OkieBoat: Scarphing Plywood

Sunday, August 29, 2010

Scarphing Plywood

I have to admit I was hesitant about taking on this project of building a boat.  The feeling was similar to entering cold water prior to a swim (and I don't like being cold).  My strategy was to just jump in whole hog and absorb the full shock immediately; so that, I could spend the rest of the time enjoying myself.  It's too early to tell if the strategy is valid, but here was the first plunge.

For some reason I felt most uncertain about these things called scarph joints, particularly for plywood, and most particularly for thick sheets of plywood.  That being said, I felt if I attempted scarphing the 3/4" hull bottom first, then the rest would be downhill.

The hull of the boat is longer than the 8' plywood panels that is composed of.  So in order to get longer panels, or planks for that matter, two pieces of wood are bonded together at a scarph joint.  Or so I had read, since I certainly hadn't tried one before starting this build.

Did I mention this log should  be looked at as a cautionary tell?  Learn by not following my example.  I'm pretty sure I broke some kind of record - the most time & effort spent on a scarph joint with fairly ugly results.

So one option would have been to order oversized panels.  Another option would have been to order standard panels that were pre-scarphed.  Other options would have been to purchase a circular saw scarphing jig, or maybe a power planer, or a belt sander.  But no, I decided to do this the hard way with primarily just a block plane.

The plane I chose was Standley's bottom of the line low-angle hand plane; even for bottom of the line, it still cost $50.  Since this is the only plane I've ever been this intimate with, I don't know how it compares to other planes.  I'm satisfied with the plane, and very glad I didn't step-up to the Standley sweetheart addition, since I accidently dropped this one on the concrete floor of my garage within 30 minutes of use.  Well that forced me to learn how to tune it.  And after hours and hours and hours and more hours of use, me and the box plane became friends.

Planing the scarph joints for the 3/4" panels took so long, I decided to go another route for the 1/2" side panels.  That time saver, which still took an amazingly long time, was to crosscut a series kerfs in the plywood with a circular saw.  A separate cut was done each millimeter.  The depth of each cut was changed ever so slightly each time.  Also I had to remeasure and align the guide for the saw every time.  Fortunately my neighbor (Thanks Gary!) let me borrow this fantastic metal guide that clamped to the outside edges of the panels.  This method would not have been practical without that guide.  I don't even think I saved much time this way versus just using the hand plane, but the depths across the panels were more uniform, and the straight glue stripes seem to confirm this.  The joints on the 1/2" panels have very straight glue stripes as opposed to the 3/4" panels.

One thing I did, which in hindsight I would do again, was to use resorcinol glue instead of epoxy.  Again, time will tell if this was a good choice.  Thanks to Larry Pardey for this decision.  There is an appendix in his book Details of Classic Boat Construction that discusses the pros and cons of various adhesives.  Also, I'm glad Matt Layden noted it as an option on his Paradox plans.  It sparked me to investigate further.

I really had to go out of my way to buy resorcinol.  I believe it is actually the glue used to bond plywood laminates together in marine and exterior grade panels.  So it is widely used by lumber companies, but it was very hard for me to acquire on a small scale as an amateur builder.  I tried a few local hardware stores and lumber yards; they couldn't even order it.  Eventually I heard about a few (very few) websites where I could order it in the U.S.  A couple of these sites were geared toward amateur wooden airplane builders, and I'm not talking about model airplanes.  Apparently there are not only amateur plane builders, but there are actually people who will get in and fly in a plane built out of wood by an amateur.  Not me brother - I can swim if this amateur screws up, but I can't fly - no thank you.  The site I ordered from was a chemical company out of Ohio called CP Adhesives (product CP-0900).  It appears to have been made and shipped from New Jersey.

To me the resorcinol appears to be easy to use.  Store it below 70 degrees Fahrenheit to maintain a good shelf life.  Use it at above 70 degrees for curing.  The hardener looks similar to brown flour.  Prior to adding the hardener, the glue looks like a dark red wine.  After mixing in the hardener it gradually goes from a wine/grape juice consistency to a sticky jam substance; it doesn't have a hot flash like epoxy.  I applied it with a paint brush.  It doesn't require very much glue.  I used gloves, safety goggles, and a respirator as recommended, but just for the few minutes to mix and apply.  There was no indication of any vapors.  Drips were easily cleaned up.  The brush and mixing bowl rinsed out with just water. [The water used to rinse the bowl and brush was then used to irrigate a small section of my yard, to prevent the direct runoff from possibly affecting the next town's water supply.]  The irony of this stuff is that it dilutes easily with water until it cures.  Once it cures, it is waterproof.  After curing it became a dark rusty brown color.  One thing I would like to note, is that the instructions recommend an enormous amount of clapping pressure, which I did not come close to achieving, but the bonds appear strong.  Again, only time will tell.

The weather was in my favor.  We had about a month of 100+ degree days.  Plenty warm for the glue to cure.  The batteries in my circular saw didn't respond well to the heat.  Fall is just around the corner though, then winter (my least favorite weather); maybe we'll get lucky and have some Indian summers.  Bring on the global warming baby.

via okieboat.blogspot.com

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