BoatBanter.com - Cutting scarfs

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Started working on my Tolman Skiff. I began by cutting the bottom
pieces from 1/2 marine ply but these have to be scarfed together to
make longer pieces. The specified angle is 8.1 degrees and the
directions describe a scarfing jig to be made with a circular saw but
they require a larger saw than mine. However, I did buy a "Sawzall" to
deal with some rot on my house. I tried various ways to do this with
little success. Finally, I simply cut inclined cuts about 2" apart all
along the scarf area angled at the 8 degrees. Then I used the Sawzall
to cut the scarf cutting each of these out. This gave a reasonable
first and second attempt. Then, I used the sander to make it nicer and
make sure the layers of ply formed parallel bands. I think my first
two scarfs are ok.

Just a caution. Having made quite a few successful scarfs of 4' wide sheets
of plywood, I managed to screw up the last set badly. My sin was to mix too
much wood flour in the epoxy. The result wasn't fluid enough and I ended up
with voids at the edges of the scarf joint. For scarfing, the mix should be
a fluid, not a paste since there is limited clamping force available, even
with my scheme of driving drywall screws thru the joint into a backup 2x6.
You do want lots of epoxy squeezing out all the edges (and a drop cloth to
protect the stuff underneath your work area.) (I'm working with 3/8"
marine ply sliced into strakes for a lapstrake hull so the voids can't hide
easily.)

I'm fixing the bad areas by playing dentist; cutting the voided area back to
solid wood and epoxy with my 4" grinder and then filling the cavity with
epoxy thickened with milled glass fibers. A layer of fiberglass on the
outside finishes the job with what I think is "good enough." I can lift a
20' long strake at one end and wave it around without it breaking. When the
strake bends, the curve is smooth and uniform thru the scarfed and unscarfed
areas.

8.1 degrees is a 7:1 scarf, while 8:1 is the roughest I've seen
recommended, 7.125 degrees. On the other hand, some people use butt joints
with fiberglass on each side, so ????

Roger

http://home.earthlink.net/~derbyrm

wrote in message
oups.com...
Started working on my Tolman Skiff. I began by cutting the bottom
pieces from 1/2 marine ply but these have to be scarfed together to
make longer pieces. The specified angle is 8.1 degrees and the
directions describe a scarfing jig to be made with a circular saw but
they require a larger saw than mine. However, I did buy a "Sawzall" to
deal with some rot on my house. I tried various ways to do this with
little success. Finally, I simply cut inclined cuts about 2" apart all
along the scarf area angled at the 8 degrees. Then I used the Sawzall
to cut the scarf cutting each of these out. This gave a reasonable
first and second attempt. Then, I used the sander to make it nicer and
make sure the layers of ply formed parallel bands. I think my first
two scarfs are ok.

Understand that a plywood scarf should be 12:1.

OK, I went out and bought a power plane. It is WAAAAAAAY easier with
it and it makes better scarfs.

Says Who?
Plywood scarfs (9mm panels) on my boat are 8:1 and I have had no
problem whatsoever with that. Every scarfed offcut I destruction tested
broke elsewhere but the scarf joint.
12:1 scarf is for the strakes to make a hollow mast and other high
load spars.

Klaus

A belt sander will do this job just as well and is much more widely useful
than a power plane.
In either case, finish the scarph surfaces with a sharp handplane. Check
with a straightedge.

"Jonathan W." wrote in message
...
Brian Whatcott wrote:
On 15 Oct 2005 16:00:36 -0700, wrote:

OK, I went out and bought a power plane. It is WAAAAAAAY easier with
it and it makes better scarfs.

Now I am getting moody. I looked round town at the last mention of a
power planer - and drew a blank (population 20,000)

There is not meant to be any power tool that I don't own.
That's what my wife says, at least

Brian Whatcott Altus OK

If you can't find one locally, how about:

http://cgi.ebay.com/MAKITA-ELECTRIC-...54745518QQcate
goryZ42283QQrdZ1QQcmdZViewItem

might have to cut and past that url :)

Jonathan

--
I am building my daughter an Argie 10 sailing dinghy, check it out:
http://home.comcast.net/~jonsailr

Jim Conlin wrote:
A belt sander will do this job just as well and is much more widely useful
than a power plane.
In either case, finish the scarph surfaces with a sharp handplane. Check
with a straightedge.

The problem with using an edged tool on any surface that's been sanded
is that the inevitable grit that's left behind does a number on the
cutting edge. That's one major advantage of using a power planer rather
than a belt sander. The others are reduced sawdust and greater ease in
maintaining a flat surface. Belt sanders are great for rounding things
over. ;-)