KNOWING YOUR PROP

Knowing Your Prop

Knowing Your Prop

This information posted with permission from Carlos Andrade (Shark Racing)

 

Propellers

 

The page bellow is an attempt to stress the importance of the propeller in r/c boat propulsion. It results from my lectures on the Web and information posted on Jim's board, mainly by Jim Nissen, Paul Govostes and Mike T. When you finish reading this, hopefully you will be interested in learning more. Go to Paul Govostes page: www.rcprops.com He is one of the best on this matter, the only problem is that he doesn't modify props for sale; you can't buy one of his props, you just have to look to them on his page and dream.

When I started thinking on a page like that I posted some question at Jim's Board; the answers just made me sure that I had a lot to learn. So, when I finished the page I asked Paul if he could look at it and correct my mistakes. Well, he not only corrected the mistakes but ad some good information. It's just a matter of justice register that this is a Carlos/Paul page and I am most honored with such company. Paul, thank your very much for your kindness and for sharing your knowledge with us.

 

Introduction:

The prop is responsible for transforming the movement of the motor’s power. It accomplishes this task by displacing a mass of water when turning. One of the first laws of physic states that with each action there is an opposite and equal reaction of the same intensity, and of contrary direction. As a result, the water movement is displaced backwards, and the prop is pushed forward. There is a rigid link to the hull, that results in the boat moving forward in a speed proportional to the volume of water being displaced. The prop rests in the water and moves forward, by the amount of it's pitch – in theory. As water is not a solid mass, the prop slips and the forward movement is less then the theoretical forward motion.

So, the larger the pitch, the higher the speed, as a result of increased forward movement at the same time unit. This is true to a certain point. Sometimes, the pitch is so great, the engine can't reach it's maximum RPM. And less RPM, means less forward movement within the same time unit, = less speed. Compared to your car, a higher gear reaches higher speed – in normal circumstances. On a stepped hill for instance, the load on the engine causes a reduction in RPM and a decrease in speed. Under these conditions, the 3rd gear may be faster. Like the car gear box, smaller pitch has more acceleration and less top speed, while greater pitch has more top speed with less acceleration.

 

Conversely, too little pitch makes the engine over-rev, with possible costly engine damage.

As your boat's engine doesn't have a gear box, the prop has to be carefully chosen to overcome these restrictions.

Diameter is also of prime importance when choosing the right prop: larger diameter = more thrust, and greater capacity for moving a heavy hull. Increasing the pitch generally demands a reduction in diameter, lessening the drag and increasing speed. Again, this has a limitation: a minimal diameter is required to guarantee the necessary thrust.

Moreover, the diameter helps control slippage, larger diameter is less prone to prop slippage, until the larger diameter causes too much drag and reduces engine speed. In a way, diameter directly effects speed, but it's major influence is on acceleration.

 

The number of blades is also important. The ideal prop should only have one blade, eliminating drag of the other blades that are not producing useful work. As this kind of prop would be impossible to balance, 2 bladed props are standard. On some type hulls, 3 or even 4 blades may be used. Mainly on boats that demand lift at the transom, because they run too louse. Outriggers are the best example of this kind of hull. It is also commonly accepted that a 3 blades prop has a smooth acceleration, because 1 blade is always into water. And more: considering the same pitch, a 2 blades prop may have 1 mm more in diameter and will load the engine the same as a smaller 3 blades prop., But a 3 blades prop has more prop walk and torque roll.

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Blade Thickness (the thinner the best). A thin and sharp blade cuts the water and reduces prop-walk tendency (walking to the right as the transom goes to the left, because of the props turning effect).

 

Preliminary Definitions:

Blade: is the prop part that cuts the water and thrusts it back. In R/C speed boats we use 2 or 3 bladed props, with very few 4 blade props.

Hub: central portion of the prop, where the blades and prop shaft are attached, The Hub provides most of the strength of the propeller.

Diameter: prop size, measured between the tips of the blades. Or 2 times the measurement between the tip of the blade and the center of the hub. Larger diameter props are normally used on large and heavy hulls. With R/C gas boats, 65 to 80 mm is the standard. Like pitch, a larger diameter means more water is moved per each revolution.

