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seabreeze
Deckhand

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18 Posts
Initially Posted - 06/26/2009 :  08:03:43  Show Profile
I am installing a holding tank system for my wc and got lots of good info from the archives, but I have one simple question: I read how stainless steel thru hull fittings are the best and ordered one from wholesale marine for the outlet from the macerator pump. But when it arrived, on the package it says "not for underwater applications"! So what's the difference? Is this just to protect them or are some stainless steel fittings better than others?

Second question: I have a 3/4" marelon thru hull fitting from the PO, but the hose is 1". What would happen if I put in the 3/4" with an adaptor at the end of the 1" hose? Would it affect the pump?

Thanks
Jim
C-25, 1982, SR,SK
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Nautiduck
Master Marine Consultant

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USA
3704 Posts
Response Posted - 06/26/2009 :  09:08:56  Show Profile
If the pump calls for a 1" through-hull I would not use a 3/4" version. I would use a bronze or Marelon through hull though - not the stainless steel version (and I love SS! ). I have used Marelon through-hulls and have been very satisfied with them. This is a case where using 5200 as the sealant/adhesive would be appropriate too although 4200 would also be fine. Adding a through-hull is an important piece of work and worth using the exact right equipment.
Edited by - Nautiduck on 06/26/2009 09:12:04
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Dave5041
Former Mainsheet Editor

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USA
3758 Posts
Response Posted - 06/28/2009 :  15:16:22  Show Profile
Stainless does not like oxygen gepleted water. Bronze or marelon.
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OLarryR
Master Marine Consultant

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USA
3467 Posts
Response Posted - 06/29/2009 :  03:59:49  Show Profile  Visit OLarryR's Homepage
Dave is right regarding oxygen depleting.

It may seem strange that while stainless steel fittings are common and superior for all boatworks above the waterline, then why not for the thru hull ? However, stainless steel is really not great for seawater and especially for any area that has stagnant seawater.

I recall (over 20 years ago) working on a project to replace piping in a ..... plant that had SS piping, welds and especially SS piping flanges that were getting eaten alive by seawater (especially the flange faces). The fluid medium had varied amounts of silt and concentrations of seawater that was eating up the piping but mainly the joints. Many years later, the word I got from my friends (after I left my company) was that on that job, the polyethylene coated carbon steel piping system we spec'd out and on-site for directing all aspects of the installation was holding up great ! The piping was made up with short flanged lengths so that each section including the flange faces could be polyethylene coated - the fluid would only touch the polyehylene.
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seabreeze
Deckhand

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18 Posts
Response Posted - 06/29/2009 :  10:50:02  Show Profile
Thanks, guys. I have ordered the marelon fitting.
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.