Shapes of karabiners
There is an almost bewildering array of shapes of karabiners available. Shape, size, colour and other individual branding characteristics all combine to overwhelm the novice buyer. Some new climbers seem to find one design that they “like” and then buy lots of them. This may not be the best approach, however.
On your climbing rack, consider carrying a selection of different shapes and sizes of karabiners. They all have different characteristics and therefore different pros and cons. You can then choose the most suitable shape of karabiner for the task at hand. But equally, don’t overdo it and carry several of each size/shape ‘just in case’ you need them.
These are the three main shapes of karabiner to be aware of:
Offset-D Shape Karabiner
In terms of design, the strongest shaped karabiner is a straight-forward ‘D’ shape. This is because the two curves of the ‘D’ keep the load directly against the main ‘back-bar’. However, the disadvantage with a straight ‘D’ is that the opening at the gate is quite narrow. Enter the ‘Offset-D’ (in the centre of the photo). This is almost as strong and the load still lies close to the back-bar. But the wider gate opening caused by the offset makes it easier to use.
The Offset-D is multi-purpose but lends itself to holding a single strand of rope. It is the perfect choice for connecting components in a main belay.
HMS Shape Karabiner
The HMS shape has nothing to do with Royal Navy ships. The name comes from the German word, Halbmastwurfsicherung. Roughly translated, this means a ‘half-a-clove-hitch-belay’. In the UK, this is what we commonly call the ‘Italian Hitch’ or occasionally the ‘Munter Hitch’.
Karabiners with an exaggerated, bulbous shape at one end are perfect for using the ‘Italian Hitch’ to belay another climber. They became known as HMS for this reason, but also get called ‘pear-shaped’.
Even within the general HMS category there are numerous variations. The photograph shows (top left) a DMM Boa. This is an enormous HMS shape that can easily accommodate 2-3 clove-hitches. On the lower left is the DMM Sentinel (sadly not in production any more) which is described as a mini-HMS.
The disadvantage with HMS shapes is that they can easily turn around when unweighted. Then they become cross-loaded across the gate, or put into a position in which they are taking a three-way load. Either of these scenarios compromises the strength of the karabiner.
HMS karabiners suit any use where multiple strands need to fit in the wide end. For example, an Italian Hitch or perhaps one or more clove-hitches. However, only one item needs to be attached at the thin end. This could be a single strand or rope, a sling or a belay loop on harness.
One typical use would be for using an Italian Hitch as a belay method. Another common use is as the waist attachment point in a typical UK trad main belay where two or three clove-hitches might be used.
Oval Shape Karabiner
The oval shape forces the main load to the centre of each curve, upper and lower. Compared to the D-shape karabiner, this is an inherently weaker design. This is because the load is away from the back-bar and exerts some leverage. (But don’t panic – they are all built to withstand any of the loads you are likely to find climbing or mountaineering several times over).
This shape makes them a less good choice for any of the scenarios mentioned so far. But they excel in uses where we specifically want a load to be evenly distributed or to run smoothly through the karabiner.
Typical uses for oval-shaped karabiners include any set-up in which a pulley may be used. For example, assisted and unassisted hoists in crevasse rescues. Many people also like them for racking wires (bottom right in the photo) as they allow the wires to sit centrally without bunching up.