A folding ("pocket") knife with a smaller blade makes a good accompaniment and back-up to your primary fixed blade knife, but really serves as your primary knife, if you do not normally wear a belt knife. Since most of us don't, it could be all you have to work with, should you fail to get your survival kit out of the aircraft. It isn't really a substitute for a fixed blade survival knife since there is no way it is as strong, but since it could become your primary survival blade, this is no place to skimp. Choose the best you can afford, a quality make with a positive locking blade.
Folding knives with no positive locking mechanism are downright dangerous. You surely don't need a serious knife cut on your hand or finger(s) while trying to survive. Even experienced woodsmen are regularly injured by non-locking folding knives. It is not uncommon in these instances for the cut to go all the way down to the bone. A word to the wise. . .
Locking folders are divided into two categories, "lockback" and "linerlock," with a few exceptions. Lockback designs rely on a spring loaded steel bar to drop into a notch on the upper back end of the blade as it is unfolded all the way. To release the blade, you must depress the back of the locking bar, on the far side of a pivot, to raise it out of the notch, allowing the blade to fold.
A linerlock design relies on a spring steel or titanium side piece of the knife (the "liner") which is set to spring sideways into the space previously occupied by the folded blade, locking into place behind the blade when it is opened. This bears against the whole rear of the blade, or a substantial portion of it. You must press the liner back out of the way, usually via a notch cut in the fixed liner opposite the liner lock, to release the blade and allow it to fold back.
The lockback is much less expensive to construct, tolerates greater sloppiness in construction and is a bit more foolproof in operation, but isn't generally as strong as the linerlock, though there are exceptions to this rule, as to most. The linerlock is found on more expensive knives and adds a good deal of strength to the knife. However, some linerlock designs can be a bit finicky to operate and less than entirely reliable about locking the blade in place, especially if fouled with dirt.
In addition, it is far easier to inadvertently cut your self while closing a linerlock. Your finger starts out in the blade's arc back to the folded position. If, as sometimes, happen when moving quickly, that finger isn't moved out of the way quickly enough, the blade can close on it. Ouch!
I prefer a good linerlock over a lockback, because of the strength advantage, but either will do the job provided you respect the fact that, by its very nature, a folding knife is weaker than a fixed blade. This is the big compromise required in order to make a compact package. It's always a good idea to test the locking mechanism before using the knife, keeping your fingers clear of the sharp edge, just in case.
A knife designed for one-handed opening is generally the best choice. The best one handed designs, for our purposes, have a finger hole in the blade (a Spyderco innovation, now available on many other knives), a small depression or a relatively small protrusion or stud close to the handle. This makes some survival tasks, like splitting wood, easier. A large protruding opening mechanism located well along the blade can get in the way at times.
Almost all locking folding knives can be opened one handed, even those not specifically designed for the purpose. Even most non-locking pocket knives can be opened by using your teeth if need be, but that can be dicey and potentially bloody. The point is, there are enough good ones out there that you shouldn't allow yourself to have to deal with something like that. Select a locking folder which can be easily opened with one hand.
The handle should be large enough to grip firmly with a sturdy and robust drop point or short clip point blade approximately three to four inches in length, give or take a bit. Larger folding knives approaching a four inch blade length and some heavier knives generally aren't going to fit comfortably in a pant pocket and do better in a belt pouch. One advantage of a belt pouch is the knife can't inadvertently slip out of the pocket. I find a two and three quarter inch to three inch blade perfect for my uses.
Essentially, the same guidelines regarding knife features apply as for a fixed blade knife. While you won't find a guard or hilt on most compact folding knives, you can at least look for a handle with a pronounced integral forward lip or forward finger groove to aid in preventing your hand from slipping onto the edge.
A lanyard hole or ring is essential. It's a good idea in a survival situation to clip or tie the knife's lanyard to your person when not using it. Knives have been known to slip out of pockets unnoticed, even those with pocket clips. A knife with additional tools or blades can be useful, but the primary blade should still lock in place.
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For those venturing onto the water, your needs are slightly different. First, forget about that high carbon steel. Water, and especially salt water and HC steel don't go together. Most offshore knives are made with 440A, not the best for holding an edge, but adequate and pretty much impervious to the effects of salt water.
The alternative to conventional stainless steels are some special stainless, such as Boker's "rust and corrosion proof" T15 T.N, and non-steel materials. Spyderco's Salt series have H1 steel that's impervious to salt water. These include colbalt alloys such as Stellite and Talonite, and titanium, all essentially impervious to salt water.
Another difference between land and sea is the need for a marlin spike (primarily used for working knots loose) and a shackle opener. Most specialty marine knives have the marlin spike included, some include a shackle opener. In all other respects the attributes desired of a good sailor's knife are no different than any other knife that could become your survival knife. The best production mariner's knives I've found are made by Myerchin Marine Classics. Their only failing is that none of their folding knives include a shackle opener.