Designing a useful and attractive boat from a single sheet of plywood - that was at the same time easy and quick to make - was not easy. The final design took only an evening's work, but learning how ... took a little longer! I set myself a number of goals for this task over the years I spent learning about canoe design and boats in general. My inspiration was the website of Hannu Vartiala at http://koti.kapsi.fi/hvartial/ which is a great source of information for anyone interested in the design challenge: for anyone interested in making a pretty, lightweight and all-round delightful small canoe, feel free to try this one!
Use only a single sheet of plywood, 4 x 8 feet or 1.22 x 2.44 m. That is the standard size available in Canada, in some other countries the sheet size is slightly larger and it's certainly permissible to increase the size of the boat!
Make an elegant looking canoe that goes as well as it looks. There are some remarkable boats that have been built using one sheet of plywood, some builders aimed at the biggest boat, or the longest boat, or the greatest displacement which is a measure of the load-bearing capability of a boat. They did a great job but my aim was to make it look good and go fast.
It had to be reasonably easy to build, suitable for a first time builder who had a nodding aquaintance with wood and the tools used to work it, but a satisfying boat for an experienced builder to knock off in his or her spare time. Oh yes, quick too!
There is an activity during the build of a boat called lofting. It involves carefully scaling up planks to full size and precisely marking out planks and stuff. ; not for me! The boat has to generate its own shape from the simplest possible measurements, and preferably as many straight-line cuts as I can get away with. I am not lazy (well not very) but I do have a history of making mistakes when measuring things and then cutting, only to regret ...
Dora, the last canoe featured in my "Canoes" page, took 1-1/4 sheets of plywood so that was a starting point for the new design. Dora is a delight to paddle, fast and easy, but she has a couple of shortcomings: she tracks (maintains a straight path) so well that she is difficult to turn, and she wants to turn into the wind if there is any, which makes her less pleasant than she could be to paddle across the wind. I wanted to deal with those problems in this boat. Rocker, or a curve in the bottom when viewed in profile, will make a boat easier to turn, and raising the height of the bow will cause the wind to press more strongly there, tending to offset the tendency to turn downwind.
The Bass-Ackwards Design Method
In the BAD method I decide what the planks are going to look like first, then get the boat shape from that. Well, sort of. For example, I can make the sheer plank developments straight and use a constant flare angle, to meet goal #4 above. When I bend the sheers to make a curve - seen from above in the Plan View - and lean them over so they are no longer vertical - which I call the flare angle - it creates a similar but flatter curve on the bottom when seen in the Profile View This is called rocker and is also replicated along the top edge of the sheer planks, which is known as the sheerline. If I allow the wood to bend naturally, bending it around one or two forms, it automatically generates a nice looking - or fair - curve so the boat looks nice too, and that smooth shape glides through the water easily, thus meeting goal #2. These curves are a blend of circular arcs and hyperbolas - which are called conics for the mathematically inclined.
I have to fix the windsocking that showed up on Dora; A little extra side area up at the bow will do that nicely, so I modify the straight sheer plank development by curving it up a little near the bow, which meets goal # 5.
Finally, as far as the design is concerned and to meet goal #1, it all has to fit onto a single sheet of plywood. Well, so far I have the sheer planks, but they are longer than the plywood sheet so they have to have an extension added at the bow end, which is where I do the curving bit - the rest of the plank is straight for easy cutting. At this point I have to budget my use of the plank, especially across the width, as follows:
-Sheer planks 2 at 6 inches -Extension pieces for sheer planks at bow, 8 inches (allows curving up by 2 inches) -Bottom, 12.5 inches -At least 4 saw cuts, say 1/16 inch wide, total 0.25 inch -Error allowance: no cut can be perfect so I allow 1/16 inch each, total 0.25 inch
- so far we have a grand total of 33 inches, leaving another 15 inches from the 48 inch wide plywood sheet, which will make two bilge planks of 7.5 inches width. These planks go on last and are trimmed to fit, so I reduce their width by 0.3" along each edge to allow for trimming, leaving 6.9" to define the sectional shape of the hull.
Choosing angles for the sheer and bilge planks will decide the shape of the boat; I will use the same 1 in 6 angle that I used in Dora for the sheer planks, and 30 degrees for the bilge planks.
So far goals 1, 2, 4 and 5 have been met. As for goal #3, Dora was quite easy to construct so I will use the same approach for this canoe.
A peek at the design
The images below give an idea how the canoe is going to turn out. With my approach to design and building it isn't necessary to make drawings, mostly I work with rough sketches, but I like to use an application called Free!Ship to make sure I am not going to make an ugly boat before I start cutting wood! Profile shows the hull viewed from the side and Plan View is from above - only half the hull because both sides are identical. Bodyplan looks at the hull from astern, and the higher bow outline can also be seen. The Free!Ship application also helps me with stuff like calculating where she will float when loaded, and how stable she will be.
Cutting the Planks from the Plywood Sheet
This image is only roughly to scale. It shows the two sheer planks, two bilge planks and the bottom plank, as if they were removed from the boat and flattened. These are called developments. Superimposed is the 8 foot x 4 foot plywood sheet in black. The sheer planks are about 10.7 feet long and require the addition of extensions that extend beyond the edge of the plywood sheet. The bilge planks - which look like segments from an orange - also require extensions. The extensions are cut from the strip cut off the bottom of the plwood sheet. The leftover plywood pieces can be discarded or used to make small decks.
The dimensions are listed in the BAD Design Method paragraph above. The 8 inch extension strip and 2 sheer strips each 6 inches wide are cut off first. The extension strip is divided into a piece 2.75 feet long for the bilge plank extensions and the remainder is divided into 2 equal pieces by a cut at 30 degrees for the sheer extensions as shown in the above figure. What is left, the Remainder, is for the bottom and bilge planks and is set to one side after the center line and bottom length are marked off. Each sheer extension is attached to a sheer strip to form 2 sheer planks, and the sheer edge trimmed to form a smooth curve.
The build process is identical to the building of Dora so there is no need to repeat it here. After the sheer planks are bent, upside-down around a midships form, and glued at each stem, the Remainder is placed on top of the sheer planks, centered fore-and-aft, and positioned beamwise until the bottom length marks each cross the top edge of one sheer plank. One the bottom edges can now be marked using the sheer plank as a guide and the process repeated to get the shape of the other edge. after which the bottom can be cut out.
The hull has been built but, as for several other planned boats, finishing it has been delayed by health problems (Aug 2011). I hope to get back to work soon . . .