Mixing Metals Beneath the Waterline - Facts / Myths ?

[delmarrey]

And as an FYI---- see the prerdy red color on these items. That's the color bronze takes on when it's loosing its tin/silicon inert metal. If the thruhull fitting shows this color, it's time to change it!

And I'm glad someone mentioned that seacocks are to always be used below the waterline!


.

[svHyLyte]

Originally Posted by delmarrey

And as an FYI---- see the prerdy red color on these items. That's the color bronze takes on when it's loosing its tin/silicon inert metal. If the thruhull fitting shows this color, it's time to change it!

And I'm glad someone mentioned that seacocks are to always be used below the waterline!


.

Del--Hopefully that strut is/was not on your yacht! Back in the early 80's we lost the strut on our old Cal 2-29 to wastage like that. We ended up having a very long, slow, cold sail home, absent being able to use our engine.

[delmarrey]

Originally Posted by svHyLyte

Del--Hopefully that strut is/was not on your yacht! Back in the early 80's we lost the strut on our old Cal 2-29 to wastage like that. We ended up having a very long, slow, cold sail home, absent being able to use our engine.

Nope! it's been replaced with a Buck Algonquin strut.

[um saudade]

Main Sail: Thanks for the excellent pictures!


My take on resolving the issue with running threads on thru- hull fittings:

Test the length of the fitting for proper fit in the hull. Then run a thread chaser over the end of the thru-hull and put a taper on the threads, not going down any further than the depth of the die. Now the thru-hull has strait threads at the point where it is compressed on the hull material with the jamb nut and tapered threads where the valve attaches to the fitting.

The fitting will seldom leak if drawn down on straight threads. The issue is whether sufficient thread to thread contact is made for the joint to have the mechanical strength it requires when an big gorilla tries to force a stuck valve. While putting a taper on the treads does reduce the wall a tiny bit, the additional couple of threads inside the fitting make the mechanical strength of the joint much higher and the joint is made up with less stress. With a straight thread going into a taper body the body gets distorted very easily and this can lead to fatigue and early failure.

I have never seen a thru-hull with an insufficient wall thickness to not be able to do this and I have not yet seen a boat sink because of this re-threading procedure.

Having said this, I am not advocating the use of regular valves as seacocks below the water line. There are places on boats that have very limited space to get to the valve/seacock and flanged cocks simply do not work.

While I have done pipe joint connections for years with all different kinds of fittings I was not sure if there was an actual number for the length of thread to be screwed into the fitting. I always figured about 75 to 80% or so. But checking the book (Machinery’s Handbook was handy) I find a chart that shows a numerical figure for each size of pipe.

As an example from the book’s tables, ¾ pipe has about 1 inch of taper threads on the pipe and the book states that the proper depth of thread is 9/16 of an inch. This is a little less than my 75% figure so I am doing ok with my thread tightening procedure. Remember that if you try to drive the threads too far into the fitting you increase the potential for fatigue failure at a later date.

I have a spot on my engine cooling where, do to space limitations, I am forced to use an automotive type (square) street elbow made of brass in my raw water cooling. I just change it out every two years, though it does not seem to have deteriorated all that much. I have wondered how long it might last but have no urge to test it out and have it fail when I need engine power the most.

If anyone has a vendor who sells the automotive type of fittings in bronze I would love to talk with them about replacing this weak link in my engine system.

Um Saudade

[Maine Sail]

Originally Posted by um saudade

Main Sail: Thanks for the excellent pictures!


My take on resolving the issue with running threads on thru- hull fittings:

Test the length of the fitting for proper fit in the hull. Then run a thread chaser over the end of the thru-hull and put a taper on the threads, not going down any further than the depth of the die. Now the thru-hull has strait threads at the point where it is compressed on the hull material with the jamb nut and tapered threads where the valve attaches to the fitting.

The fitting will seldom leak if drawn down on straight threads. The issue is whether sufficient thread to thread contact is made for the joint to have the mechanical strength it requires when an big gorilla tries to force a stuck valve. While putting a taper on the treads does reduce the wall a tiny bit, the additional couple of threads inside the fitting make the mechanical strength of the joint much higher and the joint is made up with less stress. With a straight thread going into a taper body the body gets distorted very easily and this can lead to fatigue and early failure.

I have never seen a thru-hull with an insufficient wall thickness to not be able to do this and I have not yet seen a boat sink because of this re-threading procedure.

Having said this, I am not advocating the use of regular valves as seacocks below the water line. There are places on boats that have very limited space to get to the valve/seacock and flanged cocks simply do not work.

While I have done pipe joint connections for years with all different kinds of fittings I was not sure if there was an actual number for the length of thread to be screwed into the fitting. I always figured about 75 to 80% or so. But checking the book (Machinery’s Handbook was handy) I find a chart that shows a numerical figure for each size of pipe.

As an example from the book’s tables, ¾ pipe has about 1 inch of taper threads on the pipe and the book states that the proper depth of thread is 9/16 of an inch. This is a little less than my 75% figure so I am doing ok with my thread tightening procedure. Remember that if you try to drive the threads too far into the fitting you increase the potential for fatigue failure at a later date.

I have a spot on my engine cooling where, do to space limitations, I am forced to use an automotive type (square) street elbow made of brass in my raw water cooling. I just change it out every two years, though it does not seem to have deteriorated all that much. I have wondered how long it might last but have no urge to test it out and have it fail when I need engine power the most.

If anyone has a vendor who sells the automotive type of fittings in bronze I would love to talk with them about replacing this weak link in my engine system.

