Updated Summer 2022

See also:
About rudders
About decks
Skeg-wheel trolley!

Short answer: Yes.

A skeg is like a fixed ‘rudder’ under the back of the boat and makes it easier to go straight (track). Only the very cheapest single-skin vinyl IKs don’t have skegs (Grabner, one of the most expensive IKs, is an exception; though it’s an option). If you’re IK doesn’t have one, it’s easy to glue one on (see below).

Some flat-floored IKS have up to three (imo, a gimmick) and many skegs on Chinese-made IKs are unnecessarily tall (too deep) which makes them snap-prone. Just about all skegs can be easily removed by hand, because in shallow rivers you might want to do so to avoid grounding. You can as easily buy a spare and trim it with a hacksaw.

Years ago Gumotex introduced a slip-on, black plastic tracking fin (skeg, above) which was near identical in shape to one I’d had made to replace the old-style oversized alloy skeg (left). A smaller skeg made better clearance and still tracked fine. But metal bends; tough plastic is much better and that’s what all their boats come with now.
I’ve fitted these plastic skegs to older Gumotex IKs and other IKs. The kit is about £25 + glue, and the plastic skeg is pretty much unbreakable. Just make sure you glue the mounting patch on really well; it helps if your boat is made from a matching rubber fabric as the supplied Nitrilon patch or make your own patch from same fabric. But I have glued PVC to rubber successfully – use good two-part glue. The pictures below help you see where to position a skeg.

I fitted the Gumotex plastic skeg to my Grabner Amigo IK (above) and at sea used it all the time. But on the shallow River Spey (below) this boat didn’t handle at all well without a skeg, possibly because the tailwind pushed the kicked-up stern around. It was really quite annoying because a few years earlier my broadly similar Sunny managed the Spey just fine without a skeg, so skeg-free tracking clearly varies from boat to boat.

Paddling without a skeg
If you’re an experienced paddler you’ll have acquired the knack of going straight without a skeg – handy for paddling shallow rivers where the skeg would ground. A little more paddling finesse and constant small corrections are required, especially if powering on.
It’s good to learn this technique before you need to: fix your eyes on a tree or marker on a distant bank and paddle as gently as you like towards it, not looking away and keeping the nose of the boat in line with the marker. By using very light strokes you’ll see it can be done if it’s not too windy when again, a skeg helps with tracking (going straight).

I even found I could paddle a ten-foot Solar 300 (above) without a skeg. Once you know you can go straight without a skeg, it’s just a matter of adopting the same finesse but with a bit more power. Only when you attempt the speeds of a Viking longship will bow deflection or yawing get too much because to paddle faster and still go straight you need a skeg. Out at sea or on busier rivers, I always use a skeg.

I’ve often thought a hinged retractable skeg would be a good idea: it would pivot backwards when dragging in shallows, then drop back down when there’s enough depth. It seems SUPs need skegs and in the US, FrogFish have made such a thing for boards, but you hear the spring can be a weak point.
If your kayak has a rudder mount (or you can make one), another way of doing it is fitting a swing-down skeg similar to kayak rudders. It works the same way as a rudder with a looped cord swinging the skeg up over the stern, or down into the water. The pivot skeg shown top right is made by Advanced Elements for their AirFusion IK and costs about $/£80. Or have a look here.

Packrafts

On shorter, wider, slower solo packrafts the consensus used to be that skegs made little difference. Especially when unloaded and with a full-weight paddler, the bow yaws merrily left to right as your paddling pivots the boat from the back. Or so I used to think until I tried the skeg on my Rebel 2K. Up to then I’d been ambivalent about them – using the same boat fully loaded a few weeks earlier on a fast flowing river, yawing had not been an issue. But unloaded (and with my generous 95kg of ballast) yawing was notably reduced with a skeg. Speed however, was the same or was too small to measure.

One reason some packrafts may manage without a skeg is that way back in 2011 Alpacka invented the ingenious extended stern (right). It helped limit yawing much like a skeg, and effectively positioned the paddler more centrally, like a kayak, while also adding trim.
This idea has been widely copied by just about everyone since and it definitely works, compared to the original Alpackas. But as mentioned, once there’s a good load over the bow, yawing is reduced in any packraft. Anfibio sell a detachable shallow skeg and glue-on patch for €21. My Rebel 2K came with a skeg and I must admit I use it most of the time now. But it does not have a fully extended stern like the Alpacka above right. Then again, my longer Nomad tracked great without the skeg.

Frontal skeg?
Now I have a longer 2.8-m Anfibio Sigma TXL and I am curious to see if adding a frontal skeg will take a difference. Seated centrally or two-up, this boat does not yaw from the boat like an unloaded solo packraft, but it does wander a bit (track poorly), especially with the inflated floor pad). Results soon.