Re: Hell Gate
Cartopping an 18' boat ? Wow. Yes, if you can contact someone at Columbia and get their blessing and invitation for the day, that would probably be ideal. There used to be a "Dyckman Street Boat House" on the upper Hudon side as well, I have no idea what the current state of that is.
The harbor ferries, the big ones that went out from the Battery, ride on rails and because they are so unmanueverable, they basically can't do anything to avoid traffic. They own the harbor. All the little commuter ferries? They may not like it, but they're subject to the same rules of the road as everyone else, because they are not constrained by draft and "I've got a schedule you know" doesn't appear in ColRegs. I'd say you'll have plenty of time to see then and avoid them, they're all fairly small anyhow.
And of course, since we still have geese at the airports...you know, just keep looking over your shoulder for inbound aircraft. (G)
Everything inside the bulkhead line (end of piers) is private on the Hudson side of NYC, but the folks at the Intrepid Musuem never used to mind folks ducking in to admire the ships up close. FWIW.