Evolution has done its best for us. However, evolution is conservative -- it uses what it has on hand, and it (if indeed it were an an entity, which it is not) is happy enough if a modification does not kill us, does not detract from our survival, and of course better still, aids our chances of survival.
Aids survival, that is, at least until we have produced offspring and lived long enough for our offspring to have offspring. Some theories suggest we should survive a little beyond being grandparents to help our offspring with their offspring for a few years. After that, evolution is done with us, and we should quietly die (preferrably after digging our own graves and being of minimal trouble to our kin group during this last stage of aging). Well, life is hard but them's the rules, and I didn't make them, though no doubt I will try to foil them by living for as long as I can).
All this has led to a defficiency of hands.
Yes, you heard me right. Evolution did not foresee that we would be sailing boats alone, or climbing ladders while carrying too many things, and etc. But, contigency is the wild card of evolution, which, when dealt, makes evolution's facial expression crinkle up in interested annoyance.
Now that the textual minimalists are pissed off, I must get closer to my point; simple: Many of us have experienced the sudden need for a third hand (seldom a third foot, but I am not so sure about that). You can buy a third hand from any store stocking carpenter tools, usually. You can build something more specific to your needs, too. Here I offer one for sailing.
But not just for sailing; I imagine you could attach a third hand to your belt/clothes for other uses. We do that even now, and it is called the carabiner or spring-slip, although we can improve even that very useful stage in the evolution of the third hand. But here is the specific context of my third-hand:
Often we need several hands when we are solo-sailing. These are called "cleats" and they have proved excellent companions. Sometimes we need cleats that allow quick-release features. These are often called "spring-loaded cam cleats" and they are great things.
I merely extended the idea. I wanted a cam-cleat thingie that I could use for any chance need on a boat. That is, some cam-cleats are dedicated, as you might see from a photo on my proa instructuable -- its bolted to the hull and fastens down the sail shunting line and does nothing else. But I had other, more varied needs on the boat, and for that I devised a non-dedicated third hand.
It hangs around, literally, until asked to do something. It might hold my sheet line when I need to fix something else with two hands, but I can quickly jerk the sheet loose to avoid a capsize. It might hold my canteen (all my boat stuff generally has a short line on it for cam-cleating). Or it might hold moy occasionally needed steering paddle or kayak paddle. You can see one of them hanging just below the gunwale in the photo below, left center.
Please have a look.