In "How To for Dummies" terms

[Statue]

Would someone please explain in simple terms how to choose the correct impellar?I don't get it. If a AA requires the most horsepower, doesn't that mean that it is moving the most amount of water? If that's the case, why is it that most of the "hot boats" on here seem to run B impellars? With a prop, HP + prop pitch = increased to speed, how does an impellar work.
Thanks, Wes

[Jetaholic]

They run "B" impellers to get more top end acceleration.
The lower the letter, the stiffer the impeller, and the more load is placed on the engine. A "B" impeller requires less HP because it doesn't load the engine as much, so it's easier for the motor to spin and easier to get your RPMs up, which will give you better top end acceleration response. Whereas the stiffer impeller, while yes it does move more water, it also puts more of a load on the engine, which makes the engine have to make more torque to spin it, which makes the engine spin the pump harder, and gives you more pulling power for when you're pulling skiiers.
Acceleration is all in the RPMs. Lower RPMs=more torque/higher RPMs=more HP.

[OverKill]

Could not of said it any better. Aplause

[Froggystyle]

Short version...
All impellers start out life as AA or AAA impellers depending on brand. As such, all sizes of the same brand will have very similar characteristics. They have the same ramp shape, diameter, shaft etc... What you do to create the smaller sizes is to trim material evenly off the back of the impeller vanes with a lathe to reduce the amount of pitch, or how many inches you will move forward per revolution with no slip. The bigger sizes will carry water for longer per revolution.
The most efficient size is the "A" impeller. It has the best characteristics of hydrodynamics from everything I have read, and while it isn't as steep a pitch as the AA, it does it better overall with greater efficiency. Something about the larger size having a dirtier final 1/4 inch of pitch.
Since we can't change diameter, cup or many other characteristics about the impeller, all we can do is make them smaller or larger to accomodate different boat setups and engine packages.
Let's say you have a 425 hp big block attached to an "A" impeller. For the sake of the explanation, let's say you are turning 4900 RPM with it. Your motor has a redline of 5900 though, and you are making a lot more power over 5,000 that the "A" impeller will not let you get up into. By cutting the impeller, or swapping out to a "B" cut, you will lose efficiency, and gain 400 RPM that won't make any impact on your top speed. You will just be turning a bit higher RPM at the same speed. If, however, your additional 400 RPM gets you another 80 horsepower, you will likely be able to pull more than 400 RPM out of it. Let's say that by doing the B cut you are now pulling 5600 RPM's and with an increased top speed more than likely. You had a net gain of 300 RPM in performance in this example. Your bottom end probably got a little firmer as well, with better pull through the midrange.
Let's put in a "C" cut though now. As you can imagine, you will gain another 400 RPMs over the "B", which would put you at 5700 RPM's just by the math, but it got you into even more power... On paper, you could be pulling a net gain of 500 RPM's, or 6200 RPM's and even greater top speed...
...except you are dinging your rev limiter at 5900. So, you only actually have a net gain of 200 RPM's, and at a greatly increased fuel cost, lower hardware reliability and more heat. You cut too far... If you built the motor to live at 6200, you can probably have the biggest overall gain here...
For the sake of the argument, let's say you put a AA in now. You will probably lose your 400 RPM from the math, so you are turning 4500 RPM now, but you will lose speed too, because your engine falls off in power quickly below 4900 RPM. Likely, you will lose another couple hundred RPM from the power loss, for a net loss of 200, and a total RPM of 4300 and lower speed.
Most people run about an A/B (somewhere between an A and B cut) or a B so that smaller motors can run into their horsepower peaks. A couple boats ago I had a blown 19' Daytona that would spin 6400 RPM with a "AA " impeller. I was looking for ways to pull more RPM out of it to keep the motor together, and as such was going to have to use a custom built "AAA" impeller but never ended up doing it. 6400 was a little high for my taste on that motor.
If you have any specific questions, feel free to drop me a PM or an e-mail at wes@tridentboats.com
I am not claiming to be an expert on this, but I do have some pertinent experience...

[Statue]

That's great information, thank you for the clarification.
Wes

[inwo]

Short version...
What you do to create the smaller sizes is to trim material evenly off the back of the impeller vanes with a lathe to reduce the amount of pitch,
I am not claiming to be an expert on this, but I do have some pertinent experience...
Is that what you meant to say? It seems trimming would not change the pitch.
Or are they (aa) variable pitch design with the trimming removing the higher pitch portion.