Pitch: theoretical distance the prop travels on each rotation. This is theoretical because water is not a solid medium and the prop slips. 10 to 30% is normal, the lower numbers are found only on hi-performance props, specially prepared.

Constant Pitch: the pitch is the same across the entire propeller’s working surface, or blade face.

Progressive Pitch: the pitch is lower at the leading edge and increases progressively along the trailing edge.

Consider a prop that runs in a liquid media with a constant pitch, the tip of the prop rotates faster than the hub. Conversely, if the angle at the tip is lower, the water speed on all prop surfaces will be the same. In this case, the performance is far better. Progressive pitch props offer better planning performance.

NOTE: The ability to turn a large diameter or high pitch prop at a desired RPM is determined by the amount of torque provided by the engine. Bigger is not necessarily better.

Aspect ratio: (a definition by Paul Govostes): The relationship between pitch and diameter, is measured as the "Aspect Ratio". (Pitch divided by Diameter).
(eg) The Octura 1475 has 4.13"Pitch/ 2.95"Dia. The relationship between Pitch4.13" divided by Diameter 2.95" = (1.4 Aspect Ratio). Higher Aspect Ratio = More Pitch/ Less Diameter. And lower (AR) = Less Pitch/ More Diameter.
Generally, a faster prop has a higher (AR), more Pitch, with less Diameter. However this holds true to a certain extent, as adequate Diameter is necessary to sustained push!

Leading edge: the side closest to the transom.

Trailing edge: the side furthest from the transom.

 

The bellow drawing (adapted from an original posted by Wayne Rathbun) shows a propeller parts:

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Same parts, on pics this time:

6.jpg

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High lift: a prop which tends to lift the transom when the boat is running.
Low lift: a prop which does not have this tendency.

With Octura props, the X series (an X before the number) indicates a low lift prop. Props without the X are high lift props.

Ex.:X470 Octura prop
X= low lift prop.
4= pitch ratio
70= 70 mm diameter

1667 Octura prop
lift prop
1.6 AR
67mm diameter
So: pitch divided by diameter= (AR) or pitch/67=1.6 or pitch=1.6 x 67= 107.2 mm diameter or 4,2"

Cavitation: is water vaporizing due to the extreme reduction of pressure on the back of the propeller blade. Many props partially cavitate during normal operation, but excessive cavitation can result in damage to the prop's blade surface. Note: Cavitation is often confused with Ventilation.

Ventilation: often confused with cavitation, it's normally induced by an external source, not by the prop itself. It occurs when air is directed on the prop surface. Eventually, it may help by reducing cavitation. On real boats, this is sometimes accomplished by directing the exhaust over the prop. On R/C boats this is not a common practice; nevertheless, it looks like some guys in Swiss are trying this. Look at the pic bellow:

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Cupping: Is a curvature added at the props trailing edge, normally very slight, towards the blade center, shaped like a spoon. It reduces cavitation and maximize thrust, increasing the effective prop pitch. Additionally, it lifts the bow and narrows the thrust cone. It looks like a magic medicine, the solution for all our problems but, like any medicine, it has to be taken carefully. Besides, cupping has to be equal in all blades, or you will have an imbalanced prop, that may damage your engine.

Camber: Arching curve from leading edge to trailing edge, like a spread out cup. It makes a progressive pitch at the prop.

Rake: Is the angle of the blade attachment to the hub, and the degree that the blades slant forward or backwards. It's normally use to correct ventilation or cavitation situations. Furthermore, rake lifts the bow and so the speed. Typically, low rake props are used on sub-surface drives with heavy hulls. Higher Rake blades with surface drive application, tend to re-direct and condense the thrust cone further aft.

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Questions on which prop to use are recurrent at Jim's Board. The selection bellow is a compilation of props indicated.

Manufacturer: not necessarily the manufacturer of the original prop. In some cases, the listed manufacturer is the one who provides the prop modification.

Use: no indication means that the prop was listed as a general purpose prop, one that you should have just in case.

Additional features: additional modifications made to the original prop.

