Yesterday I went fishing with an older friend. Everything went well until we got back to the ramp. He backed the trailer down the ramp, and I motored the boat up onto the roller trailer. The boat is a 16 Lund with a 30 tiller Evinrude. My friend got out and attached the straps and safety chain. I was still in the boat as he eased up the ramp with me and the boat. Suddenly there was a loud scraping sound and the boat slid backwards onto the ramp. Fortunately he stopped, but not until the boat scrapped the concrete for about 5 or 6 feet. This model Lund has three keels about 1.5" spaced across the bottom. I had tilted the motor up, but the skeg did drag, however, there is no bending or outward sign of damage other than to the bottom edge's paint. The Garmin SI transducer was tilted up, but appeared okay.The safety chain latch snapped and broke off. My friend did not engage the pawl of the winch, but did latch it to the bow eye. Altogether a big mistake was made. Last night I mentally replayed what had happened, the stress on the motor, transducer and boat obviously are major concerns. Just the stress of getting the boat back on the trailer, even though it did not fall completely off, is a concern. So my efforts from here on should be directed towards wisely checking the damage out before I schedule another fishing trip. I am concerned about leaks, and first thought about backing the boat still on the trailer into water and looking to see if water enters. I got to thinking that maybe a better way would be to place the drain plug in and wait until a dry day, fill at least the stern with an inch or two of water and look underneath the boat for leaks. Is the transducer damaged? Maybe, but the only way to tell, I assume, would be on the water. How about the motor? Just crank it up and go? Maybe not. Is it possible the boat flexed and caused metal fatigue? Right now I have more questions than answers, but have done nothing other than just look at it. I do not want to make a mistake. I sincerely would appreciate your input.