View Full Version : A technique for long(er) lived galvanized fastening
05-08-2003, 03:07 PM
I realize I've been awfully noisy, but with my project coming up I need lots of advice!!! smile.gif
I am currently going to refasten bad fasteners on my boat with hot dipped galvanized screws of decent quality. I got to thinking about the factors that cause them to waste away, and I realized one of the main ones was the plank/frame interface.
What if you core drilled out the old fastener, made up a dowel with a blood groove, epoxied it in place but left it proud by the same thickness of the plank that you have left as meat to hold the fastener? Now the hole in the plank may have to be a bit wider, but you could sink a fastener into that dowel and assuming it did not collapse, you would have some wood in that interface instead of metal.
So, for example, if your fastener was 1/2", the dowel would be 3/4", so that gives us 1/8" of good wood all around the new fastener. If the planking was 2", the dowel would stand proud of the frame by 1", and the plank would be bored out to 3/4". The fastener's head would need to be 1", a washer could be fitted and flat, non chamfered screws could be used. Under the galvanized washer you could put a simple barrier washer, like neoprene, or even just a scrap of tar paper. In this case you could even use lag screws. This also means the counterbore will be 1" deep in the planks.
See what I mean? All of this assumes, of course, that you have enough meat in the planks to do this.
[ 05-08-2003, 04:13 PM: Message edited by: TimothyB ]
05-08-2003, 06:27 PM
If you were to core the old wasted fastner out of the plank and frame, then epoxy the hole, and drive a dowel into the plank and frame, I fail to see what a screw driven into the dowel is gonna do.
I think you would be better off if you were to just dowel the plank and not the frame and move the screw next to the dowel, something like this
If I'm not mistaken fastners in carvel construction are usually driven offset so not to split the frame no? I'm not a plank on frame guy but this was allways my understanding. Perhaps someone will chime in and straiten both of us out..
05-08-2003, 08:46 PM
From reading this and your other post, it seems to me that you're creating a lot of work for yourself. In fact, there's something rattling around in my head that says white oak, being acidic, will have iron fastenings corroding in it faster than other woods such as, say, douglas fir would (Giffy Full has a iron nail pulled from Principia's hull he says rusted more on his shelf than it did during the fifty years it spent holding the boat together. Principia is built entirely of doug fir). With that in mind, I'm not sure that the "plank-frame interface" is necessarily your problem, at least not in the sense of how you're trying to deal with it. Knowing this forum, there probably is someone who could come up with a scientific study detailing the corrosion rate of iron in various woods and tell you exactly what's going on, but I don't have that info at my fingertips. Mike is right, carvel planked boat are (usually) properly built with the fastenings offset in the frame. If the builder was good, he figured in the necesity of future refastening by driving his fastening only slightly offset, so the next ones could go at a greater angle. The next time (assuming the rest of the boat is still around), they would go on the opposite diagonal. Cripes, you have (I'm assuming) 4" of futtock to play with, which is plenty of meat to use for years to come. I wish I had pictures of the schooner Timberwind's frames, which are so nail-sick that some planks had to be held on with through bolts to the ceiling, there being no place a nail would hold. You'd certainly feel better about your frames :D :D
05-09-2003, 11:39 PM
Locust trennels set in epoxy?
05-12-2003, 09:42 AM
Thanks much for the advice so far folks! Here is some clarification as to my crazy and seemingly non essential work.
This technique is actually lifted (I found out later) from a WB article about an English boatbuilder who has done lots of repair and renovation. He uses oblong wooden plugs to deal with iron sick framing and wasted wood. No, it does not add strength, but then it doesn't remove it either. The main advantage I think here is that the -plug- is not oak.. it is locust. That means that the fastener will last longer. If the plug is only slightly larger than the fastener, then really all you are doing is adding a non structural bearing to the load imposed on the frame, especially with the plug being epoxied in.
Of course, he just drives in new fasteners same size as the old ones. He was talking about taking out screws too. In my case, I am removing BIG galvanized cut boat nails and want to replace with screws, so I need to over size a bit.
Locust trennels? I'd LOVE to do that but I think I would need to have a structural engineer or boatwright look at my framing and planks and tell me if that were good. I would do that in a heartbeat if I were only coasting, but we want to take the boat offshore. I know trennels can be as strong as metal fasteners, but since the boat was never designed to be fastened this way, I'd want a pro opinion before I would do that (which I will likely get before wholesale fastener replacement).
I honestly do NOT want to just refasten into the frames. Some of them need plugs due to inconsistent fastening jobs leaving some voids. And really, I am not going to be refastening the whole boat right now! I am just going to replace those fasteners that need it first, then will do 'quadrants' at a time.
There are fasteners that are offset as well, many of them. So I suppose I'll have to fab a hydraulic nail puller. I saw a diagram for a good looking one that used two hydraulic truck jacks moounted on a frame, with the puller welded to 1" flatbar, that was in turn welded to the tops of the two jacks. I dont think that many nails will be able to resist this setup smile.gif I wonder if there are commercial hydraulic nail pullers? Hmm.. I'm going to check with wood salvage places since they have to deal with that sort of thing all the time. (drift bolts in old wooden construction)
Still thinking.. smile.gif
PS: Just remembered to mention, the other thing this technique does is keep the metal OUT of the plank/frame interface. That interface is where the metal wastes away the quickest, and so if the plug stands proud of the frame and slots into the plank, you are eliminating that area from the equation. The fastener is then entirely encapsulated with wood and there is no interface that is not sealed positively by some barrier coating. I think I will make a drawing and post it.
PPS: Drawing posted here:
[ 05-12-2003, 04:46 PM: Message edited by: TimothyB ]
05-12-2003, 09:49 AM
Assuming the scantlings would support it and the time-and-cost factors work out in a particular design, I have to admit I find the idea of locust trennels very intriguing....
05-13-2003, 01:38 PM
Wondering if anyone had more comments?
05-16-2003, 12:01 PM
I wonder how a screw in an expanding plastic jacket would work out, instead of a large dowel. Something like a Hilti wall screw which has an angled lip around the plastic plug to match the counterbore!
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