Shaping new foam rudder foils for an old catamaran

[Bobcat]

Due to logistical problems ... Don't ask! I need to fabricate two new rudder foils from a double layer of 25mm structural foam without the benefit of a router and jig.

When shaped, the core will be reinforced each side with 6 layers of woven/biaxial glass fabric (in epoxy) before fairing and coating with "Coppercoat" anti fouling.

I am looking for an appropriate way to shape the compound foil core which will become a balanced, roughly trapezoidal shaped rudder of about 22" span, 18.5" at the widest chord line and 2" thick

Maybe others have sailed that route? Any advice much appreciated.


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[out42]

I haven't done exactly what you're doing but have done similar in enlarging rudders on two outremers with similar sized blades.

I ended up making mine out of softwood and simply using an electric hand plane to bring it down to approximate shape and size and then a normal hand plane. Took about a day each. Then epoxy with microballoons and glass over that to get a smooth finish rounded off with long(ish) board fairing. Another two days - well, four, if I'm honest.
Slow, dull, works. Better still, when you do it wrong first time you have plenty of opportunities to re-shape half-way through.
If I was to start again, I would use foam as you are doing and simply use the electric plane and then layers of carbon/epoxy instead of glass/epoxy. Carbon is relatively cheap for the amount you are looking at.
Oddly, it was no trouble getting the chord line position and slope correct with a few measurements and then by eye.
Has it worked? Yes. Is it exactly right? Don't know, but it works better than before.

[barnakiel]

Another neat option is to place harder (say 3 mm plywood) ribs onto the rudder webbing then add foam in blocks in between. Now you just remove excess foam with anything long and flat. Minimum of 2 'ribs', one each end. But I found it easier to work with a rib every 4 inches or so.

b.

[UNCIVILIZED]

Some guys, who are better fabricators than me, have shaped custom parts out of foam by using the hot "wire" method. In this case the wire was actually a thin, wide strip of metal, bent to the precise shape that they needed, & fabricated into a jig.
After which, they figured out the correct amount of current that was needed so that they could slowly feed the foam through it without maiming the foam. And then the foam was sanded & coated with laminate.

I also would think that carbon fiber makes sense, & am wondering as to the choice of biaxial. What's the thinking behind using it? Aren't the primary load pathways on such a board oriented more along different axis then +/- 45 deg.?

For more knowledgable feedback on this topic, you might try boatdesign.net As well as looking towards some of the one-design, & box rule racing catamarans. The ones where guys almost exclusively sail on DIY built boats.
Stumble might know a bit about this topic, & the afore mentioned info sources.

Also, I'm thinking that unless you truly can't get a hold of one, you'll save money by purchasing a router, & building a jig for it with which to shape things. As the tool will cost you far less than if you goof, & destroy that kind of foam.
Though there's nothing which says that you can't build a jig for a power planer to do the same job, or a jig for some other tool for that matter.

I say as much, as when Kurt Hughes had just recently finished building his personal tri for himself, when I first met him, he'd farmed out having the rudder built (well, rudder #2), to the pro's at Speedwave. I thiink it was Speedwave.
When I asked about it, his comments were along the lines of $ being saved vs. were he to try & build another rudder himself, in order to get it right, & then goof. Then it'd wind up costing more than the pro built one, & not be as strong, nor fair on top of things.
As the boat already had the first rudder which he'd built, still in place. And he was hauling the boat to install the Speedwave rudder. That, & that for foils that see so much load & such high speeds, so that things need to be very precisely machined.

[barnakiel]

I would only use carbon if the rudder is a foam blank. Otherwise many rudder blades depend on the internal structure for strength and stiffness.

If you are building a performance dinghy rudder, then carbon can be a fine choice.

b.