The Sea Chest  

by David Nichols - Austin Texas - USA

 

Chapter 8: Scarfing

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David Nichols has written a new book and has graciously consented to let us post one of the chapters before it comes out. It will be titled "LAPSTRAKE CANOES: Everything You Need to Know to Build a Light, Strong, Beautiful Boat" and will be published by Garth Battista at Breakaway Books, the publishers of books by Jim Michalak and Max Wawrzyniak.

I’ve found that there are some boat building tasks that first time builders find very scary and scarfing happens to be one of those tasks. If the truth be told, I found it scary, as well. However, after cutting a great many scarfs, it’s difficult to see what was so intimidating.

Briefly, scarfing is nothing more than joining two pieces of wood or plywood together to form a longer continuous plank or bigger piece of plywood. This generally involves cutting a matching slope on both pieces and then gluing them together with an adhesive such as epoxy (Figure 8-1).

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Figure 8-1

Scarfing is nothing more than joining two pieces of wood or plywood together to form a longer continuous plank or bigger piece of plywood.

The slope itself is expressed as a ratio like 8 to 1 (8:1) or 12 to 1 (12:1). For example an 8:1 scarf in ¼ material would be 2 inches long or an 8:1 scarf in 4 mm (5/32) material would be 1 ¼ inches long. It should be pointed out that an 8:1 scarf is the bare minimum and will require careful handling to keep the plank from breaking before it is attached to the boat. Actually, any scarffed plank should be handled with care, turning so the scarf gets little or no stress, but an 8:1 scarf will require having extra support on each side of the scarf as the plank is put on and taken off the molds. This kind of special care will lessen the chance of a snapped plank.

I prefer to use a 12:1 scarf as the minimum length, but will often use a scarf that is about 20:1 in thinner material. This means a 12:1 scarf in ¼ inch or 6 mm thick material should be 3 inches long. A 20:1 scarf in 4 mm (5/32) is 3 1/8 inches long. I’ve found that a 20:1 slope is much less likely to have hard spots, spots that are stiffer than the rest of the plank, after glue up.

And for glue up it would be hard to find a better adhesive than epoxy because the thickened epoxy can fill the small gaps where the slope doesn’t match perfectly. This doesn’t mean you can be careless and sloppy, but it does give a bit of latitude with the match. The closer the better, of course, and the perfectly cut slope is the goal.

The quest for the perfect slope has lead to quite a few methods for scarfing plywood and many involve some kind of shop made jig. There is a jig for a router, a circle saw, and one jig I have heard about, but not seen, that involves turning a 4 X 8 sheet of plywood on edge and cutting the scarf on a table saw. Many of these shop-made jigs are detailed in various issues of WoodenBoat Magazine and can be found there by those interested.

However, shop-made jigs can be too time consuming for the one time builder and most don’t have a high enough ceiling in their shop to upend a sheet of plywood. But all is not lost because there are several jigs you can buy, one method involves a tool you already have, and the last method doesn’t use any tools.

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Figure 8-2

A scarfing jig.

The first jig you can buy requires a slight modification but otherwise is an out of the box jig. Figure 8-2 shows the two parts for this jig - a Bosh 1276 belt sander and its sanding frame and the plywood base.
The plywood base is designed to capture the sander as it moves across the edge of the plywood being scarffed and the aluminum sanding framing raises the sander at an angle to cut the scarf. Notice the plywood base has two plywood guides in front and back of the sander/sanding frame. The front guide is permanent but the back guide is removable to allow the marine plywood to be secured in the scarfing unit. To make reassembly easy, I’ve drawn a line where the removable guide is refastened each time and the other line is for the forward edge of the plywood. This line is 13 ½ inches from the face of the back/removable guide as indicated in the photo.

It will take a 60 X 48 inch base to scarf plywood 48 inches wide. The 60 inch width will allow enough space on each side of the 48 inch wide plywood for the sander frame. The base in Figure 8-2 is 48 X 30 inches and I find this works very well as I seldom scarf a 48 inch wide piece of plywood. Also, I only have about a 1/8 of an inch play between the sanding frame and the two parallel guides. This seems to be about right for the frame to move easily back and forth without jamming.

