Originally Posted by zz4gta
I have a hard time imagining keels 'flexing'. Turbulant flow around or off the trailing edge of a foil yes. But having thousands of pounds of lead or cast iron 'flex', even under 15+ knots, I just don't see that happening.
When I was in the Marine Corps, our Boeing rep told me a story about the company the developed the torque gauges for the H-46 helicopter. The torque is determined by measuring the twist in a 1.5 inch solid shaft over a span of about 8 inches. When the Boeing rep toured their facility there was a 10 inch thick piece of marble with sensors on it. They could measure the deflection in that marble when he set his coffee cup on it. Everything flexes. When the frequency of the flex matches the items natural resonance, the flex compounds itself, and causes a hum.