You can lock or just use the ramp in Old Hickory below the dam. When water is low it has 8" gaps in the concrete so be in a large wheeled vehicle.
Also push the boat out as far as rope will allow because if water is dropping you will return to boat on dry land somedays.
The other options are Peeler park at Neely's Bend in Madison or Heartland Ramp on the Stones in Donelson. Both are a few miles from the dam.
∑ Left Bank Recreation Area (1250 Cinder Rd., Old Hickory, TN 37138)
∑ E.N. Peeler Park (2043 Neeley's Bend Rd., Madison, TN 37115)
∑ Stones River Access (3135 Heartland Drive, Nashville, TN 37214)
∑ Lock 2 Park (2650 Lock 2 Rd., Nashville, TN 37214)