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Figure 8-3

Modifying the scarfing jig.

Figure 8-3 shows the only modification necessary to the frame. I’ve used two wooden ‘L’s to attach a ¾ X 2 X 15 ½ inch block to the back of the sanding frame. Another option is the small 90 degree corner braces, available at any building supply store, shown in Figure 8-4. Making the block about 15 ½ inches long seems to give enough bearing to control any twist in the sander/frame unit as it moves back and forth across the edge of the plywood.

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Figure 8-4

Another option is the small 90 degree corner braces, available at any building supply store.

A smooth back and forth motion is necessary because if the sander stops even for a moment, it will cut a gouge. I use a 40 grit belt on the sander and it will cut a scarf in 4 mm plywood in very short order.
Figure 8-5 shows a scarf cut in 4 mm plywood. Notice that the ply lines are fairly straight and parallel. This indicates that the slope is relatively even and that’s something you will want to look for with any method you use. When I first started using this method I would take a scarf like this and epoxy it together with its mate. It worked well but I did find it was difficult to keep each board straight.

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Figure 8-5

A scarf cut in 4 mm plywood.

Because of this I included a couple of more steps before glue up. In Figure 8-6 the sander has been removed and a guide screwed down 3 1/8 inches back from and parallel to the front edge of the plywood (a 20:1 scarf). Now, put a block plane against the guide and plane down about 1/32 of an inch, maybe a little less. This creates a slight indention that the edge of the other sheet will rest in and because the edge of both sheets is about 1/32 inch thick, it will be flush with the surface. This line is also a visual reference for aligning and keeping both sheets square during the glue up process (Figure 8-7).

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Figure 8-6

The sander has been removed and a guide screwed down 3 1/8 inches back from and parallel to the front edge of the plywood

I’ve used this system for a number of years and it has always worked very well. I keep the jig in a corner of my shop and bring it out when needed. However, in the beginning, with the addition of another accessory, I used the sander as an upright stationary sander. This helped because I didn’t have an expensive tool sitting idle a good deal of the time.

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Figure 8-7

The slight indention that the edge of the other sheet will rest in.

One off-the-shelf scarfing unit may be the least expensive; particularly if you already have a 7 ½ inch circle saw. The 875 Scarffer, made and sold by West System Epoxies, bolts directly on most circle saws. It requires that holes be drilled in the base so the two pieces of aluminum can be bolted on and removed easily. If you decide to purchase this unit, you will want to check with West System to be sure your circle saw is compatible with the unit.

I’ve used this scarfing attachment and it does the job. Certainly for the builder that already has a circle saw it is a good option. The attachment will allow plywood up to 3/8 of an inch thick to be scarffed with an 8:1 scarf. I find that the saw blade leaves a somewhat rough surface but this is a minor issue. This is a small jig that can store in a drawer when off the circle saw and that’s a real plus in a very small space.

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Figure 8-8

The base sheet has been temporarily fastened to the table and a piece of 4 mm plywood tacked to the base sheet.

If you want to build the shop made version of this jig get WoodenBoat issue 175. Bill Thomas wrote a great article on how to construct both the jig and its integral hold down system.

While the 875 Scarffer and the shop made jig by Bill Thomas use a circle saw, the John Henry scarfer-planer uses a power plane. This unit has a rigid frame that screws to the bottom of a Makita 1900B 3 ¼ inch power planer or the larger unit that screws to a Makita 1911B 4 3/8 inch power planer. The 1900B will scarf material up 3/8 of an inch thick and is more than adequate for any of the boats in this book.

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Figure 8-9

The planer with the guide runner (left side) pushed against the base sheet and positioned at the edge of the plywood to be scarfed.

In addition to the rigid frame, this system also uses a base sheet much like the sander scarfing uses. In Figure 8–8 the base sheet has been temporarily fastened to the table and a piece of 4 mm plywood tacked to the base sheet. Figure 8-9 shows the planer with the guide runner (left side) pushed against the base sheet and positioned at the edge of the plywood to be scarfed. I’ve added a spacer to the guide runner and removed all the spaces from free side (right side) runner.

