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 dont 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 doesnt 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 doesnt bond onto the floor or work surface.

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Joining Plywood Jigsaw or Puzzle Joints

I apologise for my long gap between posts, life has been very full for awhile.

Jigsaw joints have become a popular method for joining 8ft plywood lengths into the long panels needed for building most boats. It is an easy process for those who are building from a kit but it is impractical for most people building from scratch. If building from plans or from patterns then you will have to make every cut by hand using a jigsaw and the accuracy necessary for a close-fitting jigsaw joint is just not possible to achieve.

For those who have not seen a jigsaw joint, the form is exactly the same as the interlocking curves of a jigsaw puzzle, except that the interlocking curves are laid out in a straight line rather than in a grid pattern. This forms a very accurate and secure junction between the two parts, unable to slide apart and needing no clamps, while the glue sets.

Jigsaw joint configuration.

The two parts have glue applied to the bonding surfaces, then they are laid on waxed paper or plastic to prevent adhesion to the surface below, aligned properly then a rubber mallet is used to force the fingers of the upper part down into those of the lower part. Care must, of course, be taken to ensure that the surfaces of the assembled joint are perfectly flush or there will be a permanent deflection at that point. The photos below, courtesy of Chesapeake Light Craft, show the joint before and after joining.

Applying glue to the bonding surfaces

Cleaning up the completed jigsaw joint

Tight joints as described above assume that you will assemble them on a large table or a workshop floor before moving the panel to the boat. This is OK for dinghies or for professional builders who have enough space and the staff to manhandle large panels from the shop to the boat and to position and clamp them accurately, all after applying glue to the framing of the hull and to the panel, trying to get it into exactly the right position first go so that it doesnt slide on that slippery glue and spread it everywhere except in the joint where it is needed, then clamp it accurately so that it can finally be fastened. You get the picture, of enormous potential for disaster.

For amateurs building bigger boats it is just not practical to assemble the panels before installation. Panels 8ft long are very convenient for one person to handle, without having to call on the wife, kids and neighbours to help with the task. I built the structures and interior of my Didi 38 "Black Cat" entirely single-handed. There was not a single piece of timber in that boat that I was not able to carry and set up without any assistance. But I made scarph joints by hand and I glued them in position.

Building with jigsaw joints and gluing the joints in position requires a slightly different approach. It is not practical to hammer a tight joint into place with a rubber mallet. The stringers and other supporting timbers are springy and will bounce away when hit, making it difficult to get the fingers properly flush. Another factor is that of longitudinal alignment accuracy. If the first piece is slightly out of longitudinal alignment on the framing then the accuracy of the jigsaw joint will force the other pieces to continue that misalignment. An error of 1mm on the first of 4 panels will grow to an error of 4mm by the end of the 4th piece.

The way to get around this is to machine in some tolerance into the joint. When the jigsaw joint curves are added to the CAD drawing of the panel, the curves are used to break the panels into the pieces that will fit onto 8ft sheets of plywood. Then the jigsaw curves must be offset into each panel by 50% of the tolerance that is decided upon. This means that each piece is minutely shorter than the designed length and the fingers of the joints are minutely slimmer. The result is a joint that can be easily pushed into place without hammering and which also has a small amount of wiggle room to allow for correction of minor installation misalignments.

This more loose joint may need a temporary doubler on the back between stringers so that small screws can be used to pull the fingers flush if the panel is curved at that point. Dont forget to use a waxed paper or plastic bond-breaker to prevent adhesion of the doubler to the panel.

After the glue has set you can use a belt sander to clean up the surface for a nice finish and a joint that will disappear in the painted boat.

For info on our boat designs, please visit http://dixdesign.com/ and http://dudleydix.blogspot.com/.

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Joining Plywood Scarph Joints

This is the first of a series of posts about the methods of joining plywood. It is not a course to teach how to do this work but a basic explanation of how it is made and the benefits and drawbacks. Refer to that excellent book "The Gougeon Brothers on Boat Construction" to see various methods for making joints on plywood and solid timber.

I am starting with my preferred method first, only because this has been the subject of discussion between me and a couple of other people recently. This is the scarph (or scarf) joint, more technically known as a feathered scarph because the slope tapers off to a "feathered" edge.

