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Scarfing

Included in the BUILD MANUAL for both the W17 and the W22 are many small tips to help construction. One of them shows a router tray used to make a neat job of scarfing, and I thought, even builders of other designs might appreciate to have this information; so here it is. Scarfing is something that intimidates many first-time builders but if one accepts to take a few hours to make a few simple parts and get set up properly before starting, one can get good results.
I'm not going to say there are no issues because there are. But if we identify them, they can be dealt with. The main ones I've encountered are caused by either the working bed not being solid or flat, or the relatively thin plywood lifting off the work surface.
But here is a method that will eliminate most of the problems and permit you to handle the job while totally alone.

First you need a router. I have a reconditioned Craftsman model from Sears (USA) that works great. It only takes ¼" (6 mm) shanks but turns 25,000 rpm using 6.5 amps. Using a router with a ½" chuck would be even steadier but the smaller one works fine if the bit is sharp and you don't really need more power in this case. As you can get flat bottom mortising bits up to a maximum of 5 times the shaft diameter, mine uses one of 1¼" diameter (I use a Freud Model 16‑106.).

In the case of the W17, the boat is built on a 16' × 2' platform about 20" (500 mm) off the ground and that's also a good base for scarfing all the hull and deck panels, so I'll assume you have a place somewhat similar. Whatever you use, draw a straight line down the center of it as a datum line to work from.
At one end, fit two wedges about 24" (600 mm) apart that have the same slope as required for your scarfs—typically 1:8. Set them up like this, screwing mostly from underneath. It's important that both are the same height (see later).

Then construct a wooden 'tray' for the router as per the sketch below.


I used a good 8mm 5‑ply for the base with 1" × 3" sides. Make the inside width ½-1" larger than the diameter of the router base. Then drill a hole for the bit. If your tray is very long, you might conceivably be able to drill this in the middle, but I preferred to drill mine off center; 16" (400 mm) from one side, so that I can turn the tray around to go closer to that edge, without the tray coming off the wedges—something you DEFINITELY don't want to do—or your ply would be ruined!

[You may prefer to make the tray a little longer and then fit two strips under the ends to act as stoppers to prevent moving the tray off the wedges. I just 'eyeball it' but yes, it's a risk I'm OK with].
Then mount the router securely to the inside of the tray. I use two wood blocks screwed into the base, making sure the screws do not protrude at all.

My boatbuilding webmaster tells me that he would remove the plastic router base and then screw the router directly to the tray with csk machine screws. If you don't need the router for another use, this might be a good way to go.

I oil the underside of the tray to permit it to slide well on the wedges. Some furniture polish might work too.


You will also need what I call a 'plywood pressure plate' but we'll get to that later.

Now to the potentially tricky part. How to position and align the plywood? Follow this and you'll be fine.
The first step is to set the depth of the cutting bit. Mine works at about 15 mm below the tray surface, but it will depend on the height and position of the wedges, so you'll need to test it. First, make SURE that your table top has no nails or screws in the planned scarfing area that your router tip can hit. That would be disastrous to it and could even cause an injury.

Now set the tray on the wedges and without running the router, move it up and down and side to side, to get a feel for what you will be doing. Slide the tray down the wedge until the cutter hits the table and mark where the tray comes to on the top of each side wedge. Now push the tray up a little, start the router and slowly bring it down to that mark. The bit should take a small amount of wood out of the table top and leave a small curved shape with one straight edge. Do the same on both sides. See pic.

Then draw a line along the straight part, between the two marks. This will mark the lower edge of where you set the first sheet of ply to be scarfed. It's important that this line be at 90 degrees to the centerline of the building platform and ply being milled. If it's not, it's because one wedge is slightly higher than the other and needs planing down.
Now remove the router tray for the time being while you set up, say four, sheets to be scarfed. Make sure you have the datum line marked on each sheet, as this will be your guide for aligning them when you bond the joint. Place the end to be scarfed with its datum line over the one previously drawn down the center of the Building Platform. If your two wedges are absolutely perfect and identical, then the other end of the plywood datum will also lie on the centerline of your platform. If it does not, then lower the wedge slightly on the side opposite to that towards which the plywood twists. Once a new test shows that all is square with the centerline on the Building Platform, tape or pin down the other end of the sheet. That should position the first sheet.

The sheet it will eventually join to, needs to be turned over as the scarf cut will be on the underside of this one. I always like to have my top feather edge arranged to face aft, so that nothing is encouraged to get under it when moving through the water, so think about this when laying the sheets in place.

I generally do four 6mm sheets at the same time—but I would discourage you from stacking up more. You need to set each sheet back the width of the scarf, so if you prepare a small strip of wood the exact width of the planned scarf, you can lay that in place as a ready guide to this set back and remove the need to measure each one. See pic.

Make sure that all the datum lines line up with the first sheet, so that the scarfs will be cut square on each sheet. Either tape or use a small pin through the ply at the opposite end to the scarf to temporarily hold the ply in place.

Once all the four sheets are placed, then you need something to hold them down. Clamping a 2" × 3" across the platform works for that, but in most cases, will not hold the ply down close enough to where the cut will be made, so we need something else.

Here is where my 'plywood pressure plate' comes in. It's a type of plywood spring that is bonded to the sloped top of the 2" × 3" clamping bar, so that it presses down on the plywood very close to the line of cut. Using plywood for this is safe as should you accidentally move the router tray too far up, it will simply cut a little of 'the spring' away and not damage the bit. Here's the design and dimensions that I recommend.

Tightening the side clamps will apply significant pressure on the plywood and make sure it lies flat on the platform surface. I then add another 2" × 3" clamp further up the sheets and also clamp or screw on a couple of blocks over the top edge of the wedges to limit the upper movement. I'd also add a strip at the bottom edge to avoid cutting too much of the platform. Finally, here's the setup—this one was with a single pressure plate that needed additional hold-downs at the side (see later).


Then place the router on the wedges with the cutting bit off to one side of the plywood. Start the motor and working along the top, slowly bring the cutter sideways into the plywood and keep moving the tray slowly across the surface, making SURE that both ends stay on the wedges!
Then come down an inch (25 mm) and come back across the plywood. Do not rush. Allow the cutter to do its work slowly and cleanly. After going back and forth a couple of times, you'll need to stop and brush away all the wood dust as there will be a lot of it.


[You will note that I'm using a smaller pressure plate here—good for narrower work. But as it did not work as well as the wider one, I had to supplement this smaller hold-down with a screw and small pad at one side. The triple plate is a better solution for wide parts like this.]

To mill the other side of the plywood, just turn the tray around so that the router is now on that side.


This method gives a nice clean and straight cut (see pic).
At the very most, you might need to pass a sharp plane across the bevel just to be certain it's straight.


Just as a warning, if you fail to hold the plywood down flat on the working surface, then it will lift up into the router cutter and more will be shaved off.
This is what it will look like and you'll have little choice but to trim the end back ½" and cut it again. Hopefully, you have enough material on the sheet!

Another article will explain how to position and clamp up the mating pieces to complete the joint.

Read more Construction Tips & Techniques.


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