Hardware Bedding

Discussion in 'Materials' started by We're Here, Dec 3, 2011.

  1. We're Here
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    We're Here Junior Member

    I was looking at some stainless handrails thru-boldted three years ago to a gelcoat surface. I noticed rust bleeding onto the gelcoat from around the perimeter of the handrail bases. It could have been that the rust was coming from either under the stanchion base or from the side of the base but my sense of it was that it was from the former.

    My question is whether there is a better alternative to the bedding compounds usually used with this sort of installation. It seems likely that when the fittings were ratcheted down, nearly all the bedding compound was squeezed out and that the bedding-fitting seal finally failed and crevice corrosion then took over.

    I wonder if one of the physical membranes (with adhesive on both sides) might be a better alternative in certain installations of this nature? I've seen these used to install fixed front windows and it is my guess that the membrane was deemed more reliable (by the window manufacturer) than simply depending on unknown skill levels of third party installers. I'd further guess that when a window leaks, the boat owner is going to be speaking with the window manufacturer rather than the installer. Thoughts?
     
  2. gonzo
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    gonzo Senior Member

    It is possible to install a flexible gasket. There is no reason for it not to work. The bolts will still need beddign compound. The problem is that most fittings are not properly designed with a groove or cavity to hold the bedding.
     
  3. michael pierzga
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    michael pierzga Senior Member

    When you deprive stainsteel of oxygen it rusts.
     
  4. alan white
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    alan white Senior Member

    If you don't tighten the bolts completely, so that bedding compound doesn't squeeze out completely (but hardware is evenly levelled), you can allow it to cure (may take a day or two) and then do the final snugging of the bolts. This tends to create a better gasket since the material is under pressure and will therefore expand somewhat if the hardware becomes loose. It will also create a means of keeping bolts and screws tight from vibration, much like a lock washer.
     
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  5. michael pierzga
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    michael pierzga Senior Member

    Careful when you re torque a fastener after the bedding has set or you will beak the fasteners waterproof joint.

    A common method to avoid bedding bog starvation under a joint is by using a combination of 3m weatherstrip tape plus bedding goo. The weatherstrip bog joint is very desirable on fittings that you do not want to develop a strong bog bond with' .

    Of course the absolute best technique is to first bed the fitting in thickened epoxy. Use release agent on the hardware, break free after cure and you now have a perfectly "machined " surface to bed the hardware to . you will achieve even loading around the complete perimeter when you torque the fasteners.

    Another bedding trick to avoid bog starvation are thosee transparent adhesive drops that you commonly use to bed windows. They maintaine a constant bog thickness under the hardware.
     
  6. We're Here
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    We're Here Junior Member

    We're in agreement about the cause (of the rust) but I'm using its existence as evidence that the sealant bond (to the handrail stanchion base) has been broken. If it hadn't been broken, I doubt that there would be any rust under the base at all (unless the ferrous qualities of the stainless are reacting to the sealant itself).

    I wonder if using some sort of a barrier to prevent the migration of the sealant would work. Perhaps a piece of nylon or bronze window screen cut to fit the stanchion base and then let fly with the sealant?
     
  7. michael pierzga
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    michael pierzga Senior Member

    Ive never heard of screening under fittings .

    Stanchion bases are always problematic. High load, small footprint, cambered surface on deck and very many times an irregular surface under deck for the nuts and washers to load against. If the hardware "moves" because its fasteners are not supplying clamping pressure the fitting will leak. How many fasteners are on your base. Many of the modern , single , blind fastened hand rails are a nightmare to keep watertight.


    Be sure to use plenty of sealant bog on the fasteners and in the hole. Be sure that the fastener hole is oversize, for instance 6.5mm hole for a 6mm thru fastening, then chamfer the top of the hole so that as you drive the machine screw you dont strip the bedding bog off. Use a drilling guide to make the hole perpendicular to the hardware and on center.

    Many times an alloy "backing plate", set in epoxy bog is a good way to stabilize the stanchion base fasteners and keep the fasteners clamping pressure in line with the hardware being mounted. .


    If you have the hardware off and in the workshop run the base over a polishing wheel to remove scratches or any impurity that might cause rust bleeds.
     
