Mystnyx Houseboat Project: Building a shanty-boat in your backyard.: April 2018

Well, some more good weather means it is time to think about putting together in the inside frames. These will serve as the backbone of the boat and provide rigid support. The question is should I attempt to scarf the pieces of plywood together? I would need 3 scarfs to get the plywood long enough.

There are many ways to machine scarf bevels on plywood panels. The best method depends on how many scarf joints your project requires. For example, if you need to scarf only two sheets of 3mm plywood, using a block plane and sanding block is a good low cost option.

In my case I have 3 scarfs, and two sides to each scarf, so that is 6 scarfs a side x 2 = 12 total scarfs in 9mm plywood. In addition, I may want to scarf the plywood outer sides and possibly the bottom.

There is also a bit of debate on how long a scarf should be in boat building, I am going with an 8:1 scarf, so that is a scarf of about 3". There is numerous ways to achieve a scarf, you can build or buy a jig for a circular saw, planer, belt sander or router.

Circular Saw Scarffing Jig

The Gougeon Brothers are given credit for developing a circular saw scarffing jig for plywood. It first appeared in Robert M. Steward’s Boatbuilding Manual (1970).They Gougeon Brothers now have a great book on boat construction available as a free download from West Systems and is a worthwhile read: The Gougeon Brothers on Boat Construction (1979) 5th Edition. While it promotes West System epoxies, it could be read generically.

The Gougeon Brothers Jig is not for the faint of heart, but it works very well. It can be a bit tricky to make and use:

Gougeon Borthers Original Plywood Scarf Jig from Steward's Boatbuilding Manual

Based on the Gougeon Brothers design, West Systems makes a commercial device called the "Scarffer". It is an attachment, and basically allows the circular saw to work on an angle to cut a scarf. It is mainly designed for 1/4" plywood but can work on thicker wood with a hand planer.

The Scarffer is an attachment by West Systems that
allows you to use a circular saw for making plywood scarfs.

The dis-advantage to the "Scarffer" is that it can only do ¼" thick Scarfs, due to the limitation of the saw blade size. When cutting plywood thicker than ¼", the blade won't reach all the way through the wood. The scarf must be completed with a hand plane

That is not necessarily a huge issue, as in this case it is only 1/8" material left, and could easily be planed.

When Scarfing boards bigger than 1/4"
you need to hand plane the rest of the material

You can get a West Systems 875 Scarffer from Jamestown Distributors at at cost of $93.16 USD + Shipping or in Canada from Steveston Marine and Hardware for $122.39 CDN + tax and shipping. It can also be picked up locally from any of their locations.

I am not as comfortable with using the circular saw, particularly on end. So I tend to shy away from this type of jig, but it gets alot of positive reviews and results. There are other handmade circular saw jigs, but for the sake of $100 for the potential danger and safety issues, I would highly recommend using a commercial product.

Planer Scarffing Jig

Another for popular method for scarfing plywood is using a planer scarfer, this kind of attachment or jog works great for dimensional lumber. There is another commercial variety available from John Henry, known as the John Henry Planer-Scarffer. It allows a standard power planers, specifically Makita 1900B, Makita 1912B, and the Porter-Cable Model 9125 planers to be used to scarf plywood. John Henry says, that although it is designed for these planer models it is not necessarily limited to them and may fit other models.

The John Henry Planer-Scarfer attachment, Patent No. 5066177

This looks like a great device and gets some good reviews, but cost and available is a factor. Sometimes used ones can be found on ebay or a new one from John Henry. The main advantage to the Planer-Scarffer is that it is capable of doing very wide pieces of plywood.

In the end, I decided since this was an experiment and I may not scarf the bottom or the sides depending on results, I would build a "router" style jig from parts on hand to "test" how feasible a scarf was... I mainly wanted to scarf the bottom and the middle frames.

Router Scarfing Jig

The router-type scarfing jig basically places the router on a angled plane that can be moved up and down. It is more difficult to do a full 4x8 sheet as limited by the jig size. There are many different type of way to accomplish this, but the basic premise is the sa,e.

A router-type scarf jig as depicted in Popular Woodworking

We decide to try a build a jig with materials that I already had on hand. I did have a spare router - and found a good article in the Woodenboat forums.

The basic premise is that the router is mounted on two pipes, which sits on a sloped jig - 8:1 or 10:1, and the plywood is wedged underneath. It allows adjustment for thickness, but it limited in width. It requires very basic components most of which I had. I did buy the electrical conduit and PVR pipe, so I spent about $15-$20 in materials. This seemed safe and easy, and was a good budget for experimenting.

The designer gives a glowing review: "I built a router scarf jig. It takes 120 seconds for a 6cm 10:1 scarf in 6mm birch plywood. Perfectly flat, smooth, down to a feather edge. Completely idiot proof, and leaves a perfect 10:1 scarf that will match the next one. You can't do anything wrong. You just have push the router front to back until you've rubbed out the wood. The jig makes a skilled procedure, an unskilled one that gives a perfect result, and saves time if you've got alot to do. It also saves my back from alot of manual planing"

You can get the exact instructions on how to make one in the Woodenboat Forum article: http://forum.woodenboat.com/showthread.php?133907-Scarfing-Jig-Ideas

Here are a few pictures I borrowed from forum post to give the gist of how it works:

Mount the router on a set of rails, and
get a long skinny router bit

Make a Sled Stand

The sled that the router slides on and insert your plywood

Adjust the bit to the correct height and
then just slide the router along the sled

We'll, finished up the scarf jig, and put it to work today. It clamps on my tablesaw, bottomless frame so it could do wider pieces. Learned a little on first run what not to do but literally does a nice scarf in seconds.. can't wait to do more and see the glueup. I'll feather the edge a little more with fine sandpaper.