 

Tips on how to improve your prop: (all posted on Jim's Board)

Jim Nissen:

Trailing edges should be sharp. Props are like rudders and the sharp trailing edge acts like a wedge rudder's trailing edge. It helps to incorporate air into the trailing edge and this reduces drag from the blade.
Carlos,

I found the easiest way to do prop mods is to just do it! Pick up a stock prop you really don't care to ruin. Start by getting it ready to run. Thin and sharp. Then one simple method to do lakeside tests is to make up a bending pliers. Weld a large ball bearing to one jaw of a standard pliers. Smooth out the other jaw so it doesn't have any sharp edges yet is still fairly flat. You would be surprised how little bending will drastically change the props performance.

Remember to bend the trailing edge and blend the bend into the blade so it's a gradual ramp. Once you have something close then you can take it home and clean it up.

How much pitch is OK? You need to measure it. I'll start out by saying I need to get a real prop measuring tool. I built up a dial indicator stand but I am having difficulty getting the setup to measure the same back to back. Paul G. shows a real nice stand on his site. Required if you ever intend to make another one like the one you bent. Keep good records in a notebook on what hull, setup, speed, etc... as this is really critical later on when you get home and try to remember what worked well.

Other notes: I used to use pliers and the ball bearing thing but I also have a prop anvil with three different delrin blocks machined for various blade profiles. Much better blade to blade consistency. Cup can be put into blades rather easily. I have used a small hammer with this setup but they tend to dent the prop back sides. I'm looking for a small rawhide hammer as I hear these are the ticket to prop mods! Just do it. I learned up to this point by ruining several props and making some "junk" props real stars.

 

Paul Govostes:

Hi Jim/ Paul G here,
Great detailed prop post on "Just Do It". However I want to clear-up any possible confusion on trailing edge sharpening. The trailing edge shouldn't be sharpened like the leading edge, it should be done just like Jim said: " a wedge rudder's trailing edge". Here’s a link to a prop pic, that clearly shows what Jim has explained.

Attached pic:

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Pitch: There are several different ways to increase blade pitch. The easiest and often most effective approach is to start your bend about 1/4>3/8th" below the (Trailing Edge), at a point about 60>70% out from the Hub. Gradually bend to end up with a smooth increased blade ramp, than re-measure your pitch. The blade should be bent at 2>3 different points, while gently adding pitch. Always re-measure between bends. I also bend or contour the (leading Edge) to add or remove pitch at the lead. I don't recommend bending the leading edge, unless your experienced with leading edge re-shaping. This is where you can screw-up a prop, very easily.

Rake; a higher blade rake does help to redirect and condense the thrust cone. Deep Back-Cutting, starting at the Hub along the trailing edge, tapering to Zero at the tip; will also increase Rake. However, if your main goal is high Rake, it's best to start with a high Rake Prop. Rake isn't that important with our surface-drive model boats. It's very important with (full size), sub-surface race boats, because all blade surfaces are submerged.
I hope this has helped answer your questions.
Good luck & fast running!
Paul G

 

NOTE: Marty Davis is referring to water testing a prop with a Dye substance that shows areas of the blade in (RED); that may be removed to increase prop efficiency and performance.

Marty Davis:

"OK...the first thing that you should look for is red along the trailing 1/4 of the prop. If there is any, you have bent the pitch into the prop without a very gradual bend. If you don't have a perfectly gradual bend, it will make areas that will cause cavitation. The second thing to look at is the tongue area of the prop. There will be red in areas of the tongue. That indicates areas that you can remove. The tongue itself will be clean, which shows the opposite, that it is biting to much. The tongue as you know is the lowest pitch part of the prop and it is the thing that holds back the prop (especially on X series props). REMOVE THIS AREA ON X SERIES PROPS and the boat will run much faster."

Mike

After sharpening and balancing props I run the file across the top edge of the blade to square it up. A round edge will alter the flow of water off the prop, so you want a nice square edge to break the flow cleanly.

Paul Govostes:

Prop Polishing More Speed

Generally speaking, a prop that is high polished on both the (anterior & posterior) face & back of the blades, including the hub; will achieve the highest top speed. However there’s some advantage to a rough or less smooth surface, on the “back” of the blade. It gives more bite for short course acceleration, however this can cause drag at higher speeds.
I believe there's little to no difference between a satin finish on the back of the blade, vs high polish on both sides, until very high speeds are reached.

