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Wood screws into epoxy


Joel

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My Weekender keel is laminated from 3 pieces of 3/4" marine plywood. I am going to put 2 layers of fiberglass over it and cap it with a 3/8" UHMW strip. Here is my plan for mounting the UHMW:

1) mount the strip using 1-1/4" wood screws to mark hole centers

2) remove it

3) redrill the holes to 1/2"

4) fill oversized holes with epoxy/wood flour mixture

5) apply 4200 as a bedding compound

5) remount the strip screwing into a pilot hole in the epoxy plug

I made up a test piece filling some oversized holes with the wood flour/epoxy mix. It really doesn't hold the screws very well. I can pull them out fairly easy.

Any ideas for a better epoxy mix, or other mounting method?

PS: I already know plywood is not the material of choice for a keel. Seemed like a good idea at the time :wink:

thanks, joel

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Drill out the holes oversized, and then coat Machine bolts with vaseline and place them in straight epoxy poured into the holes. Center the bolts with small shims, like toothpicks placed on three sides of the bolts. Let the epoxy set and back out the bolts. You now have an epoxy plug that won't pulll out that is threaded to match the bolts and the holding power is incredible. When you add wood flour to the mix, you weaken the ability of the epoxy to make a good solid plug. Using wood screws makes too coarse a thread, the holding power is in the threads set into the epoxy plug.

This is the way some darned big pieces are set into place on big wooden boats and they will be there as long as the wood surrounding the epoxy plug is intact. Also the way that cleats, and anything attached to a wood cored fiberglass deck, etc. is accomplished by the pros. You don't have to worry about moisture being drawn into the plug or around the plug. It is many times more solid than the wood that is around it and the holding power is absolutely solid. The holes are usually tapered by rocking the drill bit a little so that the plug is wider in diameter at the bottom of the hole.

A good sized syringe with a long tip makes filling the holes quite easy.

For more infomation you can go to the web and look up the method under the Geugeon Brothers Epoxy Fabrication manuals.

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I agree with Barry, except that I'd use colodial Silica in the mix. Gives a SUPER strong plug.

Cross arm bolts on high performance plywood catamarans are held on that way, plus the Gougeon's use the same technique to hold ballast keels on- no nuts- just bolts in epoxy.

If you want to REALLy set them solidly, put a nut on the bolt way out close to the tip, drill the holes large enough to accomodate the nuts, then shove that into the hole. You can back the machine screw out of the nut and the epoxy plug, and reinstall several times.

I'd just counter sink the heads- certainly wouldn't let them stand proud.

For the holes not on a horizontal plane, you must thicken the epoxy enough ( with sillica) that it won't run. You can also put a screw sized hole in a chunk of duct tape and then tape around- that'll hold the epoxy in til it cures. The syringe idea works well also. But the epoxy must be thickened enough to not run back out. Be sure to pre wet the inside of the hole first and let it sit for a few minutes, but NOT long enough to cure.

I installed a bow eye into the Weekender I built using that method- you can pick the entire boat up with that bow eye :lol:

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I also drilled and tapped cured epoxy. This works nicely but if it isn't a through hole you have to drill the hole a bit deeper than you need so you can tap it far enough down.

Don't get the vaseline too thick if you stick the screws into the epoxy before cure. You need the epoxy to form to the screw's threads and not around a blob of pertroleum jelly.

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Joel,

If you use Charlie's idea to thread a nut onto the end of the bolt and embed that nut into the epoxy, I wouldn't use stainless steel bolts. Stainless threads are notorious for fouling (even on fairly large bolts) if you repeatedly remove and reinstall them more than once or twice. My dad is a steel fabricator and he makes it a rule to only use stainless fasteners on applications where he is confident he'll never need to remove them again. (And if he does need to remove them, he replaces any fasteners that show the least hint of binding.) Otherwise, galvanized steel is the safer way to go if you expect to remove the fasteners periodically.

Also, that is true about stainless steel needing air to prevent corrosion. I don't know if crevice corrosion can occur when you permanently seal stainless under epoxy (where water will never get to it). But it's definitely a problem if the stainless is consistently exposed to moisture and consistently deprived of air... which is exactly why vinyl coated stainless steel cables are notorious for snapping... because the moisture gets inside the vinyl coating, sits on the stainless wire, and the wire corrodes because the stainless can't "breathe" through the vinyl coating.

