Okay, first of all, a knife is a crappy self-defense tool unless you are willing to spill blood, and lots of it. Call it crazy, but most people would rather get raped, beaten, killed than cut someone else open. But, let's assume the lady in question has the ability to protect herself with a knife as well as the gumption to use it, okay? My first recommendation would be the Emerson La Griffe (or one of Fred's own custom models). The blade length is small, but the knife offers a lot of advantages due to its design (see it at Emerson's webpage). First off, it has the index finger ring. This insures that the knife will not slip forward and cut the user, be knocked out of the hand by the opponent, or be lost while making a wild slash (remember, life and death fights are generally not highly controlled environments). This also allows an experienced user to grapple the opponent and use open-hand techniques if needed, but that is the realm of the trained and experienced martial artist. The other nice thing about the ring is that you can pull a larger knife while still holding onto the La Griffe, or even a gun. very cool.
Second, the hawksbill shape, although short in this case, is wicked. The La Griffe comes from Emerson with a wickedly sharp edge, and the point is like a needle. This will be great for poking or slashing your opponent, as the knife will want to rip deep into their flesh. On the other side of the coin, the La Griffe has a short blade because the user can defend themself more easily in court. At under 2" in length, without skill or luck (bad or good, depending on how you look at it), you will be hard-pressed to render your opponent fatally wounded. At the same time, you can do an enormous amount of damage to them all the same. This is a great knife for people who want a non-lethal edged alternative. A 20" Ang Khola would be great, but you could accidentally take an entire limb off with a single swing, so choose your weapon wisely! In my review of the La Griffe, which I posted in the review forum and should be archived there, I stated that the La Griffe is a great self defense tool for people who don't know how to defend themselves. If you must carry a knife for fending off attackers, this would be the one to get, in my opinion. That said, they're illegal in any state that illegalizes fixed blade carry, and although the sheath is well suited for concealed carry, again, most states do not allow it.
The three best defense tools are your brain (don't get in a situation to begin with), your legs (run like hell), and your voice (scream bloody murder). A good can of powerful mace is also probably better than a knife, too. But, there you have it, if one must be carried to feel safe. The other nice thing about the La Griffe is that it's coating is very stealthy. I did an experiment walking my pooches one night and walked with the knife deployed for use in the park. Not one person I passed noticed it, and I made no attempt to hide it from its regular position in my hand.
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My Custom Kydex Sheath page:
http://www.geocities.com/CollegePark/Lab/1298/kydex.html
Palmer College of Chiropractic
On Two Wheels
agocs_s@dd.palmer.edu
Madpoet (Mel Sorg, Jr.) Tribute page:
http://www.geocities.com/CollegePark/Lab/1298/madpoet/main.html