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Old Apr 07, 2005, 11:36 AM
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RGinCanada's Avatar
Guelph, ON
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Do-It-Yourself Brass Props

As I was about to order another CDN$30.00 (plus shipping) prop for my scale boat, I began thinking to myself (always a dangerous sign) "There must be a way to make my own!".
After surfing the web, I found "Towboat Joe's" website, http://www.dragg.net/jbrown/ and Hey Howdy! he had instructions on (you guessed it) building your own prop. His method didn't exactly suit my needs, but it put me on track to try my own design.
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Old Apr 07, 2005, 11:37 AM
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Guelph, ON
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Application and required tools.

Well, I knew the prop I wanted was 1.5" in diameter. Its for a 31" displacement hull, and I have installed a 5/32" diameter prop shaft.
(It just so happens that 5/32" is close enough to 4mm that you can safely thread it with a M4 die).

I had just purchased a deluxe tap and die set for CDN $70.00 (don't worry, cheaper sets are available on ebay for $9.00, and they plenty good enough for brass, copper and aluminum). Along with a pair of scissors and a propane torch, I had all the tools I need to make the prop (Well, I used a drill press too, but that wasn't strictly neccessary).

I assembled some K&S sheet brass (nice stiff stuff left over from another project), a length of 3/16" brass tube, and two short lengths of 7/32" tube. (This is the telescoping tubing sold in most hobby shops)

A tin of soldering flux and [EDIT: changed solder, coreless is cleaner] a spool of coreless electronics solder were scrounged up from different corners of the shop.
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Last edited by RGinCanada; Apr 09, 2005 at 05:32 AM. Reason: Updated process
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Old Apr 07, 2005, 11:39 AM
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Guelph, ON
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My highly technical prop design approach....

I fired up AutoCAD (any 2D design software will do nicely), and created a ton of different props with various shapes and surface areas, and number of blades. I picked one that I thought looked the coolest, and printed it out. (Yeah, I know propeller design is critical. Yada, yada, yada. I have more than enough motor, so a high efficiency design isn't all that important. Remember, I'm pushing a lumbering displacement hull through the water at 15kts scale speed)
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Old Apr 07, 2005, 11:40 AM
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Guelph, ON
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Rough assembly.

It was time to burn down the garage. (Uhh, I mean solder the parts together).
First, I cut a slightly oversized 1.5" disc out of the brass sheet, and drilled a 3/16" hole in the center of it.
Next I formed four rings out of solder, two with an ID of 7/32" and two with an ID of 1/4". (Wrap it around the tubing, and cut it)
As the final preparation step, I roughed all the pieces up with 150 grit sandpaper, and coated the works with flux paste.

The "propeller blank" gets assembled by slipping the smallest tube through the hole in the sheet, and then "sandwiching" the sheet by sliding the larger diameter tubes over the smaller tube.
The solder rings are placed on both sides of the sheet, and at the outside ends of the large tubes.

The works are placed in a vise, or other non-combustible holding device , and heated with the torch until the solder melts, and any excess drips away. Important: while the blank is still nearly red hot grab it (with a pair of pliers, unless you want a permanent "propeller" shaped scar on your fingers!!) and quench it in cold water. This will keep the brass from annealing and becoming overly soft.

At this point, I needed to drill out the inner tube to remove excess solder. If you are more careful than me, and use a smaller diameter solder, This probably won't happen to you.
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Last edited by RGinCanada; Apr 09, 2005 at 05:34 AM. Reason: Added pictures, updated process
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Old Apr 07, 2005, 11:41 AM
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Finishing

I printed and cut out the propeller design (never mind cutting out the blades, just the circle), and carefully cut a 1/4" diameter hole in the center. I then glued this as a template onto the blank. (You'll want to wash the blank first). I roughed out the shape with a pair of tin snips, and smoothed everything down with needle files.

One neat thing about this way of creating props is that the blank can be a left or right hand prop until you form the curve into the blades! I found a socket that had just the right amount of curve for my prop. I bent each blade the same way by placing the socket sideways in the vise, laying the blade on top of it, then resting a scrap of wood on top of the blade. A few taps with a hammer on the scrap of wood formed a consistent, smooth curve in the blade. Each blade was then twisted with pliers to get the amount of pitch I wanted. You can use a fancy high-tech measuring device (like a ruler ) to make sure all the blade tips are offset by the same amount if you want, but I just eye-balled it.

The final step is trimming the hub to a suitable length and polishing all the soldering crud off of your shiny new prop!

