[radiorental]

Why a bail of straw? Reminds me of the forgotten reason why the onion was thrown into the varnish

"Primo Levi was working in a varnish factory. He was a chemist, and he was fascinated by the fact that the varnish recipe included a raw onion. What could it be for? No one knew; it was just part of the recipe. So he investigated, and eventually discovered that they had started throwing the onion in years ago to test the temperature of the varnish: if it was hot enough, the onion would fry."

[DocTomoe]

Often, these things are utilitarian, not mystical. So my educated guess: Back in the day, the main problem was river mariners getting hurt by bridge arches that were lower than expected, and the bale of straw was a 'soft buffer' - better to get your head hit by a swinging bale of straw than a rock-solid bridge.

[Sharlin]

More likely that a bale of hay was simply a conspicuous but lightweight thing that was easy to get your hands on back in the day.

[radiorental]

I was wondering about that but it would only work for the sailor standing in the right place on a boat sailing dead center of the river where there is typically two way traffic.

This doesn't seem like a utilitarian solution, more of a signal with a symbolic intention?

[JimDabell]

Reminds me of grandma’s cooking secret:

https://www.snopes.com/fact-check/grandmas-cooking-secret/