The Port of London Authority’s River Bylaws of 1978 has a wonderfully simple and illustrative guide to signals on the Thames used to indicate movement, purpose, and size.

This is Byelaw 27(1)(b) for example:

By night a ferry shall carry amidships in addition to sidelights, and the forward and stern lights prescribed by Rule 23(a) a blue light over a white light not less than 2 metres or more than 3 metres apart visible all round the horizon at least 1 mile.

Byelaw 29(2) caught my eye:

When the headroom of an arch or span is reduced, but still open to traffic, the following signals shall be suspended from the centre of that arch.

By day – a bundle of straw large enough to be easily visible.

Might be about time to update that signal. I guess we can be thankful it has already been updated from the old practice of hanging screaming convicts or rotting animal corpses.