Neil, I have an '82 M-15 and the Thetford 135 works just find for me. The weight of the water in the tank (for flushing) seems to be enough weight to keep the toilet from going anywhere. Granted, I have not been in blue water with 15' swells, so I can not testify to rough sea conditions, but on a typical 4 day sail with moderate weather, I've had no problems. Skip Campion M-15 1982 #208 'Wild Guppy' -----Original Message----- From: ndorf@surfbest.net To: montgomery_boats@mailman.xmission.com Sent: Sun, Jan 3, 2010 9:22 pm Subject: M_Boats: Advice Sought: Porta-Potti for '85 M15 Quick question, I want to put a porta-potti in my M15, and I have read all the nformation on the forums and M boat related sites. I understand that he Thetford 135 more or less fits, once coaxed with a soak in some ot water, then weight and wait. My question is, given the expectation that the fit will be tight in he sunken well where the 135 will sit, will I still want the Marine" version of the 135--which has a secure-down kit--or will the tandard one do just fine? I'm not sure there is really much difference in price, and my concern s also that, without actually seeing one of these, would the ecure-down kit actually get in the way? Any advice? Thanks, and Happy New Year to all, Neil Dorf oseville CA 15 1985 ______________________________________________ ttp://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats Remember, there is no privacy on the Internet!