Alex Comb

The open form holds special appeal to those who don’t want to build more than one of a single wood-and-canvas design or store a more conventional heavy solid form in the shop or garage.

When European Americans decided to replicate the native bark canoe in the late 19th century, they faithfully reproduced some aspects of the craft while using construction methods that were decidedly different from those used by Native Americans. Not only did they replace the highly prized bark of the paper birch with cotton canvas, but they also inverted the construction process. Natives built the canoe from the outside in. Europeans built the canoe from the inside out, developing a labor-intensive solid form that would withstand the rigors of production building (see WB Nos. 141–143). Indeed, canoe factories have built hundreds of canoes on a single such form.

When I started building wood-and-canvas canoes and boats in the 1970s, I used the solid form but found that it had limitations. For instance, once the form was built, it was very difficult to make significant changes in the boat’s shape. While the Native American could build a canoe any length and any width he wanted with varying amounts of rocker, those of us following the solid-form method were pretty much stuck with the size and shape we had created in the form—a form that took many hours to build and that could not be stored easily when not in use. I wanted a simpler form that would allow for changes from boat to boat.

In the years since I began building, I have been refining an “open-form” method for developing and building new boats. The sport boat shown in this article is my Fishdance design, which I can build in a variety of lengths to satisfy the disparate desires of my customers. The purpose of this article is not solely to teach you how to build this particular boat, but rather to use the boat to illustrate the process I use to build a one-off design in wood and canvas so that you have another option when you come to build your preferred design.

The Open Form

The primary difference between the open form and the solid form is that the form itself consists of station molds that are not completely covered with stripping, as in the solid form (see sidebar, page 87), but rather has ribbands that are spaced a few inches apart. One obvious limitation of this approach is that there are no metal bands to automatically clench the tacks that hold the planking to the ribs, so a handheld backing iron is used instead. There is also a potential problem in that the thin cedar ribs common in this type of construction could be easily crimped when bent over ribbands so I bend mine over a 1⁄8″ strip of polycarbonate plastic. I’ll discuss these points in greater detail later when we delve into the boat’s construction.

Before building the form, it’s a good idea to steambend the stems. I like to keep my stems drying in the bending jig for three weeks or more. With the stem pieces bent and held in place, let’s turn our attention to the form itself.

Building the open form begins with the molds. This is the same as building a solid form. If your plans show the finished dimensions (sizes) of the molds, you can build them directly from the plans or patterns. The Fishdance and many canoe plans enable you to do this. However, if your plans show only the lines of the finished canoe to the outside of the hull, you will have to subtract the thickness of the ribbands, ribs and planking from the hull dimensions to get the actual mold pattern dimensions. This exercise is explained fully in boat building books such as Robert M. Steward’s Boat building Manual and others.

Alex Comb

The keel backer provides longitudinal strength to the form and will negate the need to brace each station mold individually.

Backers and Braces

With the mold shapes established, transfer patterns onto plywood sheets and make sure to include whatever reference lines—waterline and sheer, for example are marked on the patterns. You’ll also need to include some details that are unique to this building system namely, notches for the so-called inwale backers and keel backer. The inwale backers, as their name suggests, provide a solid clamping surface for the inwales during construction and the keel backer does the same for the keel. The keel backer also assures proper alignment of the molds, and it eliminates the need to brace each mold individually.
Cut out each station mold and mount it on a level building platform. A good platform can be made from two 2×6s with top edges that are dead straight, and of a length that will accommodate your chosen design. I prefer a building platform for most canoes to be 16″ wide, so I cut pieces of 2×4 into 16″ lengths and mount them onto the 2×6s at the station lines and attach the plywood station molds to the 2×4s. I mount my station molds on 14″ centers, but your plan may have different requirements.

With a canoe that is symmetrical about the ’midship station I would place the center station in the middle of the platform and measure the station intervals from it. I mention a symmetrical canoe because that is the most common type of craft that employs wood-and-canvas construction. The Fishdance design is a sport boat basically, a canoe with a transom. It is not symmetrical about the ’midship section, so the station molds are all mounted to one side or the other of the station lines either on the side toward the closest end of the boat if you want to bevel the stations, or on the side toward the center of the boat if you don’t.

