Adam Carlson's 1/16th Japanese Type-A r/c Submarine, Part-5

A Report to the Cabal:

My contribution to the 1/16 Japanese Type-A midget submarine project was the design and fabrication of the two axially aligned, counter-rotating propellers. I was able to employ, directly, the photos of the real submarines from the Japanese book Adam brought over as part of the documentation package. As marine propulsor fabrication is my strong-suite, that job was a natural, and I jumped right in the day Adam arrived.

Building such a two-unit propeller propulsor is basically no different than building a more traditional single-unit propeller propulsor: The blade geometry is established (in this case by plan and photographic interpretation), the tip diameter and pitch are already established, and a master of the blade -- in this case two blades: the forward propeller a larger, right-hand turning unit; and the after propeller a smaller, left-hand turning unit -- cut out in such a manner as to establish a constant helical twist, the helical form driven by the specific pitch of the propeller, that work done with the assistance of a propeller blade angle chart.

As Adam was busy working the other masters of the Type-A, I measured the tip angles of the two propellers off the photo, graphed the pitch-radius station chart, built the propeller blade blanks, made rubber tools from each, cast the required number of metal blades, assembled the metal blades to the hubs, and finished off the work by applying fillets where each blade intersects its hub ... and plenty of round-file and careful sanding work.

At this point I have two nearly completed propeller masters that will be used to make the production rubber tools needed to cast complete propeller units for the eventual models that Adam, myself, and later customers will enjoy.

Though partially obscured by the propeller guard cage, this photo was instrumental in working out the pitch of both propellers.

The process was simple (assuming, of course, that both propellers are of the 'constant pitch' type): The angle of the tip of a propeller blade is measured, the circumference of the tip calculated (6.28 X radius), and a graph laid out, its horizontal line length equating to the tip circumference (the distance the tip describes in one revolution of the propeller), to the extreme end of the circumference line an angled line, the angle observed at the tip of the prototype, is laid down and run up till its other end intersects a perpendicular line originating from the other end of the horizontal circumference line. The triangle thus formed represents the pitch of the propeller (the length of the vertical line). Since all radius points along the span of a propeller blade of a constant pitch type propeller have the same pitch, it's a simple matter to plot the radius points (converting them to the circle circumference unique to that radius point) on the horizontal line and to then connect each radius point via an angled line to the established pitch point atop the vertical line.

Believe me, its more difficult to give a word description to the process than it is to simply perform the task on graph paper!

Here we see the work as it stood as of last evening. Test fitting the two counter-rotating, axially aligned propellers about a common 3/16" shaft (that will later be enlarged to 1/4" for the forward propellers, and reduced to 1/8" for the after propeller). Careful examination and lofting of the propellers illustrated in the Japanese book went a long way in my successfully capturing the look and utility of the original to model form. The forward propeller has already undergone the fillet work at the propeller-hub interface.

It's clear here that each propeller is held on its respective shaft with a boss-nut which is tightened or loosened with a spanner-wrench -- the holes set in the lighter colored boss-nuts are where the wrench studs set during assembly/ disassembly of the propulsor components. This shot is useful not only for illustrating propeller attachment means, but it also shows how the trailing and leading edge of the blades have been machined away at the end of each hub, resulting in pronounced flats which widen near the hub fillets. Something I'm replicating on the model propellers at this time. Note the large fillets between blades and hub.

And here you see the prepared propeller blade chart used to plot Renshape blanks. Those blanks cut with bandsaw, files, and sandpaper to adopt the helical twist called for. To the left you see the forward clockwise turning RenShape 20 blade master ready for some putty work. To the right is a partially worked RenShape 40 blade blank that will eventually become the after, counterclockwise turning blade master. I would have preferred to have used the RenShape 20 for both units, but I ran out of that stuff right in the middle of the propeller job ... Damn!

The completed blade masters were set into lengths of acrylic cylinder which served as containment as I poured the BJB Industries, Inc. TC-5050 silicon RTV mold making rubber. The two single-piece rubber tools encapsulated the blades which were mounted on thin brass wire to suspend them about an inch off the mold board as the rubber underwent its state change from liquid to solid. After the rubber had cured hard, it was taken out of the containment, and a slit, along the leading and trailing edge of the propeller blade master carefully made, permitting extraction of the master and later cast metal pieces.

Assembling the blades to the hub of the after propeller: I cast four white metal blades for each propeller. I then mounted one blade on its respective hub, using a piece of plastic mounted on the assembly jig -- cut to the correct angle for the tip radius. Note that I used some modeling clay to temporarily hold the blade in position as I eyeballed the blade for correct rake and pitch. Once happy with how this first blade sat within the hub it was CA'ed in place (using baking soda as a grout and CA as the adhesive to bond metal blade, grout, and hub).

That first blade was then used to create an Evercoat filler 'crutch' between the lower face of the blade and the surface of the mold board. This crutch would insure exact registration of the three following blades as they were positioned and glued to the hub. The same process was repeated for the other propeller.

And this is the work as it stands as of this morning. And it will remain at this stage as I work to get caught up on long neglected obligations.