A strip built boat is made of small strips of wood so if you were wondering where the name came from, now you know. The method is often called cedar strip, because the wood used is typically cedar. It does not have to be. Any straight grained, light-weight wood can be used. I have used redwood, pine, and mahogany as well as Alaskan cedar, western red cedar, northern white cedar, and atlantic white cedar.
The reason you want straight grained is because you will be bending the wood around forms. This stress will break wood with poor grain. For this reason you also want to avoid knots. They cause a weak spot and are also hard to smooth when you start fairing the boat. You want light weight, because you want a light boat. If you don't then ignore this advice.
The strips are typically cut out of a 3/4" thick board. The 1/4" thick strips are cut off the edge of the board with a table saw or band saw resulting in 1/4" by 3/4" pieces. These strips can be used as is or you can mill them further and make cove and bead strips.
Cove and Bead
Cove and bead strips like the one on the left have a 1/4" diameter groove cut in one edge and the other edge is rounded over to a 1/4" diameter. The strip thus becomes like a tongue and groove board. The bead nestles neatly into the cove ensuring a tight joint.
There are several advantages to this. The first is a light-tight joint, you won't get any embarrassing light shining through your hull. The surface stays smoother because the joint assures that neighboring strips stay at the same elevation. Because of this, you don't need as many staples. Finally all the above happens while you are stripping around bends.
If you just leave the edges "natural" you will end up with grooves between the strips. Structurally this is not too big a problem. The grooves tend to be small and they will fill up with epoxy. It doesn't look good however, and the boat will be lighter and stronger if you don't have the grooves. Since the grooves fill up with epoxy, you are adding the extra weight of the epoxy unneccessarily. If the groove does not fill up there is room for the structure to collapse.
To fix these problems you can plane the edges. With care this will create nice tight joints. There is nothing wrong with this technique. I've built my boats this way for years. With a little practice you soon become proficient at planing in a rolling bevel that changes as the shape of the boat varies. It does take practice and patience to do a good job. Luckily perfection is not required. Like I said about the natural edges, any gap will fill with epoxy.
I've also found that with some of my more compex shapes, especially those with hollow sections, that the cove and bead actually makes the strips want to lift off the forms. The geometry of the cove and bead makes them most comfortable when there is only a small angle between adjacent strips. With more extreme curves, the bead may lift out of the cove creating a gap. This gap may not be visible until after you have done some sanding. Because of this, when I want the tightest joints between strips, I will often take the time to individually hand bevel each strip instead of using the faster and easier cove and bead.
The advantage of cove and bead strips is the cove accepts the bead even when it comes in at an angle so a tight joint is assured even around fairly tight corners. The disadvantage of cove and bead strips is you need to make or buy them. You can find several sources for cove and bead cutting router bits. You then run the strips one-by-one through the router for the bead and then the cove. Even with good tools this is not the easiest task. Again it takes some patience. You need to be careful not the make the edges of the cove too fine as they will be delicate. The average router bit is 1" in diameter and this small size can result in tearout and ragged edges.
It is hard to say whether cove and bead strips are worth it. They make the boat go together faster, but you pay for it with some additional time up front if you are cutting your own. You will pay a bit of a premium if you buy pre-milled strips, but chances are they will be better than strips that you can make yourself and you save quite a bit of labor time.
Length of Strips
The tendancy is to try and use full-length strips. It seems logical that one strip that is long enough to span the full length of the boat would be easier than piecing together several shorter strips. For some parts of the boat this is true, but not always. You would also think that avoiding joints in a strip would creating a stronger boat. This is not something you need to worry about.
Short strips are actually easier to work with sometimes. A 18' strip needs to be handled with some care. It is hard to move around a shop. A 9' strip is alot easier to handle. When you get to the bottom of the boat you need to taper both ends of a strip to fit into the remaining space. The shaping of the taper requires some precision fitting for tight joints at both ends. It is easier if you fit one end of one strip and the other end of another, then fit a joint between them somewhere in the middle.
Short strips need to be joined together to fill the same space as a long strip. This can be accomplished with several different scarf joint. The simplest is just a butt joint. Cut the ends of the two strips square and push them together. This is easy and does not substantially weaken the boat once it is covered in glass. The only reason to do any other kind of joint is cosmetic. You can cut a joint diagonally across the strip or diagonally through the strip. What you do depends on what you want it to look like.
The grain of wood is defined as either "flat" or "vertical". In flat grained wood the growth rings are approximately parallel to the surface of the board and in vertical grained wood the growth rings are perpendicular. Either variety can be used for strip building but vertical grained is easier to work with.
In the figure above the growth rings are rising to the surface as they would in flat grained wood. Different regions in each yearly growth ring are of different densities. The different densities sand at a different rate. With good sharp sand paper such as (a) above, this is not a big deal. The difference between the soft and hard is not significant because the sharp grains of sand can still cut the hard sections. As the grains get dulled (b), they can no longer cut the hard stuff easily, but they still cut the soft wood easily. As a result you can get a ripple in the surface. This ripple will show up in the finished boat.
Vertical grained wood is easier to use because the grain does not rise to the surface in the same accute angle as it does in flat grained wood. As a result the hard part of the growth rings is much easier to cut through. The grain is also much closer together so the sand paper can not reach into the soft part of the wood as easily which is what causes the ripples.
You can avoid the rippling problem by using a plane to do most of your fairing, but even planing is easier in vertical grained wood.
Notice in the above illustration that vertical grained strips are cut from flat grained boards. This can be a problem, because generally the best wood is cut into vertical graind boards because vertical grained boards are less likely to warp. While warping is not a problem with strip-built boats, if you want vertical grained strips, you will want to find flat grained boards.
The illustrations used on this page are extracted from my book "The Strip-Built Sea Kayak"