Jet boat acceleration
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Jet boat acceleration

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    Senior Member TNYoungblood's Avatar
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    Default Jet boat acceleration

    Hey guys I got to wondering something this weekend At what point/distance does a jet boat stop accelerating ?? I'm sure there are many variables that 1 would have to add, BUT for the most part what is a good distance where you have reached max speed
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    When the horsepower curve crosses the impeller curve the boat stops accelerating. Simple as that.
    MrE

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    Senior Member Alter Ego's Avatar
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    I know from experience that it all depends on the impeller, just like changing the rearend gear ratio in a car. you could have the same motor peak mph at two entirely different times, and speeds depending on the impeller cut. My boat for instance, will be runnin all it is going to do in about 800 ft,

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    Senior Member cruzer's Avatar
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    Quote Originally Posted by MrEracer View Post
    When the horsepower curve crosses the impeller curve the boat stops accelerating. Simple as that.
    MrE
    Wrong. It's when thrust = drag, the boat quits accelerating. Otherwise, why does the rpm go to max as soon as the pedal hits the stop. I guess its because the boat has already hit top speed? (not)

    Edited to better express what I was trying to say.
    Last edited by cruzer; 06-21-2011 at 07:56 AM.
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    Member hollbrow's Avatar
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    Default mine does

    My RPM pretty much does go to max when I WOT- other than a slight variation due to pump loading. The difference in acceleration is the hull coming to an equal but oposite equilibrium with the impulse of the water from the nozzle minus friction.
    Last edited by hollbrow; 06-21-2011 at 05:56 AM.

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    Quote Originally Posted by cruzer View Post
    It's when thrust = drag, the boat quits accelerating.
    You couldn't make it any clearer than that!

    Circ

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    Senior Member bp298's Avatar
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    Quote Originally Posted by TNYoungblood View Post
    Hey guys I got to wondering something this weekend At what point/distance does a jet boat stop accelerating ?? I'm sure there are many variables that 1 would have to add, BUT for the most part what is a good distance where you have reached max speed
    max speed and acceleration are two entirely different things. my 23' cruiser, wih mighty junkyard truck engine, accelerates like a banshee to about 30mph. then, another mile and a half later, finally hits max speed. i don't call that "acceleration" because there's no sense of the boat accelerating - it's just inching forward gaining a little as it goes along. that's a big heavy underpowered boat.

    acceleration is when you go from a dead stop to 90-120 (pick one) in 6-7 seconds. going another 3/16 or 1/4 mile to finally get to a "max speed", it's just inching ahead, picking up a little at a time. even though the boat may be at a high speed, there's no real "feel" of acceleration, even though it might pick up another 10-20 mph over that longer span.

    and yes, when thrust equals drag, the boat won't go any faster.

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    Senior Member TNYoungblood's Avatar
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    Quote Originally Posted by bp298 View Post
    max speed and acceleration are two entirely different things. my 23' cruiser, wih mighty junkyard truck engine, accelerates like a banshee to about 30mph. then, another mile and a half later, finally hits max speed. i don't call that "acceleration" because there's no sense of the boat accelerating - it's just inching forward gaining a little as it goes along. that's a big heavy underpowered boat.

    acceleration is when you go from a dead stop to 90-120 (pick one) in 6-7 seconds. going another 3/16 or 1/4 mile to finally get to a "max speed", it's just inching ahead, picking up a little at a time. even though the boat may be at a high speed, there's no real "feel" of acceleration, even though it might pick up another 10-20 mph over that longer span.

    and yes, when thrust equals drag, the boat won't go any faster.



    I would think that a boat is accelerating until it reaches max speed .. I was really just wondering if a jet is still gaining speed at the 1320' mark or is doin all its gonna do at say the 800' mark ..
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    Senior Member Widetrack's Avatar
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    TN, if you haven't reached max by 900' you will be behind! J/K!


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    Senior Member TNYoungblood's Avatar
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    Quote Originally Posted by Widetrack View Post
    TN, if you haven't reached max by 900' you will be behind! J/K!




    Well that depends on what your max speed is don't it
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    Senior Member sturat's Avatar
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    As stated by me before I know jack $hit real well so take this for what's it's worth..
    My understanding is that a properly set up jet will only gain a few MPH the second half of the track again i know jack $hit real well..
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    Senior Member bp298's Avatar
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    Quote Originally Posted by TNYoungblood View Post
    I would think that a boat is accelerating until it reaches max speed .. I was really just wondering if a jet is still gaining speed at the 1320' mark or is doin all its gonna do at say the 800' mark ..
    most i know of will see increases in speed between 800' and 1320'. and they might even see more speed increase if they went further. but compared to how quickly they accelerated to 800', whatever the increase might be is almost insignificant. if a boat's running 137 at the 1/8 and 144 at the stripe, the increase in speed over that time and distance is nothing compared to the acceleration rate, from a dead stop to 137,if that makes sense. even if the boat was running 150 or 155 at the stripe, the rate of increase is still very small compared to initial acceleration.

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    Well, you're pretty quick to call me wrong. When thrust=drag you have reached max speed. Prior to that, you are still accelerating (even if slowly). Maximum thrust is reached when the hp curve crosses the pump curve, and a steady state RPM is reached. Wait, I think I said that already, or maybe we are both saying the same thing only differently...

    As to your comment about max rpm being reached when the pedal hits the stop, I think you need your pump rebuilt...

    MrE


    Quote Originally Posted by cruzer View Post
    Wrong. It's when thrust = drag, the boat quits accelerating. Otherwise, why does the rpm go to max as soon as the pedal hits the stop. I guess its because the boat has already hit top speed? (not)

    Edited to better express what I was trying to say.

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