Mounting bow and deck rails, machine bolts or lag bolts?

Discussion in 'Wooden Boat Building and Restoration' started by missinginaction, Mar 21, 2012.

  1. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    The railings on my old Silverton were originally installed using 3/4" lag bolts screwed into the plywood and sealed. Over the years water made it's way into the plywood and, well you know the rest. I also noticed that the ply tended to crush a bit from side loads placed on the rails in spots.

    Since I want to do a good job I thought that I'd mount the railings on some hardwood pads that I made up out of scrap. This approach would spread the load and minimize the number of holes that I'll need to put into the deck.

    I'm thinking of a fewtechniques to mount the rails. Any feedback would be greatly appreciated.

    1. Install a single long machine bolt through the center of the "pad" and through the deck securing from below. Apply 3M5200 under the pad as well. Install the railing to the pad using 3 lag screws properly bedded. Pilot holes sealed with epoxy and bolts installed while the epoxy is still "wet". The idea would be to use 3/4" lag bolts so as to avoid screwing into the plywood deck.

    2. Install the pad to the deck with 5200. This approach would not use a through deck bolt. Attach the rail to the pad with 3/4" lag bolts as noted above. This method would not require any holes to be drilled into the plywood deck. The issue here would be that I'm depending entirely on the 3m5200 to hold the pad fast to the deck.

    3. This approach is more work. It occurred to me that I could use machine bolts and blind nuts (also called t-nuts) to secure the railings. I'd have to relieve the underside of the pad 1/8", install the blind nuts under the pad and then install the railing using three 3/4" machine bolts into the t-nuts. The pad could be attached to the deck as in 1 or 2 above.

    Note that I could also use Sika 206 G+P as an adhesion promoter under the pads.

    I'll post a couple of photos below that hopefully will shed a bit more light on this. As always thanks for any replies.

    MIA
     

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  2. alan white
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    alan white Senior Member

    The solution that sounds best is the one that through-bolts right through everything. The other options sound weak and even scary in the case of the pads being secured with adhesive alone.
    The proper location for pads is below, inside the boat. For some reason, I am thinking you have a reason for avoiding this option due to...?
    Hardwood pads like you've made will take a beating exposed to the elements unless you go to great lengths to seal them. Then, if checking occurs, you still can have water ingress into the pads.
    If you have to put pads on the deck itself, make them from quality plywood and glass over them (make coves to allow the glass to lie down nicely).
    However, I would consider the inside pad idea instead.
    I'd use long bolts through epoxy-jacketed holes. Those holes can be drilled all at once at least 1/4" bigger than the bolt diameter.
    Try this: Make pilot holes with a 1/8" drill bit to ensure accuracy of the bolt centers. Drill the jacketing holes using the pilot holes. Accuracy will allow a uniform epoxy jacket around the holes.
    The pads should have been bedded in already.
    Now, instead of setting the bolts in epoxy, set birch dowel stock of same diameter as the bolts into the holes. Wrap the dowels in wax paper first. If there's three bolts to each stanchion, make a block of wood with three dowels sticking at 90 degrees out a few (six) inches. Set the wrapped dowels in the holes and pour neat epoxy around them. Tap the dowels/block up and out after the epoxy sets up.
    If in doubt about perfect centering, make the jacketing holes a bit bigger.
    Bed with polysulphide--- around the bolts and on all faying surfaces.
    This should never leak or allow damage to the plywood.
    Just my two cents. I would be amazed, however, if PAR doesn't come up with a better solution.
     
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  3. PAR
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    PAR Yacht Designer/Builder

    When you want it strong, through bolt it. If you want it stronger, use bigger through bolts.

    This said, you can replace the lags with machine screws, set in epoxy, which has tremendous holding power. Bolts or machine screws bonded like this (fine thread is preferred) are really tough, assuming a sufficiently large bonding area (150% of the bolt shank diameter, at least). You'll likely yank up huge chunks of deck with rails bonded like this.

    The pads you've made shouldn't be bonded, just bedded in polyurethane or polysulfide. 3M 5200 is a pretty aggressive adhesive for this task, maybe 400 would be a better choice.

    You have a single, center bolt hole to take advantage of, so you might as well employ it. If it was me and I didn't plan on using the rails for tieing things off, like pianos hanging from tackle in the rig, a single, bonded bolt or a through bolt would be my first options. If penetrations are wished to be avoided, then bond the pad to the deck with thickened epoxy and the blind nut idea is a good option. I don't like this approach because you can't remove the pads.

    Personally, I'd like the idea of the rails tearing loose, before the deck gets a big hole ripped in it. If this is preferred, then just screw the bedded pads on from below, then bonded machine screws from above to hold the rail bases or grab rails to the pad. Those rail bases don't look like they could tolerate a man falling against them without ripping out the stitch welds, so no need to make a bullet proof mount for a piece that will buckle or break when seriously used.
     
  4. michael pierzga
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    michael pierzga Senior Member

    Your pad will split and turn into a maintenece issue.

    Thru bolt only. no pad.
     
  5. bntii
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    bntii Senior Member

    Decent approach though skip the "still wet aspect" for the sealing of holes. One way to go it to over drill by some amount, pour/trowel in epoxy then drill out the center for your fastener. This creates a bushing which seals the deck for damage when the bedding fails. Alternatively buy some G10 tube and set in deck to same effect. This can/should be done on all fasteners- either the big center fastener or each single if pads are skipped.

    No- do not depend on 5200 in this manner- the rails will not hold.

    As a hybrid to #1, sure. Also, note that fasteners can be tapped into high density epoxy filler, though I would no do so for life critical components like safety rails...


    I am seeing two issues for you-

    One: maintenance. You need a approach which effectively seals the deck from damage and keeps the water out of the ply between re-bedings even if water is reaching the fastener.

