New Kayak design, Part 2

Topsides Projections 

The hull topsides shape is created by adding surface projections and secondary chines.  The goal is for the result to form a smooth curvature rather than the angularity of a single hard chine.  For this purpose, a parallel projection is used with a constant ratio between the changes in the x, y, and z dimensions.  The first projection will use the ratio x:y:z = 7:1.8:1.5.  All projections will be directed from the major chine toward the center section of the hull.  The second projection will use the ratio x:y:z = 7:1.8:3.5.  The third projection will be vertical in the z dimension with z=9 at the sheer line.  From this an entire table of offsets can be created listing offsets for all the frames above the major (initial) chine.

Where do these numbers for the projection come from?  The half angle at the bow is 0.9" increase in beam (y) in the first 3.5" of length (x). Because we are plotting length (x) at 7" intervals, we double the numbers: 1.8" increase in y for every 7" increase in x.  This ratio will give us the desired plumb bow.  Increments of height (z) vary as we go from keel to sheer:  keel is horizonal; deadrise is 0.5" increase in z for 1.8" increase in y up to the major chine; 1.5" per 1.8" increase in y in the first topsides projection, 3.5" in the second projection, and vertical in the third projection.  Together, these values help create a curved midships frame profile from the bottom to the sheer.

If we connect these points with straight lines, we have a faceted surface with five projections from centerline to sheer.  If we connect these points with a French curve or flexible batten, we can create a somewhat (the major chine does not completely blend in) smoothly rounded surface with the exactness of mathematical curves and projections.  Using narrow planks for sheathing the hull will preserve the rounded cross section.

Bottom Projections

Next, the offsets for the area below the major chine are developed.  For a narrow hull, like a kayak, a flat bottom, transitioning to a gradually steepening deadrise to provide both stability and a rounded cross-section is a good choice.  A flat plank keel is the starting point and the foundation for the entire boat.  However, if the keel is too wide, the hull will rock and pound in waves.  Laterally, on both sides of the keel will be a constant x:y:z slope of 7:1.8:0.5 (15.5 degrees deadrise).  All projections below the major chine will be directed toward the ends of the hull. 

This is a complete table of all the dimensions used to create the ten frames.  These are exact dimensions* resulting from projections from the major chine and the two additional chines for the topsides.  (*except for four numbers at the sheer obtained by graphic solution)

Stem Profiles

The next step in creating the new design is to project down from the major chine to the midline (y=0) to define the stem profiles, bow and stern.  Most of these numbers are not exact but can be calculated to whatever accuracy is desired (usually to 0.01" accuracy). The equations used for these calculations are listed. 

Plank Keel 

We can also calculate the offsets for the plank keel.  At my lumber source, I was able to get a plank about 10 feet long and 8 5/8 inches wide; then bought a second plank because the proposed keel would be about 140 inches long.  The original hull design was for a narrower 7.2" wide plank keel.  To take advantage of the additional width, I changed the projected distance from the major chine to the keel at the hull midpoint from 1.5" to 1.35" which is reflected in the table listed below.    

The bottom portion of the table above shows the calculations for the plank keel offsets.  The width (y) is the same fore and aft, but the length dimension, measured from bow and stern stems, is reduced by 20% for the stern portion.  As a check on calculations, intersections of frame dimensions at the keel should be consistent with the listed keel width at each "x" frame position.  Also, the "x" intersection of each stem profile with the keel should be consistent with the ends of the keel where y=0. 

Keel Rocker

It can be seen that we are designing the entire kayak with accurate numbers before starting building.  Only one more step to include:  It is recommended to include "rocker" (a slight upward curve) in the keel of our kayak for improved performance.  This will change the keel taper and length and induce a small change to the frame heights toward the ends of the hull.  I used keel rocker of 0.1", 0.4" and 0.9" over the 14" frame spacings toward both stems.  Total keel length will be 140.5 inches with the bow portion ending 9.3 inches short of the absolute stem and the stern section 7.7 inches short of the stern.

The numbers given are enough to build the entire kayak hull.  I already have the keel cut out and tapered.  Although using plywood cutouts for frames would have been easier, the frames are roughed out by half-lapping sections of solid wood and bonding them together.  Much needs to be done to finish the frames, and work on the stems comes next.  But for now, I will be taking a break.