Straight-sided kayak

 I saw an advertisement in WoodenBoat magazine (page 71, Fall edition) with a picture of a kayak whose stems were flat sided with no visible curvature, yet it looked nice.  I thought to myself, 'I wonder how you would describe that mathematically', and then I started toying with numbers and approximate dimensions.  After all, with the launch of my recent canoe-type hull, I have no boat projects to fuss over.

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.  

A nice size for two persons, but my wife had her own kayak today.

What are the advantages and liabilities of a hull shape with all shear curvature confined to the center section?  I can think of several possibilities, but the best way to find out is to build such a hull.  I have calculated all the numbers needed for such a design, my only remaining question is optimum hull length, re-spacing the frames (and recalculating the stems and keel).  Molds could be substituted instead of frames for a cleaner interior, but I hate to throw away carefully shaped molds, and frames add strength to the hull.

This sketch shows the deck outline for a hull 28 inches wide and 12 feet long (to the inside of the planking).  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.

If anyone is interested, I could publish the offsets, 'numbers', needed for the frames, keel, and stems.  Kayaks are not usually symmetrical; it is not difficult to re-space and extend one end as the bow.

Index of Entries, March 2026

  Find a topic of interest in the lefthand column and note the associated date (month/year), then click on the associated year in the righthand column, and you should find your chosen entry nearby.

Planking the Canoe                         02/26

A Small Canoe                                11/25

First Signs of a New Build             11/25

Designing a New Hull Shape          9/25

What should I Build Next?              9/25

Planking the New Kayak                6/25

Putting the Pieces Together             3/25

New Kayak Design, Part 2              1/25

Creating a New Kayak                    12/24

A 'Leaner' Kayak                             12/24

 Boating is more Satisfying....          8/24

What is Next?                                   6/24

Careers and a Hobby                        6/24

A Smaller Kayak                              12/23

Finishing the Planked Hull                7/23

Rules for Developable Hull surfaces 6/23

Finalized kayak Design                      1/23

Starting a New Design: Kayak           1/23

Now I Have a Boat to Look at            8/22

Next Step in Design Evolution          11/21

A Real Sharpie                                    3/21

Another Guide Boat                            2/21

Finishing the Guide Boat                    12/20

Finishing the Planking                        5/20

Creating a Developable Surface         5/20

Solving the Hull Sheathing Conform. 4/20

Planking the Guide Boat                     9/19

Garboard Sheathing                            7/19

Viking Ship Museum, Hull Fairing     6/19

Defining the Shape                              3/19

Creating a Boat 'Skeleton'                    3/19

Starting to Cut Wood                            2/19

First Steps for the New Design            12/18

The Improved Guide Boat                   10/18

Getting Ready to Build                         7/17

Completed New Model                         9/16

New Design Model, Part II                   8/16

Creating a Model for a New Design     8/16

Beautiful Boats of Scandinavia             7/16

Vietnam Memories                                10/15

Along the Adriatic Coast                       10/15

Grand Tetons N.P. Boating                     8/15

Modified 'Next Boat' Design                  4/15

Sail Design                                              2/15

Next Boat?                                              12/14

A Day at the Lake                                    9/14

Resting on its Design Waterline              7/14

At Flaming Gorge, Utah                          7/14

What a Day!                                             5/14

The New Boat Looks Complete               3/14

Why I use Rosin Paper                             3/14

The Boat Windshield                                2/14

Winter Progress                                         2/14

A Voyage to Remember                             11/13

Instrument Panel                                        11/13

Now for the Accessories                            10/13

Sample Developable Design Calculation   7/13

To the Deck and Onward                            7/13

Slow but Steady Progress                           5/13

Finally, the Hull is Turned Over                 1/13

Hull Sheathing                                            7/12

The Origin of Boat Bowls                           4/12

Another Year, Another Boat                        4/12

First Season Report                                     9/11

A Trial Run                                                  6/11

Finally in the Water!                                    5/11

Out of the Shop                                            12/10

Ready to Leave the Shop                             10/10

Finishing the Stern, Onto the Windshield     7/10

Wood Decking                                               7/10

Interior Details                                               6/10

Finally, the Hull is Upright                            4/10

Planking the Hull                                          11/09

Initial Hull Sheathing                                      8/09

The Design Becomes a Reality                       5/09        

Hull Design Using Simple Mathematics       12/08

Design Evolution                                             7/07

New Direction                                                 7/07

Model Lobster Boat                                       11/06

Updated Drawings                                           9/06

Maiden Voyage                                                5/06

A Semi-planing Powerboat                              5/06

Guide Boat Pictures                                         5/06

Adirondack Guide Boat Concept                     5/06

A Real Disappointment                                    5/06

Panama Skiff                                                    5/06

Kayoe or Canak?                                              5/06

Where did this Unusual Interest Originate?     5/06

Planking the Canoe Hull

When fitting each plank, clamps can be spaced at wide intervals, but when doing final bonding, clamps must be closely spaced to get good adaptation of their mating edges.
 

