Corrosion resistant bolts for attaching appendages to hull

Discussion in 'Metal Boat Building' started by ldigas, Nov 15, 2011.

  1. ldigas
    Joined: Feb 2011
    Posts: 189
    Likes: 5, Points: 18, Legacy Rep: 60
    Location: Zagreb, Croatia

    ldigas Senior Member

    I need a few M12 and M16 bolts. The old ones that were in place have failed. Haven't seen them, but friend who owns the boat says one of them literally broke in two, then they had to drill through the remainder of the bolt. Which is weird, but who knows.

    They are for attaching something to the steel hull, i.e. will be in seawater. What material should I look for and where can I buy some? Does anyone knows?

    I know that "plain" stainless steel does not fare that well in sea water. So, what does?

    Every advice would be appreciated...
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 487, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The designation M12 and M16 is just the thread size, in this case metric 12 mm and 16 mm respectively. What material? Are they hex bolts, cap screws, etc.? Thread count? Most importantly what are they holding up, expected sheer and tension loads and what are they threaded into?

    Stainless holds up fairly well in salt water, though some grades are better then others as you've guessed. 303 and 304 are the most commonly used stainless alloys, but they aren't as well suited for salt water as 316 or better yet 316L.

    There are other, typically application specific stainless steels to select, such as higher chromium content , formability and/or heat resistance, etc. So, what are you trying to do, specifically?
     
  3. ldigas
    Joined: Feb 2011
    Posts: 189
    Likes: 5, Points: 18, Legacy Rep: 60
    Location: Zagreb, Croatia

    ldigas Senior Member

    Sorry about that. Yes, they were hex bolts for an allen key, fully threaded (thread all along the length). A bit weird choice in my opinion, but who knows. I generally haven't seen allen key bolts until now on boats. Maybe they're more common in some other parts?

    They were holding the propeller skeg and the stern thruster. In the case of the thruster they were threaded into a solid steel plate, 28mm thick.

    I'll go look for the 316/316L equivalents in my part of the world (Meditteranean, Adriatic sea).

    But generally, yes, you gave me an answer.
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 7,076
    Likes: 1,009, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Concur with PAR.

    Although standard 316 or 316S16 will suffice. The "L" in the 316L is low carbon, which is required when welding to prevent carbide precipitation. So if not welded, 316 will do, and is cheaper than 316L.
     
  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 487, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    "Allen" key bolts are called socket head cap screws. They come in a few different head configurations, such as button and counter sunk, but all use the Allen socket.
    [​IMG]
    Hex head is the tradisional 6 sided bolt head that you put a wrench on.
    [​IMG]
     
  6. gonzo
    Joined: Aug 2002
    Posts: 15,438
    Likes: 1,010, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    If you include your location it will help. It wouldn't do you any good if someone gives you and address in Rome if you live in Singapur.
     
  7. ldigas
    Joined: Feb 2011
    Posts: 189
    Likes: 5, Points: 18, Legacy Rep: 60
    Location: Zagreb, Croatia

    ldigas Senior Member

    Well, I'm in Zagreb currently if anyone's familiar with the surroundings, but I don't expect you to find me an actual shop! Can handle that part for myself. I already got the advice I needed, mostly regarding the material.

    So, thanks to all. You're more than a friendly bunch when it comes to a constructive advice :)
     
  8. Passin Thru
    Joined: Oct 2011
    Posts: 17
    Likes: 2, Points: 0, Legacy Rep: 29
    Location: VA

    Passin Thru Junior Member

    Titanium Bolts will never corrode but Stainless Steel which is different from CRES (Corrosion Resistant Steel) will depend on a number of things. I have seen SS corrode in Aircraft and boats. Titanium is very expensive but takes the worry away.

    http://www.ssina.com/faq/index.html
     
  9. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 208, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    The stock answer here from a corrosion engineering perspective is to bolt with material as close to the parent material as possible.

    I'd use a high tensile steel bolt. Grease the thread and shank with waterprrof grease such as Shell Nautilus, torque the bolts evenly and use a locking mechanism. Paint the exposed material with epoxy. Then protect it all with a sacrificial anode ( should be there anyway).
     

  10. michael pierzga
    Joined: Dec 2008
    Posts: 4,862
    Likes: 115, Points: 0, Legacy Rep: 1180
    Location: spain

    michael pierzga Senior Member

    A steel bolt is a good solution. SS also works when installed correctly. If your bolts are corroding ,they are leaking. Stop the leak...keep the bolt dry.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.