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I used to coat screws with a very expensive mold release wax I bought from RAKA (not knowing what else to use). I'd smear it on, then carefully and painstakingly rub it off so I'd have clearly defined threads for a good, tight fit into the epoxy plug. But it gradually dawned on me that wax is wax--just something epoxy won't adhere to. Now I light a candle, dip the screw into the hot wax, and then hold the screw above the flame for a second to thin and distribute the wax into the threads. Done! Lots faster and a good clean fit.

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We used silica on a number of large bolt applications where we did place the nuts in the bottom of the holes with the bolts in them. We also used hot petroleum jelly because it would only leave a thin film but enough that the bolts would back right out. We used any number of things to make up the shims to center the bolts but wooden skewers seemed to work well as did plastic toothpicks.

Yes, we counter sunk the heads in the epoxy rather than coating over them. Where they needed to be hidden, we plugged over them with wooden plugs and then just finished them off.

We used lots of duct tape to make covers with just a small hole in it to thread the bolts thru for vertical applications then press fitted the duct tape to the surface. Only occasionally was any leakage an issue and then it just flattened out quickly with a sanding board.

Never used stainless bolts on any of these applications, always used well galvanized bolts and nuts and occasionally fender washers in large holes as well. Talk about solid, you could lift the boat with just a couple of these into a good solid framing member. Bow eyes are a breeze and you can lift most bows clear of the water with them on lighter boats. I put the eyes for the rudder box in with this techniqe. I would expect that the boat and the rudder box will be long gone before I have any problem with the eye bolts pulling out.

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in regard to the use of the stainless bolts and machine screws-

I agree, but folks- we're talking about a trailerable boat here, where the fasteners will be exposed to air all the time the boat is on the trailer.

And they will be counter sunk into UHMV, which ISN'T gonna hold any water.

So in his usage, I see no problems at all.

If the fasteners were to be under water always, then I'd of course go along with Ray- Silicon Bronze.

Modern Galvanized fasteners iin salt water, won't last all that long by the way- not unless you can find some REAL hot dipped galvanized stuff, which can get tough to do in some areas.

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I mostly agree with what you're saying Charlie, but one caution: if there are *ANY* cracks, gaps, or chips (either now, or that develop down the road) which permit seepage of even a TINY bit of moisture to get in around the bolt, you WILL have problems with crevice corrosion in a salt water environment. (In fresh water, I suspect you would be fine using stainless on a trailered boat). Actually, I suspect you would have problems even without the cracks if the boat ever sits in saltwater for more than 1 or 2 days straight in a row. The only "protection" for stainless in salt water environments is to limit its use to above the waterline or to fully seal all exposed parts.

That said, my more pressing concern was about the thread-binding problem with stainless, which I can almost guarantee will happen if you repeatedly remove and replacee a stainless screw/bolt. It's actually quite remarkable. Sometimes you change a stainless fastener 1, 10, or even 100 times with so sign of trouble; then suddenly, with no apparent reason and no visible damage to the fastener, the threads will bind. I would definitely prefer galvanized steel or, even better, silicon bronze fasteners.

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That said' date=' my more pressing concern was about the thread-binding problem with stainless, which I can almost guarantee will happen if you repeatedly remove and replacee a stainless screw/bolt. It's actually quite remarkable. Sometimes you change a stainless fastener 1, 10, or even 100 times with so sign of trouble; then suddenly, with no apparent reason and no visible damage to the fastener, the threads will bind. I would definitely prefer galvanized steel or, even better, silicon bronze fasteners.[/quote']

One of the manufacturer's I worked for used stainless bolts on water pumps, and the tendency for stainless to gall caused them to go to nickle plated brass nuts. There's no problem mixing brass and stainless.

We found the problem greatly exacerbated when using power tools ... it was almost like the metal "spin welded" itself when using pneumatic or cordless nut drivers, and when we asked a metallurgist, he confirmed that's exactly what happened. This quote is typical of the info we found on it:

Galling occurs when the stainless steel oxide surface film breaks down as a result of direct metal contact. Solid-phase welding can then take place (whereby material is transferred from one surface to another). The symptoms of galling include surface damage and seizing and freezing up of equipment. Galling commonly occurs when using stainless steel nuts and bolts together, where the contact points are subjected to high tightening torques.

From http://www.assda.asn.au/asp/index.asp?pgid=18732

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So far I have only used silicon bronze fasteners on my Weekender. I am planing to use stainless screws to mount all the rigging hardware and to mount the UHMW strip on the keel. I like the bronze fasteners but, I think they are too soft for some applications. Here is a link to a product I have used at work. It seems to work OK

http://rshughes.com/products/079340_77124.html

joel

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