Using stainless steel screws in wooden boats

Discussion in 'Boatbuilding' started by metin_mehel, Mar 21, 2014.

  1. metin_mehel
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    metin_mehel mech.eng.

    Hello,
    Is using stainless steel screws prohibited in wooden epoxy coated boats?
    Thanks
     
  2. JSL
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    JSL Senior Member

    not prohibited but do not do it.
     
  3. daiquiri
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    daiquiri Engineering and Design

    As previously said, it is not prohibited but requires some care. Commonly used stainless steels suffer the crevice corrosion. It is a situation where a metal is in contact with an electrolytic conductor (like a screw inside a wet wood), which separates it from a direct contact with the air. The electrolytic medium will create areas of locally dissimilar concentrations of oxygen in contact with the metal (so-called "differential aeration cells"), which in turn creates a difference in voltage between dissimilar areas. Kind of like putting zinc and steel adjacent to each other, in a wet ambient.

    AISI 304 (or grade A2) fasteners are the worst performers in these conditions, and are imo out of consideration for use in naval constructions. The AISI 316L (grade A4) fasteners perform much better, but still require attention to following steps:
    1) The wood has to be dry (max 10%-12% humidity).
    2) Each screw requires a pilot hole to be drilled first, each hole then soaked with epoxy. Besides isolating the screw from water and humidity, soaking the pilot hole with epoxy will considerably increase the grip and the fastening force of the screw.
    3) Finally, the hull has to be completely encapsulated with resin, in order to isolate it from both the water and the air humidity.

    An even better and more reliable choice would be the Duplex steel fasteners (like 2205, or grade FA) which have very good mechanical properties (yield strength typically 2 times higher than the 316L) and are much more resistant to crevice corrosion than the AISI 316L. It will cost somewhat more than 316L, though.

    Cheers
     
  4. Stumble
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    Stumble Senior Member

    Instead of duplex I would go with titanium at about the same price. But frankly screws are almost always a terrible idea on boats. They don't do very well with vibration, so every time the engine starts every screw on the boat looses a little strength.
     
  5. daiquiri
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    daiquiri Engineering and Design

    I really wouldn't worry too much about that. Wood is a very forgiving material when it comes to vibrations, a properly made wooden hull tends to absorb them pretty well. You can see a plenty of very old wooden workboats in the marinas, with inboard engines and no vibration-induced problems with fasteners. Most problems related to metal fasteners in wooden hulls are related to the crevice corrosion which eats the metal from the screw surface. In the process, the diameter of the screw decreases over the time and loses the grip on the wood. Prevention of water absorption and rotting (both of hull and screws) is a much bigger and costly issue than the vibrations. ;)
    Cheers
     
  6. SukiSolo
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    SukiSolo Senior Member

    I am not sure from the question whether it is meant using S/S screws to glue and screw in the build process (and either remove or leave in place), or to fasten after the build is completed? One tip is if you wish to remove them, break the bond with the epoxy when about 2/3rds set, otherwise they will be pigs to get out. Best done by unscrew a bit then rescrew in until fully set.

    In the case of the former, I suspect as long as the screws are effectively isolated from water by the build schedule they should be OK. Damn tough on chisels if you have to replace the timber around them later....

    Fastenings after build should be like Daiquiri says, but be warned if you epoxy a screw in you will not get it out except by drilling. On small dinghies and the like I prefer to 2 or 1 pack varnish the hole because it will come out 10 years later!.
    If it was say an engine mount stud or similar it would be different and there are different ways of ensuring captivity, to double security.

    Be aware that rolled self tap stainless screws have a higher tensile strength than machined 'wood' screws so you can add that in to shear calculations.
     
  7. rwatson
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    rwatson Senior Member

    Nah, just put a soldering iron on the head for a minute, then you can back it out like it was in butter
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Interesting hint, I wonder if that idea works on other stubborn fasteners, maybe makes it harder to extract in other materials. I assume the heat softens the epoxy ?
     
  9. rwatson
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    rwatson Senior Member

    Yes, anything being held in place by epoxy will benefit from a local heat source.

    Heck, epoxy itself comes off when its really hot - say splatters on metal or wood.
     
  10. PAR
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    PAR Yacht Designer/Builder

    I've been burying SS fasteners in encapsulated wood for decades, with few problems. The key is to bond them in and not expect them to be anything more than steel. Simply put, if entombed in goo, they work fine, if they're exposed in any fashion, they'll cause issues.

    Heat works on most stuck fasteners.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Titanium is mentioned above as suitable, isn't it subject to crevice corrosion too ?
     
  12. SukiSolo
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    SukiSolo Senior Member

    Maybe I just haven't got the heads hot enough on S/S screws, partly because poor heat transfer of the material. I've certainly used heat to get a few metal items out of timber beddings before now.

    Melt point of epoxy varies a lot from around 150 deg C through around 230 deg C (for West and SP epoxies) up to a little over 350 deg C. Wood chars at around the same point (225-230) as the West/SP epoxy melts though it should release a bit earlier.

    Now why does this board not like it when I put in Alt248? - I thought that was Ascii, Ainsi and Unicode for degree symbol!.
     
  13. michael pierzga
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    michael pierzga Senior Member

    You can use stainless as long as it doesn't become wet.

    On an epoxy wood boat all fasteners should be dry so it shouldnt be a problem.

    Stainless steel Countersunk heads can be trouble if the hull skin is thin. If you feel that the head cant be permanently buried under a thick layer of epoxy , you should use bronze.
     
  14. michael pierzga
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    michael pierzga Senior Member


    Use a powerfull electric soldering iron with the correct bit as a tip. For slot heads i use a copper tip and file it to the correct size. It transfers heat better

    Get it red hot..insert into the slot..the epoxy in the slot will poof vaporize...after a few seconds you will see another poof around the head perimeter...remove the hot tip from the screw and use a regular screw driver to break free and back out.

    Most come out without trouble. Epoxy softens at low temp.
     

  15. Stumble
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    Stumble Senior Member

    No. At tempratures in excess of 300c it is subject to a form of poultice corrosion. Other than that there is no corrosion in the marine market that actually effects titanium.
     
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