M_Boats: depth for launching

Out here in the west we are having some water level issues at launching ramps. Just wondering if anyone has measured the depth over the axle necessary to float a 21st century Montgomery 17 with a stock Pacific trailer. I hate to drive several hundred miles to find she won't float. Someone told me that 4' is a rough figure, but I have always been too busy at the ramp to confirm this number. Thanks much. Tom Jenkins M17 Scintilla, Hull 626