R/C'ing the Revell 1/72 GATO Class Static Submarine Kit, Part-12

A Report to the Cabal:

The Revell kit propellers are nice little pieces of work. However, the blades are simply too thin of section to survive a hull ahead or astern bell, leave alone the normal handling accidents these models suffer from time to time. I decided to build masters, tools, and eventually produce cast white metal propellers for the r/c version of this kit. Some things you just gota do!

I'll walk you through this process with the minimum of comment. If you want more information in the form of long-winded text, I refer you to these articles -- plenty of good stuff about propeller theory and fabrication techniques. And don't be put off by the second article, though dealing with pump-jet rotor fabrication, the majority of that discussion is germane to this subject too:

http://vabiz.com/d&e/CABAL/Building%20Masters%20For%20A%201_48% 20SEAWOLF%20Pump-Jet,....htm

http://vabiz.com/d&e/CABAL/Improving%20the%20SubTech%201_60%20ALBACORE %20phase-2%20Kit,%20Part-3.htm

The set of white metal propellers I'll eventually fabricate, based on this master work, will join the other WTC-2.5/GATO package items -- all intended to permit the kit purchaser to convert the Revell 1/72 GATO into a practical r/c model with little other outside sourcing.

(However, I don't plan on making these propellers available separately -- there are just too many jerks out there too willing to copy/steal my (and others) work. If I do decide to make 'em available separately I'll price them outrageously high in order to keep the riffraff's paws off 'em. We'll see).

Making a marine or aero propeller at first seems a very difficult task. No! It's easy! Trouble is: making a reasonably efficient propeller is HARD!

Doing it right means you have to understand the principles involved and be able to apply them as you turn concept into hardware. You identify the propellers diameter, its pitch and the total blade area (as pointed out above, if you know pitch or diameter you can work out the other two numbers), employ a propeller blade blank forming and carving methodology that will produce a master who's every radius point (a given point along the span of the blade) has the same pitch as all other radius points along the span, and to be able to build a propeller blade blank that, when carefully carved, will embody not only the correct helical twist to assure a constant pitch along the span of the blade, but will also embody the correct foil section at each point along the span of the blade.

I'll show you how I do it and will give you a little insight into propeller (specifically, marine propeller) theory and operation, sprinkled with some item specific nomenclature. About a year ago Adam Carlson gave me a copy of one of those old MAP (Model and Allied Publications) books titled, Model Boat Propellers, ISBN 0 85242 712 3. A good read and keeps the discussion non-math laden with chapters such as, 'Basic Propeller Theory' and 'Propeller Design.' The rest of the discussion is electric/glo speed centered and of not much utility to those wishing to better understand submarine propellers. Recommended reading.

The word 'propeller' denotes any mechanism (or inductive device for you MHD fans) that induces the fluid -- in our case an incompressible liquid, water -- to move. Propellers include such devices as oars, paddlewheels, sculling rudders, webbed feet, pumps, Archimedian screws, and the Voith-Schneider cyclic wheel. The propeller most of us are familiar with is a derivative of the Archimedian screw: Blades rotating about a central shaft, the blades arranged (usually) equally around the shaft, their spans perpendicular to the shafts axis. The shaft turns, the blades rotate within the fluid setting it in motion. The blades are so shaped and angled as to create an overpressure on side and a negative-pressure on the other; this differential pressure causes the fluid to move axially in relation to the propeller shaft.

Why are propeller blades shaped the way they are? In marine propeller parlance there is the 'pressure/face (aft) and the 'suction/back' (forward). The pressure/face is typically flat or very slightly convex of surface, the suction/back has a substantial convex curve to it. The severity of these curves and blade thickness increases near the hub of the propeller, this to speed-match all portions of the blade and to increase the strength of the blade at its typically narrowest section, where it intersects the hub. Keep in mind that the inboard portions of the blade propeller travel slower than those areas near the tip, therefore the need for greater angle, the objective of this uniform helical twist along the span of the blade is to (attempt) achieve a uniform pressure differential over the entire span of the blade.

In a non-slip environment the distance a propeller blade will travel (axially to the propeller shaft) in one complete revolution is called pitch. Pitch is a linear measurement. Pitch is not an angular expression! Pitch is the theoretical distance traveled in one revolution. However, as a practical matter the true advance of a propeller through the water is some fraction of the pitch. Without slip, without the propeller blades assuming a positive angle of attack to the fluid as they spin through it, there would be no creation of a pressure differential across the pressure and suction sides of the foil and no thrust.

Pitch is one of the three major defining measurements by which we describe a propeller. The other two are the propellers diameter and total developed blade area. Know those three things and you can make a reasonably efficient constant pitch marine propeller.

Ideally, the pitch at each radius point up and down the span of the blade is the same. Therefore the blade has a constant rate helical twist to it, the degree of twist a function of the pitch of the propeller. I've just described a 'constant-pitch' type propeller. And that's the type I'm building here.

Some rules of thumb for submarine propellers that have served me well: Make the pitch equal to the diameter. and make the total developed blade area (less the hub) between fifty and seventy-five percent of the disc area (a circle defined by the tip diameter). So, If you know diameter, you have your pitch; if you know the diameter, you have your disc area; and once you know the disc area you subtract the hub area take, say, sixty-percent of that and divide that number by the number of blades you want, and presto-chango you know the developed area of each blade.

Simple!

The hard part is working out the geometry of the blade blank so the finished carving has the right helical twist and foil shape from root to tip.

On this job -- two replacement propellers for the 1/72 Revell GATO kit -- I knew the diameter, and I knew the blade shape. I simply worked the helical twist to suit the pitch (which is equal to the diameter).

