Building Rudder Plug

[catenahalf]

I have drawn up an elliptical rudder I want to make a plug of. The planform is about 15"x62" including the head stock. The section is a NACA0012. My plan was to build separate halves by making male half sections placed about 6" apart running from the root to the tip. The section templates would be a 1/4" shy of the final foil cross section, to allow for 1/4" thick wood strips running the length of the blade; like stripping a hull. Followed by sanding, bondo, sanding, bondo...

The thing I'm trying to resolve in my head is getting the "nose" shape correct with the 1/4" strips (and the trailing edge, now that I think of it) . Do I just strip the major part of the board and do the leading edge (and TE) separtely? I have a pretty decent collection of woodworking tools (bandsaw, tablesaw, planer, jointer, drum sander, routers etc). Just not sure of the most efficient use of them is for something like this. I can build stuff with corners and flat surfaces pretty easily. If the foil were a simple taper things would be easier, but I really want an elliptical shape.

The elliptical shape makes it hard to machine without a CNC cutter. Having a CNC cutter do 2 male halves would be perfect, but I'm trying to accomplish this without too much outlay--I presently have more time than money for this :). I would also like to learn some plug and mold making skills while I'm at it.

Any advice is greatly appreciated!

[Landlubber]

catenhalf,

Divide your rudder into many sections, say every 4". Plot these out full size, as it is only 15" wide you can do it on an A3 (I think).

Now with foam blocks 4" thick, stick a piece of section on every piece, rough shape with the bandsaw, glue em all togethet them plane down to the paper templates, the shape will be perfect if you take your time doing this.

Once the rudder shape is complete, simply coat with poly bog made from resin and glass baloons for ease of sanding them finish off as good as you wish the product to be. The plug is very important to be perfect, as every piece made from it will only reflect the amount of work that is in the plug in the first place. Have fun.

[catenahalf]

Thanks Landlubber,

Here's what I understand from your post (and a few additional questions):

I'm going to print and cutout half sections and glue them to the foam (incidentally, I have a 2x4ft x 4in blue foam used in hotwiring). These will be roughed out on the band saw with a vertical cut, so the half section is 4in tall.

Next, lay these out on their side (centerline) and glue them together following the pattern from a planform template.

Then, I will carefully sand the foam pieces to the point where the paper half sections begin to show.

Bog them up and fair; do the same for the other half then bond them together?

Will these be tough enough to do a final fair as a unit and to mold from?

How much "skin thickness" shoud I allow for in the paper templates?

How would you suggest I keep track of the leading edge centerline for positioning the parting board?

I like your suggestions; a lot less tools and setup involved in your method.


Thanks for the input!

[Landlubber]

Yeah, it works well, have had to do it on numerous occassions, the thickness of the skin is up to you, it is only a plug don't forget, the mould has to be strong, the plug is only used once then hung up on the wall to show everyone that comes into the workshop how clevere that you are....

300g csm is enough for such a small job, go thicker if you feel you need to, the thickness is only about 2-3mm anyhow. In boatbuilding it is known as "moulded depth", the inner dimension of the object being built.

the centreline is marked on the paper templates. a straight edge aligns then easilyas they are all laying on the table when glued together.