Tuesday, December 2, 2008

Tech Tip Tuesday

When you build a canoe, you have several decisions to make about the boat. One of the more important decisions is about the stems of your canoe. It is possible to build a several ways:

  • no stems
  • only an inner stem
  • a combination of inner and outer stems
No Stems

Some builders try to save weight by building a canoe without any stems at all. Gil Gilpatrick follows this building method. As a Maine Guide, he uses canoes that he's built using this method for his guiding business. Considering the abuse that these boats take, this method must be strong enough.

This method depends on the fiberglass coating to provide the strength that you need in the ends of the canoe. As you are building the canoe, the strips meet at the ends and one strip is beveled and the other passes by. The strips are glued together to hold the shape of the canoe until the builder can fair and glass the hull. This method requires glassing the hull inside and out. Glassing up in the ends is difficult and messy.

Other than the glassing, I have some concerns about keeping the hull together during the building process. There just isn't a lot of area for the glue where the strips meet in the ends. Also, the fiberglass and epoxy is flexible. If you have an impact, the fiberglass and epoxy flexes and the soft cedar underneath will crush. The fiberglass and epoxy usually recovers, but the wood fibers do not, leaving a delaminated area where the 'glass and epoxy aren't in contact any more.

Oh yeah. Did I mention that it is difficult and messy to glass up inside the stem? (Yeah, I meant to repeat that...)

One Stem

It's also possible to build with only an inner stem. The inner stem provides a larger bonding surface for the glue than strips alone. It is more rugged than without, but takes a bit more time and effort. To save weight on the small canoes my students build, I recommend softwood for the inner stem - usually poplar or basswood. On small canoes, we do not glass over the stem, but cut the fiberglass about an inch away from the stem and paint the wood with epoxy to seal it. The same delamination issue from impact still remains as you don't have an outer stem.

Inner and Outer Stem

This is my preferred method. I use the same softwood inner stem mentioned above, and either a softwood or hardwood outer stem. I feel that this provides a much nicer cosmetic appearance. Also with a hardwood outer stem, the crushing issue from impact is much less of a problem. The choices of hardwood let you choose from different colors to give many different cosmetic appearances.

In addition, there are several areas along the stem where beginners can have difficulty getting good joints. The stems cover these areas and provide a bit of camoflauge for the bad joints while also giving a pleasing appearance, cosmetically.

Building Stems

If you are lucky enough to have some unique pieces of wood that have a curve matching that of your stem (crooks or knees...) You can saw the stems out of solid stock.

Most of us aren't that lucky.

Another way to deal with the issue is to steam bend the stems. This isn't a bad or difficult method, really, but steam-bending is a subject unto itself. When Jerry Stelmok bent stems for his canoes, they were symmetrical, so he bent them out of one wider piece of stock and cut them in half with a table-saw. (This was done to avoid twisting of the stems.) The one thing to keep in mind is that steam-bent stems have a tendency to relax, so it's best to keep them clamped in place until ready to use.

The way I teach my students to make stems is to laminate them from thin strips of stock. First, I cut thin strips that will bend around the forms without breaking. As I cut the strips, I keep them in the same order that they come off the board. I want to laminate them this way. There is a reason - when it comes to bevelling the strip later, keeping the grain direction constant makes the cutting easier as the grain doesn't change directions and tear out. Here is a pack of strips fanned out:

The stem patterns are covered with a layer of tape so that the stems will not be bonded to the forms.

The strips are then bonded together. We usually use wood glue as the stem will be coated with epoxy. You can use polyurethane adhesives, but epoxies would be overkill.

Starting at one end the strips are clamped to the pattern. (there are holes in the pattern to make this possible) I usually use spring-jaw clamps to keep the strips from sliding side-to-side when slippery with glue. You want to be careful to make sure that the stem isn't twisted.

As I've said before - you can never be too rich, too good looking or have too many clamps.

Once the glue has set, the stem can be removed and will hold the shape once removed from the pattern.

In a later post, we'll talk about trimming the stems and bevelling them.

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