Impeller choices

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Tikotiko

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« on: October 14, 2018, 02:10:47 PM »
I want to learn more about impeller choices. What is the difference between them all.

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Flusher

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« Reply #1 on: October 20, 2018, 07:35:41 AM »


What is the difference between them all.  SoCal Jet Boats mobile app

That is a huge topic!  Limiting this to just Berkeley and Berkeley clones:

The critical dimension that ultimately defines the life (and value) of the impeller is the wear ring surface diameter.  This dimension is an outside diameter of 7.225 inches.  This corresponds to a wear ring inside diameter of 7.250 inches, producing a total radial clearance of .025 inches.

This clearance is aggravated by by running on the trailer and also boating in water with a high abrasive sediment content.  Along with the erosion of the radial clearance, performance also erodes as slipping increases.  Impellers  made from softer alloys, such as aluminum and bronze or wear rings made from bronze, plastic, and urethane tend to wear faster than those made from stainless steel.  Modern aluminum impellers are typically hard anodized to resist wearing as well as corrosion and pitting from cavitation.

The next important dimension is the length of the wear ring diameter.  Some early Dominator impellers have a shorter wear ring length that can usually be cut longer as these older wear rings are no longer available.  Otherwise the length of the wear ring surface is 1.063 inches typical across all Berkeley and Berkeley clones.

The diameter is the first dimension that we inspect.  What can you do when your impeller is undersized?  Undersized wear rings are available from most manufacturers such as American Turbine and Place Diverter.  We frequently bore undersized wear rings to achieve an ideal radial clearance.  If you have a lot invested in a good impeller that is showing a lot of wear, this is the only practical way to bring the radial clearance back into spec.
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

Flusher

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« Reply #2 on: October 21, 2018, 01:31:11 AM »
CUT SIZES

The typical Berkeley JB/C/G etc., Dominator 12s (10.250" diameter small bolt circle), American Turbine AT-309, and others have a register to locate the bowl to the suction that has a 9.125" diameter.  A Legend bowl has similar dimensions except the suction bolts are 3/8-16 UNC instead of 1/2-13 UNC. Realistically, the largest impeller that can be installed is an A2 or AA. The Large Diameter of a AA impeller is 9-3/32" (9.09375) and is easily identified by the trailing edges of the vanes extending beyond the Large and Small Diameters.

There are actually two different A cut impellers, both sharing a 9" Large Diameter.  The difference between the two is the Machining Angle of the cut of the trailing edges relative to the pump shaft.  For the large A impeller, that Machining Angle is 30-degrees relative to the shaft, (60-degrees to the face of the impeller pictured in the diagram).  The small A impeller is cut to 33.5-degrees reducing size at the Small Diameter.

Whole letter sizes are in 1/4" increments at the Large Diameter, i.e. B is 8-3/4" Large Diameter, C is 8-1/2" Large Diameter, etc.  Letter sizes in between whole letter sizes are in 1/8" increments, i.e. a BC impeller, the most common size of impeller in drag racing, has a Large Diameter of 8-5/8".  Then, to complicate things further, there are plus and minus sizes.  When we are really trying to nail the RPM, we will specify a Large Diameter decimal inch size rather than a so-called letter size.

Going the other way, for very high horsepower applications, you can install up to 9-1/2" impeller in the large bolt pattern Dominator bowl and suction.  According to American Turbine, any larger than that and the impeller vanes will hit the bowl vanes.

It appears to me that the castings for the large bolt pattern Dominator bowl is the same as the small bolt pattern bowl.  The differences in machining between the two is the register is machined to a larger 9.500" diameter, the bolt circle is a larger 10.500" diameter, the suction surface is machined for an (Triangle Crush Groove) o-ring suction seal instead of a gasket, and there is no provision for an o-ring for the transom cover.


HORSEPOWER

To describe how impeller cut works with engine horsepower, we must first define what horsepower actually is.

