By George, I Think I've Got It!

Fussing and fiddling had finally gotten me my OWN design:

• 13 1/2 feet long,
• with 13-inch-high sides for adequate freeboard heavily loaded,
• 26 inches wide at the bottom of the center frame (for a modicum of stability)
• 33 inches wide at  the top of center frame (for modest rocker and not too wide to paddle easily),
• double-ended to be paddle either way and so as not to drag much with ends immersed,
• with exterior chine logs to fasten bottom to,
• inwales to stiffen the sides without adding width,
• canoe-style seats (in addition to the center frame) to help the hull hold its shape

(Exterior chine logs are not only easier to make, but might also help the boat keep from sliding sideways in a breeze.)  Two people can paddle it comfortably.  If someone goes solo, reverse the boat so you're on the forward seat facing aft (the usual "stern" being the new "bow") for better one-person balance.  (Not an idea original to me, by the way.)

I started measuring the sides on ply this morning and came to an abrupt halt: this boat is so close to being a two sheeter!  If I continued, once I cut the 13" sides from the long edges, the resulting <22" wide center is a few inches too narrow to cut the bottom from--the more so because exterior chine logs add 1 1/2 inches to the width of the bottom.  At the same time I sat Stephen on the deck and measured up his side: even one-foot-high sides would make it difficult for him to paddle.  (Granted he wouldn't usually be sitting flat on the bottom, but either on canoe-style seats or cushions.)

The plywood layout plan implied by my original design.  Look at all the wasted wood!

So: reduce the sides to 12 inches.  Still too little left for the bottom--even if I narrowed the center frame to 24", since the ply thickness and chine width must be taken into account. 

Ah--but cutting out the sides leaves three feet or more untouched at one end of one sheet: can I work out a way to get enough of the widest part of the bottom to fit in that few feet?  Lots of drawing, and figuring, and measuring of models later, I have a plan:  keep the bottom of the center frame at 26", making total width of bottom 28" at the middle (assuming 1/4" thick ply and 3/4" wide chine logs).  Take advantage of the stems' rake of 45 degrees (1' in from ends).  Work out how the big end of one ply sheet and the small end of other piece are combined: 4 1/2' total from edges to bottom of stem cuts.  Divide this half, making center of bottom be 15" in from edge of bigger remains piece.  Now turn to the balsa model and figure out how many inches the bottom would narrow in the 27" from middle of bottom to where the bottom would be constricted by cutting the sides.  Leave a teensy bit of slack for the saw kerf and a wandering cut.

Drawing: the overall rectangle is two sheets of plywood laid end-to-end. The sides are cut from the top and bottom edges of the ply, and the two parts of the bottom are cut from the middle beginning at the outside edges.  the leftover bit in the middle of the two ply sheets could form modest decks at the ends, with advantage advantage taken of the wedge-shaped cutouts for aesthetic affect.  The top shows the finished boat in profile and end-on.

DONE--26" full width of bottom would narrow to about 22" at 2' from the middle.  That's a little short of constriction (at 27") and still leaves a bit for errors or a more gradually-narrowing bottom!  The remaining pieces of ply would amount to 3', which (after removing 4" in the middle for a bottom butt strap) could become fore and aft decks of 12" and about 20" on centerline, but much longer on the sides because of the taper of the bottom at the ends.  Little coamings following this inside curve could make this boat look rather pretty!  There should be enough left for side butt straps, and maybe seats!  One final structural test: lining up the joints and framing, the sides are butted together over 1' forward of the center frame, and the bottom is butted together about 2-3 inches forward of the center frame, so no joints align to make a weakness that extends through the entire hull.

Boat Design for Dummies

(No, not the book, if there is one.)

I have been invited by a kind pastor to run a summer program for upper elementary kids at her church.  Since I have pretty much free rein, I have decided to make it "the Boat Science Experience": we will investigate floating and sinking, buoyancy, stability and the like, building and testing paper models along the way, and finishing with the building of a real paddling boat (maybe even more than one).  To this end, I have been investigating designs for boats that would hold two kids or one kid and one adult, be a good enough performer not to be frustrating, and be simple and cheap to build.  A Michael Storer (Aussie) design called the Quick Canoe 150 seemed to fit the bill, and I bought a set of plans.  The plans--all metric--require a good deal of meticulous measuring and drawing of curves, which might be good practice for using metric units and doing careful work.  Or not, with kids this age.

Then I started asking around on-line for advice on building boats with kids and got a lot of good responses--some of it related to the ins and outs of doing this kind of class (keep it SIMPLE), some of it related to the choice of designs.  I was impressed by the simplicity of the designs suggested: some are made of two perfectly straight-cut sides attached to a trapezoidal center frame and bend around to form the ends of the canoe/kayak.  Though the wood is straight, the boat develops lovely curves because the sides are held at an slight angle (called "flare").  This makes the boat wider at the top than at the bottom (as most boats are) so that the ends curve gently upward both at bottom and sheer (top edge).  (The fore-and-aft curve of the bottom is called "rocker.")  The ends of the sides are cut at an angle to accommodate this flare and giving the boat more or less pointy ends as the designer wishes.  The greater the flare, the more the ends would curve upward--that is, the more the rocker and curve of the sheer.  Gunwales, sometimes inwales, and one internal frame and maybe seats give the hull enough stiffness to hold its shape.  The bottom could be experimented with to establish a shape and measurements for pre-cutting, or simply cut to fit the sides.  The designers of these "straight cuts" boats--all amateurs--could alter beam and length independently or scale the whole hull up or down as desired without altering the drawings that much.

This was a revelation.  Here were nice-looking hulls very simply drawn and built.  You could play with such designs easily with pencil, ruler, cardboard and scissors.

Top ("plan") view: from the right, the first two boats are  the published designs Wacky Lassie and Lazy Weekend Canoe, the others are my modifications.  Actual lengths are about 13 feet for most.

Except for Wacky Lassie, all the designs have a similar amount of "flare" (angle of sides from the vertical)--

--which gives them a similar "rocker" and sheer line (curve of bottom and gunwale)

Now-- I have always been of the opinion that boat design was for pros and really experienced amateurs.  I would never trust my money and time to a design of my own--I would want to be sure the thing would GO once I had spent all that money for materials and put in the months of labor.  But for a simple boat like this, I'm beginning to change my mind.  You see, the designs I've looked at aren't quite what I want: good capacity but not too much length, wide enough at the chine (bottom edge) not to be too tiddly but narrow enough at the sheer for a child to use a double paddle, and light enough for a couple of pre-teens to handle and car-top.  Hmmm...