    Two: your deck is showing compression failure as a result of load and softening from water intrusion. One thing to keep in mind is effective bushing, either through cast in place epoxy or G10, is that compression loads are carried by the bushing as well as the deck. This keeps things from moving in use so much and grants longer life to the bedding. If used in all fasteners as provided in you bases, a tripod is created in deck which carries load in a similar manner to your pad. In conjunction or its absence.

    Bedding issue in general- provide some manner of keeping sufficient bedding in place under things. This allows the material to flex and not break bond in use. I do this in wood components by milling groves, counterbores etc
     
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  6. missinginaction
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    missinginaction Senior Member

    OK, no 3m 5200. You've all given me some different things to think about.

    I've got an idea. Excuse me while I go and find a 1/2" Forstner bit and a 2 3/4 inch hole saw.......

    I'll be back.....

    MIA
     
  7. bntii
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    bntii Senior Member

    OOPS- I miss read your first post to using lags and cast into the wet epoxy and see that Alan covered most of what I had said as well.

    Yep- what PAR said- strong this way though I think we both like to see through bolting, fender washers or backing plates
     
  8. missinginaction
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    missinginaction Senior Member

    Here's an experiment for mounting my rails....

    I made up a jig to line up 3 machine screws (1/4 - 20) with the stainless steel feet on my railings, 120 degrees apart. These 1" machine screws are cast into 1/2 inch cavities that I made with a drill press and Forstner bit. The idea is to fasten the railing onto the pad from above. A center bolt and oversize washer would secure the pad to the deck and be bolted through the deck. Only one hole would be required in the deck.

    Should the pad ever need maintenance or leak, the railing could easily be unfastened from the pad and the pad removed from the deck.

    First photo is the bottom of the railing "foot", second is the pad and jig, third is the jig placed on the pad to line up the machine screws while the epoxy cures.

    I'll see how this works out after the resin sets up.

    Comments?

    Regards,

    MIA
     

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  9. PAR
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    PAR Yacht Designer/Builder

    The exact same thing could be done to the deck, so that you can mount the pads and rail bases to the boat, all in one shot, no deck or cabin top penetrations. It's not as strong as a fully bonded through bolt, but again, you have to be careful what you wish for.

    You could also take this approach a step further and bond the pad to the deck or cabin top, then drill for the bonding holes, though the pad and nearly through the deck/cabin top. This increases the "purchase" on the fastener (stud), as well as spreads the localized loads imposed by the rail bases. The side benefit is no deck or cabin top penetrations. Probably pretty darn close to the strength of a through bolt, if the studs are the maximum size permissible by your rail bases. In the event you rip one of these puppies out, you'll take a good hunk of deck or cabin top with it, which is as good as you can expect with a through bolt. Naturally, the wooden pads will become a well bonded part of the deck or cabin top, but so what, you'll have hell to pay to remove them neatly, what ever technique you use to mount them. Consider them sacrificial load spreading rail base pads. Varnish them up, then in a few years, when you've grown tired of this, paint them to match the deck, then when they're really ugly and filled with epoxy a few times to handle dings and dents, you knock them off with a chisel and make new ones. Good thinking Bart . . .
     
  10. missinginaction
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    missinginaction Senior Member

    Understood PAR.

    The only issue that I see is getting the depth right when I'm boring the cavities into the deck or cabin roof on the boat. The pads would just be drilled out but the deck/cabin roof would have to be done free hand. Maybe I could rig up some kind of stop collar on the shaft of the Forstner bit. You see, I drilled the cavities in the mock up pad on a drill press so it was easy to get the depth equal for all three holes.

    Well, after the epoxy set up I tried out the pad and you can see for yourself that it came out fine.:D

    While I was playing around with this it occurred to me that I could add a fender washer to the top center of the pad under the retaining bolt head. I found a supplier for 1/8" thick 8-18 stainless fender washers for 1/4 - 20 thread. The washer has an OD of 1 1/4" and would fit perfectly in the center of the pad. I just need to order another Forstner bit to bore 1/8" into the pad and that washer will be a custom fit.

    Either way ought to be plenty strong enough, don't you think?
     

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  11. PAR
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    PAR Yacht Designer/Builder

    You could make a depth gauge for the bonding holes in the deck and cabin top, by drilling through a block of softwood, then using a miter saw to hack off the length you need to serve as the depth gauge. A piece of tape, arranged as a "flag" on the bit shank, will let you know when you're deep enough. Naturally, you'll want to make some test bores to get the depth right. You could also rig a steel rod to the side of your drill, maybe with a big hose clamp. It will bottom against the surface at the depth desired, again with some setup tests.

    I still think the center hole holding the pad to the deck and the studs in the pad, holding the stanchion base to the pad, is introducing more complication then necessary. I'd prefer to see the three studs mounted in the deck or cabin top and come all the way through the pad and stanchion base, to be dogged down with lock nuts. Of course all the holes are bonded (both deck and pad), so no water can get past the bonded fasteners and rot wood or deck cores. In fact, it can be done the other way around, by casting the bonding holes with the fastener threads. This eliminates the lock nuts and the bonded holes in the deck become epoxy blind nuts. Also in this same vain, possably a little more bullet proof, is to actually cast blind nuts in the bonded holes. Coating the threads of the machine screws with wax, then inserting them into the blind nuts and using the machine screws to hold the nuts in position as the goo cures. Once cured, you removed the waxed machine screws (fine thread) and the bonded blind nuts are one with the deck, ready to receive a fastener.

    I think all three approaches can work. I tend to try to whittle things down to the simplest form, the least amount of parts or leak points etc. You've done enough engineering on the fly, to handle this little deal. Pick one and you'll do fine.
     
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