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Figure 8-10

The base sheet needs to just touch the attachment plate for the depth of the cut to be correct.

The purpose of the spacers is to fine tune the angle of the cut and the depth of the cut. The base sheet needs to just touch the attachment plate for the depth of the cut to be correct (Figure 8-10). If the cut is too deep you’ll get an edge that looks like Figure 8-11.

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Figure 8-11

If the cut is too deep you’ll get an edge that looks like this.

I had to add an additional thin cardboard shim to get the depth of cut just right. The runner guide comes set for a 9 mm base sheet but I was using a 12 mm base sheet. Because of that I added a spacer to the guide runner but I still needed to add two more thin shims. Also I pulled the spacer from the free side (right side) runner because I wanted a scarf with a ratio greater than the 8:1 factory setting.

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Figure 8-12

For the first few tests I was getting a snipe at the end of the run.

For the first few tests I was getting a snipe at the end of the run (Figure 8-12). I corrected this by adding a piece of scrap on that end for a few practice scarfs. In short order I was able to get consistent 18:1 scarfs like Figure 8-13 without using the scrap at the end of the planer run. The snipe seemed to be caused by how the planer was held and how pressure was applied rather than something out of adjustment.

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Figure 8-13

In short order I was able to get consistent 18:1 scarfs.

I did add one last step before glue up, however. In Figure 8-14, a guide for a block plane was added 2 ¾ inches back from the edge of the scarf. This gave a nice clean line and recess for the edge of the opposite scarf. The drill and driver point to the line (Figure 8-15). It also served as a reference line to keep the two pieces square and straight.

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Figure 8-14

Here, a guide for a block plane was added 2 ¾ inches back from the edge of the scarf.

The addition of the last step is purely a personal preference and is not necessary for the planer to cut good scarfs. In fact, any of the three over the counter scarfing units do a good job. Are they worth the money to buy them? Absolutely, but for the one time builder the expense may not be justified.

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Figure 8-15

The drill and driver point to the line.

So if all of the over the counter scarfing jigs seem like too much money and the shop-made jigs too much trouble, then turn to your tool box and pick up your ready to go scarfing unit – the plane. That’s right, either a block plane, which I like best, or a 9 to 10 inch bench plane. I use a Bailey #3 and a Record #4 that both belonged to my father and either a Stanley low angle or Buck block plane.

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Figure 8-16

The 4 mm plywood has been staggered back 1 ¼ inches from the edge of the sheet below and the scarf is then cut.

Before you start to think how impossible it would be to cut a sloping scarf by hand, let me say that a good many boat builders cut scarfs just that way and by choice. I had a conversation with a British boat builder a number of years back and he felt it was so easy and fast there was no reason for him to go to the trouble of building a scarfing jig. He explained that he could cut the scarfs in the time it took to set up the jigs.

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Figure 8-17

It takes a little practice but acceptable scarfs can be achieved after just a few practice runs.

I’m not sure that I really believed him until I cut a few with a block plane. And sure enough, it was fast and easy. However, your plane must be sharp, very sharp or it will not go smoothly or quickly. A few strokes on a stone between scarfs will do wonders for the quality of the scarfs.

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Figure 8-18

4 mm, flush with edge of the table and strike a line 2 inches back from the edge for 12:1 scarf.

Most builders seem to prefer stacking the plywood and cutting several scarfs at a time. One, it’s faster and two, because the stack forms a ramp that helps cut the angle. In Figure 8-16 the 4 mm plywood has been staggered back 1 ¼ inches from the edge of the sheet below and the scarf is then cut. A pencil line at 1 ¼ inches will help keep the slope matching its mates. Notice the bottom sheet is flush with the table underneath. This is important - otherwise the plane can’t stay at the correct angle. It takes a little practice but acceptable scarfs can be achieved after just a few practice runs (Figure 8-17).

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Figure 8-19

Plane the forward edge down and work the slope back to the pencil line.

