Monday, February 18, 2013

Joining Plywood - Butt Joints

The butt joint is the most basic method for joining two pieces of plywood end-to-end or side-to-side to make a sheet large enough to suit a large bulkhead, make a hull side panel or anything else larger than the material that is available. Within the label of "butt joint" there are various ways to do the job. Click on the diagrams to enlarge them.

A butt-strapped joint is a simple end-to-end joint, with the meeting edges glued together and with a piece of plywood on the inside to reinforce the joint.

A basic butt-strapped joint

With traditional adhesives you would have had to clench-nail through the panels and butt-straps to be sure that they would stay in place but modern adhesives are stronger than the wood, so the fasteners become redundant after the glue has cured. I would use short temporary steel screws that I would later remove and re-use for the next joint, then fill the holes with epoxy.

The strap width will vary depending on the thickness of the plywood being joined. About 15x the plywood thickness is good, so:-

135mm wide for 9mm plywood
180mm Wide for 12mm plywood
5 5/8" for 3/8" plywood
7 1/2" for 1/2" plywood

Be sure to chamfer or radius the edges of the strap before fitting it, which neatens the appearance of it on the inside of the boat and removes sharp edges that don't hold paint well. If you are fitting butt-straps between stringers in the bottom of the hull, stop the strap about 10-12mm (3/8" to 1/2") short of the stringers to leave a channel for bilge water, or you will create small traps where water can lie.

This joint is very easy to make but it does have drawbacks. It adds unnecessary weight and it spoils the neat interior surface of a boat that will have the inside of the hull exposed. It also adds a hard spot that affects the smooth curve of the hull surface, particularly if the joint is made on a flat surface before the panel is installed rather than being made in place with the panel already curved. There is a chance of the outer surface of the joint showing a hair-crack over time, so it is worthwhile to glass-tape the outside of the joint, as described below.

This joint is good for bulkheads, where a nice piece of hardwood trim can be used in place of plywood to cover and reinforce the joint on both sides. In that case the hardwood trim could be about double the thickness and half the width of the plywood butt-strap.

Taped joints are the simplest way to join plywood in small boats, with sheet thicknesses of 4-6mm. Easiest is to butt the two pieces end-to-end then laminate a length of 50mm (2") wide glass tape onto the outside. When it has cured, turn the panel over (being careful to support it properly so that the unreinforced side doesn't crack) then laminate a similar strip of glass onto the other side. Sand the edges of the tape to feather them into the plywood surface.

Taped and Flush-Taped Butt Joints
The main problem with this taped joint is that there will be a slight mound at the joint, which can be difficult to fair out so it will show, particularly with a gloss finish. The way to get around this is to recess the glass tape flush into the plywood surface. To do this, slightly bevel the both surfaces of each piece of plywood with a hand plane or a sanding machine, forming a slight slope about 1-1.5mm deep at the edge. See Step 1 in the diagram above.

When you bring the meeting edges together they will form a shallow V into which you can lay your glass tape. build up the glass tape so that it completely fills the V. When both sides have been glassed and have cured, sand the tape down flush with the plywood surface, producing a very neat and almost invisible joint.

For all of these jointing methods you will need to lay waxed paper or a sheet of smooth plastic under the joint before you start gluing or glassing, so that it doesn't bond onto the floor or work surface.

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