Most amateurs have a fear of making scarph joints that fits anywhere between mild trepidation and total panic. In reality a scarph on plywood is relatively simple work to do. If you can handle a hand plane with reasonable accuracy then you can scarph plywood. A scarph joint is no more than a sloping butt joint between the ends of two sheets or lengths of timber, generally sloped at between 6:1 and 10:1. The 1 is in the thickness of the timber and the other number is in the length of the timber. For scarphing plywood I would generally use about 8:1 slope. So, in 6mm plywood it would be 48mm and 12mm plywood would be 96mm. These numbers are not finite rules, so they can be rounded off to 50mm and 100mm respectively for simplicity as long as both halves are planed to the same angle. In imperial measurements an 8:1 scarph on 1/4 plywood will be 2" wide and 4" on 1/2" plywood.

The Anatomy of an 8:1 Scarph Joint on 12mm Plywood

It needs only a tape measure, straight edge and pencil to mark the scarph, in addition to the plane. Once you have the plywood panel cut to required shape to fit the boat, you will measure in from the edge that will be scarphed by the width of the scarph. Do this at three or more places along the edge, which will show up any errors in your measurement if the straight edge does not touch all points. Draw a pencil line with the straight edge to define the edge of the slope.

If you have a hand power plane, this will make short work of removing the excess material but dont try to get up close to the line or you will risk messing up. Stop at least 3mm above your line then use a sharp jack plane to take it down to the finished surface. The closer you get to the finished surface the finer the plane must be set. Remember that you are working to create a feathered edge right at the edge of the board, so you dont want to remove any material that will destroy that edge. Clamping your board to another sheet of plywood or a firm table, with the edges aligned, will support the feathered edge so that it doesnt move away from the plane when you work.

I cant emphasise "sharp" strongly enough for the plane blade. It must be as sharp as you can get it and you should hone the plane blade again before each scarph is planed. You will be planing across the end grain of alternate layers, which will rip instead of being sliced if the plane is blunt, ruining the accuracy.

Use the edge of your plane or a straight edge to check that your slopes are straight. If you can see light under the straight edge then the surface is not straight. If the surface is concave then the middle of the joint will be hollow. If convex then the feathered edge will not lie against the other sheet so you will have gaps .

If you dont have a hand plane but have a skill saw, you can buy a "SCARFFER" attachment that bolts to the base plate to set the slope. You can see this attachment in use in the Gougeon book mentioned above. I did the same by cutting a block of wood to the required angle and bolting that to the base of my skill saw.

If you have neither a power plane nor a skill saw then you can still do it all by hand. Start with the plane set coarse until you are nearing the line, then sharpen the plane and set it more fine for the final finishing.

Be careful not to make the most common error, which I have done a few times. That is to plane both slopes onto the same side of the plywood instead of onto opposite sides. You dont realise your error until you bring the two pieces together and they dont match.

Is it a problem if your scarphs are not entirely true? That depends on how big the error as well as what glue you are using. Epoxy has better gap-filling properties than resorcinol and also gives a stronger joint if there is a film of epoxy between the two surfaces that are being glued. Resorcinol gives maximum joint strength when there is wood-to-wood contact, so epoxy is more tolerant of bad joints than resorcinol. Still, dont use the fact that you are using epoxy adhesives as an excuse to accept shoddy workmanship from your own hands.

When gluing the joint, remember that end grain is thirsty and will suck glue out of the joint, so dont skimp on the glue. Let the glue lie on the surface for a few minutes so that it can soak in, then apply some more if needed.

Scarph joints on plywood are a lot stronger than the wood that is being glued, so there is a natural safety factor. Also, you will generally be attaching the panels to a wood structure that has stringers and other longitudinal timbers crossing the joint and reinforcing it. You are very unlikely to ever break that scarph once the panels are glued onto the boat.

Making scarph joints is satisfying work once you get the hang of it. It produces a nice smooth surface both inside and outside, much nicer if your boat will not have hull liners covering up all of your handiwork. Scarph joints are neat, strong and can be made by hand. I cant think of any disadvantages to this joint, aside from the unnecessary fear of making them.