Small versus big prop – An interesting question posted at Jim’s Board and very conclusive answers.

Although the original question pertains to an impeller enclosed into a pump jet, the answer still applies do our props.

Small fast prop or big slow prop.

What are all the advantages and disadvantages for going either option?
It seems that if you had a choice and wanted to go as fast as possible, most people would choose a tiny wee prop and spin as fast as possible. If that is the case, what’s the disadvantage of going slower with more pitch?
My jet pump design has got a constant diameter so the only alterations I can make are to change the rpm/torque characteristics or change the pitch of the impeller.
Is the reason for going fast and small only valid for surface drive props or would they also be valid for an enclosed impeller? I'm probably going to try out different pitched impellers anyway, but I would like to know any theory as well.
Cheers bye
Peter

A. From my limited understanding jet pumps should be rather tight tolerances on the impeller outside diameter or circumference. The pitch would depend on your engines ability to turn it and the intended velocity desired. I will assume maximum velocity is desired so try 3 or 4 impellers of the closest tolerances and different pitches. Also may want to limit the twist of the water leaving jet so as not to spray but to shoot strait back. Others will help better
have a nice day
Sean

B. Bigger props have less slip, but there is a point where they can take over the handling of a boat too. Also because of less slip they can accelerate better, depends on a lot of things, one being weight of the hull.
Mike Gilman

C. Finding the right prop configuration is generally a black art. You've done a super job of designing and building your jet, but since you are breaking new ground, I suspect you may be boxed in by certain decisions made early in the process like size of the passage. You may eventually want to build several jet assemblies to try. Maybe some of the fast jet-ski builders can give you some hints on impeller design. We have it much easier just dealing with struts and props, for sure. Our method in choosing props is to find one small enough not to bog the engine down (i.e., allow it to achieve the RPM where it develops peak power) but large enough to prevent over revving, Then increase pitch to give the boat high enough "gearing" to put the RPM and power to best use. But low enough pitch to allow it to maintain speed thru the turns. Then shape the blade to maximize thrust versus theatrics (rooster tail). Most fast props have progressive pitch, meaning the pitch increases gradually as you move farther back on the blade. Factor in lift to get the right hull ride characteristics. Lift is great for a hydroplane but is usually unwelcome in a mono. Then we snoop around to see what everyone else is running and try to buy or build something "as good" and "better" by tweaking around their design. Strange things happen when a prop is spun at 16,000 RPM half out of the water, and any prop that is "efficient" at 6,000 RPM would kill performance at 16,000.
Mike T.

D. Peter because your Prop (jet ) is enclosed diameter really cannot be changed much. I think you should be trying to get your motor to rev at about 14k, I think it is a stocker right? 13,5-14 is good for a stocker in a mono with a prop. The way I see it you have two choices, pitch and number of blades. More blades will decrease revs but increase efficiency; more pitch more speed but could bog it down. Your second problem will be keeping your impeller loaded as the boat skips across the water; I am not sure of what impeller design affects this.
Good luck and more pictures.
Mike Gilman

 

Final thoughts

After those tips on HOW to improve the propeller, I start thinking on WHY. Why those mods improved the prop? Of course, I post my doubts at the board (Jim’s, of course) and guess what? I received lots of very useful information from David, Denny, Mike T., Gene Gillman and, last but not least, Paul G. himself. Lets try now putting that information in order.

He had me reduce the diameter and round the tips slightly: to reduce transom lift and prop walk.
The tips were also slightly cupped: to direct the trust cone instead of making a big rooster tail, I now get a lower spray of water. Seems to be giving more "push".
Also performed was a back cut: this was to unload the engine from having to turn a high pitch prop. Seems to only affect the lower rpm range (take off and turns).
Recently, I got bold and performed a small barr cut on the same prop since it seemed that there was too much load on engine with my prop. Seems to have helped.
My 2 cents worth
David

