Aligning forms for a small boat building project can be an exacting process. I'm not about to explain how to align building forms perfectly, although I will offer a few tips along the way. Rather I intend to list the many ways forms can be misaligned so that the individual builder may devise his or her own ways of measuring and checking that will chase the demons of misalignment away from their strongback.

The Strongback, External & Internal
There, I've introduced a new word, the strongback. The strongback is the foundation to which the forms are attached. There are as many different types of strongbacks as there are boat builders, but in the world of kayak and canoe builders they tend to fall into two major categories: internal and external. Canoe builders and some kayak builders use external strongbacks; that is the foundation lies outside the finished shape of the boat.

But kayaks, being small and light, lend themselves to internal strongbacks where the foundation is contained entirely within the boat. Once the shape of the craft is established it is separated into hull and deck pieces (Is there any reason why someone doesn't split their kayak into left and right halves?) and the strongback is removed.

The most important property that a strongback can have is rigidity. A strongback that bends, sags, sways, warps or twists between the time the first form is attached and the finished boat is removed will defeat all the builder's efforts at form alignment.

Now for some terminology. Illustration "A" gives us a basis for discussing misalignment. The important concept is the three axes, Roll, Pitch and Yaw. The Roll axis is parallel to the longitudinal centerline of the boat and perpendicular to the forms. The Pitch and Yaw axes are parallel to the forms.

A basic element of boat building is form spacing. Usually, but not always, forms are spaced evenly along the roll axis. For strip-built kayaks and canoes a spacing of 12 to 18 inches is common. This information is always included on the plans, even if for a 16-foot kayak the plans just show 17 forms and say space evenly. (Forms 1 & 17 would correspond to the tip of the bow and the tail of the stern)

To Bevel or Not to Bevel
That is the question. For canoe and kayak builders beveling is important because their boats come to a point at either end. Beveling, or sloping the edges of the forms so that they are parallel to the hull surface at the point of contact, is not usually necessary for strip-built kayaks and canoes. But the choice to bevel or not is important because it determines where the form will be placed. Using our example of the 16-foot kayak above, with the forms spaced at 1-foot intervals, It's easy to take our tape measure and mark our strongback every 12 inches and say these are our form (station) locations. And we'd be correct. What is not as obvious is that each form has two sides and is probably between one-half and three-quarters of an inch thick and that only one side of the form can lie on the station marks on our strongback. Here's the rule:

Rule 1
If the forms are beveled the larger side of the form lies on the station mark on the side of the line towards the nearest end of the boat.
If the forms are not beveled the form sits on the station mark on the side of the line away from the nearest end of the boat.

In either case of Rule 1 the side of the form that is on the station mark is said to be the "controlling" side of the form.

So the first task is to position the controlling side of the form on the station mark. There are several ways to arrive at a form mispositioned relative to it's station mark but as likely as not it will be too close to the bow (The First Way of Misalignment) or too close to the stern (The Second Way of Misalignment).

If we were clever
We marked the centerline of our boat on the strongback before we began placing our forms. If we were extremely clever we also marked the centerline on each form. The task at hand is to place the form at the correct position along the Pitch axis. In other words, the form should not be too far towards the left or port side of the boat (The Third Way of Misalignment), nor should it be too far towards the right or starboard side of the boat (The Fourth Way of Misalignment).

If we were clever enough to mark the horizontal centerline of the boat on the strongback we probably also thought to mark the vertical centerline. But wait! Our boat is symmetrical from left to right, but not from top to bottom. There is no vertical centerline. Instead there are waterlines, which don't really have anything to do with the actual waterline of the finished boat except that they are roughly parallel to it. Waterlines for kayaks and canoes are typically spaced every 1 or 2 inches. A good choice for a waterline to align forms to is one that passes through most of, if not all, the forms.

Using an internal strongback it should be possible to mark the chosen waterline on the strongback itself. In the case of the external strongback the waterline will not lie on the strongback, but at some distance from (often above) the strongback. If the top surface of the strongback is true, the alignment waterline can be positioned an arbitrary distance away. Choose a distance that is easy to remember, like 10 inches. In reality, we are establishing that the top surface of the strongback is coincident with a waterline, namely the waterline chosen for alignment plus or minus (depending on the direction the waterlines are numbered) 10 inches. If the top surface of the strongback is not true, establish a true horizontal line to measure from, perhaps on the side of the strongback.

What we have done is establish two more ways to misalign a form by mispositioning it along the Yaw axis. It can be too high relative to the waterline (The Fifth Way of Misalignment) or loo low relative to the waterline (The Sixth Way of Misalignment).

The Second Half Dozen
The first six Ways of Misalignment are all linear errors, that is they can be determined by measuring with a ruler or tape measure. The second six Ways of Misalignment are rotational errors.

Returning to the station line marked on the strongback and the controlling surface of the form, it is entirely possible that the controlling surface is not perfectly vertical, or in the case of a strongback that is not perfectly horizontal, parallel to the Yaw axis. Visualize the form attached to the strongback at it's base. Now grab the top of the form and pull it towards the bow of the boat. The form rotates around the Pitch axis with the top displaced towards the bow (The Seventh Way of Misalignment). Now pull the top of the form towards the stern (The Eighth Way of Misalignment). This assumes that the forms are 1) perfectly rigid, that is they do not bend when we pull on them, and 2) that they were cut to the correct size and shape (another big assumption) and were flat to began with.

Stand at one end of the boat and look at the waterline that we have choosen to measure from. Is it positioned exactly 10 inches above the reference point on our strongback? Is it positioned exactly 10 inches above the reference point on our strongback at every point along it's length. Is the waterline level from left to right or is the right side too high (The Ninth Way of Misalignment) or the left side too high(The Tenth Way of Misalignment). These are examples of rotating the form on the Roll axis.

In the beginning we established station marks on our strongback. Extend these station marks out to the extreme left and right sides of the form. Does the controlling face of the form lie on the station line across its full width? Or is the right side (The Eleventh Way of Misalignment) or the left side (The Twelfth Way of Misalignment) too far forward? This is rotation around the Yaw axis.

Understanding the ways a form can be misaligned is the first step towards devising methods to insure correct alignment.

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-Mike Scarborough