The total cost to make a custom prop was around $0.75, and it took me (aside from drawing time) about an hour to finish.
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Last edited by RGinCanada; Apr 09, 2005 at 05:47 AM. Reason: Adjusted the process
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Old Apr 07, 2005, 12:09 PM
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McAlester
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RG,
Sounds easy enough, and certainly simple enough. Have you tried your prop yet and how'z the ballance?
- 'Doc

"Easy" and "Simple" are relative!
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Old Apr 07, 2005, 12:20 PM
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Lt Doc:

Ahhh, you got me there!

While I have installed the prop onto the boat for photos, I haven't "tub tested" it yet. The beautiful Raboesch prop I bought for my last boat (This one's twin) drove it through the water waayy too fast, and this one creates a breeze when running in air, so I think it'll be OK.

The balance seems fine too, but I can always fine tune that with a file and some sandpaper.

Ray
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Old Apr 07, 2005, 01:54 PM
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United States, MN, Brainerd
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Great post!

What a great "how-to" post. A good model of a simple way of sharing skills and methods. I would never even have considered making a prop - your detailed post makes it look easy. Now where's that sheet brass......
Thanks!
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Old Apr 07, 2005, 10:03 PM
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Monterey Bay California
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I agree! well done, Ray! Thanks for posting this topic! looking forward to the pctures!
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Old Apr 08, 2005, 12:52 AM
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good idea thanks
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Old Apr 08, 2005, 10:00 AM
KC8WPF
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Euclid, Ohio, United States
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You can make a simple prop pitch gage using a block of wood, a peg, and some triangles of wood or plastic. I use a piece of plywood bigger than the prop. Look at the diagram below, it shows three triangles; divide a radius line into three equal lengths. The number of triangles and divisions of the radius line dependes on the size of the prop; a small prop might need only two triangles; a larger prop might need 4 or 5. In the center of the ply, drill a hole equal to the prop shaft diameter. Insert a peg into the hole, the peg is the same diameter as the shaft. Put the prop on the peg. Glue the triangles to the baseboard on the divided radius line. You can now tweak the prop blades to match the triangles.
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Last edited by CG Bob; Apr 08, 2005 at 10:18 AM.
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Old Apr 08, 2005, 12:54 PM
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Hi CG Bob:

I wonder if you could clarify the use of the prop pitch gage a little. Forgive me if I seem a little thick, but my boat experience is near zip.

I understand the plywood base with the peg, and I understand how to create the triangles. What I don't quite get, and I'd like to understand is:

a) why is 'H' the blade width at the hub?
b) which way do I glue the triangles on ?
c) what part of the prop blade should match the triangles
(maybe the angle of the blade at that chord?)

I'm hoping that I'm not the only one that this wasn't immediately obvious to .

BTW, I decided the prop blades on my first prop were too thin (I bent one tightening the prop onto the shaft), so I'm now making an nearly identical prop with 0.032" thick blades. I should have it finished by tomorrow, and I'll post pictures of the entire process then.
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Old Apr 08, 2005, 03:17 PM
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Euclid, Ohio, United States
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Quote:
a) why is 'H' the blade width at the hub?
Look at your prop artwork - the blade width at the hub is narrower than the blade body or chord. You don't really want "H" to be equal to the length of the prop hub - look at my scratchbuilt prop on the USS SKIPJACK.

Quote:
b) which way do I glue the triangles on
The hypoyenuse of the triangle is what you match the prop blade to. The bottom line on the triangle drawing represents the plywood base. You can glue the triangles on for either a right hand or left hand pitch prop.

Quote:
c) what part of the prop blade should match the triangles
The prop blade should lie flat on the triangle. This is merely a jig to put the pitch and curve into the blade.

I'll make a little mock up and post some pictures later.
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Old Apr 08, 2005, 03:54 PM
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Hi CG Bob,

Thanks for the follow-up (and nice prop on that Skipjack!). If I understand correctly, 'H' controls the amount of pitch built in to the prop, increasing this value increases the pitch.

Using the thickness of the blade at root is a handy rule-of-thumb.

(By George, I think I got it! )
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Old Apr 08, 2005, 04:56 PM
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You're right, "H" is the prop pitch. The blade angle changes from the hub to the tip, so the pitch stays constant. If the tip blade angle was the same as the hub, the tips would race ahead of the blade roots - causing cavitation. Here are some pictures of the blade angle gage. This is a right hand pitch prop; for a left had pitch prop, turn the triangles 180 degrees.

One of my workbench references is the "Propeller Handbook", written by Dave Gerr, published by International Marine; ISBN 0-07-138176-7. Cost was US $19.95.
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