The question "How usable is a knife with a short tang? Is there a minimum length required for a serviceable knife?" came as the result of the request in our August 30, 2017 e-mail for topics that you would like answers to.
A narrow tang knife, one in which you can't see the tang because it is enclosed in the handle, must have a tang that is at least two-thirds the length of the handle. This is needed to provide strength to the handle. If the tang is shorter, the user is relying on whatever the handle is made of for the strength, which is usually not terribly strong.
If you will look at the narrow tang knives which we make in our shop here in Rogers (examples include A. G. Russell Shopmade and Morseth), or the narrow tang knives which Bob Dozier makes, you will see that the tang goes all the way through the handle with some kind of a nut on the butt end to add extra security to the handle. I have included a picture of a Morseth knife which we made years ago to show our customers how our Morseth knives were, and are constructed.
I dare say that most narrow tang knives that are handmade are made this way, while most factory knives are not. The best brands will make the tang at least two-thirds the length of the handle. Cheaper knives may have shorter tangs.
Notes from A.G.