Hi Les and all,
As Chief Magneteer, can I inject a little sanity into any proposal to tow a ferry floating in real water with magnets?
One of the hard facts about magnetic attraction is something called the "Inverse Sqaure Law" which means the distance between the attracting magnets is critical to the attractive force exerted by the magnets. If you double the distance between the magnets you only get 1/4 of the attractive force that you had previously. If you halve the distance between the magnets you get 4 times the attractive force. Lets apply this and some other basic physics to a floating barge in water.
For a barge floating in water. the bouyancy of the barge has to exceed the weight of the barge for it to float at all. If you bring magnets into the equation the magnetic force exerted has to be enough to attract the barge toward the bottom hold it for towing yet be not enough to sink the barge when added to weight of the barge.
If the barge is bobbing about the distance between the magnets is varying. If the magnet force available to do the towing is varying you run the risk of:
1 - sinking the barge and it becomes stuck to the magnets on the bottom of the pond if the attractive force becomes too great
2- losing the magnetic tow effect and the having the barge drift away if the attractive force is reduced too much (more likely, especially if waves are introduced)
If you were to "sink" your ferry so that it ran along the bottom of the tray and the ferry had some wheels underneath it that ran along some tracks:
1 - would allow the barge to be moved along a fixed track on the bottom of the tray and with an identical path under the tray with some type of haulage (chain, locomotive on track, string with magnet) with the magnet(s) attached)
2 - the closer proximity of the magnets on the barge and towing mechanism results in a lot stronger magentic attractive force being available to tow the barge with less risk of a break away and the resultant "barge drifting off into the sunset" scenario.
3 - because the height of the barge deck above the bottom of the tray then becomes a known quantity, the loading docks at each end can be accurately built which then results in "first time, every time" accurate docking with rail allignnment
4 - The stronger magnetic force will also hold the barge in position at the dock which also reduces your chances of "splashing" a locomotive or car when shunting
5 - provided that you ran identical length cars with the same wheelbase the railheads on the tracks on the ferry could be attacked with a grinder at known distances along the deck to give you restraining dimples that the wheels would sit in during transit. This would reduce the chance of having a car or 2 going "overboard" during transit.
6 - I would caution against an automatic timer circuit for triggering the ferry, because shunting may not have been completed when the ferry departs the wharf resulting in a "SPLASH". However a simple "on demand" DC reversing circuit on the haulage track woud be sufficient to run the ferry journey hands-off if a loco with attached magnets is used as haulage power.
7 - add some muddy dye to your water to camourflage the underwater tracks
Thank heavens I haven't got to make this one work - but I could...