Optimum Prop RPM???

[Bubba1235]

Ok, so I have read a ton of stuff on prop design but the thing I have not found is what the target RPM of the prop should be. Surly there is a theoretical limit to prop RPM before the prop begins to cavitate by forming a vacumm in front of the prop. From what I have seen this number is somewhere between 3000 and 4000 RPM but can find nothing to confirm that.

PS: I am not at all concerned with engine RPM or gearing as that is all about how to spin the prop. What I AM looking for is where the limit of props are in general.

 

[steelespike]

Re: Optimum Prop RPM???

Most straight drive inboards are 1 to 1 so prop spins at motor rpm.
I'm sure some high performance motors are red lined at 6,000 rpm or more.
There are also high speed boats with step up gearing turning in excess of 10,000 rpm at the prop. Some of these step ups were around way back in the 1930s.Even a stock inboard from the 30s could turn 3,600 rpm with no special equipment,props etc.I guess my point is prop rpm is almost unlimited
at least in relation to your 3000 to 4000 estimate.

 

[Dhadley]

Re: Optimum Prop RPM???

What you're describing is a vortex of air that forms on a sterndrive or outboard lower unit. As the case goes thru the water, the nose pushes water away and then it comes back to the case. The faster you go, the further back toward the prop the vortex moves.

Once the vortex reaches the prop blades the prop is spinning in air rather than water. This is what "blow out" really is. Many people think "blow out" is the point at which the prop vents. They are two very different things. Real blow out is not a fun thing to experience.

Since blow out is a function of gearcase shape rather than strictly a propeller issue we work with the gearcase shape to get around the vortex. Every gearcase has a blow out speed. A typical V6 case with a blunt nose has a blow out speed in the low to mid 70's. We can increase that by installing a nose cone. That changes the shape enough to raise the speed considerably.

Todays prop, by themselves, don't usually have a problem staying hooked up as long as the vortex isn't there. Props that are designed for speed often see (prop) rpm of well over 4500 rpm. IE - a prop on a V6 case where the motor is turning 9000 will see over 4800 rpm.

 

[QC]

Re: Optimum Prop RPM???

Assume I have a shaft (like an inboard) that I can spin anywhere fro 0 to 10,000 RPM and power is not an issue.

Are you contemplating a CVT type gear box? If so I have wondered about optimum prop speed as well. I imagine prop design would alter the answer considerably . . . Seems like expected top speed would also be a huge factor.

 

[Frank Acampora]

Re: Optimum Prop RPM???

In aircraft propellers, what you are referring to is not the speed of the prop itself, but the speed of the blade tips. Once the blade tips reach the speed of sound--about 6-700 feet per second at sea level--prop efficiency drops way off.

Now from a strictly limited but practical experience: With my Chrysler engines there are two gear ratios 1.72 and 2.0. 1.72 to 1 is used on the 4 cylinder engines and 2 to 1 is used on some 3 cylinders.

Take a given pitch propeller on the 1.72. Go up 2 pitches with the 2.0 and the theoretical speed of both is almost the same. However: If you take a four cylinder engine running a 17 pitch prop and swap on a 2 to 1 lower unit with a 19 pitch prop, with all other factors remaining the same, speed will increase about 5 MPH. The slower turning higher pitch prop appears to have better efficiency translating into more speed. PROP RPM range here is 2500-2800. Thus: a 17 pitch prop turning at 2800 RPM delivers less thrust than a 19 pitch prop turning at 2500 RPM EVEN THOUGH THEORETICAL ADVANCE IN INCHES PER MINUTE IS SIMILAR.

Again, this is practical experience limited to only one engine brand and one prop brand. It will in all probability not hold true under all conditions, for example, surface running props.

 

[steelespike]

Re: Optimum Prop RPM???

No question the unlimited hydros both the old piston engine hydros and present day turbine jets I believe turn in excess of 10,000.The piston engined
boats using Allison aircraft engines turning at something like twice there design rpm into a stepup gearbox.I believe the turbine has to step down due the the turbines very very high rpm.Can I tell you what prop; no but obviously custom built.I'm pretty sure the smaller hydros and non I/O off shore racers run close to if not over that 10,000 rpm figure.Some with step up gearboxes.