Assuming an upright hull position:  As the topsides curve becomes more vertical toward the sheer, the lower edge for each new plank needs to be tapered (to partial hull length) where it meets the previous plank.  The hull is somewhat rough now.  Adjacent planks seldom meet in a perfectly matching curve due to variations within the wood itself.  It will need to be sanded and filled when the planking is complete.

Planked up to the sheer line.  When you are bonding with epoxy, or any adhesive, use plenty of clamps.  The final length 155.25 inches, width 29 inches, and midships depth 10.5 inches.

Next, the hull will be turned over, and I will begin the process of sanding and filling until a smooth surface is achieved.  Then, the interior needs to be finished but not to such a polished degree.  Last will be completion of the small stem decks and sheer.

You can see the pattern of planks.  Initially, full-length 1 1/2" wide planks were used (for the first projection).  At the initial chine, a narrower 1" wide plank was used due to increased frame curvature.  Partial-length 1 1/2" wide straight planks were used to fill-in remaining frame curvature as the topsides curved upward.  The final two planks were 2" wide because, as the frames approached the sheer level, the frame edges were almost straight.

Realize that this is a mathematically created (mainly algebra) developable surface boat design.  No computer or drafting board were used to create a table of offsets.  The only change is that, after all highly accurate offsets were plotted, the various plotted points on each piece (keel, stems, frames) were connected by smooth curves rather than short straight lines.  Then narrow solid wood planking was used instead of plywood.

  

Three coats of varnish on the inside and two coats of white paint on the exterior.  I ordered white by mistake; I was thinking light blue but didn't recheck my online order entry.  Oh well, the small stem decks and interior will be varnished wood, and I'll add an accent stripe at the sheer.

Bonding the entire deck in one session.  Nice to have plenty of clamps.  It is rough now, but when the epoxy cures, I will trim it, sand it, and add a small splash rail to each end.  The blue stripe you see is painters' tape to protect the white hull from oozing resin.

All that is left to do are fine sanding, paint, and varnish.  Not quite 13 feet long; not quite 29 inches wide.  Easy for one person to lift and carry.

I saw this partial deck design in a book of old canoe designs and liked it.  The deck is alder which should darker significantly when varnished.

Just a few more coats of varnish and it will be complete.  I like the choices I've made in the design but still ask myself what could be done to improve it.  When I get the canoe out on a lake, I will know better.  How should I construct the seat(s)?
  

A Small Canoe, Fairing the hull frame

 With the essential frame completed, I decided to continue the canoe sequence in a new entry.  The next step will be to fair the hull frame in preparation for sheathing with narrow planks.  Most of the fairing will be easy; however, the stems will require drastic reduction from their present rectangular cross-section, instead tapering to a point at their outside edge.  I usually do that using an angle grinder with a coarse flap disk, checking frequently by laying a plank section along the ruling lines.  The designed slope from the major chine to the keel ends is X:Y:Z= 11/3.6/0.8, but practically, I will be using the adjacent frames as my guide. 

The chine stringer has been freshly bonded in place.  Four cockpit edging pieces are resting in place but will require trimming and will not be finally bonded until after other decking is completed.

Fairing Done!

For me, fairing of this hull frame was a big deal.  Everything is bonded together; so, if I messed this up, I am in deep trouble trying to fix it.  Fairing includes not just beveling the frame edges to fit the curve of the hull but also reducing the 1 1/2-inch-thick blunt stems to sharp ends around a curve with changing bevel.  The keel edge needed a rabbet to receive the first plank; easy to do in the parallel mid-section, but difficult near the stems around a curve with a constantly changing bevel. 

Keel shown on left with future plank butted up against it.  A rabbet must be created to accept the plank in smooth transition and provide bonding surface.
 