[bp]

another way to think about it might be that the pump is a horsepower absorber; from a given rpm, it will require additional horsepower to pump more water. depending on the pump and impeller, the additional horespower required (to increase flow) will increase linearly, but at a point, the horsepower demand (from the pump) per rpm will increase at a faster rate.
which is why it's important to know your engine's horsepower curve, and where peak horsepower is.
as an example, a pump with an AA impeller will absorb "x" horsepower at 4000rpm. given the same everything, an A impeller will require "x" horsepower at 4200, a B impeller will require "x" horsepower at 4400, etc., (x being the same hp value). but, a person might find that they can only turn an AA 4100, but can turn a B 5100. the reason the B can spin faster is that the engine horsepower curve is staying ahead of the impeller demand curve. for the AA to increase above 4100, it required more hp than the engine made at that rpm; ergo, it hit a wall (hp curve below impeller curve). but, with the B, there was enough horsepower to increase rpm above that point in the curve; the rate of horespower increase through that rpm range was greater than the increase in demand.
it is most likely that, at some rpm point prior to "peak" horsepower, the engine horsepower curve will begin to flatten; i.e., if it's increasing at 25hp/100 rpm, at a point, hp increase might slow to 25/300rpm, perhaps 500 rpm prior to peak. it's generally not effective to try and trim the impeller to go after that last 40hp gain. in other words, the engine might peak at 750/6600rpm, but it made 700 at 6050. the B curve might overlap at 6060, and be very efficient, but to get to 6600, it might have to be trimmed to a D or smaller, giving away a lot of hp on the bottom end where the curves are way out of alignment.
the bottom line is that you need to know (or have a real good idea) of what your engine horsepower curve looks like to select the correct impeller for your application.

[Froggystyle]

Is that what you meant to say? It seems trimming would not change the pitch.
Or are they (aa) variable pitch design with the trimming removing the higher pitch portion.
You are thinking in terms of pitch being measured like a prop, where the angle is what you change. What pitch refers to is how many theoretical inches with zero slip the prop or impeller will move you forward in one revolution of the unit. If you took a 28 pitch propeller and cut the back half off of it, it wouldn't be a 28 pitch anymore, even though the angle is the same. It would move half the distance, becoming a 14 pitch or something. There is less material to carry the water in both cases, so you have a reduction in pitch.
May not be the easiest word to understand considering most of us know props as pitch, but it is correct in essence...
After re-reading that, I am now not sure it is correct, because in a theoretical world with no slip, it wouldn't need to hold the water on the blade, so the pitch wouldn't change.
In any case, you are making the vane shorter, which has the same exact effect as removing pitch from a propeller.

[flat broke]

Is that what you meant to say? It seems trimming would not change the pitch.
Or are they (aa) variable pitch design with the trimming removing the higher pitch portion.
I don't want to speak for Froggy on this, but I'm guessing that he was referring to the the machine angle of the trailing edge of the blade of the impeller. If you look at the size charts for say a berk impeller, it will give an O.D. measurement for the small diameter side at the rear of the impeller and indicate the resulting angle you shoud arrive at.
When looking at the power absorbed by the impeller, the curve will be exponential as you increase or decrease the surface area, but that is not to say that the physical pitch is progressive.
We've gotten off topic from the original question that started this thread, and to that end, BP's advice about knowing your actual HP before guessing which impeller to use is the most pertinent tidbit for the original poster.
Statue,
Don't forget that this topic has been covered in depth many times in the past. Using the search feature will also bring up threads with input from valuable contributors who don't waste their time on here anymore. Doing searches for "impeller matching" "impeller sizing" etc. should be fruitful, and might even expose you to different areas of setup and theory that are either interesting, confusing, downright hillarious, or all of the above.
Chris

[ck7684]

Basically, the impeller should be matched to the operating RPM's of the engine. If you have an engine that makes all it's power at a higher RPM, it may not have enough torque down low to utilize a larger impeller. Look at F1 cars. They get pushed by the crew just to get moving, even though they make gobs of power...but once under way they can really put it to use for high speed...