I need to replace a couple of slightly leaking thru hulls on my 1984 Oday 28. The configeration currently in the boat looks to be a thru hull
mounted on plywood
backing plate (now rotten) then a bronze ball valve attached directly to the thru hull
. I'ver been reading up on the prefered NPS thru hull
with a true flanged seacock also with internal NPS threads, but don't quite understand the preference for NPS threads as these are not a mechanical seal as are NPT threads. The set up I have was used extensively for many years and I understand the advantages of the flange adding strength and the matching threads and the need for the thru hulls to be furnished as NPS as they might need to be cut shorter, but I'm struggling with a couple of things. First the fact that I don't really want to add several more holes in my hull
. In addition it seems that NPT threads would provide a better long term seal. I've looked for the so called combination thread thru hulls, but can't find them anywhere. (I know Groco will make them on special order). In addition my 17 year old ball valves are working just fine. I have looked at the flanged Groco adaptors and see there is a method of attaching to the backing plate, but again I'm not too sold on NPS threads. I've read everything I can find, but can someone please explain why NPS would provide a good seal as oposed to NPT. BY the way I played around with some seacocks and thru hulls at West the other day and the NPS to NPS seem loose while NPT to the NPS seems to make up nicely.