One question I get asked a lot by new climbers is how many screw gates they should carry, as it’s not uncommon to see some climbers carrying over a dozen screwgates while others carry only one or two. The answer I can give is only a personal one, and in no way implies that those that carry more are wrong.
First off screwgates offer much higher levels of security than a single snap gate karabiner and are also generally stronger and larger in size; important if they are the focal point for large potential loads (such as on belays) and holding bulky knots and ropes. The screwgates disadvantages are they are heavier and in some situations; like winter or alpine climbing, they can freeze up badly, plus they can also lead you into believing that their security is total, which often isn’t the case.
One mistake people make is assuming that a screwgate is crucial in any high-security situation and that nothing else will do, such as in belay setups, or belaying itself. This isn’t the case, and a well-placed snap gate or two snap gates with opposed gates can offer just as much, or even more security.
When there is a static (non-moving) load on a piece, say at each connection to a cordelette, or bolt belay, then it’s fine to use snap gates, as there is virtually no chance of either the gear or the sling/cord escaping from the karabiner. The difference comes when the load is moving around, such as one of your main connection point when belaying, abseiling or when hanging on belays. In these situations, a screwgate offers less chance of accidental separation. Even then this isn’t guaranteed, as screwgate collars do come undone unless really cranked shut (which can lead to stuck gates if screwed down under load). If total security is needed then the only option is either to use two karabiners with gates opposed or a miallon. Using opposed snap gate karabiners works really well in situations where gates often become frozen or are difficult to operate with gloves on, and I often use doubled wire gates in these situations.
So how many should you carry?
Weight is always an issue so I try to keep my screwgates down as low as possible, and will usually only carry two very strong HMS screwgates, adding a smaller D shaped screwgate if I’m using a magic style plate (to be used in its auto-locking mode). The HMS shape isn’t strong, being much weaker then a D shaped crab, so it’s vital that you buy a forged design that maximizes strength. Another factor is its large size, as this gives plenty of room for Italian hitches and other knots when tying yourself to the belay. With these two karabiners, I feel that I can achieve a high level of security, switching to opposed gates when I need further security.