Don't use a ramp. Back up to a convenient tree and use an appropriate attachment system (rope or nylon straps) to attach the back of the boat to tree. Using adequate support to protect the boat jack the stern up enough to take the weight off the back of the trailer. slowly drive away until the boat just passes the balance point, then put some adequate blocking under the keel, and finish driving away, leaving the boat supported by the blocks....then reverse the process with the new trailer.