Venice, Italy has them, same with Amsterdam.
http://www.tripadvisor.com/LocationP...ce_Veneto.html
Carful... in the states we do use them around construction/dredging operations and traffic lights are used for locks and draw/swing bridges.
But it seems the ones you have in the states aren't at junctions and maneging canal traffic, I was thinking of traffic lights at river/canal junctions, acutally managing conflicts between vessel approaching 90 degrees apart. If you have any traffic lights on the water, a boat sitting at a red light would need to be anchored or moored to protect against drifiting through a red light. Also, as boats (being without brakes) often need paths allocated to them in dense traffic, marine traffic signals could not run in cycles like road traffic lights do. Boats don't have brakes and run at much longer headways than land vehicles of similar size, such that only one boat would be allowed per green light on each (allocated) path. If say two boats were approaching a canal junction 90 degrees apart, the merit of each would need to be decied before either arrives at the junction, either aribitarily by a traffic director, or a pre-determined system of priorities.
Minimal drift is ambiguous without qualifying. Drift to stop, holding a position, bearing vs course vs direction vs heading vs track????? :noidea:
Hence the question "what is minimal drift?"
Sir Isaac Newton and the laws of physics might disagree with you... The boat contains a certain amount of kinetic energy. Moving through the water imparts drag, which subtracts energy. At some point your drag absorbs all kinetic energy and you stop. Otherwise you just invented the first perpetual motion machine. Keep in mind drag from water IS an external force... Think of it the other way, you are saying at some point that drag goes away entirely. That is the only way it could continue moving for infinity. Outside of a vecuum, drag never disappears.
It is absolutely the same as a vehicle that you let coast on a perfectly flat piece of concrete. Its going to coast for a long time, but it will eventually come to a complete and dead stop. The mass of the vehicle and the medium it is traveling through (air or water) make zero difference, drag always wins eventually.
Dynamic properties of water mean that it is not "absolutely the same," and the medium in which it performs does make a difference. When a boat coasts, its rate of deceleration relative to the water decreases as it slows, the slower it is moving relative to the water, the less water it displaces, so it does in fact get progressively slower relative to the water without quite reaching a complete stop relative to the water.
because a vessel hovering over a river bed must go upstream in relation to the water flowing over the bed
Uhhh, no. A vessel 'hovering' over a riverbed is by definition stationary in relation to the riverbed. True, water may be flowing under the hull, but it's water moving, not the hull. Practical definition would have 'upstream' be the direction water is moving from, 'downstream' is the direction water is moving to. The vessel isn't going either direction if it's in a true 'hover'. Be that as it may, mayhap you can define which direction a vessel is moving when hovering over the bed of a true tidal river ... :faint2:
It may be stationary in relation to the riverbead, but as I said, it is heading upstream in relation to the water flowing over the bed. There is a difference between hovering above a riverbed and being tied to the ground, but not between hovering in one place above the riverbed and between going downstream at twice the speed of the water, apart from direction, both boats maintaining equal and opposite power and the same amount of steerage.
