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Spray HPK – Hull Construction

Posted By Dan Caouette

With the forms cut out it was time to start the real construction of Zack’s Spray HPK.  What follows is mostly standard kayak building stuff.

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The forms were attached to the strongback per the drawings.  Sixteen spaces at 38 cm per space.  To hold the tops of the forms at the correct spacing I tacked on some scrap strips leftover from a previous project.

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This beautiful clear vertical grain (CVG) western cedar got ripped into the 4.5mm (3/16″) thick strips.  Some of the pieces had perfect vertical grain.  Cutting them as-is would have resulted in flat grain on the strips. So I to ripped them into billets to turn the grain 90 degrees and then ripped them to thickness. Doing so also allowed me to have strips of various width for different curves on the kayak.  They ranged from 3/8″ wide to 1″.  Most were 3/4″ and 7/8″ wide.

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Stripping the hull.

I started at the sheer with a double thickness strip, rabbeted and beveled to accept  the deck.  The forms had small cutouts to accept these.  It form an easter-egg joint later.

For this build I’m trying Björn Thomasson’s method of strip building: The strips are all cut square and stapled dry to the forms.  Glue (epoxy) is then worked over the entire surface.  The epoxy seeps into the joints post-gluing the strips together.  The epoxy fills the gaps between the square-edged strips.  The staples are then removed, the exterior is sanded and fiberglassed.  I’ve always wanted to try his method.  It’s a very quick way of making a kayak.

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The hull is completely stripped.  Ready to apply the glue.

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Glue was just worked over the entire surface.  Hard to believe this works…

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When the glue had cured I started pulling the staples.  The staples pulled out really hard.  I ended up snapping almost a quarter of them.   I first blamed the glue.  On further trials led me to fault my plywood instead.  Come to find out I just had really tenacious plywood. Björn recommends particle board.  The staples don’t hold as hard as the fir plywood.

I did have a few areas where the glue didn’t penetrate into the seams and thus came apart during the staple pulling and sanding operations.  Speaking of sanding…

One of the biggest negatives of this process is that the surface sanding is much more difficult.  The glue penetrates the outer surface of the wood toughening it up.  I used three times the number of P40 sanding disks I usually use to rough fair a hull.

The other issue is the number of strips that came out of a alignment during the gluing process.  Several moved upwards of more that a 1/16″.  With a strips only 3/16″ to start with, this doesn’t allow much left for sanding.  The worst ones were cut free and reglued.  Spot tacking the strips together with CA adhesive (ie super glue) would have hugely helped with this issue but would have greatly slow down the strip-laying process.

A large majority of the gaps were also not filled by the glue.  After rough sanding I spent some time filling these gaps with putty.

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For final sanding I always use a longboard. This one is my new favorite.  It’s made for automotive body work.  It came in a kit of three (24″, 18″ and 9″).  It has a rubber base to attach the self-stick abrasives with the upper part made out of medium-density foam rubber.  It’s very comfortable even after an hour of finish sanding.

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Two layers of 3.2oz tightweave fiberglass cloth were laid on the hull.  The ultra-light builds often get only one to save weight.  The extra layer of cloth adds a some weight (maybe half a pound) but will greatly improve the resistance to abrasion and toughen up the hull for those Teneessee waters.

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The cloth was wet out with Entropy Resin Super Sap INF.  It’s a very thin resin line that’s generally used for infusion.  There is no required post-curing so it also works great for hand laminating.  It really penetrates the tight weave cloth.

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The stems get a few extra layers of bias-cut cloth.

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The hull exterior gets two fill coats of epoxy.  For these coats I switched to Super Sap CLR/CLF resin.  It’s a bit thicker than the INF.  Both were lightly tinted red.

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After curing overnight I trimmed the glass and flipped the hull onto some cradles.  The cradles are cutoffs from making the forms.

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The forms were removed and the interior was sanded to P80.

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You can really see the complex curves in the sheer line.  The dip is placed at the paddle catch.  It helps narrow the deck slightly.  Very aggressive looking!

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Fiberglass was cut for the interior.  To help eliminate waste I mostly used cutoffs from glassing the hull. The overlaps while strengthening the hull are strategically placed as not be be seen.

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Interior glass wet out with some more Super Sap INF infusion resin and left to cure.

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While the resin cured I trimmed the forms to the deck profile.  After the resin cured overnight I reinstalled them inside the hull.  A few dabs of hotglue hold them in place. You can see the double-thick sheers.  They were beveled to match the curves of the deck.

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Ready to start stripping the deck!

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2 thoughts on “Spray HPK – Hull Construction

  1. wow i would have never tried that but i have a painted project coming up where gaps wont matter. So a million thanks. Hey do you think it adds much weight? also can you sand away the stuck staples?

    1. Filling the gaps does use a bit extra epoxy. The total volume though is quite small. I figure it adds a few ounce of weight to the build. 1/2 pound worse case.
      You could sand away stuck staples but removing the forms would be more difficult. Also, staples are hard on sandpaper. It’s best to remove them.

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