Tuesday, February 23, 2016

To Kerf Or Not To Kerf

That was the question.

My approach to getting a nice curve on the bow curve up has been working out this way:

I PL Premiumed and screwed the first lay of ply athwartships (love that word even though blogger spell check doesn't... athwartships).  the half inch ply complied well enough.  However the curve lacked for some fairness and had some flat spots.  Put it all down to my exuberant work pace and wanting to actually sail the boat some time before I croak.

I wanted to have the next layers as best as I could running fore and aft.  There was no way to bend 1/2 inch sheet of Douglas Fir 1/2 inch ply in this manner without kerfing it.  But kerfing the ply across the longitudinal fibers would defeat that strength!

My decision was to rip the plywood into 8 foot long planks about 4.5 inches wide.  I experimented with some different widths - and used them all - but the 4.5 inch strips seemed easiest to conform.

About the flat spots.  There were two that ran athwartships... full width across the boat.  One of them was negligible and I was able to force the strips into full contact, but the other one caused the strips to bridge a void about 8 inches long.   I decided that this was ok, because the next layer will be in full contact with these fairer strips in that location.

The material I used was all left over from planking on the first layer and I ripped them up with my cordless handheld circular saw.  That is why the joinery looks a little rough.  I also planked the first 20 or so inches athwartships so that my 8 foot strips would be stepped aft, so that when I plank on the third layer of strips they will end that distance forward of the strips in under them.

I will rip up the last layer of ply for this area at home on my tablesaw so it should look a lot neater.  I will make the strips in two widths though, 4 inches and 4.5 inches so I can always select a piece that will cover the gap or joint below it.

The Raptor nails are working out well, however I've had to drive a screw here and there, which I will remove once the glue cures.

I have also decided that I will rip all of my 4 by sheets into 2 by 8 sheets for all the flat sections.  They will be much easier to handle, and knelt on to conform to the flatness.

As a completely unrelated aside, our Canadian made Cubic Mini wood stove arrived a few days ago.  It is very well made!  I am quite impressed.  Can't wait to use it.  I have an earlier post with a link to their site... worth checking out if you need a wood stove for your boat.  It would be nice to see some finished, value added products exit our borders instead of raw logs, ore, bitumen, and water.  But I'm not bitter...


  1. Kerfing, good topic. Been thinking about that off and on.

    You know, if I had to kerf a kitchen cabinet (think 70's mod furniture), or a cabin roof, I could probably live with it. Boat hulls? As in, the part of the boat that takes the most abuse, weight, and wave workouts? And double that for bows where the kerfing happens? The hull that keeps my house from becoming an artificial reef? And do it by purposely severing plys in a thing that's good because it's called "ply"wood? Let's just say that I have serious reservations.

    I know everyone's doing it, including people with eons more experience than I have. But ... just don't like it or trust it. That said, this might be a case of an imaginary perfect being the enemy of good enough. Summary: I understand the decision to go "patchwork".

    Now, about this ... "One of them was negligible and I was able to force the strips into full contact, but the other one caused the strips to bridge a void about 8 inches long." ...


  2. What happened? Did our momentary tangential joviality of late break some boatbuilding blog levity taboo and scare off the huddled masses? ;-)

  3. I doubt that :-) As for me I've been givin' 'er on the hull. There'll be some new posts soon that I hope will tease out a few comments!

  4. Looking great, Alan, and going together fast!

    In general, I would't worry about kerfing across the long grain. Stresses are carried mostly in the skins of the finished composite, so kerfs on the inner faces don't present a strength problem so long as they don't line up with other kerfs or seams.

    One way to think of it is as a planked bottom x however many layers, with a few veneers of plywood, rather than caulk. Essentially, your cross planks are just deeply kerfed (all the way through).

    Still, your approach does no harm, and sure breaks things up into smaller, more manageable bites.

    Dave Z

    1. Thanks Dave. I would recommend this method of doing the 'big bend'. It breaks the job down into manageable pieces. You can work a bit, then stop and pick it up again later. I thought about kerfing a whole sheet, gluing it and then trying to make it do what I want. For me it just seemed easier to do it this way.