Can sombody explain this to me?
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Can sombody explain this to me?

  1. #1
    These colors don't run BLWN BUX's Avatar
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    Default Can sombody explain this to me?

    I am a bit confused about Jet pump physics and what some folks have clamed about the results of changing impellar sizes.
    My question is.. why a larger impellar would have any better holshot over a smaller one.

    I understand the fundimentals about impellar sizes and how they load an engine .
    Smaller impellars reach a higher RPM befor they absorb the engines power.
    From what I can tell, impellar size selection are usially changed to get and engine to rev into its peak hp range.

    So if a given jet boat will turn an "A" to 5400rpm (apr 500 hp), and turn a "C" to 6000 rpm (apr 500hp)..Don't they pump about same amount of water just at different RPM's?

    And if you are pumping the same amout of water through the pump then
    why would one impellar have a better holeshot over another?

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    Just Me snoc653's Avatar
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    I'll take a stab at it.

    Go back to the theory that Torque is quick and HP is fast. The bigger impeller requires more Torque (stronger push of the water) so it's quicker.

    A Jet pump has to develop pressure to build speed. Basically, the faster it can builds pressure the faster it can accelerate. The extra torque required to turn a big impeller is what drops the RPMs. It is actually the pressure differential that accelerates the boat.
    So many projects, so little time

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    X2 at taking a stab at it.
    First, it is torque and only torque that turns the pump. Hp is a measure of the speed at which the torque occurrs or work accomplished. Second it is the velocity of the water as it leaves the pump nozzle that determines thrust. Pressure is a byproduct of fluid resistance to the flow. There is some force generated by the pressure difference but it is minimal when compared to the force generated by the change in water velocity. Assuming a perfect world with no inertia of the rotating parts and no fluid losses, both examples assuming the same nozzle diameter you shared would be exactly the same. In the real world, the combination of engine and impellor that moves the greates amount of water the quickest after you hammer it will be the one that gives you the best holeshot. From here we can begin the discussion of suction pressures, the function of inducers, loaders, and grates as a function of hull speed through the water but I have not only scared myself with all of this technical stuff, I am becoming confused too. ( :

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    Senior Member slow67's Avatar
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    Also, a smaller impeller is farther away from the bowl veins, so it takes longer to build bowl pressure, also affecting holeshot.

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    Paging BP 298

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    Sleeper has a chart somewhere


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    Senior Member bp298's Avatar
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    Quote Originally Posted by Sleeper CP View Post
    Paging BP 298

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    can't take it anymore jon. somebody asks a question, then people that have no idea what they're talking about fill the thread with...

    and then, more just from reading 3 posts, i have a headache.

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    Just Me snoc653's Avatar
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    Quote Originally Posted by bp298 View Post
    can't take it anymore jon. somebody asks a question, then people that have no idea what they're talking about fill the thread with...

    and then, more just from reading 3 posts, i have a headache.
    So enlighten us oh knowledgeable one. I'm always willing to learn and we're always looking for new principles in the laws of physics. Or is this a case a of "If you can dazzle them with brilliance, baffle them with "?
    So many projects, so little time

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    Senior Member zacho!'s Avatar
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    Quote Originally Posted by bp298 View Post
    can't take it anymore jon. somebody asks a question, then people that have no idea what they're talking about fill the thread with...

    and then, more just from reading 3 posts, i have a headache.
    Im scared to comment now, BUT I will say that a proper impeller in my buddys v hull jet made an incredible difference with his 396/iron head with small NOS. From AB to B was a world of difference, gaining around 700 RPM. It's no good outta the hole BTW.

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    B1 Racing cs19's Avatar
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    Quote Originally Posted by zacho! View Post
    Im scared to comment now, BUT I will say that a proper impeller in my buddys v hull jet made an incredible difference with his 396/iron head with small NOS. From AB to B was a world of difference, gaining around 700 RPM. It's no good outta the hole BTW.
    That's very abnormal when just going from an AB to B, probably something else going on which would explain the holeshot suffering.

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    Senior Member lbhsbz's Avatar
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    Quote Originally Posted by zacho! View Post
    Im scared to comment now, BUT I will say that a proper impeller in my buddys v hull jet made an incredible difference with his 396/iron head with small NOS. From AB to B was a world of difference, gaining around 700 RPM. It's no good outta the hole BTW.
    I'm willing to bet that the first one wasn't an AB or the second one wasn't a B.....or more changes took place.
    GN7: "If you were to have ever had two brain cells you could rub together and make heat, you be dangerous"

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    Senior Member bp298's Avatar
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    ok. 2 tylenol - head feels better.

    Quote Originally Posted by BLWN BUX View Post
    I am a bit confused about Jet pump physics and what some folks have clamed about the results of changing impellar sizes.
    My question is.. why a larger impellar would have any better holshot over a smaller one.
    it won't, unless the hp absorbed by the larger impeller is greater than the hp absorbed by the smaller one. engines have hp "curves". hp is either rising or falling. it doesn't remain static from one rpm point to the next.

