a boat from foam-fiberglass laminate panels

I have found three ways to figure the miter angles:

The first is to split the angle by turning it into two congruent triangles. This can be done with a compass, setting it at a distance from the angle joint, then extending its scriber end some distance past the angle joint, then scribing an arc. Do the same on the other side of the angle joint, making sure to put the pivot end of the compass the same distance from the joint as you did on the other side. Scribe another arc. Where the two arcs cross is where the line that splits the triangle goes.

Another method works on the same principle. It does not require a compass. With this method, you measure a given distance from the joint angle and leave a mark there. The you do the same on the other side of the joint angle, making sure to leave the 2nd mark as far from the joint angle as it was on the 1st. Connect these two marks with a straight line, then measure the distance between them. Divide that by 2, and measure that distance from one of the two original marks, and make a 3rd mark there. A line connecting the 3rd mark to the joint angle will divide that amgle into two equal ones.

The third method requires less measuring. Make a cardboard strip that is as wide as the material is thick. Lay this along one side of the joint angle joint (on the inside), so that one end extends past the angle joint. Scribe a line along the inside side of this piece of cardboard until it reaches the other side of this angle. Do the same on the other side of the angle joint. From where these two new lines cross, draw a line to the angle joint. This will show your miter angle.