I was able to bevel the stems at sheer level on my table saw before bonding them in place, but the curved section is still blunt, squared off.  This requires major reduction by hand to get the desired changing bevel.  First, I used an angle grinder with coarse flap disk, then a belt sander.

Th almost finished product of shaping the stem edge.  Any final adjustment will be done as I fit each plank.

A section of the keel rabbet, curved with a changing angle.  I couldn't think of a way use a fence for guidance, so it was done freehand, slowly and carefully with multiple passes, with a router and finished with a hand plane.

Major tools required:  Milwaukee angle grinder for the initial reduction of the stems.  Rigid router for creating a rabbeted landing on the keel edge after initial reduction with a power plane.  I could use a fence on the straight mid-section but had to freehand the router near the ends (carefully, a little at a time).  Craftsman router for finishing the stem bevels.  Stanley plane for finishing the keel edge.  No fancy equipment, but they get the job done.  (I forgot the power plane.)

With the routed edge to the keel, future initial planking will fit solidly and supported when it is time to bond it.  Now I need to create the planking.

I found a nice 1 1/2-inch-thick poplar board from which I have been able to slice many 0.21" thick planks.  The planks were first ripped on a table saw and then run through a planer for final uniformity. These planks are slightly thinner than what I usually use, but the frames here are slightly closer together (11-inch spacing) providing better backing.  Here you see the first full-length plank being trial clamped in place prior to epoxy bonding.

The last full-length planks are clamped into place.  A next full width plank would cross the major chine and, thus, would be part of a different projection at a steeper angle.  Alternately, I decided to use a one-inch-wide plank to complete this projection.  Because of the significantly steeper deadrise slope of the next plank, it will be only partial length. tapering at the ends and only about eight feet long.  Subsequent planks will also be partial length as the topside frame angle sequentially increases.  At the sheer, the final plank will again be full length for the hull.

First signs of a new build

 This week, I scarphed together two lengths of soft maple.  Why maple?  Because I can get it in approximately 8-foot lengths of 1/2" thickness by 8-9" width at an okay price.  It is a hard wood destined for a canoe-styled hull.  I used an 8/1 scarph ratio which is more than required.  The scarph was made initially with a hand power plane for gross reduction, then followed with a belt sander and medium grit.  The resulting surfaces were flat and mated well.  Epoxy adhesive was followed by clamps.  I have three hand belt sanders.  Why didn't we have them in junior high shop class!  Planes can produce 'chips'; belt sanders never do.

   With epoxy and paint, also kept indoors, weather will not be a factor for the maple in this hull. Once scarphed together, I ran the combined board through a thickness planer to bring the thickness down to a uniform 7/16" (0.435").  Why that thickness?  Because I wanted a balance of weight, strength, and flexibility for the slight rocker planned.  These were rough boards when received, thickness was not uniform, and they initially were not entirely straight.  I corrected the side curve with a hand plane and frequent checking with a straight edge.  The product now is straight, uniform, and has the flexibility desired while retaining sufficient strength.  This will be the plank keel creating a foundation, tying all the frames together. 

Today, I bought lumber for the frames: knotty alder.  Why knotty alder?  It is a beautiful hardwood, but relatively light, and at a reasonable price.  Of course, I hand selected the best boards and bought excess (perhaps enough for a next hull).  The drawback is the knots present but, for the frames, I will be bonding together rather short lengths of wood.  I think I can work around the knots. 

New lumber.  The keel is maple, overall length 136".  Current width is 8".  The other boards are 1/2-inch-thick knotty alder.  As you can see, the knots are not too distracting.  I can buy 1/2"x5 1/2" boards, which I can't produce on my home saws.  The frames will be 1/2-inch thick, and most will be 1 1/2" wide, but with 5 1/2" overall board widths, I will not be limited if I want a wider piece for a frame.

There will be twelve frames, symmetrically spaced fore and aft.  These are full-size rosin paper patterns.  At the turn of the bilge, I will need to use narrower strip planks to follow the increased curve.

I chose a 13-foot length with frames spaced 11 inches on center.  The length could easily be increased with 12 or 13-inch spacing.  Knotty alder wood was used, but I was able to avoid almost all of the knots.  Fifty-six total wood pieces were machined and bonded to make these frames.  I will need to notch these frames for the sheer and a chine.  