The stacking technique seems to work best if the ratio is 8:1. I haven’t had a great deal of success cutting a 12:1 scarf by stacking and I cut these one at a time. Place a piece of plywood, in this case 4 mm, flush with edge of the table and strike a line 2 inches back from the edge for 12:1 scarf (Figure 8-18). Then plane the forward edge down and work the slope back to the pencil line (Figure 8-19). At this point, I screw a guide down a 1/8 of an inch behind the pencil line and place the plane against the guide to clean up the slope (Figure 8-20). The finished scarf should look like Figure 8-21.

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Figure 8-20

I screw a guide down 1/8 of an inch behind the pencil line and place the plane against the guide to clean up the slope.

I’m always pleasantly surprised by how easy it is to cut scarfs this way but allow yourself some time to practice on scrap plywood. Don’t expect to get a perfect scarf with the first try; it always takes a while to learn a new skill. Be patient and after a bit you will be able to use the plane to cut scarfs. As you work, just think about the money you will save by learning this skill.

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Figure 8-21

The finished scarf should look like this.

However, the rewards go way beyond the money you saved by cutting the scarfs by hand. There is a satisfaction that comes from mastering a task that has no monetary value and this is one of the great joys of building a boat.

There is one other way to join plywood together to make a long plank out of two shorter pieces and it doesn’t require any monetary outlay either. I first read about this method in an article written by Dynamite Payson for the now out of print, Small Boat Journal. Payson just butted the ends of two pieces of plywood together and put fiberglass cloth or tape across both sides for support. Unlike Payson, who suggests using polyester resin; I recommend that you saturate the fiberglass cloth in epoxy. Also, you’ll want to use a small amount of peanut butter thick epoxy between the edges.

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Figure 8-22

A butt splice with peanut butter epoxy between the two edges and a layer of fiberglass cloth before being saturated with epoxy.

Figure 8-22 shows a butt splice with peanut butter epoxy between the two edges and a layer of fiberglass cloth before being saturated with epoxy. If you choose this method use as light a cloth as you can, like 4 ounce or 6 ounce cloth, and sand the edges of the cloth smooth once the epoxy has cures. Be sure to put freezer paper under the butt joint so the epoxy and the plywood don’t become a permanent part of the table.

You will have to put the cloth on one side and then when the epoxy cures, apply the cloth to the opposite side. Be careful as you turn the splice over because the unsupported side will allow the splice to break.

There are those who swear by this method, but I’m not one of them. Given how easy it is to cut a scarf by hand and the excellent jigs available over the counter it’s hard to justify using this method and I would counsel against it. I put it in as a fall back plan, a disaster plan to be used in case all else fails, nothing more.

Once you settle on the scarfing method you will use; hopefully a method other than the disaster plan, the next step will be to cut the 4 X 8 sheets of plywood into smaller pieces or blanks. Ripping the sheets into smaller pieces or blanks allows the long pieces to be easily handled as they are taken on and off the boat for fitting. It also helps cut down on waste.

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Figure 8-23

Rip the sheets of 4 mm used for the planks and the 6 mm for the bottom for Little Princess.

Each boat has a different set of blanks and Figure 8-23 shows how to rip the sheets of 4 mm used for the planks and the 6 mm for the bottom for Little Princess. The blank layout for the 14 foot and 16 foot canoes are in the appendix with the plans.

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Figure 8-24

The individual pieces have been placed on freezer paper to keep the plywood from becoming permanently bonded to the table or surface underneath.

When the 4 mm blanks and the 6 mm bottom are ripped and scarffed it will be time to glue up the short pieces to make the full length blanks. In Figure 8-24, the individual pieces have been placed on freezer paper to keep the plywood from becoming permanently bonded to the table or surface underneath. Also, clear packing tape has been applied to each piece where the scarf starts and in Figure 8-25 clear tape has been applied to the opposite sides. This is an important step because the tape helps protect the plywood from the epoxy squeeze out. The epoxy is much harder than the plywood when it cures and having it peel off with the tape is much better that trying to sand it off.

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Figure 8-25

Clear packing tape has been applied to each piece where the scarf starts.

