Saturday, June 6, 2009

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

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