Impeller cut and speed association...

[Scarab Jet]

I am trying to figure out why it is that [let's say] a "C" impeller in a given boat gives it a higher top-end speed than [let's say] an "A" impeller... Is it because the C cut basically moves less water quicker vs. the A cut that moves more water but slower?
Or do I have the whole reasoning wrong? If so, I appreciate if someone can explain the physics behind what makes a particular cut impeller provide better top-end but slower hole-shot vs. the one that provides exactly the opposite...
Thanx in advance...

[SmokinLowriderSS]

I can't even touch this, but I might just learn somethin'. :idea:
Someone from the highly experienced "old guard" who have been running and tuning jets for decades I am sure will come and educate both of us. I have been told the same thing, by DuaneHTP, but didn't ask for an explanation at the time. I'm sure a lot depends of setup. If an engine is being held below it's best power, an impeller cut will make it faster as the engine can now wind up to it's power, allowing more pump pressure to build. Beyond that obviousness, I'm gonna sit back & get a lesson.

[Duane HTP]

Scarab, check your PM.

[flat broke]

That's a good question Scarab Jet, and while I'm sure Duane has you covered in the PM, it might not be a bad idea to address the topic here on the forum for others to learn from.
Simply put, think of the sizes of impellers refer to how much water they will move in each revolution. An AA takes a bigger gulp of water per revolution than a C. The total amount of thrust, which is the volume of water moved per revolution multiplied by the revolutions per minute of the impeller, dictates how the boat will perform. Physics are blind to the make, model, and size of our boats and components. It always boils down to applied force vs. the various resistances we are trying to overcome.
If we give each impeller a numerical value for units of water per revolution this will start to make more sense. For the sake of illustration refer to the following examples:
D cut - 1 unit
C cut - 2 units
B cut - 3 units
A cut - 4 units
AA cut - 5 units
If your motor could turn each of the impellers to 5000RPM, you would see between 5000 and 25,000 units per minute moved through the pump. It wouldn't take much to decide that you might as well move 25,000 units of thrust for the same cost as 5000. Unfortunately with each increase in units of water/revolution there is a corresponding increase in the required HP to turn the impeller. So let’s say that the HP requirements at 5000RPM for each impeller are as follows(These are not accurate numbers, but exaggerations to illustrate a point)
D to 5k - 100hp
C to 5k - 200hp
B to 5k - 300hp
A to 5k - 400hp
AA to 5k -500hp
If the motor in your Scarab makes 300hp at 5kRPM, then you would likely choose a B impeller, as it will place the motor operating at its peak HP when at WOT. Pretty simple right? Well, what if your motor made it's 300hp at 550RPM? Now we have a bit of a problem. In order to get to the peak HP, we need to spin more RPM than the B impeller will allow us to. A compromise is in order. We need to balance spinning a less efficient impeller (a C cut) faster to get to the peak HP of the motor; against the efficiency losses associated with going from a B impeller to a C. Because this is a fictitious example, we are in luck and can easily figure out what we should do. Assuming we'd be able to spin a C to 5500RPM with your motor to get to your peak HP, we know that we would be able to move 11,000 units of water per minute (2 units per revolution X 5500RPM). If you were only able to turn the B impeller to say 4500RPM with the same motor, you would be moving 13,500 units of water at 4500RPM. In this instance, even though you're not able to get to your peak HP which is at 5500RPM, the efficiency of the B impeller at a lower RPM still moves more water than the C impeller at a higher RPM. In a real world scenario, it would likely be the case that the motor in question made little more HP between 4500RPM and 5500RPM.
With that example under our belts, let’s look at your original question, "I am trying to figure out why it is that [let's say] a "C" impeller in a given boat gives it a higher top-end speed than [let's say] an "A" impeller... Is it because the C cut basically moves less water quicker vs. the A cut that moves more water but slower?
From the example above we can see that in each boat with different peak HP RPMs and HP levels, it is not always a given that a smaller impeller will make a boat move faster. In actuality the C moves an equal amount of water slower than the A impeller; as it will require more revolutions of the C impeller to move the same amount of water as the A impeller will move in one revolution.
Your question touches on what I think is the biggest gap in our knowledge about jet drives and the efficiency associated with each impeller size. If there were accurate published information saying that at a given RPM and inlet pressure, each impeller moved X GPH/GPM/whatever, we would be able to much more accurately determine which impeller would create the optimum efficiency for a particular boat. Without that information, we rely on matching the impeller to the peak power of the engine to get the most out of the equation. However, just like in our example, there are real world scenarios where a bigger impeller at lower RPM will go faster and accelerate harder than a smaller impeller at a higher RPM. Just this last weekend I was out on a boat that runs 2 mph faster with a AA Impeller compared to an A impeller. The boat looses 2-300RPM by going to the larger impeller, but the total amount of water moved as a result of using the bigger impeller is higher. This results in more applied force against the various resistances the boat sees.
While the above was a very general oversimplification, not accounting for many other factors such as running gear setup, internal losses in the pump etc. It illustrates the fact that the smaller impeller isn't always faster. It's the impeller that can move the most water in a given period of time with the available horsepower, which will ultimately be the most efficient setup.
Chris