Engines produce torque, a rotational force (https://en.m.wikipedia.org/wiki/Torque).  Torque is what is actually measured when we talk about dyno sessions (https://en.m.wikipedia.org/wiki/Dynamometer).  The units used for torque, relating to engine output and impeller requirements is Pound-Feet (https://en.m.wikipedia.org/wiki/Pound-foot_(torque)). 

Horsepower is a calculated unit of power, or the rate at which work is done (https://en.m.wikipedia.org/wiki/Horsepower).  The equation for horsepower that is most relevant to what we do is:

HP = (Torque * RPM) / 5252


IMPELLER CUT SIZE

The primary differentiator between impellers is the cut size.  From what I understand, all impellers (within the same brand and material) start out as the same raw casting that is then machined to whatever cut size is desired.

There are however, differences in the molds between stainless steel and aluminum because of the different shrinkage rates of the material being cast.  For example, a stainless American Turbine has considerable differences in appearance in the vanes compared to the aluminum American Turbine impeller.

It is the cut size that primarily defines the RPM at which it stalls any given engine.  There are other factors that can influence RPM such as cavitation and pseudo cavitation or aeration.  For now, the focus will be on cut size.

The cut of the impeller is essentially the length of the vanes of the impeller.  Berkeley pumps and their clones are a mixed flow pump.  I like to describe a mixed flow pump as a combination of a centrifugal and an axial flow pump.

A centrifugal pump impeller discharges fluid radially, perpendicular to the pump shaft.  A centrifugal pump produces pressure by accelerating the velocity of the fluid.  The fluid is then allowed to slow down within the housing.  It is at this point that the velocity of the fluid is transformed into pressure (https://en.m.wikipedia.org/wiki/Centrifugal_pump).

An axial flow pump is essentially a ducted propeller.  It it moves a high volume of fluid at relatively much lower pressure than a centrifugal.  The flow of an axial flow pump is parallel to the pump shaft (https://en.m.wikipedia.org/wiki/Axial-flow_pump).

The Berkeley (and clones) mixed flow pump discharge water at approximately 55-degrees relative to the pump shaft.  The internal shape of the pump, intake to droop, forms the shape of a venturi.  This design produces a combination of a high volume of water pumped at high pressure.

As the the impeller is turned, by the engine, the helical shape of the impeller vanes accelerates (pushes) the water along its back side.  The cross-sectional area of the impeller decreases from the inlet eye to the exducer.  This reduction in cross-sectional area helps to accelerate the water, i.e. the  same volume of water has to pass through a smaller opening in the same amount of time, increasing its velocity.  The cross section is continued into the bowl, where the flow of water is redirected and gradually allowed to expand, slow, and build pressure, before it is converted into thrust in the nozzle.

Pressure is the result of resistance to flow.  The turning impeller, by itself, produces only flow.  The nozzle area is the point of greatest restriction to that flow.

Newton's 3rd law states:  For every action, there is an equal and opposite reaction.  As the impeller vanes push the water through the pump, the resistance to that flow pushes back against the impeller vanes.  The length of the impeller vanes effectively produce a lever arm that resists the rotation of the engine's crankshaft.  The greater the length of the torque arm (larger cut size), the greater the resistance the water produces acting against the crankshaft.


WORK

Thrust, in a jet boat, is a reaction force where a mass of water is accelerated and expelled from the nozzle, producing an equal magnitude of propulsion in the opposite direction (https://en.m.wikipedia.org/wiki/Thrust https://en.m.wikipedia.org/wiki/Work_(physics)).

The goal is to size the impeller so that the torque force balances with work output of the impeller.  Peak horsepower is the point where the engine is producing the most work.  As you can see from the impeller charts, the faster a given impeller is turned, more water is being pumped, the more power it takes to turn the impeller, more work is being done.

If an impeller is too large for a specific engine combustion (horsepower output), the torque required to turn the impeller overwhelms that produced and the engine is stalled at an RPM lower than peak power.  The work output potential of an engine pump combination is never realized.

