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scarfing, again!

Posted: Thu Sep 11, 2008 7:36 am
by jlparsons
Hi, I'm planning to build a SOT in the summer next year and have decided on the sabalo. I have looked at the posts on here regarding scarfing and have two questions which I can't find answered.

I'm planning to scarf the plywood sheets first, then draw out the pieces "whole". This seems to be commonly used with other stitch and glue designs and whilst it does look more of a PITA I'm convinced it'll be worth it. Do the sabalo plans detail offsets for "all in one" pieces? Or is it half-and-half only?

Secondly, has anyone messed about with "stepped" lap scarfs? I've worked with thin ply a few times and have on occasion needed to thickness ply along an edge with a router by 1-2mm. Having done this, I'm thinking it would be easy enough to form a jig to make accurate steps in the scarfed edges rather than make a perfect gradient. In 3 ply I'd use four "steps" so the inner and outer faces would overlap plus the inner core would be stepped. I'm thinking it would be very nearly as strong as a perfect scarf joint but much easier to do and far quicker. Has anyone done this?

Many thanks for your time folks, this looks like a nice forum community you have here.

James, Wales (UK)

Posted: Thu Sep 11, 2008 8:05 am
by hairymick
G'day James and welcome aboard mate!!

Re the scarfing stuff. I have played with it a bit and found it more trouble to do well than it is worth. Matts Panel splice method is very strong. Like you, I had my doubts at first, so I did some panel splices in scrap ply and tested them to destruction. Probably not quite as strong as a perfect scarf but close to it and when the panels are all glued together into your hull, and fibreglassed, they support each other and are very strong indeed. 8)

I am hard on my boats, way harder than most plastic boat owners and haven't had one fail on me yet. :D

A sub-standard scarf is sort of like a not quite perfectly mitered picture frame. Still probably good enough but looks bad.

As far as I know, all of Matts drawings are done for his panel splice method but it shouldn't be a biggy to work out how do them for the type of joint you are thinking of. Matt is the best one to answer that though.

I would be very interested in you processes on how to achieve this rebated scarf?. :D

Posted: Thu Sep 11, 2008 9:02 am
by jem
There are some scarfing jigs out there that can help make a nice scarf. But they don't look easy to work with.

Posted: Thu Sep 11, 2008 10:53 am
by bronco302
I've lap joined thin plywood for other things before. The problem I have found is with ply something is always cross grain that being the weakest point. Also the layers have a tendency to lever each other apart when stressed. At the very best I don't think you would end up with a stronger joint than the one Matt uses, and it would be more work. jmho.

Joe W

Posted: Sat Sep 13, 2008 12:13 am
by craiggamesh
I agree with Mick and think the scarfing would me more work than it is worth. The butt joints are very strong. An example of extreme butt (edge) joints would be the dozens to hundreds used in strip built boats. All those strips aren't scarfed on edge, they are butted up to each other and glued. Then, they are covered in fiberglass. I have built two like this and I know of many other people who have built them. I have never heard of one failing, nor have I ever seen discussion of one failing in any forums. JMHO.


another rank amature weighs in

Posted: Sat Sep 13, 2008 12:23 am
I think scarfing is a carry-over from the old boatwright days of building boats.
There's nothing wrong with scarfing, but I've never seen a scarf joint that acheives the "perfect" grain match that the proponents have been trying to gain.

For the time, effort, and end result, I have yet to see the true benefit over the butt-joint with fiberglass reinforcing.


Posted: Sat Sep 13, 2008 6:04 am
by jbo_c
I scarphed for my first build just to prove I could do it. Glad it's over. I'll never waste my time doing that again.


Posted: Sat Sep 13, 2008 10:30 am
by Kayak Jack
I've used 1/8" plywood reinforcing blocks on all end joints. Strong - like bull. Easy - like 1-2-3. Accurate - like a tack driver.

If it ain't broke - don't fix it.