Carlos, Some of the modifications are easy to understand. The Barr cut (the rounded notch cut at the trailing edge by the hub), the back-cut 1/8" at the hub to 0" at the tip and also trimming back the lead lobe all reduce the low end load on the prop so it can get the rpm's up quicker. They do some other things as far as efficiency is but I am not going into that. The trailing edge of the blade must be squared off with a file to a sharp edge so the water comes off it clean on the concave side. If anything it can be angled towards a sharper edge to make it cleaner. The shapes of the blades are kind of deep secret sort of thing, but I favor the shapes that are like the ABC H-31 and 75x97. When I re-work 1475's and 482's they end up in this similar shape. I hope this helps you but Paul G. can write a book for you. He is GOOD.
Denny

The best reason I can give for cutting down the diameter is to get the pitch that comes on a larger prop, but reduce the size (thus reducing the bite) to free it up some out of the hole and through the turns. You reduce the tongue to cut off that lower-pitched part of the blade also frees it up and allows it to slip a little and keep the revs up in the turns. Meanwhile the greater pitch can kick in and give you longer legs at top speed. You also need to thin the blades a lot and keep your concave shape on the power face or all the rest of that will be a waste of time. Remember, a lot of these props were actually designed several years ago when engines and boats were a little different and need to be updated for what we are trying to do now. I have an outboard racing prop here from 1935 (the year) and it looks a lot like a stock 1475 or 1667. It's also money well spent to buy some props worked by one of the masters and keep them as examples of what you want to achieve.
Mike T.

I have had real good luck doing this too, select the next size up for more pitch, reduce the dia, thin the blades and recap tips for tighter thrust cone. Simple and it accelerates like a BANSHEE!!!!!!! I would say perfect is when the engine gets on the pipe, the rooster tail lays down and yer GONE!!!!
Gene

Hi Carlos,
The following explains: (Increased Pitch) (Reduced Lift) & (Reduced Slip) These are the 3 most important factors that improve prop performance.
With Surface Drive, you can gain additional speed with most props, including the H50, by running it deeper -reducing slip. This is the single best trick for higher speed. In general, surface drive props suffer from anywhere between 20 > 35+% slip, the effect on High RPM is greatly diminished by excess slippage. The spoon shape blade narrows the thrust cone as water exits the blades. It is re-directed further back for increased forward thrust and also reduces lift as water exiting the submerged blade is also re-directed aft. Not straight off the blade in a downward direction, which tends to raise the transom, causing unwanted "Lift".
The trick is to modify the blade shape, to accomplish the above without loading the engine, causing reduced RPM! With the spoon shape mod you win 3 fold, (Increased Pitch), (Reduced Lift) & (Reduced Slip)!
Best Regards, Paul G

What makes a prop a high or low lift? This interesting question was posted at Jim's Board. Jim Nissen himself gave the answer:

You can't always tell from just looking at a prop if it is a lifting prop or not.
It all depends on how narrow or expanded the thrust cone is exiting the prop. A wider thrust cone will expend some of the power pushing water backwards and also down and to the sides as well as up (more spray in the rooster tail). The water that tends to spill off to the sides provides some lift. Props that have very narrow thrust cones will push almost all of the water directly back. Very little to no extra water will be wasted to provide lift.
On some mono props you can see the outer edges actually have a bit of cup built into them. This helps to redirect the water so it doesn't easily spill off the blade outer edges.
If your starting to experiment the best advice is to bend very little and measure a lot! Keep a log book and if possible measure the pitch that you bend into the props. Pitch guages are available or you can make one. They really are valuable as a tool to analyze your prop performance. Also a good idea to invest some in a good prop anvil and a small non marring hammer. I also have a pliers I modified to bend pitch into props out at the lake site. With GPS's now it's really easy to tell if your moving in the right direction or not!

After reading all this, what lesson you can learn? The most important: there is no a single prop good for all possible combinations of hull/engine. The basic rules are listed, but consulting the experts and lots of experimentation and patience is fundamental.

Regards

Carlos Eduardo

2/22/02

 

A note from Paul Govostes:

From RC Props: I don't modify or sell props as a business, only as a hobby. I do them for myself, and occasionally for boating friends and racers, free of charge. I will help anyone that (Just Wants To Do It) by learning the techniques and tricks, needed for tweaking their props, for maximum speed!!

 

 

Copyright © 2000 - Carlos Andrade

 

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