Building the Bouchie Dory Part 12 – Scarfing the bottom panel
The introduction of waterproof plywood was a boon to the boatbuilding trade and there was a time when you could order it in any length you choose right from the mill but those days are long over. Now we are stuck with the tyranny of the 8’ long panel which mean that unless you want a 7 1/2’ long boat, you are going to have to do some scarfing.
Scarfing is very intimidating to the un-initiated but it needn’t be so. The secret to a successful scarf is mostly attention to detail. we are simply cutting a matching taper onto the ends of two pieces of wood. The layout must be the same on both pieces obviously. Where people probably go wrong is in a couple of areas:
- Not taking their tapered surface down to a feathered edge.
- Undershooting their planing marks.
- Not properly flattening the scarf area.
- Not aligning the two pieces properly during the glue up.
- Not applying the adhesive properly.
- Not clamping properly during the glue up.
There are a few little cheats you can rely on if you are struggling.
Your planed surface doesn’t need to be 100% flat. It does along the width of the board. The length of the scarf cannot have a hump but it can be cut with a slight hollow. I often do this on purpose. So long as you use properly thickened epoxy it will work fine and it will allow the feather edges to properly lay flat along the layout lines. If you were struggling with hand planes and reached for an angle grinder, you would probably end up with a hollow scarf surface. (I almost shudder to even suggest that but on the jobsite or on large scarfs I have done so myself) If you use hollow scarfs you should use a thicker stiff clamping board for your glue up.
Over shooting your planing mark isn’t a deal killer because you need a little space for the feather edge anyway. the most important thing in this case is that you do not loose the location of those layout lines. Mark them on the edge of your plywood at the start. You can then arrange your panel right to the layout lines instead of pulling back a hair as I discuss in the video.
You can clamp your scarf by simply screwing right through your clamping pad and the scarf area. This is a popular method but you will have some small holes to fill later. Make sure you use a thick backing block for this so the screws have something to hold onto and dip your screws into some paste wax so they don’t bond to the epoxy.
Better to have your panels too far away from each other than too close. With thickened epoxy any voids will be filled but arrange them too close and you will have a hump that either remains or you will be sanding through your veneers to flatten it back out.
If you are worried about working time, remember that wetting out the surfaces with un-thickened epoxy and gooping them with thickened epoxy are two separate operations. Don’t worry about mixing enough epoxy to do both at once. The wetting out can be done well ahead of time because it will saturate into the end-grain and it won’t matter if that epoxy begins to kick off before the fill coat.
It may seem like a great idea to stack up a lot of panels and cut and glue them all at once but I find that even with the most meticulous setup, something almost always doesn’t go quite right. I never do more than a stack of two or three panels and prefer to do just one if I have the space and time for it.
Lastly, if you are planning on a bright finish (that’s what we call varnish in the marine trade) it pays to do a couple experiments with fillers to find the right colour combination so your glue line doesn’t stick out like a sore thumb. West System’s 405 Filleting blend filler is a good start. the colour used to be a closer match to occume mahogany (the most common marine ply species) but these days it’s on the pale side. I try to capture different coloured dust from my random orbital sander for colour match purposes and I also use dry pigments to achieve a colour match. Don’t try using sawdust from below your tablesaw or from your belt sander. They tend to be far too course and it looks terrible.
This is one of the most exciting moments in building a boat because for the first time you get to see that actual mass of the boat, or at least get a realistic impression of it. This is one workday that always feels very productive at the end of it.