When I replaced the bunk boards on my M17 I replaced with 2x6 treated wood as clear as I could find. Then covered it with bunk cloth. Make sure all the bolt heads are recessed, of course, else they could score the gelcoat bottom. I have had good luck with the above procedure. I would not go any more than 2x6 lumber, especially on an M15 since they are pretty light and may not bend the 2x8 lumber to their hull thereby lessening the contact between support board and hull. Your boat will end up sitting on top of the stiff bunk board rather than nestling down into it. I found I could change my bunkboards while the boat was on the trailer. I just jacked up the boat about two inches using a jack under the keel. That allowed me to cant the boat first to port than starboard using a line run from the jib sheet winch on the cockpit coaming down to the trailer. Once tipped to one side I was able to unbolt and remove the opposite side bunkboard and replace in kind. Then just shift the operation to the other side. Good luck, Tom B. M17, #258, "AS IS"