Propeller design - RC Groups
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Oct 17, 2005, 09:55 PM
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Propeller design

Ok this is for the engineers in the group. I am looking to beter understand propeller design and apply it to my sub. I have noticed that there seems to be a back flow that is causing maneuverung problems with my boat, this lead me to look at the "optimal pitch" at the hub at the prop, and I recognzed that the pitch is way too much. I think that it is causing a cavitation and hence the back flow. Has been verified through observation. So I was doing some rough calcluations to find the velocity components on the rotating blade (ignoring circular flow) and came to the conclusion that there are two approaches that might be used. A conservation of forces approace, and just assuming that the speed along the longitudinal axis of the boat over the prop is relative close to the speed of the boat. I will post my hand calculations tomorrow and if there are any that have experience in this area, please help me out. If you are asking why am I doing this, well, because I can, and I need to find some use for the stuff that they teach me in school.

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Oct 17, 2005, 10:45 PM
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This: is a site that I used to try to do calculations for an airplane prop. I'm sure that you could fairly easily adapt it for a marine prop. You just have to change the rho. Note: I never actually managed to get the spreadsheet that I was trying to do such calculations on to work correctly, but I think the water (incompressable) calculation would be easier. I'm not sure what you will want to do with a and b... but there's about all I know about the subject.
Oct 17, 2005, 11:56 PM
CG Bob's Avatar
You also need to look at the pitch at the blade tip and at least midway along the blade length (from the hub). If you have 45 degree pitch at the hub, blade mid-length pitch could be 35 degrees, and 25 degrees at the tip. In one of the past issues of the Sub Committee Report David Merriman did a write up on prop building and pitch layout along the length of the blade. Here's a discussion on boat props.
Oct 18, 2005, 12:32 AM
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i guess this is on your diffuser propellor and I assume that diffuser propellors work similar to diffusor impellers used in pumps, in that they provide a higher efficiency than conventional design, albeit within a reduced optimal operating range, ie they are specialised to a certain duty point. It could be that when operating at the normal running speed it is working very well indeed, but at lower speed or even higher speeds will be off-cam so to speak and therefore the backflow occurs causing cavitation.

In summary i believe that diffuser designs are much more sensitive to the angle of attack than conventioanl propellor. good luck
Oct 18, 2005, 09:37 AM
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Merriman Prop Charting Article

Dave Merriman does a nice job of how to "chart" your homemade
prop in the SubCommittee Report - Issue # 11 Winter 1992
pages 66-67. Back issues of the SCR are available from Don Osler

In the piece, Merriman shows how he builds the jig and calculates
the angle at different distances from the hub - illustrated by
Dave's as always nice photography.

Kurt Greiner also has a good "how to build props for ship models"
webpage - though he doesn't go into pitch or charting.

Here is my Hunley prop sitting in the jig after the blades were set the
way I wanted them. The "T" upholstery pins hold the blades in the jig
for soldering.

When I actually soldered the blades to the hub, I put
several layers of wet paper toweling under the blades and hub
to avoid torching the basement. A firebrick is a safer way to go, however.
You can get them at art supply houses that deal with ceramicists.


Last edited by TMSmalley; Oct 18, 2005 at 05:05 PM.
Oct 18, 2005, 10:11 AM
Suspended Account
Originally Posted by Aeroengineer1
Ok this is for the engineers in the group. I am looking to beter understand propeller design and apply it to my sub...


There is a lot of wrong information being spread on the various boards about screw type propellers. I see that Tim Smalley has presented one of my articles on the subject -- that should be useful. But, a few observations and a little advice here.

Get this straight from the start, gang: There is no cavitation at the differential pressures our little, low torque propellers, used on r/c submarines, produce. A relatively high differential pressure between suction back of the blades and water must be achieved before you get anywhere near to the vapor pressure of the water (ambient pressure/depth of the water is also a determining factor of vapor pressure). No, all r/c submarine propulsor problems are a consequence of improper propeller design and/or application.

Basic rules: Keep total developed blade area to about 65% of the disc, less the hub. And make the pitch (constant pitch propellers) the same as the diameter. Not a perfect set of parameters, but a good baseline for those of us not working at David Taylor.

So few of you who make your own propellers take the time or invest the effort to produce propellers who's blades are of a constant pitch through out their span. And the term 'pitch' has been so misused of late:

The term 'variable pitch' means a screw type propeller who's blades have different non-slip axial travel distances per revolution. The term 'variable pitch' does not define those propellers which employ a mechanism to alter the blade angle. 'Constant pitch' propellers are those with the proper helical twist to their blades that insures all radius points across the span of the blades advance the same non-slip distance axially through one revolution of the propeller.

