The strength of butt and scarf joints is directly and proportionally related to the length of glue bond. Any claims of one type being inherently stronger then the other is improper and misleading. A short scarf is weaker then a long backed butt, and visa versa.

For reference, a true butt joint would be simply the flat ends of two boards butted up against one another with a bit of glue holding them together. That would be a very weak joint. What is called a butt joint in boat building is actually a reinforced or backed butt joint, made as just described, but (pardon the pun) with the addition of a third or even fourth member laying over the actual butt joint itself, and glued to both boards on each side of the actuall butting. The longer this member, the stronger the joint.

Both joints cause changes in the bending characteristic of the material being joint. The scarf to a lesser degree than the butt joint due to the additional material added in reinforcing the butt joint. However a scarf joint does not bend like the original material because of the addition of the adhesive. The effect of the adhesive is often so minor as to be nearly invisible though.

The butt joint is flexible, and does not inherently refuse to bend. It bends in direct proportion to flexibility of the additional reinforcement material, and inversely to the thickness of that reinforcement material. The thicker the reinforcement panels, the less willing it is to bend. The longe the reinforcement panels, the longer an area that is less willing to bend. In the case of thin sheets of plywood, the bending characteristics of the boards are not drastically altered. Tapering the ends of the reinforcement panels to eliminate abrupt changes in the bending character of the assembled panels, reducing the visibility of the changes, and makes the panel stronger by eliminating hard spots of abrupt changes.

Assembling a good butt joint is quite simple to do, requiring virtually no skill. Align the two panels on a flat surface, apply a glue, and lay the reinforcement panel on top. Let dry.

To do a scarf joint requires greater skill, and it is more prone to misalignment in all three dimensions. The edges of the scarfs must line up as above in doing a butt joint, but they mus also not lay above one another. A mistake very easily made when sliding the wedge shapes of scarfs over one another. If that occurs, there is no fix. You can sand and fair the incorrect joint to reduce its visibility, but it will always be there.

For the gradual curves in panels that is required in kayak building, a tapered reinforced butt joint quite adequately meets the needs and is virtually to literally invisible on the assembled hull. The reinforcement panels will be quite visible on the inside of the hull though. A properly done scarf joint, while more difficult, will be virtually to literally invisible on both sides of the hull.