If the impeller is too small, the engine will rev past peak RPM.  Even though the RPM are higher, the amount of water pumped, and therefore the amount of work being done is less than the combination's full potential.


COMPROMISES

Matching dyno numbers and impeller charts is not the end all be all definitive answer to impeller cut selection.  For example, a heavy boat, such as a mini day cruiser, will really struggle to launch or even get on plane with an engine/pump/impeller combination that would be a rocket ship in an 18' gullwing.  Sacrificing a little top speed for a better launch more than makes up for it.

Another peculiar case is that of a positive displacement supercharger.  Positive displacement blown engines produce so much bottom end torque, the hit on the impeller is more than it can handle until the pump is fully loaded.  Impellers do not load themselves, they need to be loaded and remain properly loaded throughout the run.  An reviewing the impeller charts, you can quickly see that, at low RPM, even a A2 (AA) cut impeller absorbes very little power compared to what the engine is producing at that same RPM.  The result is; the engine easily overpowers the impeller, the RPM increases uncontrollably until it bounces off the rev limiter, and the pump is not able to recover until the driver backpedals.
« Last Edit: October 21, 2018, 01:46:47 PM by Flusher »
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

Tahoe540

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« Reply #3 on: October 23, 2018, 08:12:15 AM »
Thanks for that great explanation. New to jet boats and always learning :thumbup:
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Tikotiko

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« Reply #4 on: October 23, 2018, 08:53:27 AM »
CUT SIZES

The typical Berkeley JB/C/G etc., Dominator 12s (10.250" diameter small bolt circle), American Turbine AT-309, and others have a register to locate the bowl to the suction that has a 9.125" diameter.  A Legend bowl has similar dimensions except the suction bolts are 3/8-16 UNC instead of 1/2-13 UNC. Realistically, the largest impeller that can be installed is an A2 or AA. The Large Diameter of a AA impeller is 9-3/32" (9.09375) and is easily identified by the trailing edges of the vanes extending beyond the Large and Small Diameters.

There are actually two different A cut impellers, both sharing a 9" Large Diameter.  The difference between the two is the Machining Angle of the cut of the trailing edges relative to the pump shaft.  For the large A impeller, that Machining Angle is 30-degrees relative to the shaft, (60-degrees to the face of the impeller pictured in the diagram).  The small A impeller is cut to 33.5-degrees reducing size at the Small Diameter.

Whole letter sizes are in 1/4" increments at the Large Diameter, i.e. B is 8-3/4" Large Diameter, C is 8-1/2" Large Diameter, etc.  Letter sizes in between whole letter sizes are in 1/8" increments, i.e. a BC impeller, the most common size of impeller in drag racing, has a Large Diameter of 8-5/8".  Then, to complicate things further, there are plus and minus sizes.  When we are really trying to nail the RPM, we will specify a Large Diameter decimal inch size rather than a so-called letter size.

Going the other way, for very high horsepower applications, you can install up to 9-1/2" impeller in the large bolt pattern Dominator bowl and suction.  According to American Turbine, any larger than that and the impeller vanes will hit the bowl vanes.

It appears to me that the castings for the large bolt pattern Dominator bowl is the same as the small bolt pattern bowl.  The differences in machining between the two is the register is machined to a larger 9.500" diameter, the bolt circle is a larger 10.500" diameter, the suction surface is machined for an (Triangle Crush Groove) o-ring suction seal instead of a gasket, and there is no provision for an o-ring for the transom cover.


HORSEPOWER

To describe how impeller cut works with engine horsepower, we must first define what horsepower actually is.

Engines produce torque, a rotational force (https://en.m.wikipedia.org/wiki/Torque).  Torque is what is actually measured when we talk about dyno sessions (https://en.m.wikipedia.org/wiki/Dynamometer).  The units used for torque, relating to engine output and impeller requirements is Pound-Feet (https://en.m.wikipedia.org/wiki/Pound-foot_(torque)). 

