Sunday, September 20, 2009

Making a new rudder, part 2

This post is part of a series on making a fibreglass rudder with a foam core:
Designing a rudder, part 1
Designing a rudder, part 2
Making a rudder, part 1

Calculations done, foam cut into neat rudder-sized rectangles, I had no alternative but to start shaping the foam. I did it like this...

Step 1: mark the depth of foam to be removed. I did this by setting a circular saw to the calculated depth for a given point in the profile, then running it the length of the rudder. I kept the trenches very close together at the leading edge of the foil - the first few tracks were only 2mm apart (the saw blade width), then, as the slope of the foil changed more slowly 4mm, 5mm, 10mm. On the rear half of the foil, whose slope is almost straight, the gaps increased to 20mm, then 40mm. With all the tracks cut, I sprayed blue paint into the cuts, making sure to get good coverage on the bottoms of the tracks.

Step 2: remove the bulk of the foam. I tried chisels: bad idea. What worked really well was to take a wood saw, hold it sideways and cut along the foil at a shallow angle, keeping a millimetre or two above the bottom of the trenches. In about 40 minutes I was able to remove most of the excess foam, leaving behind a nice flat surface.

Saws make short work of Corecell

Step 3: the saw was followed by a wood plane, which cut to within 1-0.5mm of the trench bottoms - but not lower, because the plane did not cut the foam as smoothly as wood, tending to leave the surface a little rough.

Step 4: I next used a surform to remove the last of the excess foam, leaving the surface of the foil flush with the trench bottoms.

Step 5: To get a really smooth finish, I followed up the surform with a random orbital sander.

With the two halves shaped and (I hoped) pretty symmetrical, I mixed up 250mm of epoxy, then stirred in enough colloidal silica (amazingly fine white powder) to make a paste with a honey-like consistency. A squeegee was perfect for getting an even spread on the flat side of the starboard foil half. Interesting to note: although the foam looks like a sponge, it certainly doesn't act like it - it doesn't the epoxy in, but leaves it on the surface - where I need it .

Rudder core halves clamped together; polythene drapes keep epoxy off the worktop

With an even spread of epoxy covering the starboard half, I dropped the port half into position, and applied my entire collection of clamps. You never have enough clamps. And that was it for the day: all going well, by tomorrow morning, the two halves will be one - permanently.

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