[MikeF]

Some more insight to the question you pose is in the Feb 2005 issue of HB mag. Cyclone did ALOT of testing that day for an article for HB.

[flat broke]

Some more insight to the question you pose is in the Feb 2005 issue of HB mag. Cyclone did ALOT of testing that day for an article for HB.

Mike, were you refering to me, or to Scarab Jet? The data I'd like to see is nowhere in that article. I doubt any jet mfg is going to spend the $$$, (and it would be lots of $$$) to do a complete test to determine the actual flow rates of their impellers at a given RPM. There are probably folks out there that have a strong enough handle on fluid dynamics to extrapolate some flow numbers based upon the physical dimensions of the impeller, losses, through the pump etc. But unless they have a chubby for jetboats, I doubt we'll ever see much published on the subject.
Which gets me to thinking. I know at Lake Powell, they have a sample of one of the turbine impellers out on display. It looks like a mixed flow design and it's huge. Do you think they have done the research to know how many gph or whatever they measure in is moved with each revolution at a given RPM? In a sense, I would almost think they have to know. If they need to spin the Generator x RPM to make X kilowatts, they have to know how much water to release right?
Just looking for a place to find information where someone might have had deep enough pockets and justification to do the research.
Chris

[UBFJ #454]

Talk with Don Bowers ... He did a lot of flow testing when designing his inducer and has a lot of information dealing with impellers and their flow rates at maximum pump throughput.

[MikeF]

Mike, were you refering to me, or to Scarab Jet?
SJet and anyone who wanted to look at the article.
Which gets me to thinking. I know at Lake Powell, they have a sample of one of the turbine impellers out on display. It looks like a mixed flow design and it's huge. Do you think they have done the research to know how many gph or whatever they measure in is moved with each revolution at a given RPM? In a sense, I would almost think they have to know. If they need to spin the Generator x RPM to make X kilowatts, they have to know how much water to release right?
Chris
I'm sure they did the research. :boxed: So much so they make $$$$$ with theirs.

[Scarab Jet]

Thanx Chris... I really appreciate such a comprehensive explanation on this subject that we all jetters can benefit from...
Your thorough explanation brings up a couple of more questions that I appreciate your response to [in advance]:
1) So basically the long standing myth that "by going to a higher cut impeller, one would automatically gain a better hole shot and loose some speed on the top end at the same time" is not necessarily always true, correct?
and
2) In your illustration, you mention the most horsepower that an engine makes in a given RPM is the number to shoot for when comparing one impeller cut vs. another... But, how about maximum torque for a given RPM that a given engine makes? I.e., shouldn't we also look at that number when selecting an impeller? Or should we just be concerned about HP number alone? Or perhapos a combination of both (somehow)?
Thanx again in advance...
Mike...

[Konabud]

I not the expert but I think the main thing is to load your engine properly at that engines max RPM (not unlike a boat propeller) Boat weight and several other factors might come into play but at the end of the day you want your engine to turn Max RPM at WOT