Tuesday, August 25, 2009

Making a new rudder, part 1

Following on from the previous posts on the design of rudder foils, our hero now begins the tricky fabrication stage.

As discussed previously, my new rudder will have a fibreglass skin (6 layers) wrapped around a foam core. Not just any old foam - Corecell, a light, stiff material that is absorbs little resin (or water) and will contain any damage to a very localised area. It won't rot, delaminate or crack, and the sheet I bought from the helpful folk at MID has a density of just 60kg / cubic metre.

I've never used this material before, so wasn't sure how easy it would be to work with. First, I trimmed the lengths I needed for the rudder itself from the original 4ft by 8ft x 25mm sheet using a jig saw. No problems, and surprisingly little dust - most of the material from the kerf seemed to stick together, hinting at one potential difficultly: since this foam is a very good insulator, heat from cutting tools disspates slowly - so cutting at higher speeds can cause melting.

Next, I used an off-cut to practice my foam-shaping skills (never shaped foam before, didn't want to start by destroying my proto-rudder). First, I clamped a straight-edged piece of wood atop the foam as a cutting guide (straightness verified against a handy glass window - float glass is very flat). Then, I set the cutting depth on my hand-held circular saw and cut a series of trenches in the foam, the depths calculated from my spreadsheet describing the NACA 0012 foil I want to make.

Next, I sprayed the yellow foam with blue paint, making certain it penetrated to the bottom of the trenches.
Once the blue paint was on, it was now "safe" to start cutting away excess foam; I tried a random orbital sander, but, especially at high revs, discs clogged too quickly. Tried a sharp penknife: not too useful. A block plane worked reasonably well, and a rasp/file came in very handy for tidying up small irregularities. The paint worked a treat - you can see below how the blue lines allow me to be sure I've not removed too much material.

Shaping the leading edge of the foil was quite easy; shaping the trailing edge - much more material to remove, but accuracy is less crucial - turned out to be more difficult. I actually tried chiseling, which worked up to a point, but was slow and difficult. Not sure about the best way to do this yet - I hope to try an angle grinder soon to slice the bulk off, then follow that with plane, disc sander and rasp for the final finish.

Conclusions so far: Corecell is very easy to cut. Shaping it is much easier than shaping wood, but the same techniques won't work.

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


Mattis said...

Hi Phibius,
I see it's been a few years, just wondering how the rudder worked out for you. I'm in a similar situation with my Contessa 32, so any more information, pictures of the build, etc. would be much appreciated.
Kind regards,

Phibius said...

I'm many months late replying, unfortunately. It actually worked out really well. Best thing I ever built. The Contessa is a really gorgeous boat!