On my Monty 17 #258 I added 2- 1.5 inch cockpit drains modeled in location after the ones installed on new Monties being built. I was going to go with some sort of backflow check but found them to be expensive so used a Blue Sea system flapper valve that screws over the transom end of the drain pipe after it pierces the transom above the waterline. They are inexpensive and have served well to stop water from sluicing back into the cockpit. I made sure they exited above the waterline to avoid water flowing back into the cockpit when the cockpit had three or four people in it. I was able to find regular PVC fittings for the 1 1/2 diameter pipe that were a slip type on one end while the other end was threaded for a 1 1/2 inch pipe thread. This allowed screwing a female PVC fitting over the male thread as a nut to hold the fitting in place both at the aft end of the cockpit and the outside of the transom. For the actual pipe I used 1 1/2 in flexible pipe like that used to plumb spas. This allowed me bend the pipe into place inside the hull between the transom and the backside of the cockpit aft wall once I had glued the fittings to both ends of it. Once installed it was a simple matter to make a gasket of #400 Adhesive and screw the female ends onto the male fittings. I had already cut off most of the female end fittings so neither stuck out into the cockpit enough to catch an ankle and to also allow a decent sized flat surface to screw the Blue Sea flapper valve on the outside of the transom. Several pictures in the Msog photosite show the finished product. I was also leery of the stock aluminum sleeves that were fiberglassed into the boat to receive both ends of the original cockpit drain so I glassed them further into the boat so that the new drain hole clamps onto fiberglass instead of only to aluminum. That way electrolysis won't eventually corrode a hole in that aluminum and flood the boat. Currently I have my boat moored in a marina. Have fun, putter on the boat, go sailing. "AS-IS", Monty 17 #258 Tom B