Kjetil--You got the girth measured correctly. It's a measure of how fat the canoe is. At 132mm (52") thats a good sized canoe. I'm going to work through the calculation in English system measurements because I am less likely to overlook something. The method will be the same for metric dimensions, but you might get a slightly different result from rounding off numbers as you convert.

Each strip is .75 inches wide, but each strip has a cove cut in one side and a bead on the other side. When they fit together they actually cover .625 inches each. I divide 52 inches by .625 inches per strip and get 83.2 strips to go around the boat at the widest part.

All your strips are not going to need to be 18 feet long. Some on the bottom of the boat will be much shorter. I always plan on enough strips to go around the boat at its widest part and as long as the boat. That gives me some extra wood to allow for breakages, waste from cutting, and being selective in matching color and woodgrain.

Each strip will be .25 inches thick, but even with a thin saw blade I will waste half a strip as sawdust for every strip that I cut. So I allow .375 inches for each strip when ordering wood. Multiply 83 strips by .375 inches per strip and I get 31.1 inches. I would order sizes that are easy to work with. Six pieces of wood each six inches wide and 18 feet long and .75 inches thick would be adequate.

You may want to consider a piece of wood long enough to cut the longest strips at the gunnels in one piece, but you could always scarf together two shorter pieces.

I think that if you substitute your actual metric measurements you should get a very accurate answer.