Pick up the depth of the keel backer at each station and, by plotting these depths on a piece of 1×4 stock, define the rocker to the keel. This piece of 1×4 will become the keel backer. Use a fairing batten to check the fairness of your points, draw a line to connect them, and then saw out the keel backer. Mark the station lines along the keel backer’s length, as this will be helpful when aligning the stations. The marks are also helpful in settling any discrepancies that may arise when the station molds are erected. Then fit the keel backer into its notches. Tap it in, check for proper alignment, and then screw it to each mold, driving the screws through the face of the backer and into the cheek of each key slot.

As in the construction of many strip-planked canoes, I attach the stem backer perpendicular to the second station of the canoe. The first station is made from two small pieces that are screwed to the sides of the stem backer. I usually use 2× stock of fairly clear pine or spruce for this. This forwardmost station mold should be mounted to the forward side of the station line (toward the end of the boat), then beveled to provide a fair landing for the ribbands. I normally screw my ribbands to these thicker station molds.

When setting up the form, it is important to keep the stations in proper alignment and to make sure that all areas of the hull will be fair. It is also important that the stem backer does not develop a twist. The stem backer is held in position by being fastened to the second station mold; it is also screwed to the building platform. The bilge ribbands will do a good job of keeping everything in alignment once they are attached, but the stem backer must be aligned and braced before these ribbands are secured. Brace some pieces of plywood against the second station mold by running them at an angle along the stem backer and then screw them to it. This will help keep things aligned.
The inwales will be clamped to an inwale backer, which is simply a strip of ash or some other straight grained, flexible wood. Screw the inwale backers into their respective notches on each station mold at the sheerline in a fair curve. If you see humps or hollows, spend the time necessary to correct them. You should end up with inwales that match each other in height.

Alex Comb

A plywood mold called the “stem backer” is used to define the shape of the bow in this boat. It would be used at both the bow and stern in a symmetrical canoe.

Alex Comb

If building a boat that has a transom, you’ll screw ribbands to the aftermost stations and also to the“transom backer” in the stern. Install the transom backer at this stage.

The Flat Ribband

White ash makes excellent ribbands. It is strong, flexible, and readily available in many regions. If you can’t find wood in the lengths you need, I would suggest gluing shorter pieces together with a 10:1 scarf joint to make up the length. In lieu of ash, oak would work, as would a variety of other woods. Run a flat ribband (1″ wide × 5⁄8″ thick) along the turn of the bilge at each station. Attach it at the middle station mold and then run it out to the ends of the boat, making sure that it follows a fair line. Mark where the ribband lands on each mold and use a rasp to make a flat spot where this ribband will land. Then attach the ribbands to the molds with plastic ties as shown in the photos on the facing page.

Alex Comb

It is important to first place a flat ribband at the turn of the bilge, as this is probably the most defining line in the hull.

Alex Comb

Drill a 3⁄16”-diameter hole through the flat ribband, angling it to ease passage of the plastic tie as it travels from the mold. Thread the plastic tie to attach the ribband to the form. Repeat at each mold intersection.

Install the Stem and Keelson

With the form set up and the two flat ribbands attached at the turn of the bilge, you can get ready to attach the stem and keelson if there is one. Remember the stem stock you steam-bent and placed in a jig a few weeks ago? Now is the time to put it to work. Set your bandsaw table at the appropriate angle (mine is about 12 degrees) and cut bevels on the stem cheeks.

Notch the heel of the stem to accommodate the ribs, spacing notches as your plan indicates. I temporarily fitted the stem onto the form to make notching the last few ribs easier. Then I removed it to the bench and cut notches for the remainder of the ribs.
On to the keelson if your boat has one. After cutting all necessary notches in the stem, screw the keelson to the stem and then install the assembly on the form. Once the keelson is attached, I plane a rolling bevel to match the deadrise angle. Now back to the ribbands.

Round Ribbands

Looking for the best method to connect the ribbands to the station molds took a circuitous course. I began by using plastic electrical ties, but found they were not strong enough to pull ribbands into the twists needed toward the ends of the canoe. Where the electrical ties broke I switched to steel wire. It did its job, though was quite tedious and, as I discovered, could result in nicks and cuts: I nearly emptied my first aid kit of Band-Aids. Then I realized I could sidestep the problem by rounding the corners of the ribbands. Later still, it occurred to me that if the ribbands were round in section instead of rectangular there would be no need to twist them, and the plastic ties would be strong enough throughout.

In laying out for the ribbands, I divide up the girth of the ’midship station mold so that there will be a ribband at least every 4″. In some areas the ribbands may bunch together and in other areas they may be spread apart. Bunching is not a problem, and greater spacing is fine if it is in an area where the curve of the boat is gentle. On the boat pictured, I only rounded the ribbands for part of their lengths because this boat is quite flat aft, but it’s best to round the whole length for a double-ended canoe.