 

[Dhadley]

Re: Optimum Prop RPM???

Many drag guys (outboard and inboard) are using 1:1 gears and turn over 10,000 rpm. Most F1 guys turn well over 10,000 as do the Mod U guys. I'd assume the SST 140 guys are over 10,000 too. I'd bet the SST 120 guys are right there too.

 

[Tail_Gunner]

Re: Optimum Prop RPM???

Triple Digit Racing - Top Fuel Drag Boat
You can find pictures of the new Triple Digit IV boat on our Main and ... and she carries over 20000 RPM prop speed behind her as she amazes the crowd! ...
topfueldragboat.com/driver.htm - 14k - Cached - Similar pages

 

[QC]

Re: Optimum Prop RPM???

I know a little about centrifugal clutches, but like a torque converter they waste some energy at low (or all) output speeds. That's why I like a CVT . . . No "slip" in the trans, mega reduction for hole shot and some reduction for prop RPM control if necessary . . . No need for OD with almost all recreational and commercial applications.

Sounds like you are running into the Vortex that Dhadley describes above. I assume that a shaft and/or hull can cause that as well?

 

[QC]

Re: Optimum Prop RPM???

There is quite a bit of data out there for a ducted type prop. But your design sounds like a coffee can with a propeller welded inside of it, right? The can spins in bearings itself. No shaft at all. Gear teeth in the "end" of the can. Sorta like a flywheel ring gear. The engine "driven" part would be like the starter pinion, but always engaged. There must be some stationary piece that wraps around the can though, right? Bearing "housing"?

 

[steelespike]

Re: Optimum Prop RPM???

Penn Yan tunnel boats used to have a primative ducted prop but prop was not inclosed just set a hollow in the bottom. creating a shallow draft inboard.
The boat did seem to squat and I think was slow to plane.Reverse was even harder to control than a standard straight drive inboard.

 

[Frank Acampora]

Re: Optimum Prop RPM???

AHA! Your problem may be that you are using a propeller design in your duct! Remember: a ducted prop is different than a free running prop is different than a surface running prop. Each has its own design limitations.

For example: let's use a 2 blade surface running prop for simplicity. It is designed to run at high RPM with only one blade in the water at a time and the blade geometry is designed to maximize efficiency in that mode. Use the same prop fully submerged where wash off one blade tends to foul the other blade and the efficiency drops WAY down. (this is a very simplistic example, not meant to be exact)

Take a standard propeller and duct it and the efficiency of the prop rises until at some RPM the resistance of the duct puts backpressure on the blades and then---OOPS--SLIP. While one prop theory has a standard prop acting as a pump, I believe a better theory is the airfoil mode of operation--but that's just my opinion and God knows, I've been wrong before. ---I got married, didn't I?

SO: Look at the 0-slip IMPELLERS on jet skis. The blades are at least 1/2 revolution in size, usually almost one rev, sometimes more. Thus, water can not bleed between the blades and "slip". So: increase your blade area or the number of blades for improved high speed thrust if you do not want to go to an impeller design.

Smatter of fact, When I was manufacturing F/M Hydro-Jets, a model Berkely jet pump for model boats, I used a cut down prop which ended up looking a lot like a turbine blade. These ran at 10,000-15,000 RPM depending upon engine size (I used an .60 cu. in. 2 HP engine) and never blew out.

 

[Jeff Walkowiak]

Re: Optimum Prop RPM???

that is very interesting reading,, I often contemplated building a p w c with a tunnel and a prop instead of a jet drive only big issue is hull resistance,, without a lower unit to get most of the hull out of the water you are constantly fighting hull resistance and wasting a lot of energy,,
It always bothered me that my p w c with a 155hp 4 stroke engine can barely hit 60 mph and that is with low fuel and smooth water, but if you had a 155 hp 4 stroke outboard on an 11foot long ,4foot wide boat it would go considerably faster.
I would imagine you setup would have similar problems with hull resistance and loss of thrust in chop.

