Tuesday, July 28, 2009

Foil Shaping

Finally some progress on the Goat! After the plywood setback I decided to work on the foils and will probably also build the mast before getting back to the hull. In this way I'll have most of the various bits ready to go once the hull is complete.
The dagger board and rudder are made by first ripping cedar and hardwood boards into staves. Then gluing the staves into blanks. The blanks are then thicknessed to 22mm. And then they are planed to a hydrofoil shape. Since my planing skills are at rank beginner level I decided that the foils were not a good first project. I want to finish the foils clear and therefore need the wooden blanks to be shaped as perfectly as possible since I will not be able to use filler to fair out any imperfections after glassing. I would love to have them CNC routed to shape but did not want to invest in the considerable shop time for a one off project. So I came up with a jig which I call the SMC router. SMC for Simon's Manual Control. The jig is made from 3/4inch MDF and it allowed me to mill the blanks to a very precise thickness. I then used it to cut most of the hydrofoil shape, finishing up with a hand plane. As a bonus I'm going to make an angled carriage for the router and use this same jig for scarfing staves for the birds-mouth mast.

Cedar 2x4s from Lowes ripped and ready to glue.

Rudder blank being glued up. I had a bit of trouble keeping the staves from sliding around while the epoxy was curing so I used the table saw table extension as a flat surface to help clamp the gooey mess into submission. Wax paper kept the table saw from becoming a permanent part of the rudder.

Rudder blank on jig ready for thicknessing.

Router carriage over rudder blank. The carriage is free to slide around but is guided by the T shaped bit on the lower left. As long as a little pressure is applied to keep this T in contact with the fence everything moves smoothly with no binding.

Router carriage.

Router on carriage. The carriage is a bit wider than the router base. This allows the router to be "locked" in position by twisting it slightly so that the flat edge of the router base wedges itself in the carriage. This worked well and sped things up considerably since the alternative would be to clamp or screw the router to the carriage for every pass.

Video of dagger board being thicknessed.
I'm very happy with the resulting blank surface finish and thickness.

Thickness of one end of blank.

And the other end. This level of precision is not too hard to achieve but the jig has to be absolutely straight. Any warp in the base or fence of the jig will be faithfully reproduced in the blank. The first pass with the router showed that I had a .5 to .75 mm warp in the middle of the jig base. I placed the jig base on the table saw assuming that it was perfectly flat. Well it's not and the slight hollow on the table was transferred to the blank. A bit of shimming got that taken care of and the next pass with the router was near perfect.

Once the blank is the correct thickness, width and length a template is used to draw the foil outline on both ends.

Then back on the jig but this time the cutting depth of the router is varied to remove wood up to the pencil line. Then the leftover ridges are planed down with a hand plane. It is pretty easy to get a very straight and uniform profile the full length of the foil since the grooves left by the router make perfect depth gages. Stop planing as soon as the groove disappears and you are at or very near the correct profile.

Trailing edge being routed to shape. I left a lip on the end of the trailing edge to support the thin section while routing the other side. The lip is easily planed off afterwards.

Rudder blank ready for hand planing to final shape. Oh almost forgot, the blank is held down to the jig with sheet rock screws. Two are located where the rope handle holes will be drilled later. The other end of the rudder was left about 1 inch too long to allow for the clamping screws. In this way there will be no sign of the sheet rock screw holes in the finished rudder.

Total time to date 56 hours.


  1. Simon, I'm SO grateful for this excellent tutorial post. Your solutions are great: leaving the trailing edge lip, screws located in the future handle holes, planning ahead with extra length... brilliant! MDF FTW!

    Rest assured, I will do my best to mimic this whole process. I look forward to working my way through your blog to what I know turns out to be an awesome GIS.

    Thanks again for sharing!

  2. Dave, I'm happy you are getting something out of this blog. This is probably the most time consuming method of making the foils since you have to make the fixtures but they did turn out straight and fair.

  3. Hello Simon, I know I'm late to the party but better late than never. I am considering a GIS myself up here in Panama City and I would like to know if you have any info (drawings and such) on how to make your router sled and table? Or links to who does and might be willing to share?

    I found your blog by accident and have already seen your boat, Very nicely done indeed!
    I am coaming your blog to see how much I can learn from your project. Like the Tank too.

    pecostx51 at yahoo dot com