A simple table-mounted router makes rounding the ribbands easy. I use a 3⁄8″-radius round-over bit and take four passes on the faces of 1″ × 1″ stock to make 3⁄4″-diameter ribbands.
When using these 3⁄4″-thick ribbands I drill holes that are large enough for the tie to pass through, whereas when I use 9⁄16″-thick ribbands I simply rasp a groove diagonally across the ribband at each station so that the electrical tie fits into the groove and can be looped through a hole in the station mold and then cinched up.

Alex Comb

Ribs will need to lie flat on the ribbands. When securing the ties, make sure they’re not proud of the ribband, or they’ll interfere with the ribs.

Alex Comb

Occasionally I find a spot where driving a screw makes more sense than using a tie. In this case, you’ll want to make sure to countersink the screw heads below the surface of the ribband to prevent them from fetching up on the ribs. Note that screws land between rib notches; this spacing will facilitate ribband removal later on.

Install the Ribs

After you install the remaining ribbands and clamp the inwale to the form, prepare the rib stock. The ribs must be steamed, and since steaming will drive the moisture out of the wood, you might consider using green lumber for this. Otherwise, air-dried works well although I have found it helpful to soak the stock beforehand. I soak the ribs in water for several days. This makes them supple and helps them to hold their shape once bent.
The ribs are bent over and attached to the open form as with a solid form. The difference is that we are bending over the aforementioned strip of 1⁄8″ polycarbonate plastic, which offers a temporary continuous curve for us to bend against.

Once the rib is bent the strip of plastic can be slid out from under the rib and the rib can be tacked into place. Normally, a 2 × 2 strongback is attached to the form at the keel line so that the ribs can be slid beneath it and held tight. The strongback is easily attached to the keel backer with some utility screws. With this method a gap must be left between the strongback and the form to allow both the rib and the plastic strip to pass. Once the plastic strip is removed, shims can be used to force the rib tight against the mold. (The sport boat pictured has a keelson already installed, so no strongback is necessary; the ribs are nailed tight to the keelson.)

Getting the ribs to take the shape of the form is vital to keeping the boat true to its design. I often use wooden toggles that can be screwed into a ribband and hold two adjacent ribs tight to the form. These are the same toggles used on the solid forms. It is important in either case to keep the ribs in place until they are dry and set. It is perhaps more critical in the open-form method since the ribbands must be removed or at least relocated in order for the planking process to begin.

Alex Comb

After clamping the inwale in place you can bend on the ribs. Use a 1⁄8″ piece of polycarbonate plastic to help support the ribs as you bend them onto the form. Once bent on, slide the plastic out from underneath the rib and tack the rib to the inwales.

Alex Comb

Before starting to plank I always leave the flat ribband at the bilge on the form and normally remove the rest, but as a precaution, I chose to only remove the first three ribbands in the bottom of the boat.


Because mine is a square-sterned boat I was able to pull the ribbands that I wanted to remove out of the rear of the form without releasing the clamps holding the inwales. Normally, in a double-ended canoe I will release the inwale entirely. But great care must be taken to return it to the original position after the ribbands are removed. Locator marks can be made on the inwale and inwale backer to enable this.

Along the way I built my transom. I left the transom off the form until these ribbands could be removed and then I clamped it to the transom backer so that the planking could be nailed to it as it was being installed. I waited until it was clamped in place to finish planing the rolling angle at the transom edge using the bent ribs as a guide.

A clenching iron that varies in thickness from 1⁄2″ to 3⁄4″ with a slight crown on one face works well in flat areas of the boat; an iron called a Martin Wedge, which tapers quite thin on one end and has a large curved end that offers a grip on the other works well throughout the boat, though because of its higher crown it is problematic in the flat areas. A canoe clenching iron can also be used, but it is not as versatile. Slide the appropriate iron under the rib where the tack is to be driven, hold it tight to the rib and it will clench the tack as it is driven. A few tacks may not be accessible for clenching because the form interferes. It’s best to wait and install these tacks after the canoe is off the form.

Once the canoe is removed from the form, the remainder of its construction and finish is the same as that of any other wood-and-canvas canoe or related craft.

Alex Comb

The open form is adaptable, easy to break down, and produces fine results.

This article was originally published in WoodenBoat No. 217, November/December 2010.