Horsepower is a calculated unit of power, or the rate at which work is done (https://en.m.wikipedia.org/wiki/Horsepower).  The equation for horsepower that is most relevant to what we do is:

HP = (Torque * RPM) / 5252


IMPELLER CUT SIZE

The primary differentiator between impellers is the cut size.  From what I understand, all impellers (within the same brand and material) start out as the same raw casting that is then machined to whatever cut size is desired.

There are however, differences in the molds between stainless steel and aluminum because of the different shrinkage rates of the material being cast.  For example, a stainless American Turbine has considerable differences in appearance in the vanes compared to the aluminum American Turbine impeller.

It is the cut size that primarily defines the RPM at which it stalls any given engine.  There are other factors that can influence RPM such as cavitation and pseudo cavitation or aeration.  For now, the focus will be on cut size.

The cut of the impeller is essentially the length of the vanes of the impeller.  Berkeley pumps and their clones are a mixed flow pump.  I like to describe a mixed flow pump as a combination of a centrifugal and an axial flow pump.

A centrifugal pump impeller discharges fluid radially, perpendicular to the pump shaft.  A centrifugal pump produces pressure by accelerating the velocity of the fluid.  The fluid is then allowed to slow down within the housing.  It is at this point that the velocity of the fluid is transformed into pressure (https://en.m.wikipedia.org/wiki/Centrifugal_pump).

An axial flow pump is essentially a ducted propeller.  It it moves a high volume of fluid at relatively much lower pressure than a centrifugal.  The flow of an axial flow pump is parallel to the pump shaft (https://en.m.wikipedia.org/wiki/Axial-flow_pump).

The Berkeley (and clones) mixed flow pump discharge water at approximately 55-degrees relative to the pump shaft.  The internal shape of the pump, intake to droop, forms the shape of a venturi.  This design produces a combination of a high volume of water pumped at high pressure.

As the the impeller is turned, by the engine, the helical shape of the impeller vanes accelerates (pushes) the water along its back side.  The cross-sectional area of the impeller decreases from the inlet eye to the exducer.  This reduction in cross-sectional area helps to accelerate the water, i.e. the  same volume of water has to pass through a smaller opening in the same amount of time, increasing its velocity.  The cross section is continued into the bowl, where the flow of water is redirected and gradually allowed to expand, slow, and build pressure, before it is converted into thrust in the nozzle.

Pressure is the result of resistance to flow.  The turning impeller, by itself, produces only flow.  The nozzle area is the point of greatest restriction to that flow.

Newton's 3rd law states:  For every action, there is an equal and opposite reaction.  As the impeller vanes push the water through the pump, the resistance to that flow pushes back against the impeller vanes.  The length of the impeller vanes effectively produce a lever arm that resists the rotation of the engine's crankshaft.  The greater the length of the torque arm (larger cut size), the greater the resistance the water produces acting against the crankshaft.


WORK

Thrust, in a jet boat, is a reaction force where a mass of water is accelerated and expelled from the nozzle, producing an equal magnitude of propulsion in the opposite direction (https://en.m.wikipedia.org/wiki/Thrust https://en.m.wikipedia.org/wiki/Work_(physics)).

The goal is to size the impeller so that the torque force balances with work output of the impeller.  Peak horsepower is the point where the engine is producing the most work.  As you can see from the impeller charts, the faster a given impeller is turned, more water is being pumped, the more power it takes to turn the impeller, more work is being done.

If an impeller is too large for a specific engine combustion (horsepower output), the torque required to turn the impeller overwhelms that produced and the engine is stalled at an RPM lower than peak power.  The work output potential of an engine pump combination is never realized.

If the impeller is too small, the engine will rev past peak RPM.  Even though the RPM are higher, the amount of water pumped, and therefore the amount of work being done is less than the combination's full potential.


