[walt123]

Using an existing thru-hull is an excellent idea. It saves putting another hole in your boat. The only crevat is how close is it to where you plan the sump? Most sailboats have a head/shower with sump located together or adjacent so the routing of the plumbing is convient and short. You also need to consider where you can locate the pump [protect the wiring] but the location isn''t much of a problem if the pump self primes has a strainer, and is easily accesible. Insure that you have an above the water line vent in the head discharge line and the connection is on the side nearest the thru-hull you should have no problems. If for some reason you cannot hook into the existing thru-hull when installing a new one put it a couple of inches above the water line and be sure to install a proper seacock. On last thing put a one way valve in the line to avoid sewer water backing up into it if you use the exixting head discharge.