HI re 60 ft in 57 ft lock, go for the 60 ft boat.the main lock in question is on the calder and hebble the centre lock in a flight of 3, yes you have to go diaganol from a corner to centre of gate, corner to corner you will not get the gate open,i have done this in a 60ft 3 inch colecraft, the only problem you could have is if you buy a low slung tug where the bow wont go over the sill when climbing the flight,going down the flight it is possible you may have to go down backwards. but do not despair you will get through,all the best shorty
how long is your boat? shorty