... Oh, you guys who have or are building a SubTech, Thor, DeBoer, Atomic Subs, Steve Neill, or D&E submarine kit ... with a few exceptions, I'm the guy who designed, built the prototype, did the tooling, and produced the model propeller you're sticking on the ass-end of that toy submarine of yours

... Me, Propeller Man!

OK, here's how the masters went together.

Other than demonstrating I don't know how to spell 'circumference' ... this drawing, a blade chart, is used to graphically determine the angle at specific radius points along the span of a propeller blade for a given pitch.

Once you establish the angle of the blade at a specific radius point (spot along the span of the blade) you can formulate the geometry of the blank from which the blade master is carved. Here you see a blank, market off and ready for the second cut to reduce it to the correct sections at the identified radius points. From there I simply connect-the-dots with knife and sanding drum. on this propeller I used only three radius referenc points.

After forming the gross twist and foil shape on the two blanks I then placed masking tape templates over the work and cut and abraded the blanks to outline -- this is the 'developed' blade outline, the actual, not apparent, look of the blade. I enlarged the blade outline a bit from that of the plastic kit propeller, the objective to slightly increase the total blade area -- the kit props struck me as slightly anemic.

One-half of a blank cut down to shape with rotary sanding drum and knife. The crosshatch marks denote what has to be removed from the other side of the blank to give form to a propeller blade master. Brass templates seen here to good effect.

Though I only needed two identical Renshape 40 blanks from which to make the required left and right hand pitched blade masters, I built four blanks. One for photo work you see here, and another, initially used to get a 'feel' for how I would work the blank -- something to learn on before committing the two actual blanks to work. Remember: when I do a photo essay like this, you are seeing mostly what goes right rather than what goes wrong. However, whenever I deem it instructive, I'll go into detail on things that go horribly, horribly wrong. It's a natural human want to revel in the agony of others and I'm here to please!

Further reduction work on a presentation blade master -- starting to look like a propeller blade. This is the 'practice' blank used for photo work and to work out proper tool selection and order of use. As I work this thing I take note of what tool is best for a particular stage of forming. For example, I found that the initial cuts on the blank were best done with the Moto-Tool rotary drum, followed by hand filing with little bastard files, then using #100 sanding sticks to complete the contouring. Initially I tried using a #11 X-Acto blade for the initial cuts, but after finding I was loosing more material off the end of my thumb than the blank, I quickly switched to the rotary drum. I'm a quick study in the presance of blood!

I first copied the actual developed shape of a kit propeller blade, then enlarged it about five percent -- I'm using a Graupner 400 on each shaft so felt I had the torque available to warrant the extra load. But, more significantly, I found the kit propellers to be a bit shy in area, there even being short-shots of some of the blades at the roots -- failure of the injection process to completely fill the propeller cavities during production, so the actual model parts themselves are suspect as to accuracy.

At some point I mounted the two propeller blade masters on Renshape 40 handles. Note that I've marked the handles with 'Pressure' on this side. Of course, I marked the other side 'Suction.' Here I'm applying masking tape templates onto each blank. The outline will guide me as I cut and grind the blanks to correct blade outline. The, L.E. denotes, 'Leading Edge.' Notice that I maintain the three significant radius points as cord lines on the blanks during the entire carving operation.

The left and right handed propeller blade masters ready for primer. To the right is another 'practice' propeller blade blank and the templates used to define its shape of the propeller blanks. The oval template is used to cut out the masking tape templates adheared to the blangs as I give final shape to the blade masters.

The finished propeller blade masters were used to make high-temperature RTV silicon rubber tools. From those tools I produced the required number of cast white metal intermediate blade masters. These needed to create the propeller masters. Behind the mounted blade masters are lengths of Lexan tube, used as rubber containment dams.

The cast white metal blades were trimmed to length and CA'ed within cutouts made in the turned Renshape 40 hubs. Here you see the propeller master assembly jig, used to evenly space the blades around a hub. The jib also insures that each blade sits on the hub at the proper pitch, rake and skew.

Here you see that the first R.H. propeller blade has been glued in place to its hub, this blade is used to form the blade crutch needed to insure symmetry of the other three blades about the hub. The L.H. hub, to the right, has it's blade aligned next to it sitting atop a hunk of clay -- a shot of CA adhesive and some baking soda will bond the blade to the hub, but only after I adjust the position of the blade so that it assumes a near perfect 'mirror' orientation in relation to the R.H. propeller blade right next to it. This is my preferred way of making left and right handed propellers.

This is how I build up a propeller blade crutch: First I butt glue a blade to the hub, using plastic angle pieces to insure the blade is at the correct angle to the hub, the blade held and shifted around on a little mount of clay stuck to the board. I then take the single blade hub to a hole off to the side of the jig and there, brush on some wax to blade, hub, and board to keep things from sticking to one another. I then build up a clay dam on the leading edge side of the blade and pour in catalyzed filler under the blade and let it cure hard.

I then pop the blade off the crutch, pop the crutch off the board, and (using the blade again, this time as an alignment tool) glue the crutch to the work station on the board, where it will be used to hold each blade as it's glued within a hub cutout.

The two assembled propeller masters ready for some filler buildup at the root-hub unions to form tight fillets as well as filling depressions caused when I cut out hub cavities a bit too wide during blade assembly.

Just for fun I slipped the propeller masters into the strut bearing foundations to check for 'look.' In foreground is one of the kit propellers used to identify the main perimeters I wished to capture on the new propeller masters: tip diameter, hub diameter, and blade shape. I developed a blade chart to give me a blade helical twist that would produce a pitch equal to the tip diameter of the propeller -- my practice when building practical marine propellers.