All of the propellers we wish to use aboard our model submarines are (should be) of the constant pitch type, and this goes for rotors within shrouds. Few are so built. We model builders, as a group, simply don't have the tables, math, nor facilities to conduct exacting empirical studies that would permit us to make the proper integration of a finally tuned variable pitch propeller/rotor to a specific hull and speed envelop.

The boards are crammed with misused terms and wrong assumptions. If you don't know, then shut up or ask for help. Don't guess and then spout such crap as fact, it muddies the water.

Put this to the FAQ if you wish, Adam.

David D Merriman lll
Model God, Keeper of the Flame
Oct 18, 2005, 04:42 PM
Registered User

thanks link fails to connect. I'm about to start a scratch prop for the type 17. David or Tim please provide a link or two so dummy here can do this. I did make my own props for the type 17. Two of the little suckers and to my surprise they work better than good.

I did a simple job with flat brass and a wheel collar. I made a constant pitch of 45 degrees and silver soldered the blades into groves I made with a dremel.

I need something to get me going in the water until David makes the custom prop later down the road here. Thanks men.

Oct 18, 2005, 05:04 PM
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Weird - It works fine if you type it in...
Oct 18, 2005, 05:35 PM
Registered User
Thanks Tim! It works now!

Oct 18, 2005, 05:41 PM
Registered User
Here's another article by Pat Tritle that's helpful.

Oct 19, 2005, 07:24 PM
Registered User
Ok, here are my hand calculations. It is kind of hard to read, but I think that it is readable. There are two different approaches listed to the problem, so keep that in mind. Also my conclusion is that the answer is a combination of the two approaches. Please let me know what you all think. If you do not understand something, send me a PM with you ph# and I can call you. I think that it might be easier to do it over the phone. I can only call if you live in the continental US. Also let me know when the best time is to call you and your time zone.


PS You might need to save the picture to your desktop and then use an image viewer to zoom in. I tried to up load the pdf, but it was too large of a file.
Oct 19, 2005, 07:47 PM
Registered User

Merriman is absolutely correct in his analysis and advice. Listen to him! Another point is that you are NOT designing a propeller. You ARE designing an impeller. The science is NOT exactly the same. If you apply propeller design to an impeller it will only be applicable to roughly the first 37% of the diameter. The pressure balance, tip losses, & flow volumes vary greatly between the two designs. Why do you think that there are those funky vanes and the reduced ID in a propulsor shroud. Look at the flow of water in a blender on low speed. The flow dynamics vary greatly from the blender to the outboard motor propeller. In addition, submarine propeller mathematics vary GREATLY from your standard power or speed props. Profile is also extremely important. You will learn some of this in your Senior fluids class(elective)

Oct 19, 2005, 07:49 PM
Registered User
Here's what I'm doing in terms of prop calculations. I'm doing a straight forward angle at radius calculation based on ideal conditions, and simply adding in an efficiency factor. You input desired speed, root diameter, tip diameter, motor speed, and an efficiency, and it kicks out angles for the root, tip, and every .1" along the length. I figure considering the redementary nature of the building, it will provide all the precision I'll need. Spreadsheet attached, zipped up.
Oct 19, 2005, 08:09 PM
Registered User

I realize that this is far from a true optomization. In fact, I myself recognize that there are some major simplifications here, but what I am trying to do is two fold, first use some of this nonsense that they teach us in school, and second have a little fun by reasoning my way through the problem to come up with some sort of a good governing theory that can be built off of in the future. As for the angle varriation due to the effect of it being an impeller, not a propeller, as you have defined, I am going to take David's advice and make it a constant pitch propeller. As for what you have said to the comparison of a propeller to the impeller calculations only having commonality to the first 35-40% of the blade length, I am looking for the pitch at the hub so this should be about the same. If I were to get within 15-20% of my predictions, great! I am not looking for 0.01% accuracy here. I am just trying to see if I am (despite simplifications) on the right track in understanding propeller/impeller theory. If you have any reccomendations for reading let me know. If you have any suggestions on my theory, even better, I would like to learn where I can improve!


Thanks I will take a look at it.
Oct 19, 2005, 09:27 PM
"Old 8140"
Old 8140's Avatar
I would purchase one from Dave!!! Pure and simple!


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