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Joining Plywood Stepped Scarph Joints

A stepped scarph joint is the mechanically produced equivalent to a hand-planed sloping scarph joint. It is normally cut on a CNC router that has only three axes of movement, namely forward/backward, side-to-side and up-and-down. The sloping face of the hand-planed scarph is replaced by a series of small steps, each comprising a small vertical face and a broad horizontal face that coincide with mating faces on the other piece to which it will be bonded.

The main problem with stepped scarphs is similar to that with sloping scarphs. That is attaining proper alignment when gluing the joint and keeping it aligned until the glue has set. A sloping scarph can and will slide in almost any direction as soon as the slippery glued surfaces are brought together, so it has to be very carefully clamped. A stepped scarph cannot slide forward because the steps prevent it but it can slide back or sideways. Again, it needs careful alignment and clamping.

Various methods have been developed by different companies to overcome this problem. These methods are not patented, so can be used by others.

Mechtronics in Cape Town, South Africa, use a dowel peg system to lock the joints. The holes for the pegs are drilled by the CNC machine and hold the joint so securely that the panel can be moved right after the joint is made, without having to wait for the glue to set. After the glue has set the joint is sanded smooth with a belt sander, which also trims the dowels flush.

Mechtronics panel as it comes off the CNC machine

Mechtronics stepped scarph joint completed.

Ertug in Istanbul, Turkey, achieve a similar locking effect by programming the CNC machine to form islands in the one half of the stepped scarph that lock into holes that are cut into the other half. This is also a very neat solution.

Ertug stepped joint details.

Stepped scarph joints are good for any size of boat but care needs to be taken in high load situations or where there may be flexing of the panel. The weakness of this joint is in potential cracking along the surface joint lines, either from stress in the panel or from bending. Joints should, where possible, be kept away from edges of openings that might align loads along the joint to promote cracking. Edges of openings must be reinforced with wood perimeter frames or with glass or carbon tapes to spread the loads away from the corners and into the panel surfaces. The weak areas of the joint can also be reinforced with glass tape laid across the surface joints. Joinery shelves or locker tops that span across the joint also help by stiffening the panel, preventing flexing that might initiate surface cracking.

The CNC operator must take extreme care when cutting scarph joints to ensure that the plywood is hard against the sacrificial backing board during cutting, or the accuracy of the stepped surfaces deteriorates. There must be a vacuum on the table to suck the sheet against the table, as a basic requirement. If this is insufficient then plastic nails should be used to mechanically fasten the sheet to the table. All waste material must be efficiently removed as it comes off the router bit so that there is no chance of it getting between the sheet and table. This is best done with vacuum right at the cutting tool. Finally, the cutting paths need to be programmed so that any puncturing of the full depth of the sheet happens as the last stage of the cutting process. If it happens earlier it will increase the chances of waste material getting under the sheet and it will weaken the vacuum that is sucking the sheet against the table.

See our range of designs at http://dixdesign.com/ .

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Joining Plywood Sawtooth Stepped Joints

Like the stepped joint that I discussed in my previous post, the sawtooth stepped joint is not something that the amateur builder will make from scratch. It may be that this joining method will be a feature of a kit that the amateur builder buys from a kit supplier, so he may do the final assembly of this joint without cutting the shape into the plywood.

This is a modification of the stepped scarph joint. The only difference is that instead of  the straight-line plan shape of the joint, it has a sawtooth plan shape as can be seen in the photo below. The form  in the photo is a true sawtooth shape, with straight lines between the changes in direction of the cutting tool. A variation is for the steps to have a wavy form composed of a series of curves instead of the sawtooth form.

This bulkhead in the photo is from a kit for the Dix 470 catamaran, cut by Exocetus Catamarans in UK.

These joints were developed primarily to get around the alignment problems of the straight variety of stepped scarph. The components that are being joined can easily slide relative to each other, parallel to the steps. The toothed or wavy form of the joint allows the two components to interlock accurately. As long as there is pressure to push the joint together when is is clamped then it will engage accurately.

There is a side benefit to this type of joint that is particularly beneficial to larger boats, with large loads in the structure. The form of the joint does a better job of distributing the loads between the two boards that are being joined. There is a much reduced chance of the joint initiating a crack along the line of one of the surface joints. Nevertheless, joints should be positioned away from the corners of openings and other stress-raisers in the bulkhead or whatever component is being made.

To see our range of designs, go to http://dixdesign.com/ .

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