More Car Covers

Boats have covers that can be on when the boat is running down the freeway on
a trailer, no problem. There seems to be NO car cover that you can put on an
open car on a trailer and run it down the freeway. The carcover mfgs say no no
no, can't be done, but they do it with boats all the time. Any ideas what the
difference is?
George Procyshyn
'58 AN5