The next step is to mix up a small batch of epoxy, about two ounces of resin should do it, stir the recommended 90 seconds, and brush on the epoxy with a disposable brush (Figure 8-26). Next add a small amount of silica and wood flour until a heavy cream/running catsup consistency is reached and brush a light coat on each side (Figure 8-27).

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Figure 8-26

Brush on the epoxy with a disposable brush.

I like to use a heavy cream epoxy rather than a thicker mixture when scarfing because I found the thicker epoxy created a hard spot in the plank. The heavy cream mixture comes closer to matching stiffness of the glue between the plies.

Also, it wouldn’t hurt to take some of the practice scarfs you cut and glue them up before you start on the blanks. This will give you a good idea of how much epoxy to put on and be a good test of your clamping technique.

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Figure 8-27

Use the line created by the plane as a reference to square both pieces.

Once both sides have a light coat of heavy cream epoxy just flip one piece over on top of the other. I like to move the top piece around a bit to spread the epoxy. Then use the line created by the plane (see Figure 8-7) as a reference to square both pieces.

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Figure 8-28

Be sure to wipe up the excess squeeze out with paper towel.

Be sure to wipe up the excess squeeze out with paper towel (Figure 8-28), check to be sure the pieces are square to each other and then screw down a 1 X 8 or scrap plywood as a clamp (Figure 8-29). However, don’t forget to put a non-stick barrier between the plywood clamp and the scarf.

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Figure 8-29

Check to be sure the pieces are square to each other and then screw down a 1 X 8 or scrap plywood as a clamp.

I think you’ll find that 6 X 1 5/8 inch course thread dry wall screws can apply an amazing amount of clamping pressure. On wide pieces, like the bottom, I’ll put several dry wall screws spread across the center to distribute the clamping pressure. It’s better to have a few holes in the center of the plank and get good even clamping pressure across the entire plank because any holes created by these dry walls can be filled later with thickened epoxy.

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Figure 8-30

This photo shows just how much squeeze out can be pulled up with the tape.

You’ll want to allow a minimum of twenty four hours, in colder weather allow more time, before you remove the clamp and work on the plank. The first job will be to remove the clear tape and as much squeeze out as possible. Figure 8-30 shows just how much squeeze out can be pulled up with the tape.

Once you’ve removed the majority of squeeze out take a random orbital sander with 100 grit paper and finish cleaning up the scarf (Figure 8-31). Exercise a bit of care with this task because the sander can cut through the top ply fairly quickly. Should this happen don’t worry because it won’t be a problem unless you plan to varnish the entire boat and I would recommend you not do that anyway.

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Figure 8-31

Take a random orbital sander with 100 grit paper and finish cleaning up the scarf.

Just be sure to support the scarf on each side by spreading your hands out a good twelve inches on either side of the scarf when picking up the blank. Turning the blank on edge will help lessen the stress on the scarf. Use another person, if possible, to help handle what now resembles big limber noodles.

Also, this will be a good time to coat the entire blank with unthickened epoxy and sand it smooth. It may seem unnecessary at this point but this will save time later. It is much easier to coat and sand the blanks while they are flat on a table or floor than on the boat.

Actually, I’ve found that the hour or so spent coating and sanding a blank smooth now will save about two hours of labor when putting the final finish on. Then you’ll really be glad you did because you’ll be in a hurry to launch the boat.

So take the time and coat each blank with a layer of epoxy, let it cure, sand it smooth, apply another coat and sand smooth. As each blank is finished and sanded, store it in a safe out of the way place until you’re ready for them.

Addendum to Chapter 8

First let me say I was impressed enough with the John Henry Scarffing attachment that I used in Chapter 8 to buy one. It is light weight and fast to use but what makes it a good choice in my view is the ability to scarf an 8 foot long piece of plywood. This wouldn’t be important on a small boat or canoe but on a large boat where the sides might come out of a 6 foot wide by 20 foot long piece of plywood this makes fast work of those scarfs.

So for those looking for a good over the counter scarffing jig the John Henry Planer/Scarffer is an excellent choice.

Contact:

John Henry, Inc.
PO Box 7473
Spanish Fort, AL 36577
(251) 626-2288

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