Straight-sided kayak stems

 I saw an advertisement in WoodenBoat magazine (page 71, Fall edition) with a picture of a kayak whose stems were straight sided with no visible curvature at the sheer, unique yet it looked nice.  The bow and stern were slender 'V' shapes.  Curvature connecting them was confined to the midsection.  I thought to myself, 'I wonder how you would describe that mathematically', and then I started toying with numbers and approximate dimensions.  It didn't take too long to create a harmonious major chine curve and choose constant projection ratios to establish offsets describing a somewhat similar shape.  Currently, with the launch of my recent canoe-type hull, I have no existing boat projects to fuss over.  I need a new project.  

I digress, finally getting my most recent build in the water.  This hull has a short parallel section amidships with continuous curvature into the stems.  Finally at a nearby lake.  A small canoe for two persons.  Performance was as expected: more stable than a kayak but less 'glide' between strokes.  It weighs about 40 pounds and is easy to carry for one person.  

A nice size for two persons, but my wife had her own kayak today, so I was by myself.

What are the advantages and liabilities of a hull shape with all sheer curvature confined to the center section?  Doesn't a continuous mild arch create a stronger structure than a flat surface?  Is a slender entry angle important for craft that move at rowing speed?  I can think of several other questions, but the best way to find answers is to build such a hull.  I am curious as to whether such a shape might be easier to build.  Planking the stems, where curvature is normally at its greatest, is usually one of the more difficult tasks.

I have calculated all the numbers/offsets needed for such a design including re-spacing frames (and recalculating the stems and keel) to slightly extend the bow section.  The straight portion of the major chine is about 40% of its entire length.  Molds could be substituted instead of frames for a cleaner interior, but I hate to build and then throw away carefully shaped molds; included frames add strength to the hull.  When water splashes into the hull interior, it is trapped between frames and easy to wipe up.

This sketch shows the deck outline for a hull about 28 inches wide and 12 feet long (to the inside of the planking).  The bow half-angle is about 14-15 degrees.  Shorter is lighter, easier to handle and store, longer would be prettier and more graceful on the water.  Something to think about.  We are planning a winter move to Florida; this would probably be a project to build while there.

I drew out the frame outlines and don't like what I am looking at.  It is embarrassingly ugly.  Let's forget the straight-line stem experiment.  But I have learned something:  Previously, I have been using curves that extend almost the complete length of the chine.  Now, I realize that I can extend the straight-line portion of the chine for more than a few inches near the stems with enhanced results.

Near the stems, at frames 6 and 18, the shape looks like a "U", too parallel-sided.  I would prefer more of a "V" shape for frames near the ends.  Now I will start over to generate a new major chine description with multiple projections to form surfaces.  I also feel that a 14-degree bow half angle is finer than I would prefer, 16 degrees would be better.

Update:  My new calculated chine description includes straight sections for about 20% of its length.  A few other coefficients were 'tweaked' for optimization, and the new result is much superior.  New frame outlines are posted below.  

Doesn't that look better?  Evenly curved instead of slab-sided.  The revised design pushes the beam out to 30 inches.  (14-degree deadrise, 16.2 degree bow half angle, 9.5 inches center depth, 13-foot length, 6.5-inch-wide plank keel).  I think I would finish it as a partially decked canoe, similar to my most recent build.  

The frame outlines (for this triple chine hull) appear as smoothed curves because I connected all the plotted offsets by lines using a French curve instead of straight lines.  If I strip planked the hull with narrow wood strips, the final hull would be similarly rounded, but I intend to use 1.5-inch or 2-inch-wide planking which will create some minor faceting to the final hull surface.  I don't have the patience for those narrower strips. 

As an aside, I had two sheets of 6mm marine plywood sitting on my lumber rack for several years.  This past week, I used them both in building a piece of furniture for our Florida home.  I don't expect to have much use for plywood for future boat building.