Could this refer to the profits arising from the operation of a chain ferry?
A chain is laid across the river and a vessel is attached to the chain so
that it is free to move along the chain. Below the vessel is a keel set at
45 degrees to the river. The sideways force exerted on the keel by the
current drives the vessel across the river and the chain stops the downstream
component of the force sweeping the vessel down the river. In order to cross
back over the river, it is necessary to have a mechanism whereby the keel can
be moved to 45 degrees the other way.
I remember seeing such ferries operating on the Rhine 40 years ago but have
no idea whether they were ever used on the Thames.