 

[Randybeall]

Re: Optimum Prop RPM???

When you get to high flow rates impeller design becomes critical! On top of that the design of intake and exhaust into the duct is critical also. It seems that just an open ring would be good but fluid dynamics is a quite complex subject and open front and rear of the flow pattern won't work. You will need some guidance of the fluid into the impeller and out in the direction of flow.

 

[QC]

Re: Optimum Prop RPM???

These two posts seem like they may be significant if I understand your design correctly . . .

Penn Yan tunnel boats used to have a primative ducted prop but prop was not inclosed just set a hollow in the bottom. creating a shallow draft inboard.
The boat did seem to squat and I think was slow to plane.Reverse was even harder to control than a standard straight drive inboard.

that is very interesting reading,, I often contemplated building a p w c with a tunnel and a prop instead of a jet drive only big issue is hull resistance,, without a lower unit to get most of the hull out of the water you are constantly fighting hull resistance and wasting a lot of energy

Assuming this is a single screw v-hull, is there any portion of the "propeller" that is above the bottom of the "vee"? If so, your blow out could be air right? As soon as the hull gets high enough to be efficient the prop could be getting above the clean flow of water?

 

[Randybeall]

Re: Optimum Prop RPM???

Take a look for pictures of the propeller on a late model necular submarine. Those props are turned at higher rpm that anyone cares to discuss. Some of the disign considerations have to do with width of blade and speed of blade in the water. As you go out from center the blade must narrow and thin to avoid cavitation. At the same time the cup shape must be correct to get the bite at low speed, without being a disturbance at high speed. Intrestingly enough the impellers in jet drives resemble the screw more closely, the only difference from a casual observation is that it is small diameter..Next thought is that by spinning the duct you have created causes an eddy current on the surface of your duct which most likely disturbs the water flow on the outer end of the blades of your prop. Thinking about the spinning ring changes several things that we don't normally think of. I am going to research jet trubine air flow and maybe find out more there. Non-compressable water does react differently than air so it may not be applicable.

 

[Frank Acampora]

Re: Optimum Prop RPM???

Reading Randy's post reminded me: Do you have the absolutely necessary straightening vanes aft of the prop? Without them, the water becomes a spinning slug that goes no-where. Without them, at some point, probably due to surface friction and inertia, instead of moving the water aft, the prop just spins it.

Without straightening vanes, my Hydro-Jets would not pump any water.

And, don't forget, because your system is more like a jet pump than a prop, it will probably have the same loss of efficiency. Basically and simplicitly, the faster the boat goes, the faster it "runs away" from the jet of water squirting out the back, so there is less reaction, proportionally less thrust, and the rpm/speed curve goes flat or drops.

 

[QC]

Re: Optimum Prop RPM???

the faster it "runs away" from the jet of water squirting out the back

Not sure if this is related, but coincidentally, my boat has an unsusual tendency to run away from any small vessel with a "jet of water squirting out the back" . . .

 

[Randybeall]

Re: Optimum Prop RPM???

Ok, so I have read a ton of stuff on prop design but the thing I have not found is what the target RPM of the prop should be. Surly there is a theoretical limit to prop RPM before the prop begins to cavitate by forming a vacumm in front of the prop. From what I have seen this number is somewhere between 3000 and 4000 RPM but can find nothing to confirm that.

PS: I am not at all concerned with engine RPM or gearing as that is all about how to spin the prop. What I AM looking for is where the limit of props are in general.

Lets look at this from the beginning with the addition of a lot of peoples information. What you are asking is how fast can a piece of shaped metal move through the water before cavitation occurs. Your design has merit, however there are a number of drawbacks. Weight, pump turbulance, necessary thrust guides, to name a few. I would suggest that you might slow down your impeller and give it a higher numerical pitch. Why waste energy when you want to make the most efficent water mover you can. How much friction does the large bearing area of your spinning ring cost you? Think about the fact that a boat is no more that a water dynometer where you can stop and fish when you get tired of trying to beat physics.
Any given blade design will have a maximum speed it can be turned before cavitation occures. Every blade design will be different. Your orignal question is answered. No one number will do it all.