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  8. gonzo
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    gonzo Senior Member

    The bedding on the base is not for waterproofing but to take on the imperfections. A chamfered hole will hold bedding around the bolts, which is where water intrusion is a problem.
     
  9. We're Here
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    We're Here Junior Member

    Excellent advice - much appreciated. WH
     
  10. rxcomposite
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    rxcomposite Senior Member

    I use 316 stainless steel rather the most common 314 variety. It rust less. Also bed it with Sikaflex sealing compound.
     
  11. Joakim
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    Joakim Senior Member

    Yes, they really are! What is a good design for a stanchion base for lifelines in a 35' sailboat?
     
  12. PAR
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    PAR Yacht Designer/Builder

    Michael called it right in one of his posts, which is to "bond" the hardware as well as the fasteners. This doesn't mean glue them down, but it does mean to make a custom fitting, precise mold of the stanchion base with epoxy, so the bedding is uniformly smashed when dogged down with it's fasteners. Bonding the fasteners should be done on all weather deck items that receive a screw or a bolt. This entails drilling out the fastener holes about 1/3 larger then their normal clearance holes and filling with epoxy. You can wait and drill this cured epoxy bushing for the clearance hole or you can insert the fastener while the goo is wet. If you'd like the ability to easy remove it, coat the fastener with a few coats of paste wax. If you don't you can still remove it with a soldering gun, but this is more troublesome, though much more secure.

    When bonding fasteners use fine thread bolts as they hold better. In fact, tests have shown that replacing screws with bonded, fine thread machine screws hold better. Lastly, counter sink the over size hole used for the bonding, on the head or hardware side and fill slightly proud of this hole. Epoxy can shrink a bit so you'll need some height to grind flush. Also counter sink all holes where hardware meets the substrate, including under washers. Apply a healthy dab of bedding into this counter sink as you set the hardware and/or fasteners. When you tighten down the part, the bedding will form fit to the countersink and mold an "O" ring gasket directly around the fastener. This is an old trick, but works very well at preventing leaks.
     
  13. Joakim
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    Joakim Senior Member

    Thanks PAR! This is very useful information, although my question was more overall design.

    What kind of stanchion base is the most reliable? There are ones with 3-4 6 mm bolts on a plate and a tube for the stanchion and "bolt stanchions", which have a 14-16 mm central bolt or ~20 mm "screw" for gluing and the stanchion come on top of it. And then some of the former ones support also to the aluminum toe rail. What is an adequate size for the plate of the former for a 600x25 mm stanchion? Is a triangular one good enough or should a rectangular one be used?

    What is the structure needed around the stanchion base? It should be single skin, but is the junction of hull and deck already strong enough or should there be a thicker laminate? Quite often boats have cracks around stanchion bases. What is an adequate laminate thickness for a 600x25 mm stanchion?

    Should the base be strong enough to bend the stanchion before anything happens to the base? In all directions?
     
  14. PAR
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    PAR Yacht Designer/Builder

    Your questions are pretty broad and difficult to answer. Stanchion bases, like most things, need to be application specific, for the loads anticipated. As a rule, most of the production boat bases are woefully inadequate for their job. A welded base is best, but most use cast. Good materials, instead of pot metal, well sized fasteners of also good material and a substantial packing plate or structural member beneath it are all necessary.

    As to size, on center spacing, tube heights, diameters, etc., again these are application specific questions, so what works on a 28' harbor queen can't be considered for a passage maker of similar length.
     

  15. Joakim
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    Joakim Senior Member

    Do cast bases fail commonly or are they just heavier? I just bought new welded AISI 316 GL approved bases to replace the cast aluminium ones, but the problem was clearly too narrow footprint (25 mm bolt to bolt and 50 mm side to side in longitudal direction), not that the bases would have failed.

    Isn't the stanchion used enough for dimension the base? If you have a 600x25 mm stanchion in a 40' passage maker or a 28' harbour queen, it still has to be able to capture a man falling off the boat and all the forces to the base come from the same tube that has a limited strength (while capturing a man or hitting another boat etc).
     
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