Tom, I went aground with my M15 on mud so soft that the boat sat level, and it caked inside the centerboard housing such that when I floated off, the centerboard was stuck and would not move either way. I got it to recede by force when retrieving the boat on the trailer. It took a strong jet of waterspray from underneath, on my driveway, the flush out the mud. I'll be launching again next weekend and then I'll know for sure that I have full up/down movement. Sand probably would not have been a problem but who can say? Bottom line; I would not want to beach my boat if I could avoid doing so, especially in mud or tidal areas. But in a freshwater lake, on a sandy surface, you're probably OK if the lake levels don't vary too much and if you are willing to take the risk of possibly lightly scratching the bottom of your boat. Neil Dorf M15 Nothern Calif.