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Hey guys I got to wondering something this weekend :)bulb 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 :D
 

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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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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.
 

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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.
 

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Hey guys I got to wondering something this weekend :)bulb 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 :D
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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Discussion Starter #9
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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TN, if you haven't reached max by 900' you will be behind!:D J/K!:)hand :nothing to add:
 

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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.. :)sphss
 

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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


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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first you said this:

When the horsepower curve crosses the impeller curve the boat stops accelerating. Simple as that.
MrE
then, you said this:

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
i'm not quite sure what it is you're trying to say. "max thrust" and "stops accelerating" are two different things. from deadstop idle, my engine is at max rpm within 0.3 seconds (and i'm slow). and no, i don't need to have the pump rebuilt since it was just at mpd 4 months ago. but regardless, it's behaved pretty much the same way for 10 years. i could post a pic if you'd like.
i'd like to know what curves you're referencing.
 

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I could post the pump curves for most impellers but every engine has it's own hp vs rpm curve so with a little imagination I will try to describe it. Take a copy of the pump curve for your impeller and a same scale copy of your hp curve and lay them on top of each other so that the rpm of both curves line up across the bottom and the hp for both curves are the same on the vertical scale. Now, everywhere the hp curve is above the pump curve, you have more hp available than the pump requires and the excess hp is available to accelerate the pump to a higher rpm. At some point on the rpm scale the hp curve crosses the pump curve. There is no more hp left over to accelerate the impeller and you have reached a steady state rpm with zero acceleration...

As a hypothetical thought experiment, lets say you used the impeller curve for a Berkeley 'A' impeller and the crossing point occurred at 5200 rpm which is below the max power of your engine. Now lay the pump curve of a 'B' impeller over your hp curve. This has the effect of moving the pump curve to the right. Now your hp curve doesn't cross the pump curve until, lets say, 5600 rpm. This is higher up your hp curve and there is more hp available at that rpm... Wa La, you will have more thrust (hp) to overcome your hull drag and your boat will accelerate to a faster speed... You might even look at a 'C' impeller and find that the cross over does not occur until say, 6000 rpm and you could hypothetically go even faster... This is a rather simplistic look at what is happening in a jet boat but it serves to explain how things work... You probably know this, but it may help others to understand it better.
MrE
 

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all that's fine. there is no curve that matches my pump and impeller, especially at my hp level.

regardless, "maximum thrust" has nothing to do with what "maximum speed" might eventually be. i'm at maximum thrust 2.5 seconds after i leave from a dead stop at idle. the boat doesn't stop accelerating at that point. it will continue to accelerate, and then slowly gain speed, for several more seconds.
to say that max speed occurs when thrust = drag may be too simplistic, but its relatively accurate. the force applied has to exceed the weight and friction from the water in order for speed to increase. there's a point when maximum force won't be enough to overcome the weight and water friction of a particular hull, and whatever speed the boat is going will be all there is. but then, do more work on the hull/hardware/setup to get the boat to go faster with the same amount of thrust.

thinking this is related solely to some pump curve is innaccurate, and incomplete. my hull weighs 730lbs bare. i know for a fact that if i installed my engine and pump in a boat that's 300lbs lighter, it will go substantially quicker and faster. the design of the hull also plays a part because the design has an effect the amount of friction. same hp, same pump, same theoretical curve, different max speed.
 

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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 ..

This thread would be much better with a few charts don't you think? ;)

So much of your question depends on the boat. I have a friend who's old Gullwing ran 105 in the 1/4 it ran 99 in the first 1/8th.

Did you have any type of boat in mind ? I think some of the 8 second 130+ mph tunnels are gaining 8-12 mph in the back 1/8th which means they are 92-94% done by the 660' ( plus 120ft run up)

S CP :D
 

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As a hypothetical thought experiment, lets say you used the impeller curve for a Berkeley 'A' impeller and the crossing point occurred at 5200 rpm which is below the max power of your engine. Now lay the pump curve of a 'B' impeller over your hp curve. This has the effect of moving the pump curve to the right. Now your hp curve doesn't cross the pump curve until, lets say, 5600 rpm. This is higher up your hp curve and there is more hp available at that rpm... Wa La, you will have more thrust (hp) to overcome your hull drag and your boat will accelerate to a faster speed... You might even look at a 'C' impeller and find that the cross over does not occur until say, 6000 rpm and you could hypothetically go even faster... This is a rather simplistic look at what is happening in a jet boat but it serves to explain how things work... You probably know this, but it may help others to understand it better.
MrE
So what impeller should I use:




And where would my GW stop accelerating ?

S CP
 

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This thread would be much better with a few charts don't you think? ;)

So much of your question depends on the boat. I have a friend who's old Gullwing ran 105 in the 1/4 it ran 99 in the first 1/8th.

Did you have any type of boat in mind ? I think some of the 8 second 130+ mph tunnels are gaining 8-12 mph in the back 1/8th which means they are 92-94% done by the 660' ( plus 120ft run up)

S CP :D
The amount of knowledge and experience you and BP are so willing to share blows my mind. I wish I would have documented every gain each mod gave me, VS what feels faster.:)sphss
 
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