View Full Version : Drilling for Spinning fasteners
11-22-2005, 08:17 AM
I am in the process of refastening my 1951 concordia yawl and have come across a few fasteners that will not come tight even after going up two screw sizes. I have been told that the best way to deal with these is to drill the hole with a larger size drill bit and plug the hole with a dowl and epoxy.
Should I use a mahogany dowl? How big do I drill the hole. The boat is fastened with size 14 two inch screws and I have been using size 16 to refasten. I have heard of variations of dowl shapes. Any help would be most appreciated.
11-22-2005, 11:11 AM
Perhaps you're over-screwing with an electric driver or not using a tapered bit and making too big a pilot hole, but have you determined that the problem is not rot in the frames?
Either way, while epoxy is a nice way to set things in the right place, you'll want to know the cause before you leap to the cure.
[ 11-22-2005, 11:13 AM: Message edited by: Ian McColgin ]
11-22-2005, 11:37 AM
How big do I drill the hole.
Um, the size of the dowel.
Hard to say what's going on, but the frame could well be punky. That's the only reason I can think of for a bigger screw to be spinning.
11-22-2005, 11:51 AM
Donít use a dowel. They will be end grain and the fastening will not be reliable. Stack some bungs in the hole with epoxy glue.
11-22-2005, 12:11 PM
Mahogoney dowels seem like very bad trennels with a pointless screw augmentation.
I rather like black locust trennels for this sort of repair. Assuming the frames are in reasonable shape and normal size, you could make them 3/8" diameter.
I set mine in a bit of epoxy both to seal the endgrain in the plank and the frame and to provide a little lube driving the trennel in. The fit should be quite snug and hold without blind or outside wedges.
The trennel should be so snug that even dry the air pressure build up in the hole will make driving it home hard, bouncy at the end. Dry, it'll go as the air will relieve through the frame's endgrain and maybe a very little the plank's. But with an epoxy slick, it won't. So make each trennel with a relieving groove (I do this with a table saw kerf before rounding the trennels down from square stock) which will be up facing when you drive the trennel home. That way the excess epoxy can escape and will also plug the relieving groove just fine. With the up facing groove, the excess will mostly travel to the end of the trennel rather than dribble down the plank, much neater after the glue sets and you cut (not a bung - no chisel swat here) the excess of the trennel off.
In really good wood, you can move fast dipping the trennel end in epoxy, wipe the very end against the inside of the pot, and drive it right in. If you have reason to doubt your frames (you do!) put a dab or really thin epoxy or CPES in the hole first to get well into the frame's grain, then a dab of epoxy on the trennel and drive it home.
There are store bough trennels that have two diameters. They are ment to be drilled such that the thick part gets a bit past the frame/plank interface providing good shere strength and then be smaller deeper in the frame thus weakening the frame a bit less. I've not actually worked with them because I've not done repairs on a boat so lightly framed where the choise was not rivets anyway. Also I'm cheap and won't buy stuff I can make for about free. In plank on frame carvel sailboats of normal scantlings, trennels that are scaled down to keep the porportion of about 1" diameter per 100' waterline length work just fine and don't harm the frames.
Black locust is the very best wood for trennels as it has wonderful shear strength, takes driving very well, has a very tight end grain that won't wick any water, and is only a little less rot proof than granite. You don't need a dowl maker. Rip out sqhare sections just a schkoudgue bigger than planned diameter and use the kerf to put that relieving slot in before rounding. Rounding's easy. Have something hard and round you can drive the sticks through. I used an old iron plate about 1/2" thick I found lying about with three graduated holes taking the corners down a bit first before the finish hole. I drive 6" long sticks through the rounding jig, cut them in half to make two long enough trennels, and relieve one end just a little so it can get started in the hole.
11-22-2005, 04:32 PM
Providing your frames are not soft, the holes can be snugged up by tapping in a tapered pin of white oak with epoxy in the hole. The fastening holes can then be re-drilled with a Fuller Taper Drill of the correct size for the fastening.
On a Concordia, I believe the fastening size would be a #12 wood screw. Going over size will weaken the frames. So stay with the size the boat was fastened with.
Most non boat builders believe that the function of hull fastenings is merrily to hold the planking to the frames. However the truth of the matter is that the first function is actually to keep the planking from shearing on the frames. Only at the hood ends and butts do fastenings come into extremes of holding planks on. This means that the shank of the screw should be just enough in diameter to give sufficient shear resistance as the hull twists. Both Loydes, Dixon Kemp and
N. G. Herreshoff worked up tables for fastening sizes in relation to scantlings of frame and planking over a hundred years ago. It is still sound advice.
11-23-2005, 07:44 AM
Thanks all for your help. Are there any good sources for Black Locust in New England? It looks like I am going to only need a small amount.
11-23-2005, 07:56 AM
Locust should present no trouble. It's actually quite likely that there's a nuisance tree near-by waiting for you to fell it or trim some limbs so first chat up a local arborist. A specialty hardwood store or any competant wooden boat yard are the other most likely sourses.
Side note on when you try the plug and rescrew: On a Concordia I helped with, we repaired the screw holes with bungs: Oak for the oak frames and a topper bung of mahogoney for the planking, set with harmonious grain alignment. You don't have to use epoxy but we did.
I was taught not trust the holding power of a screw in an oak dowel and I'd rather not aim oak's endgrain at the surface, though oak is occasionally used as trennel material.
11-23-2005, 08:42 AM
You should go visit Jim Titus. He had tons of black locust last time I visited but more importantly, he is close to you and he knows whereof he speaks. He has a very active restoration facility in one of the buildings on the Bristol Marine property off Popasquatch (sp) road next to the Bristol Yacht Club. It's worth the visit just to check out the boats.
I bet the frame is rotten but, if not, I vote that you inject a glob of thickened epoxy in the hole, let it harden then drive the screw (with approporiate pilot hole).
11-23-2005, 08:59 AM
Are we certain that the plank is tight to the frame?
If they aren't tight, is it a good idea to create a hard epoxy glob between the plank and frame?
To my mind it may create a stress riser, and if the frame isn't already cracked it soon will be.
11-23-2005, 04:00 PM
I have dealt with this in several ways, all seem to work.
Remove the plank, drill hole in frame and glue in a face grain plug.
Drill entirely thru the plank and frame, saturate the hole and plugs with glue press in an oak plug first into the frame, then follow up with mahogany plug.
Set a new countersumk hole immediately next to the screw, 1/2 inch away, not cutting across the plank, this new screw will be offset from the center of the frame. The plank will just have another hole along its length and wont effect the strength.
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