Fir is a heavier wood, but it is stronger than cedar on a size-for-size basis. If a plan calls for a cedar part to be an inch thick, but you use fir instead of cedar, you can use a piece of fir which is 3/4 or 7/8 inch thick. The smaller fir part will be lighter than a fir part made to match the larger dimensions of the specified cedar part, but it may still be heavier than the cedar part. The difference in weight won't be as great.

The strength should be compatible, but the size may not be. If your plans are drawn with notches for the stringers, and these are sized to fit 1 inch stock (cedar) then anywhere you replace a cedar part with a fir part you would need to modify the size of the notch.

So, go through your plans and check for the size of the components. It takes a little extra planning time, but you can frequently substitute materials, and not get too big of a weight change. Sometimes you are better off with a different material that what the original designer used.

As long as you are going to carf together shorter lengths of wood, why not look for an ash, maple, oak, or hickory board and make your stringers from one of these strong hardwoods? Ash is much stronger and less brittle than cedar, and has a long history of use as stringers on kayaks.

Hope this helps.

PGJ