Rule 800 GPH Shower Drain System The Rule Shower Drain System provides the ultimate pumping performance, keeping your boat clear of nuisance water. Rules series of automatic shower and sink drain systems are tested to the highest standards to provide better efficiency for the toughest applications. This shower drain system features a powerful, high capacity pump to cope with incoming grey water. This system is also equipped with back flow prevention, reducing the amount of water that can siphon