Shaping a dynamic chord length daggerboard/rudder blades

Hi,

I am building a new boat, and for this one I wanted to have a nicely shaped daggerboard and rudder blades, like so:



I wanted to use NACA 0010 for the daggerboard, and 0012 for the rudder. Usual method of shaping boards to a specific profile is using a router on a sled jig, like described in this nice article: Duckworks - Another Approach to Shaping Foils https://www.duckworksmagazine.com/09/howto/foils/index.htm

However, my boards are not just straight, they taper down towards the ends, meaning the chord length changes, and therefore this method would not work.

So, I built this little frame from plywood that has cut-outs in the shape of the required NACA foil at various sections along the board. The idea is to shape the board with a spokeshave, frequently lowering the frame onto the board, and using it as a guide, like a 3D template to mark the high spots and gradually work my way down to the correct shape. Once I'm close enough, I would switch to sanding and finish shaping by eye.



Turns out, this method is not as great as I thought. It is very awkward to hold the board up to be able to use the spokeshave - I can't put into any vice or anything because I need to check with the template frame every few strokes. And removing so much material by hand, trying not to go too low at any spot... Well, difficult to say the least. And takes way more time than I had anticipated.

I wonder if any of you can suggest any faster and more accurate way of getting that NACA foil shape over a board that is tapered towards the ends like mine. Getting these boards CNC'ed would exceed the cost of the entire boat (I made some inquiries, it is expensive as hell), and it would still need finishing by hand. Maybe some other kind of smart jig for a hand-held router? Or something else? Any ideas?

 

If you are not looking for a wood finish the easiest way is to work with filler like done on a normal template fairing job.
I guess you will glass the wood Core first. Best if the core is a fraction smaller than the final shape. Then mix some epoxy filler adding some pigment. Spread the filler generously on the board to match your grid. Place you grid in the correct position onto the filler (some release agent or similar recommended)
Let it cure. Remove your template. Now you have a reference grid on your Board. fill everything with a softer easy to sand filler. Use a sanding longboard to sand the filler down to you hard epoxy guides( hopefully red or similar and veryceasy to see)

 

That is a nice idea, thank you. But I was wondering if there was a better way to shape a foil like this to start with. It is not too late for me to re-do these boards from scratch with a better method. Preferably involving a router or other automated way to avoid all that time-consuming shaping by hand.

 

You could make a 3D model on the computer, and take the .igs or similar file to the router that, with all security, will be able to reproduce the model on wood or foam, only missing to apply several layers of fiber on it.

 

What you are describing is essentially CNC machining. As I wrote in my first post, I made some inquiries. The machining of such a part, wood or foam, is incredibly expensive. I sent the STL files of my boards to some local machinists. The cheapest offer was $300 for one side of the smaller board. You can imagine the total... Which is why I am looking for a do-it-yourself kind of a way

 

That would be a manual copy router or pantograph. The template is strip planked over positive stations.
More adventurous people carve the foils with an angle grinder.

 

That is a very neat tool. But it would require building a template first, which kind of suggests that perhaps it was better to build a framed, hollow board with outer layer of bent plywood/fiberglass instead of shaping a solid piece like I'm doing. If I were to make a lot of solid boards of the same shape, then this would definitely be the way to go. But for a one-off, it kind of doesn't justify the time investment. I mean, by the time I built that template, I would have planed the board manually like I'm doing now...

 

Yes I know, you have to build the machine first. For easier hand shaping you can use very coarse rasps, they remove wood very fast.

 

Yeah, the machine and the template itself... This would be good for mass production. Okay, I will buy some very coarse rasps. I hoped that there might be some better way to shape boards like mine with some similar jig to the one in the Duckworks page, but allowing for dynamic chord length. I guess I'm back to elbow grease