COMPROMISES

Matching dyno numbers and impeller charts is not the end all be all definitive answer to impeller cut selection.  For example, a heavy boat, such as a mini day cruiser, will really struggle to launch or even get on plane with an engine/pump/impeller combination that would be a rocket ship in an 18' gullwing.  Sacrificing a little top speed for a better launch more than makes up for it.

Another peculiar case is that of a positive displacement supercharger.  Positive displacement blown engines produce so much bottom end torque, the hit on the impeller is more than it can handle until the pump is fully loaded.  Impellers do not load themselves, they need to be loaded and remain properly loaded throughout the run.  An reviewing the impeller charts, you can quickly see that, at low RPM, even a A2 (AA) cut impeller absorbes very little power compared to what the engine is producing at that same RPM.  The result is; the engine easily overpowers the impeller, the RPM increases uncontrollably until it bounces off the rev limiter, and the pump is not able to recover until the driver backpedals.

Wow. Thank you.

mash on it

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« Reply #5 on: October 23, 2018, 09:08:10 AM »
Wow. Thank you.

Does this answer your question?

Dan'l
CJ/RR 212...under construction  "Pistol Annie"

Denon Osterman

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« Reply #6 on: October 24, 2018, 11:36:33 AM »
That was awesome. Possibly the most thorough yet concise overview of the topic I've seen!

Flusher

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« Reply #7 on: October 25, 2018, 04:00:33 AM »
MATERIAL

The performance of any impeller can be improved (or destroyed if modified wrong) regardless of construction material.

Aluminum

The first picture is of an aluminum impeller in an application that slightly exceeded the horsepower limits of the material.  This is one situation where, if you are living life right on the edge, it's better to step up to a stainless.  Nothing sucks more than being stranded in the middle of the lake from a damaged pump, the wife and kids cooking in the blazing sun, on the first day of your vacation.

Aluminum should really be relegated to applications below 500 horsepower.  It's not a matter of if it will fail, but WHEN it will fail.

The quality of the earlier castings are absolutely horrible for performance and efficiency.  The crude castings have a lot of issues with cavitation.  These can definitely benefit from a mild reworking, IF the impeller is in otherwise usable condition AND it matches the application.

Usually used aluminum impellers are beyond their practical life expectancy.  If the application justifies replacing with another aluminum impeller, the new castings from American Turbine and Place Diverter are very nice and don't really require detailing.  The shape of the hub, shroud, and vanes are pretty decent (for the application) right out of the box.

Aluminum impellers are the easiest to detail and modify because of the free machining characteristics of the aluminum.  We just feel that there are better ways to spend your money.

Bronze

Bronze impellers are commonly favorites of performance enthusiasts "looking for a deal."  I don't believe bronze impellers are currently in production, but at one time, bronze impellers were marketed as more economical than stainless steel but with greater horsepower handling capabilities than aluminum (but less than stainless).

If you have a bronze impeller, I don't think that it is necessarily worth upgrading to stainless (for moderate performance applications).  I would never specifically search out a bronze impeller.  In the event that your bronze impeller is damaged, depending on the damage, there is no way to repair a cracked bronze impeller.

Another minor issue is that the bronze castings are not nearly as nice, in the as cast condition, as most stainless and modern aluminum impellers.  Again, cavitation is an issue.  This is not really an issue after proper detailing, they can be made to work well.  Compared to a good stainless, a bronze impeller will never quite bite as hard as a good stainless.  The reason for this is the thickness of the vanes.  Thinner vanes slice through the water with much less leading edge cavitation.

Stainless Steel

First, I want to say, just because an impeller is cast from stainless steel, that alone doesn't make it magically work well.  For example, there are some extra heavy duty castings that were marketed by Berkeley as being specially designed to tolerate ingesting rocks and debris.  These castings are very heavy as every feature is made excessively thick to protect against rock damage.  The vanes are very thick and blunt.  The fillet radii are huge and extend way out into the hub and shroud.  The shroud is also very thick, closing off the intake opening with a very poor transition.  These are well suited to fishing boats that regularly run over sand bars and not to performance boats.

