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Re: [Phys-L] Impossible vehicles.

David ripostes that a sail boat's contact with the ground is unnecessary if the channel can be tacked up or down IN STILL AIR.I urge him to review the requirements for faster than windspeed travel   <g>
On Thursday, June 3, 2021, 10:13:40 AM CDT, David Bowman <> wrote:

Regarding Brian W's comment:

David's boat on a river in still air moving up or downstream faster than the
water speed seems to be less reliable in concept because it demands a
connection both to stream flow and immobile bed (mud?) or bank, but
still it is in the spirit of the land-yacht or directly upwind boat.

Actually the immobile mud is worse than irrelevant.  All that is really needed is a sufficiently efficient sailboat and a wide & deep enough river so it can be adequately tacked in a zig zag path while remaining in the central channel where the draft is deep enough and the current flow speed is fairly uniform.  If the boat gets too close to grounding out or too near the shore where the current flow speed is slower than in the main channel then the performance will be reduced.

David Bowman
Forum for Physics Educators