80 CFM at 100 HP at 5200 RPM. Just random numbers, then you could also use the RPM numbers to determine fuel consumption, as some manufacturers offer RPM/fuel burn curves.
    Just a thought..
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    No offense Yukoner, but you must be an engineer . I have several friends who are engineers and all they seem to want to talk about is why this, why that and what if .

    You offer a good question though. Two things come to my mind right quick: they don't care to do the math and OBJ makes **** near all of the jet pump units out there, as I understand it.

    Seriously, I think OBJ knows exactly what the numbers are, but for whatever reason choose not to release them to the public, or perhaps the numbers are there for one to see, but are difficult to find

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    Member Yukoner's Avatar
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    Man, is it that obvious? LOL!
    Actually, im a plumber primarily, but i also have a diploma in Mechanical Engineering Technology.
    Anyway, CFM is what moves the boat, and if you know that your present boat with an X HP engine and Y impeller goes Z MPH at ---RPM, it would be easy to figure out what HP and impeller combo you would need to get the performance you want. Much cheaper than guessing or relying on dealer recomendations on engine performance.
    Gawd, there i go again....
    But yeah, you are right, they must have that data somewhere, it would be easy to get it.
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    Quote Originally Posted by Yukoner View Post
    Man, is it that obvious? LOL! Anyway, CFM is what moves the boat, and if you know that your present boat with an X HP engine and Y impeller goes Z MPH at ---RPM, it would be easy to figure out what HP and impeller combo you would need to get the performance you want. Much cheaper than guessing or relying on dealer recomendations on engine performance.
    Gawd, there i go again....
    But yeah, you are right, they must have that data somewhere, it would be easy to get it.
    Well... its not entirely true that "CFM moves the boat". Or at least not the whole picture. Discharge velocity plays a huge role as well. Generally speaking, slower RPMs moving mass quantities of water through the pump will lift a bigger load to step, but higher velocity, higher discharge pressures will allow higher attainable top speed, and more fuel efficiency. Theres a fine line of tradeoffs in performance expectations. All of the inboard jet manufacturers that I'm aware of have their pump curves for various impellor configurations available to do exactly what you're talking about in optimizing engine performance. I've never seen such a chart for the outboards but from their website they mention designing the specific models to allow full rpm development on the powerheads they are designed for so it doesnt look like they've spent a lot of time tweaking the "in between" numbers, but again, coarser impellors in a jet designed for a 100 hp powerhead will only serve to lower the velocity and discharge pressure.

    You ought to get in touch with Alvin Millard at Artistic Aluminum in Wasilla, he rebuilds impellors and has actually re-pitched a few inboard impellors for me. He may have some curve charts for some of the outboards.

    (just dont tell him you're a "Engineer"!)

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    Good info. Sort of like the Amps vs Volts scenario.
    I guess im assuming same size impeller and nozzle. More CFM in the same unit will give you more velocity.
    Does the Scott power trim nozzle decrease the discharge area and up your exit velocity?
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    Quote Originally Posted by Yukoner View Post
    Good info. Sort of like the Amps vs Volts scenario.
    I guess im assuming same size impeller and nozzle. More CFM in the same unit will give you more velocity.
    Does the Scott power trim nozzle decrease the discharge area and up your exit velocity?
    And higher rpms generally create higher blade speed, and higher velocity, up until the point where turbulence/cavitation occurs as well.

    You've pretty much got it right about the Scott adjustable nozzle. it is a simple set up with interchangeable nozzle inserts at the end of the tailpipe assembly. These are changed by removing 3 large flathead screws and pulling the insert out, it takes about 30 seconds to switch them.

    By selecting a smaller nozzle you increase the pump discharge pressure, which increases the velocity calculation where the discharge meets up with the river water to build your thrust. This higher pressure also serves to limit cavitation and the smaller nozzle loads the engine to a greater degree, limiting your potential rpms.

    Conversely, the larger sized inserts allow a lower pump pressure, a greater flow through the pump (cfm) higher potential to cavitate, lower speed potential. To a certain point, the higher volume through the pump with the larger diameter nozzle insert will lift a greater load to step more readily, but there is a point of diminishing returns where you simply cavitate rather than lift the loads.

    The impellors offer quite a slew of options and tuneability (and confusion on my part!) With the pump having 2 impellors, and several pitch options (I think 17 through 24 with custom ones available for the 1500 hp crowd!) you can install a 17 pitch with an 18 pitch, or more likely a 17 pitch with an 17 progressive pitch and so on. Of course you start from the pump curve charts (or Scotts notes on which combinations worked best with which motor hp/torque charts) and generally hit pretty close to the perfect combination for your setup right off the bat. (in my experience and with my buddies 2 boats as well)

    My best torque occurs in the 3800 to 5400 rpm range and was convinced Scott didnt understand "my motor" so he humored and and we tried 19 progressive pitched impellors but found they needed such a large diameter nozzle insert in order to free up the motor to spin to its potential, that we simply cavitated virtually instantly what with the lower pump pressure situation I described above. I think we were using a 114cm insert with them. When we backed down to the 19 standard pitched impellors as the factory had recommended, the motor had the torque to spin them with a 108cm insert and we eliminated the cavitation, still had virtually the same load lifting capabilities as with the progressive impellors, and gained somthing like 2mph at the majority of the power settings when compared to similar rpms logged with the progressives.

    I can now run the motor up to maybe 5300 rpms if I hold it there long enough and have smooth enough water, but the preformance band is situated right where the motor best likes it with pretty instantaneous response in the 3500 to 4800 range where I mostly operate.