There's not much I can really say about stainless steel as a material for impellers.  Stainless impellers are successfully run at all horsepower levels.  The highest horsepower application that I am aware of one of our American Turbine impellers is in is around the 3500 horsepower level.

Type 17-4 stainless is the material of choice.  The number 17-4 represents alloying elements 17% chromium and 4% nickel.  When I had my prototype inducers cast, this was the material I specified. 

I have seen 304 and 316 castings also.  The 316 is miserable to cut because of the work hardening characteristics.

Other Alloys

For the super baller who has everything, American Turbine has cast titanium impellers.  EXTREMELY light weight!!!

Precision Jet Drive offers billet impellers in both 1018 and 8620 steel.  These are very nice impellers and the performance potential definitely sparked my interest.  I really wish these were available in 17-4 stainless, corrosion is the issue.


COMMON FAILURES

If you are inspecting your own impeller, purchasing a used impeller, or you are an inspiring pump guru, there are some areas you need to watch for cracks.

1.  The leading edges of the vanes, near the hub.

2.  The shroud at the trailing edges of the vanes.

I am always suprised to see how damage occurs in practice, it seems like it is rarely what I expect.  One such example is the trailing edges of the vanes that have been thinned (foiled) by an overly ambitious guru with a grinder.  The trailing edges will bulge outward from pressure building inside the impeller, not from the force of the vanes pushing the water.

Rock Damage

If you have spent a lot of money on building a really nice jet pump, most people suck up rocks only once.  Some don't learn the first time.  Yeah, yeah...  I know...   It's always the wife's fault.

So, now that you (I mean the wife) sent some rocks through the pump, what can you do?

If the damage to the leading edges (picture 2) isn't too bad, they can be cut back.  You won't change RPM cutting the leading edge back and re-detailing them.

If the rocks pass through and rattle around between the trailing edges of the impeller and the leading edges of the bowl, the trailing edges of the impeller might only get bent.  Sometimes, this can be straightened and blended.  Excessive damage could require cutting the impeller to a smaller size, increasing RPM.  It is a bad idea to repair the vanes unevenly, because that will cause the impeller to push the shaft to one side, throwing off the balance and even causing contact with the wear ring.

Damage to the bowl (picture 3), on the other hand, can be dressed out, but it will never work as good as it once did.

Rock damage is one of those things where an ounce of prevention is worth a pound of cure.

Water Hammer

If the pump is unloading and loading, the shock to all pump components can be detrimental, including, but not limited to, breaking the impeller and bowl, even twisting the pump shaft (picture 4).

Cavitation Burn

Cavitation is the process of boiling in a liquid as a result of pressure reduction rather than by external heat addition. 

My favorite video on cavitation...  Yes, I'm weird like that:  Watch "16. Cavitation" on YouTube

Hopefully you see how irregularities in the leading edges (picture 2) of the impeller vanes cause cavitation.  In picture 1, the cavitation burn is the light colored blotches on the vanes at the radii near the hub and shroud.  In some cases, the 'cav' burn is so bad that the vanes are eroded almost all the way through to failure.  The burn itself is the result of the jet stream, acting on the surface of the impeller vanes, as the bubbles collapse in the higher pressure areas.

Not only does cavitation cause damage to the pump, it also induces inefficiency that robs performance.


OTHER FEATURES

American Turbine offers impellers with a secondary wear ring (picture 5) on the bowl side.  This is to fill the void between the impeller hub and the bowl.  This option requires either a Dominator bowl, that features a corresponding bore (picture 6).  A Berkeley bowl (picture 7) will not fit, but can be machined to accept this secondary wear ring.

Most of the time, we just machine these off, particularly if a stuffer plate (picture 8 ) will be used.  I think a properly clearanced stuffer plate is a better option, but I prefer the secondary wear ring over nothing.