Um Saudade

There is good reason the manufacturers don't do this, wall thickness. If you have decided for you and your boat the wall thickness is sufficient, that is fine, but it is not a recommended practice I have ever seen the manufacturers recommend, to cut a tapered thread on the end of a straight threaded thru-hull.

They DO make what are called "combination thread" thru-hulls which do not impact the wall thickness.

This is a combination threaded thru-hull and a straight thread. Basically the tips of the combination thread thru-hull are shaved off to seat it further into an NPT valve. This si the proper way to get a thread fit on a thru-hull and they can be special ordered with "combination threads".


Interestingly enough the wall thickness of an off the shelf 1" Apollo/Conbraco thru-hull fitting is 1.54 MM after you subtract the thread depth. A US Penny is roughly 1.57mm thick! If you feel comfortable cutting any more wall thickness off your thru-hulls go for it. Be aware they are not all sufficiently thick to cut a NPT thread into and to still be able to support a 500 pound static load at the innermost hard fitting for 30 seconds.

I would never personally do that when "combination threaded" thru-hulls are available.

[roverhi]

You can mix some metals below the water line. Bronze and stainless are present in almost all boats with a stainless prop shaft and a bronze propellor. My old boats gudgeons and pintles were 304 stainless and all the thru-hulls were bronze. Still going strong after nearly 40 years. Keeping electrolysis at bay is the key so keep your zincs in good condition and replace often. Unfortunately, it's not always your boat that is the problem. Tieing up next to a hot boat or marina can also do you in.

[delmarrey]

Here is a little more info about pipe threads (tapered and straight) that is not readily available to just anyone.

Your welcome! ..................................._/)

[GordMay]

“... ABYC 27.6.1.2 “Threads used in seacock installations shall be compatible (e.g., NPT to NPT, NPS to NPS).”
This entry, which specifically prohibits the mixing of incompatible thread types, was added to the standard in 2008. While it’s inconceivable to me that any experienced boat builder or mechanic would knowingly attach plumbing components whose threads were incompatible, it’s an all-too-frequent occurrence; thus the specific prohibition ...” ~ Steve D’Antonio

See ➥ The Standards for Seacocks

[Bob]

Originally Posted by James Dieterich

I am about to install some brass thru-hull fittings complete with sea-cocks and was wondering if mixing metals below water was a problem. I have my sacrificial zincs in place and have heard that "mixing metals" beneath the water could cause the brass to degrade since stainless is also present. I am beginning to think that my sources are not altogether reliable.

James, to assist with your original question, there is a very good article about this topic on the Cruising Wiki - HERE

Hope that helps to answer your question.

[mrwright]

Originally Posted by James Dieterich

I am about to install some brass thru-hull fittings complete with sea-cocks and was wondering if mixing metals below water was a problem. I have my sacrificial zincs in place and have heard that "mixing metals" beneath the water could cause the brass to degrade since stainless is also present. I am beginning to think that my sources are not altogether reliable.

FWIW, you can't even replace screws or bolts with different metal types as doing so will turn your boat into a battery.

this is all sound advice here. best to go with high-quality/high-tech metals and parts available from a marine supplier. keep away from the cheap-o stuff and the hardware store.

[GordMay]

Originally Posted by Bob

James, to assist with your original question, there is a very good article about this topic on the Cruising Wiki - HERE

BOB:
How does one determine the author of a Wiki article?

There is a significant difference between the way I read articles written by me, my sister’s friend’s cousin, or someone I’m willing to actually believe knows of which he speaks.

[Jim Cate]

Originally Posted by GordMay

BOB:
How does one determine the author of a Wiki article?

There is a significant difference between the way I read articles written by me, my sister’s friend’s cousin, or someone I’m willing to actually believe knows of which he speaks.

Right on, Gordy!

Jim

[delmarrey]

Originally Posted by GordMay

BOB:
How does one determine the author of a Wiki article?

There is a significant difference between the way I read articles written by me, my sister’s friend’s cousin, or someone I’m willing to actually believe knows of which he speaks.

It's the Wonderful Web World of hearsay.

References

Nigel Calder, Boatowner's Mechanical and Electrical Manual: How To Maintain, Repair, and Improve Your Boat's Essential Systems — 3rd Edition.
Miner K. Brotherton, The 12-Volt Bible for Boats — 2nd Edition

[Bob]

Originally Posted by GordMay

BOB:
How does one determine the author of a Wiki article?

There is a significant difference between the way I read articles written by me, my sister’s friend’s cousin, or someone I’m willing to actually believe knows of which he speaks.

Gord

There is a "History" tab at the top of each Cruiser's Wiki page that lists all the page's contributors (by username). As of the last software upgrade, each user has a "profile" page. It all works just like Wikipedia - information can be collated/corrected/edited by any registered member and Wiki admins have the ability to accept or reject (rollback) any edits by following edits on the "Recent Changes" page (in the Wiki's L/H menu).

The World Cruiser's Wiki can only be edited by registered members which limits spamming and vandalism (edits which can be rolled-back anyway). Register on the Cruiser's Wiki to see how the process works.

We all happily quote (in the most part, unsubstantiated,) articles from Wikipedia in forum posts - no difference to Cruiser's Wiki except to say that the articles are written by dedicated cruisers and boaters in general that have a genuine wish to share information. It is a great resource that is growing very fast (currently about 7,000 pages set up for editing). The more cruisers/boaters that edit and contribute to the Wiki, the more credible this amazing resource becomes. See this section - HERE - to see the huge potential of the World Cruiser's Wiki.

Please note that I have no underlying interest in the Cruiserswiki except to
watch it grow - to benefit all cruisers around the world.