    I dont know if that muddies the water more, or clears it up some. But it was a kind of fun science experiment.

    So, to get back to your original questions about outboard jets, it seems to me that with a guy like Alvin who can custom pitch impellors, there would be an infinate number of possibilities, but have a feeling that (like Scott) they've done their research on which pitch works best with which powerhead. I imagine with a ton of experimenting and fine tuning you could improve the performance somewhat, but I bet it's pretty minimal compared to the research they've already accomplished on the powerheads they are building for. And again, I think you're right in that when you compare apples to apples, the more aggressive impellor you can spin in the same size nozzle, up to the RPM setting you want, the more flow you'll acheive, and the better the pump will perform.

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    In your conversations with Scott, did they ever get into measuring net positive suction head on the pumps with different impellers and inserts?
    Do the pumps have ports tapped into them to allow one to measure the inlet and discharge pressures?
    That would be the ultimate tool for the tuner; you could mess around with impellers and inserts, all the while measuring your suction head with respect to vapour pressure and dial it in to just above the ragged line where you get cavitaion occuring. Heck, you could tap the castings yourself for a 1/8" NPT and rig up some test gauges.
    Classic example of too low net positive suction head getting too low is bent intake grates. Boom, cavitation.
    Last summer, i was in Wasilla looking at a Wooldridge with a 200 Merc on it. We took it out on one of the lakes around town for a spin, and man, would it cavitate! As soon as you gave it serious throttle, it would just cavitate like crazy. You had to either slowly apply throttle, or put up with it.
    I had a look at the intake, and it was clear, the grate was fine. It didn't have the bolt on side fins, but i ahve run OBJ's without them and never had that problem on a straight start up. Didn't buy the boat, but i really wanted to figure out what was going on.
    On another topic, could guy run a SportJet with a Scott pump like the 751?
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    I am taking my 1984 MKIII down to Tracey this week. At least then I will have some resale value left on it....

    Rob - you said that you had success in reshaping the back of the transom - how did you pull the material tight and keep the shape without sagging? I might have Tracey take an inch or two out of it - not sure yet....

    At some point I might want to run a jet on it...but am finding that rivers are pretty bad on the rubber duckie - I haven't hurt it - but see the potential.


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    Quote Originally Posted by Yukoner View Post
    In your conversations with Scott, did they ever get into measuring net positive suction head on the pumps with different impellers and inserts?
    Do the pumps have ports tapped into them to allow one to measure the inlet and discharge pressures?
    That would be the ultimate tool for the tuner; you could mess around with impellers and inserts, all the while measuring your suction head with respect to vapour pressure and dial it in to just above the ragged line where you get cavitaion occuring. Heck, you could tap the castings yourself for a 1/8" NPT and rig up some test gauges.
    Classic example of too low net positive suction head getting too low is bent intake grates. Boom, cavitation.
    Last summer, i was in Wasilla looking at a Wooldridge with a 200 Merc on it. We took it out on one of the lakes around town for a spin, and man, would it cavitate! As soon as you gave it serious throttle, it would just cavitate like crazy. You had to either slowly apply throttle, or put up with it.
    I had a look at the intake, and it was clear, the grate was fine. It didn't have the bolt on side fins, but i ahve run OBJ's without them and never had that problem on a straight start up. Didn't buy the boat, but i really wanted to figure out what was going on.
    On another topic, could guy run a SportJet with a Scott pump like the 751?
    Thats a truly classic statement and I want you to know I plan to use it in the future!!!!

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    Default Intake pressure is yet another huge variable...

    You're right, they have done extensive testing on the pumps suctions over at scott, as well as amrican turbine, hamilton, and others I'm sure. There has also been extensive interesting race testing of this principle and how it applied to jetboating which you can dig up at www.jetboatracing.com and on the outlaw-eagle website as well.

    Interestingly enough, the hull design has the greatest ability to influence this component once you get past the basic pump housing design. How clean the water feeds into the pump inlet has everything to do with how much you can fit through the pump, and how fast you can shove it through.

    The concept of forming a "spoon" shaped intake to sort of "venturi" the flow into and through the pump for greater efficiency (in racing they mean top speed with this respect, in my world I equate that to engine life and fuel ecomony for moving the load) Additionally they've found that once you make your intake too large, you slow the boat down from the increased drag (my theory for transposed race terms equates this to higher fuel consumption, lesser engine life) caused by a too large of a positive intake pressure area. So again, there is a fine point to straddle as you get closer to optimum

    I dont think there are any pressure taps built into the housings on any of my jets, scott, hamilton, or mercury. Of course, the more taps you put in there, the more you tend to spoil the laminar flow characteristics through your pump housing so it makes sense that when they bring the product to market they'd give the end user the cleanest, smoothest, and most efficient flowpath they possibly could.

    We asked Scott about sportjet impellors, they dont plan to build any as they believe the 'standard" and the "hi-skew" impellors provided by mercury more than meets the needs for the typical user.

    Have you done any research on your Outlaw-Eagle outfit there in Red Deer? They wrote the book on the mercury sj and riverboating... Mercury consigned them to build what eventually became the 17 foot sport. Their boats consisently win the worlds championship whitewater marathon championships and they;ve studied this stuff more than most. Next to them I believe Riddle Marine in Lewiston is another authoritive source.

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    Default Hull design

    Oh man, don't go there!
    Another secret of mine to avoid cavitation is to make sure i don't run my boat aground
    Seriously though, i am sujper impressed with Scott pumps design theorys. I don't have an IB, but if i did, it would be a Scott.
    A 751 with a small engine like a Subaru marine or an Ecotec would be the way i wan't to go.
    Cheers
    Paul
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