Note:  A stuffer plate is one of those things you do AFTER you have already done everything else. From a performance per dollar spent standpoint, investing in a good impeller, loader, and bowl work are much higher priorities.


THE BUTTERFLY EFFECT

You didn't think it would be that easy, did you?  Well...  Sometimes it is.

Pump Shaft

Typically, if you're upgrading from an aluminum impeller, it's because of a corresponding upgrade to horsepower.  At this point, we consider an upgrade from a 304 to a 17-4 shaft mandatory.  Be advised, some older Dominator pumps and all new Dominator pumps come with a type 17-4 shaft standard.  Type 17-4 stainless is magnetic.  If a magnet will positively stick to your pump shaft, you're good to go.

"Looking at strength, designers should consider two parameters. One is ultimate tensile strength (UTS), the maximum tensile stress a material can endure without tearing. The other is yield strength, the tensile load per unit area required to permanently deform a material. Up to the yield point, deformation is elastic; the material returns to its original shape after the load is removed" (https://www.machinedesign.com/materials/comparing-stainless-steel-and-other-metals).

Mechanical Properties, depending on heat treating condition:

Type 17-4

UTS:  115-190 KSI

Yield Strength:  75-170 KSI


Grade 304 (18-8 - 18% chromium 8% nickel)

UTS:  75 KSI min

Yield Strength:  30 KSI min

Note:  KSI = Kilopound per Square Inch.


At idle, heavy impellers tend to make a lot of noise.  The reason for this is that there is a lot of accumulated driveline lash with two u-joints and a Parallel Key Splined yoke driving the shaft (https://en.m.wikipedia.org/wiki/Spline_(mechanical)).

The issue is that the engine does not produce a smooth rotating motion of the crankshaft, but an oscillating motion instead ( https://en.m.wikipedia.org/wiki/Piston_motion_equations).  Through each of the four cycles, the crankshaft actually bends and twists.  This motion is transmitted through the driveline to the impeller.

Steels are generally 66% heaver than aluminum.  A heavy bronze or stainless steel impeller acts like a flywheel, retaining rotational inertia (https://en.m.wikipedia.org/wiki/Flywheel).  As the crankshaft experiences positive acceleration, the impeller is likewise accelerated.  In between power strokes, the crankshaft experiences a negative acceleration, however the mass of the impeller is an energy store and doesn't change velocity as quickly.  The lash in the splines (and needle bearings in the u-joints) reverses directions until the next power stroke.

One related issue is that the hammering back and forth can distort the external splines of a type 304 shaft.  Support of the internal splines of the slip yoke is lost with the deformation of the shaft splines.  If the slip yoke cannot slide freely on the pump shaft, the axial forces will be transmitted directly to the crankshaft thrust bearing.

To give you an idea of how much force we are talking about, with a BC impeller and 200psi bowl pressure, the jet pump thrust bearing would experience 11,685.2 pounds of force pushing towards the engine.  The engine thrust bearing will be wiped dry as the oil takes the path of least resistance, out the front of the thrust bearing.

Using the equation F = P A

F = Force

r = Radius of the impeller large diameter

P = PSI (bowl pressure)

A = Area (8.625" impeller) = pi (3.14) r^2

Note:  200psi is around half of what a QE drag boat would produce

My opinion, it's not worth the risk of losing an engine.
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

Flusher

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« Reply #8 on: October 25, 2018, 04:33:13 AM »
Protruding vanes, on an American Turbine impeller, extending past the Large and Small Diameters.

Edit:  Thought I had pictures of an A2

Actually, I can't remember what size impeller this was.  It is larger than an A2, I can tell because of the flat surface at the trailing edge of the shroud, perpendicular to the pump shaft.

Proof positive that you never really know what impeller you have until you measure it!!!
« Last Edit: October 25, 2018, 04:48:56 AM by Flusher »
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

 


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