Thursday, April 2, 2015

Skin on Frame Kayak: Bending in the Ribs



Most builders of skin on frame kayaks don't build their boats on a strongback with molds.  I chose to use molds in building this boat because I was trying to exactly replicate Brian Schulz's design.  I used the molds to hold all the longitudinal members (gunnels, chines, and keelson) in place.  Then I used this structure as a jig to bend the ribs to the correct shape.  The process was effective, but inefficient and inelegant.  I'll be building future kayaks without molds.

The ribs are 1" wide and 1/4" thick hard maple.  I soaked the maple in water until it reached the fiber saturation point.  You can tell that the wood is at fiber saturation by measuring it with dial calipers.  When the wood stops swelling, it has absorbed all of the water that it can into it's cellular structure.  In the steambox, water in the wood acts as a conductor of heat from the steam.  When the wood is full of water (fiber saturation) the heat transfer from the steam to the wood is better.


Throwback.  Coopering class at Country Workshops 2010.

I've been fortunate to have several opportunities to learn from woodworking genius Carl Swensson.  I cannot say enough good things about Carl.  He doesn't teach much, usually one class every summer at Country Workshops, but seize the opportunity to learn from him.  During my summer internship at Country Workshops in 2010 I took Carl's coopering class.  He developed a brilliant technique to limber the wood that becomes the coopered bucket's hoops.  The heart of the technique is a simple compression strap limbering jig.




The jig is a compression strap with two end blocks that can be clamped in a workbench vise.  The wood is placed between the end blocks, pulled into a tight spiral, then opened, flipped, and bent into a spiral with the opposite face on the inside of the bend.  The strap takes on the tension load of the bend, forcing the wood into compression.  The result is a super-compressed blank that can be taken out of the strap and pretty much tied into knots long after the wood cools down.




The technique works great for the hoops of Carl's coopered buckets and I thought it would be perfect for bending the deep v-shaped ribs in the bow of the kayak.  The resulting super-compressed wood is mostly unbreakable and the bends are very stable and do not spring back.


Lots of clamps.

I bent all twenty ribs into place without breaking one of them.  I hope this technique is useful to other skin on frame kayak builders.


12 comments:

  1. This is exactly what I was trying to puzzle out when running through my head how to build the F1 on a strong back. Do you know if this technique works on other bending species like white oak or ash?

    ReplyDelete
  2. Jarm,

    I am working on a blog post about why I use hard maple for steam bending. The diffuse porous structure is more forgiving to the slight grain runout that is inevitable when working from sawn stock. But green white oak will do anything you want it to do. Are you working from logs, air dried boards, or kiln dried boards? Do you have access to beech?

    ReplyDelete
    Replies
    1. I have a sawyer locally who can provide green white oak at a reasonable price. That's interesting to note about the diffuse porous structure being more forgiving. More food for thought. Thanks!

      Delete
  3. Have you ever seen the need or actually used this technique with your bent chair parts?

    ReplyDelete
    Replies
    1. Ray,
      Most chair parts aren't this extreme. So far this is the only other application I have used this technique for, but it is so much fun and it works so well that I am always looking for a new way to apply it where appropriate.

      Delete
  4. Do you use the same length of rib stock for all ribs to use the same compression strap form? How long do they dry before you fit them to their mortices?

    Steve

    ReplyDelete
    Replies
    1. Exactly right. They all have to be the same length to work with my form. I think that I just waited overnight before fitting the in the mortises.

      Delete
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