There is no record of Scandinavians using patterns during the "Viking Age" but they certainly had the capabilities. They had nails, they had marking tools, they had battens, and they could make thin pine sheets but whether or not they used this method is unknown. But during the reconstruction of the Gislinge boat we use both pattern making and going directly from the plank. This will explain the pattern making method we use.
The Gislinge boat, like most Viking ships, is a "clinker-boat" or "lap-strake." That means that the planks over lap eachother. As opposed to a Carvel planked boat which the planks meet tightly on each edge, giving the hull a smooth look. The most common method of pattering off a Carvel plank is called "spilling." This is the act of making a patter in a negative space between two planks or marks. Normally when building on forms. This can also be done with a "clinker-boat" but you will need forms or molds. The Gislinge boat is build in the traditional manner. From the keel up, without forms.
Building without forms or molds poses a few problems. First, you don't know where the planks will land. And second, you don't know the angle or flare of the planks going outboard. When you build using forms, the ends of the outer most edge of the planks are defined right on the form translated from the lofting, this allows you to spilie your plank to your markings. You also have the angle of the planks right there. The forms will also be make to the lofting so the angle is already defined. All you have to do, if your lofting is correct, is lay the plank flat to the form and you have your angle. The way this is rendered in the traditional building method is by using what are called "tic-sticks."
In order to find the plank widths and angles on Gislinge boat they've come up with a rather smart and easy method. The information needed to find angles and widths for the planks come from a line plan determined from lofting a scale model projected from the original Gislinge boat.
Using the information from the plan, marks are made on the tic-sticks to triangulate the angles. Each station is marked and a center line is run from each stem. These points are used to find the langths and angles of each plank.
On one side of the tic-stick is the width of the plank and on the other is the langth from the line.
Each plank langth is represented by a mark and a number. Each station has is own stick.
After the width of the plank is marked on the pattern or plank if your going without a pattern, the tic-stick is tuned around and flipped over to determine the angle of the plank at that station.
Getting the right angle on the pattern is very important. The idea is to have the pattern lay naturally and not force it into place. If you move one section you have to make sure there isn't any edge-set in the other section. Edge-set is a forced radial bend in the pattern and this is "very not good" as Ture says. It may look ok on the boat but when you put the pattern down on the planks to make your marks it won't be the same line. The edge-set leaves a memory in the piece and will return and your line will be bogus. You don't want that.
After you have gotten all your marks lined up and your angles set you can draw your line following the top edge of the previous plank. Always remember to add the overlapping edge when you transfer your line. On the Gislinge boat we are using a 3.5 cm overlap. It really sucks if you forget to do this. Trust me.
After all your marks have been laid and you have double checked your angles and you feel confident we take the pattern off the boat and get ready to lay it out. This is a rather easy step unless you forget which marks are yours. The tendency is to use only one patten for every plank so by the third or fourth plank it can get a bit confusing if you don't make it clear which one is yours. Trust me.
After the marks are laid out, assuming that you haven't forgot which ones are yours, you can lay a batten.
After a good bit of standing and starring at the batten we go ahead and draw the line. After the line is drawn you can proceed to cut your line. Making the same scores when shaping then you can go back and cut to the line.
And with all this done you can start to shape your plank.
Whether or not if the Vikings used this method of patter making or any at that is unknown. Most could probably plank a ship by eye. But this is a very effective method, whether you're building a Viking ship or just a regular clinker ship. Have fun.