My next task will be to create the stems; they will add about 20 inches to the length seen here.  I will start with Douglas Fir 2x4" material to construct them.  The measured sheer piece when bonded will fix the frames at proper spacing.

The hull stems are now in place, ready to be bonded. Next the sheer will be better defined with a scarfed longitudinal.  I need to strengthen that area before I can turn the hull over for planking.  The half angle at the stems/ends is 18.12 degrees.  There will be small decks at the ends for which I need to add some support.

Many details were involved getting to this point.  That curved sheer strip is 157.7 inches long by calculation.  However, I didn't trust my calculations, so I made it a couple inches longer just in case I was wrong.  Then when fitting it, I wasted time recutting to the proper length of 157.7".  The intersection point with each frame was also precalculated.  Everything is held together with spring clamps currently.  A next step will be to mix epoxy and bond it all together.  At the ends where the sheer and stem and a short deck beam come together, the calculated intersections were perfect.  The task was to cut the actual wooden pieces to fit exactly with all the intersecting angles.  Hull length is currently 154 inches, but that is to the inside of the sheathing, when done it should be about 2 inches longer.

The curved laminated section of wood placed from deck beam to sheer illustrates where the edge of the small fore and aft decks will be when complete.  I plan to use a herringbone design of planks for decking.

I have doubled the sheer stringer and added another wood strip to define the major chine.  Next step will be to fair the hull in preparation for planking.  The lower portion of the stems currently have a rectangular cross-section and will undergo major reduction to a "V" cross section (like it appears at the sheer level).  Using a short straight section of planking, laid in the direction of the design mathematical projection, as a guide, I have found that an angle grinder with a coarse flap disc is best for achieving the needed reduction. 

The plank keel will also need to be routed to create a landing for the first plank, the garboard.  This will all wait until after the upcoming holidays.  

    

Designing a New Hull Shape

This hull started as a concept in my mind.

The initial boat type, pictured in my mind, was of a slender canoe which could be paddled with a double-bladed paddle.  Speed was not a goal, reduced weight (partial deck) and slightly more forgiving stability were the goals.  The first thing was to picture the midships cross section; a bottom slightly wider and flatter.  I 'played with numbers' in an equation, used to define the major chine.  I could alter the coefficients of X, Y, Z in parabolic equations to get the right length (X), width (Y), and upsweep (Z) that I was seeking.

Next, I needed to decide the X:Y:Z ratio in the bottom projection; get the midships deadrise desired with progressively steeper deadrise approaching the stems.  In order to achieve this, the major chine needed to be reconsidered.  Position and spacing of frames needed to be included as well as width of the keel.  The Y:Z ratio is determined by the desired deadrise angle amidships.  The X:Y ratio is determined by the bow half angle of the major chine.  

Creating the topsides is perhaps most critical.  At this point, I had to start over with all new numbers for the equation for the major chine; however, there were multiple choices/combinations to pick from.  Incorporating three more projections to create a more rounded topsides shape, I discovered, would result in a hull that was wider than desired.  I used a paper sketch to help decide the coefficients for my topside projections.  I had to choose a major chine equation with less beam (Y).  Iterative process, but I was making progress.

When I started to achieve harmony between all the equations and coefficients, I had more aspects to consider.  Just how slender do I want this hull to be?  What should the overall length be?  What should the sheer look like?  How much depth amidships?   Height at the stems?  Should I consider creating a transom at one end; should the hull shape be symmetrical or should one end be slightly more tapered?  But those factors are not requirements- they are choices to create the specific boat you want.

With the offsets already generated, I needed to make two more projections: first to see a lateral view of the stems and secondly to see the shape of the plank keel.  All offsets need to be charted on paper before I can visualize the result.  (Hey, all I use is a TI pocket calculator: no computer screen.)

Full-size paper patterns for the frames, stems, and keel are complete, a little drafting practice.  Not sure they are interesting enough to post (pencil on rosin paper), but the cross-section shape is satisfyingly rounded.  I plan to add some partial decking at the ends.  The major dimensions, as finally constructed, are 155" (13 feet) long, 29" wide, and 11" deep.  Weight is about 43 pounds.

Thus far, my description has only been in generalities, should I post a table of numbers?  It will be some time before I can start building.  I do already have the plank keel lumber on hand.  Next task will be designing the frames and developing a list of materials. 

Stacked frame patterns drawn on rosin paper. 

     

I ordered the white paint by mistake, but I like the appearance of the final product.