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.

Designing a new rudder, part 2

By measuring photographs and doing a little maths, courtesy of the NACA 0012 formula, I was able to discover what size and shape my new rudder (transom-hung, constant foil cross section) should be. Next problem: what materials?

My old rudder was a single piece, almost certainly a tropical hardwood, dense and strong. Marine plywood was an obvious option, except that no local suppliers had real marine-grade plywood (non-marine grade, which I have used for cabin furniture, can have voids and gaps within interior laminates, hidden weaknesses which could be fatal in a load-bearing structure). My old rudder was a single piece of tropical hardwood, which I also tried to source: a local supplier had lots of beautiful teak and iroko, but no planks wide enough to make a rudder in a single piece - they simply aren't there to be had (they've all been made into rudders already?). It might be just as well: a solid piece of timber can be sundered by a single stress-grown crack, not such a problem with laminates.

Abandoning nature, I talked to the very helpful Liam Phelan of mid.ie, and began to investigate the possibility of a synthetic foam core (Corecell) wrapped in fibreglass. To get an accurate spec on which foam and how much glass, Liam suggested I talk to Martin Armstrong, chief technologist at Gurit, a firm which supplies composite materials to pretty much everybody who builds composite structures - submarines, wind farms, huge racing yachts, aircraft, etc. Martin is a busy guy, but he spent half an hour talking an amateur sailor and novice builder through the materials and techniques necessary to fabricate a composite rudder.

First, the core: A550 foam (Corecell) for the rudder core; a single 8ft x 4ft x 25mm sheet would suffice. I wasn't sure how easy this would be to shape, but Martin reassured me that it is far less dense than wood, while also having no grain; normal wood working tools would suffice, it could even be sanded into shape; a surf form might be handy. Only one problem to watch: being an excellent insulator, it is really bad at dissipating heat, so power tools should have fresh, sharp blades to minimize friction.

Layers making up my composite rudder

Then, the exterior, from which will come much of the strength; Martin specified six layers of glass cloth:

Layer 1: 290g 4-harness satin, laid at a 45 degree angle, and with a 100mm overlap both sides at the leading edge, and a similar tail at the trailing edge
Layer 2,3,4: uni-directional 500g fabric running top to bottom (no overlap)
Layer 5: 290g 4-harness satin, 45 degrees again
Layer 6: 290g 4-harness satin, 0 degrees

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

Designing a new rudder

So, our beloved Briongloid, a 6.6M fin-keeled sailing yacht went adrift from her mooring, and spent an uncomfortable day bouncing on pointy rocks. The pounding reduced her wooden rudder to matchwood - so it's time to make a new one.

How big, and what shape? From a profile scale illustration of a Pandora International (our boat's model) I figured out the height and width - about 1.65 metres * 0.37 metres. Now, I just needed the cross-section's shape.

It turns out that the best shape for a rudder is a foil - like the shape of a bird or aircraft wing, the magic of the foil shape is that it generates lift (unlike, say, a flat surface, which only creates drag). Back in the 1930's, the boffins at NACA, the forerunner of NASA, investigated different foil types to find the best shapes for different aeronautical (and incidentally marine) applications.



For relatively slow-moving displacement craft like our yacht, their "NACA 0012" foil is the best fit; by creating a Google Calc document based on the NACA 0012 formula, I generated the cross-section above (y and x axes are not in proportion). Note the very round leading edge and thin trailing end.

Many fins and rudders taper from one end to the other, and give the leading edge a crescent profile; this tapering reduces drag by about 4% - for me, not worth the much-increased difficulty of shaping the foil.

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