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Topic Review (Newest First)

  • 06-12-2011, 06:14 PM
    DEL51
    Bob,I have never heard of an axial flow cetrifugal pump. I have heard of axial, water flows along the impeller axis. Centrifugal, where it entes the center and exits the outer diameter. But not both. Is "axial flow centrifugal" also called a Mixed flow? Thanks,DEL51.
  • 06-12-2011, 01:26 PM
    Brendellajet
    So what you are saying is that a boat that can't reach its max power will be faster if the impeller was trimmed to allow an increase in RPMS?

    What if the HP curve is very flat? For example my motor makes 800 @6000, And 825 @6800, is it worth the expense to go after the extra hp? Currently turning 5800 (775hp).
  • 06-11-2011, 02:26 PM
    Sleeper CP
    Thanks Bob.

    A wealth of knowledge as usual. And yes by "heavy" I mean like yours or mine at 780 lbs out of the mold.

    S CP
  • 06-11-2011, 10:42 AM
    bp298
    Quote Originally Posted by Sleeper CP View Post
    Thanks Bob.

    So even in a heavy boat an "C" impeller could holeshot better than a "B" if the "C" is allowed to spin to the peak HP rpm of the engine ?

    S CP
    jon, define "heavy". are we talking about your boat/engine combo? mine is similar to yours; hp curve is the same, bare hull is 730. and if i change from the B to the C, holeshot is better, boat is quicker and faster. because more hp is being absorbed by the pump to produce greater thrust.

    the only caveat is the same one that applies to any jet. the instant high hp is applied from a dead stop, the suction goes negative because there is significant volume removed and no speed to replace it. the key is in how that's resolved...
  • 06-11-2011, 12:05 AM
    Sleeper CP
    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....
    Thanks Bob.

    So even in a heavy boat an "C" impeller could holeshot better than a "B" if the "C" is allowed to spin to the peak HP rpm of the engine ?

    S CP
  • 06-10-2011, 08:53 PM
    zacho!
    With pump changes comes U/W changes to get overall performance. M[/QUOTE]
    who would have thought...
  • 06-10-2011, 08:27 PM
    ol guy
    Quote Originally Posted by zacho! View Post
    could be...he said it was much worse before 3" insert
    BIG SHOCK THERE! Kinda explains the theory. With pump changes comes U/W changes to get overall performance. M
  • 06-10-2011, 07:41 PM
    UNASHAMED
    Tom has a chart, maybe he'll post up. I'm going from an ab to a b so I'll let ya know what the difference is come 4th
  • 06-10-2011, 07:24 PM
    zacho!
    could be...he said it was much worse before 3" insert
  • 06-10-2011, 07:19 PM
    ol guy
    Quote Originally Posted by zacho! View Post
    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.
    Could be how he is loading the pump after the impeller change. IE: if you change gears in a car do you adjust tire size for bight? M
  • 06-10-2011, 11:25 AM
    bp298
    Quote Originally Posted by zacho! View Post
    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.
    could be any one of a myriad of things. design/detail of the second impeller vs the first. pump clearances with second impeller vs first.
    setup. at 6100, the engine may be making more hp than at 5800, so the pump is trying to process more water, so needs more supply than it was using before.
    if it had a good holeshot, and now doesn't, it's not likely the impeller exit blade length has anything to do with it...
  • 06-10-2011, 09:41 AM
    zacho!
    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.
  • 06-10-2011, 07:05 AM
    Bodean's Hammer
    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.
  • 06-09-2011, 11:53 PM
    bp298
    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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