    Quote Originally Posted by BLWN BUX View Post
    I understand the fundimentals about impellar sizes and how they load an engine
    are you sure? quick review. berk style pumps are axial flow single stage centrifugal pumps. the impeller discharges water outward away from the center, at high velocity. velocity is converted to pressure downstream in the bowl volutes.
    when an impeller is "cut" to a smaller size, the diameter of the exit vanes is slightly reduced. this results in a slight decrease in velocity and volume at a given rpm. meaning, slightly higher rpm is required to achieve the same velocity and volume.
    keep in mind, the force that makes the boat move is the result of mass x velocity. mass as in mass flow rate, lbm/hr; velocity as in ft/sec/sec2. the trick is in balancing the two values. mass moves mass, so there has to be sufficient flow rate to move the boat. and the mass has to be at sufficient velocity to have an increase in speed. but this is greater detail than your questions ask.

    Quote Originally Posted by BLWN BUX View Post
    Smaller impellars reach a higher RPM befor they absorb the engines power.
    no. a pump that is sufficiently loaded is always aborbing hp. an impeller is sized depending on the hp curve; at what rpm does the engine make what hp?

    Quote Originally Posted by BLWN BUX View Post
    From what I can tell, impellar size selection are usially changed to get and engine to rev into its peak hp range.
    very close. and it depends on what you would consider "peak hp range". but yes, it's desirable in performance boats to size an impeller such that close to engine peak hp will be absorbed.

    Quote Originally Posted by BLWN BUX View Post
    So if a given jet boat will turn an "A" to 5400rpm (apr 500 hp), and turn a "C" to 6000 rpm (apr 500hp)..Don't they pump about same amount of water just at different RPM's?
    in any "given jet boat", it is extremely unlikely that the engine will make exactly 500hp at 5400 rpm, and also make 500hp at 6000. hp is either increasing or decreasing. in two different jet boats, one engine may make 500hp at 5400, and in a completely different boat, 500hp at 6000 (although 500hp at 6000rpm doesn't make a lot of sense).
    but yes, if both are absorbing exactly 500hp, both will move the same amount of water.

    Quote Originally Posted by BLWN BUX View Post
    And if you are pumping the same amout of water through the pump then
    why would one impellar have a better holeshot over another?
    the pump that absorbs the most hp, and makes the best use of the hp in mass and velocity, will have the best holeshot. bigger or smaller is irrelevant. if i remove my b and install my c, the boat leaves harder and is quicker and faster. because it's a hp "curve", and i am well below "peak" with the b, the c absorbs more hp.

    and like cs said, if someone changes an ab to a b and the engine increases 700rpm, there's something wrong....

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    "BEER BUDGET RACING" Bodean's Hammer's Avatar
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    Quote Originally Posted by bp298 View Post
    ok. 2 tylenol - head feels better.



    it won't, unless the hp absorbed by the larger impeller is greater than the hp absorbed by the smaller one. engines have hp "curves". hp is either rising or falling. it doesn't remain static from one rpm point to the next.


    are you sure? quick review. berk style pumps are axial flow single stage centrifugal pumps. the impeller discharges water outward away from the center, at high velocity. velocity is converted to pressure downstream in the bowl volutes.
    when an impeller is "cut" to a smaller size, the diameter of the exit vanes is slightly reduced. this results in a slight decrease in velocity and volume at a given rpm. meaning, slightly higher rpm is required to achieve the same velocity and volume.
    keep in mind, the force that makes the boat move is the result of mass x velocity. mass as in mass flow rate, lbm/hr; velocity as in ft/sec/sec2. the trick is in balancing the two values. mass moves mass, so there has to be sufficient flow rate to move the boat. and the mass has to be at sufficient velocity to have an increase in speed. but this is greater detail than your questions ask.


    no. a pump that is sufficiently loaded is always aborbing hp. an impeller is sized depending on the hp curve; at what rpm does the engine make what hp?



    very close. and it depends on what you would consider "peak hp range". but yes, it's desirable in performance boats to size an impeller such that close to engine peak hp will be absorbed.



    in any "given jet boat", it is extremely unlikely that the engine will make exactly 500hp at 5400 rpm, and also make 500hp at 6000. hp is either increasing or decreasing. in two different jet boats, one engine may make 500hp at 5400, and in a completely different boat, 500hp at 6000 (although 500hp at 6000rpm doesn't make a lot of sense).
    but yes, if both are absorbing exactly 500hp, both will move the same amount of water.



    the pump that absorbs the most hp, and makes the best use of the hp in mass and velocity, will have the best holeshot. bigger or smaller is irrelevant. if i remove my b and install my c, the boat leaves harder and is quicker and faster. because it's a hp "curve", and i am well below "peak" with the b, the c absorbs more hp.

    and like cs said, if someone changes an ab to a b and the engine increases 700rpm, there's something wrong....
    Damn! now thats an education. Thanks for the question and the answer.
    Last edited by Bodean's Hammer; 06-10-2011 at 10:27 AM.

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    Senior Member zacho!'s Avatar
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    Quote Originally Posted by cs19 View Post
    That's very abnormal when just going from an AB to B, probably something else going on which would explain the holeshot suffering.
    I stand corrected. After talking with him a minute ago,he said he picked up 300 rpms, 5800 - 6100. What a difference that made on his boat, dunno whats up with cavatating outta the hole.
    Last edited by zacho!; 06-10-2011 at 08:56 AM.

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