A good topic to start a virtual bar fight. :laughing:
If joining face grain to face grain, I would not expect the dowel to be adding to the joint strength.
If joining face grain to end grain or end grain to end grain, then the dowel may help with the joint strength. I can imagine some minimal length would make sense, but after some length, additional dowel penetration may not make much difference.
I normally use purchased dowels which are 1 1/2in long and normally have 1/2 the length in either piece of the joint.
I mostly use dowels for alignment not to strengthen a joint.
A well fitted dowel can be as strong as an M&T. It is basically a "loose tenon". The difference being that in an M&T, one end of the tenon is part of one of the members. Both are a butt joint fit, and the faces mating help from the two parts rocking on each other.
IMO, the diameter of the dowel is more of a consideration in strength than just the length.
Let me add my two cents. [I've been using dowels for more than 20 years; tried M&T, but quickly came back to doweling.] From triple dressers to tall chests to exterior doors ... never had a problem.
When properly sized and glued, the wood will yield before the dowel/joint fails.
The surface area of the dowel's embedment [the circumference of the dowel x its imbedment] provides the strength to tensile [pull-apart] forces ... so longer is better. For resistance to shear forces [like those which are produced when stepping on the rung of a ladder], the material and diameter of the dowel are major factors. 90% of the dowels I use fall into three categories: 1/4 x 1 1/4; 3/8 x 2; and 1/2 x 3.
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