: I am scarfing together boards for keel, chines, and gunwales. I
: don't have a belt sander or a table saw. And I guess I'm too
: dumb. I couldn't understand the instructions for building a
: circular saw jig, as they were all geared toward plywood (and
: several of them required a table saw... why would you.. I mean,
: if you had a table saw?) Anyway... I'm cutting them with a hand
: plane, the old fashioned way.
: On to the question... to calculate the 8:1 or 12:1 ratio, does it
: change depending on whether you're using a width:width scarf or
: a thickness: thickness scarf? For example, on a 5/8"x
: 1.5" x 10', if I'm scarfing across the 1.5" do I need
: to make the scarf 12" long? Whereas it would only be
: 7.5" if scarfing the thickness? Apologies for the naive
: question. I'm new to woodworking, as well as kayak building.

Don't worry about using the plane, it will work fine if it is sharp and a lot of professionals do that.

I prefer to scarf across the thickness because it is less work, wastes less wood and is easier to clamp. It's also a stronger joint because - for the same ratio - there is more glue area. An 8:1 joint across the thickness of a 5/8 piece would be 5" long.

Reasons to go longer are:
The scarfed member will be bent severely at the joint
The 2 pieces slide apart when the joint is clamped (or you can use a panel pin)

Reasons to scarf across the width are:
The joint may be less visible viewed edge-on than at an exposed feather edge (not everyone will agree with that)

If you want to scarf across the width the same ratio will work, but if the plank is bent the glue may creep due to shear forces. You may more glue area for strength; increase the ratio by x root(width/thickness)