Tuesday, June 23, 2009

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

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