{short description of image}




Scarfing

Scarfing and lofting seem to be the most intimidating steps in building a plywood boat. Hopefully I will dispel those fears. On this Mudpeep I spent a grand total of about four hours scarfing, lofting and cutting out the plywood panels so they were ready to stitch together.

Unless you have access to sheets of plywood long enough to make the panels of your boat, you will need to scarf one or more pieces together. In this case I am using three 4'x8' sheets of plywood to make the panels for the hull. I will cut one in half and add each half to a full sheet ending up with two sheets a little less than 12' long. The plans call for a full 12' so I will just make the boat a couple of inches shorter. If you want to build this boat without decking, then one 4'x16' panel will be enough for all the bottom and side panels.

Scarfing plywood is the process of beveling the ends of two sheets of plywood and gluing them together to make one longer continuous sheet. The length of the bevel should be a 10:1 ratio to the thickness of the plywood. The Mudpeep calls for " thick panels so the plywood should have a bevel of 2 1/2". Uniform bevels on the ends of the plywood sheets are important for a good bond at the glue joint.

I have found that the best way to get even bevels is to cut them all at once. I will start with a flat surface and lay down the first sheet of plywood. Since the end of the plywood does not always lay perfectly flat, I will screw it down to the work surface. Measure back about 3" from the end and anchor it with three screws, one on each side and one in the center. Lay the next sheet of plywood on top of the first, but set it back 2 1/2" from the first and screw it in place. Do this with all four sheets of plywood and on the fourth strike a line 2 1/2" back. I now have an evenly staggered stack of plywood. By removing all the "steps" on the stack of plywood I will end up with one continuous bevel.


Scarfing Plywood


I find that using a power plane and finishing with an 8" sander/grinder works best for scarfing. By running the power plane down over the steps I can uniformly shave off the ends of the plywood. I take off about 90% of the wood with the plane and finish up with the grinder and 36 grit sandpaper. Use the lines that appear in the plywood as a guide to achieve a uniform bevel. When the end of your stack of plywood is smooth, flat and the lines are straight you are ready to glue up the panels. Ideally the ends of the plywood should feather down to a thickness of a piece of construction paper, but in most cases it will be sharp edge.


Scarfing Plywood


You need a flat clean surface to start your glue up. I like to use strips of 10 mil plastic as the barrier between panels. Because the epoxy can squeeze out a fair bit you may want to go with 24" wide strips. Lay the first full sheet of plywood down so the lines of the bevel are facing up and slip a layer of plastic under it so you do not glue it to the working surface. Mix up some epoxy, 9 to 12 ounces should be enough for the whole job, and apply a generous coat to the first panel with a bristle brush. Apply epoxy to one of the half sheets of plywood and lay it on the first sheet so the lines of the bevels are facing each other. If you are unsure of your scarf joints being uniform, mix a little wood flour or Cab-o-sil into the epoxy to create a honey like consistency. This will help fill any voids you may have. When you lay the half sheet down, do it slowly. If it "flops" down, the plastic underneath may get blown into the joint. I will always slightly pull on one end of the plastic and move it side to side to verify that it is laying flat.

Before tacking the panels in place you need to ensure the scarf is aligned properly. Slide the panels apart until you can just see the second ply of the plywood on the scarf cut, you don't want to glue the plywood panels this far apart, but it will help you gauge how much room you have to work with. The other extreme is where the scarf cut starts on the first ply and I go by where the sanding marks from the grinder ends. Split the difference of these points and your positioning will be fine. Depending on the thickness of the individual plys in the plywood, you should have between 1/8" and 1/4" to work with. If you plan on painting the hull of your boat there is no need to be this fussy about alignment. I am going with a bright wood finish on the hull and a poor scarf can be unsightly. Make sure both pieces of plywood are straight by running a straightedge or string along the edge of the two plywood sheets. You will need a perfectly straight edge as your baseline when lofting. Drive in a finish nail in on each edge of the plywood half way between the start an end of the scarf joint. This will keep the scarf joint from sliding apart when weight is later added. Lay another strip of plastic down and repeat the procedure with your next panel.

After the panels are tacked in place another strip of plastic should be laid over the scarf joint and a 10" or 12" wide piece of plywood over that to evenly distribute the weight. I put a few hundred pounds of iron over the plywood as you can see I the picture below. If you don't have weight around, you can also use duplex nails by driving them through the scrap plywood on top down into a sawhorse. This will act as a clamp and are easily removed later. The panels will need a day or two to cure. You can tell the epoxy is fully cured when it becomes brittle. There should be some squeeze out on the plastic that you can test. If it bends before breaking it is not fully cured.


Scarfing Plywood


Lofting


The Mudpeep is an easy design to loft. The baseline is not only the reference point you measure from, but is also one of the edges of your bottom and side panels. With a more complicated design the baseline is only a reference that you would use to loft both edges of the panel.

Before you start drawing on the panel it is very important that the panel is flat, the baseline (edge of the plywood panel) is straight and the end of the plywood is square in relation to the baseline. The first step in lofting is to mark the stations on the panel. On this design they are at 12" intervals. I prefer to run a tape along both edges putting a mark every 12" and then using a straight edge to draw the station lines the full width of the panel. The stations should be marked on the panel 0 through 12.

I will start with the side panel. The sheer line of the side panel is also the baseline from which I will be measuring. At station #1 you find the numbers "1-4-5". The first number represents feet, the second number represents inches and the third number represents 1/8ths of an inch. In some cases you will find a plus or minus mark, this tells you to add or subtract 1/16th of an inch. The station one measurement 1-4-5 translates to 1' 4 5/8". Measure in this distance from the baseline at station 1 and put your first mark. You will notice there is a station 11a, these sub-stations will help you better define the shape at the ends of your panels. In the case of the stem area you will need to measure up from the baseline at 4" intervals and draw lines from station 0 to station 1. The stem measurements at 4" intervals will determine the shape of the bow.


Stitch And Glue Construction Lofting

When all the lofting marks have been made, a finish nail should be driven into the marks and a straight, even grain wood batten should be used to insure the lofted lines are fair. Place the batten along the outside edge of the nails and sight down it. You should find the batten touches all the nails and it has a nice fair curve. Of course this rarely happens and you may have to re-check your measurements. If after re-measuring there is still a slight discrepancy, try bending some of the nails a little to shift the batten. When the batten is fair, mark the line. After the keel, chine and stern lines have been made; you can start lofting the bottom panels using the other edge of your plywood panel as the baseline.
Stitch And Glue Construction Lofting

Previous

Next


This is the Devlin design "Mud Peep", for more information on this design and others offered by Devlin Boat, go to devlinboat.com





{Home} {Stitch And Glue Construction} {Construction Projects} {Epoxy and Fillers} {Glossary of Terms} {Tools} {Links} {Mail}
boatbuilder.org -All rights reserved