(Updated August 2008: new applications in new boat)
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.
Step 1: Comfortable, Streamlined, Both?
My first third-hand was for my dory, and I cut it out of 3/4 inch marine ply and installed two cam-cleats and a brass screen-door handle as a line-guide (to help prevent the lines from angling out of the useful grip-range of the cam-cleats). See picture below, the round one, with the ghosts of the two camcleats I once had installed. This worked well. It held my jib sheet and main sheet when the wind was light and with no risk of capsize from gusts.
Note that the third-hand always has a dedicated line with a loop on it (I call it the attachment line; clever, eh?), for tying to a traditional cleat or some other member of the boat. Sometimes I held the third-hand in my hand, and sometimes I attached the third-hand to a cleat as I lazily ghosted along paying attention only to the tiller.
On windy days, no, (no, no, no!) -- I held the lines in my hands (sometimes steering with a leg crooked over the tiller) because short semi-dories capsize easily and are a real bummer to bail out (waves keep refilling it; I once sailed mine back to the ramp mostly full of water and without a rudder, which had floated away, but that is a tale for another day).
Anyway, the round third-hand was comfortable to hold and was later pressed into service for my sprit sail. The sprit sail has two important lines, the snotter and the brail. The snotter pushes the sprit up into the peak of the sail to give the sail its full shape. The brail bundles the loosened sail to the mast to depower the boat, beach/anchor it, or stow the sail and take down the mast and lay inside the boat (the sprit rig excells in these functions, making it very useful in a small boat that is rowed almost as often as it is sailed; see Tim Anderson's canoe spritsail here on Instructables). The third-hand excelled at caring for my small sprit rig, whose set-up tensions were low enough for these small cam-cleats to handle.
Later, when I built my proa, I wanted new third-hands for occasional use. I built the more slender version in the left of the photo below. I gave it a somewhat artistic (I hope) shape which also offered a good grip. Yet, you might guess that it is less-holdable than the simple round version. True, but these third-hands were attached at each end of cockpit (Proa 101 lesson: you switch ends when you tack/shunt a proa, old stern becomes new bow, so you need duplicate things at each end of the boat). They dangled on their attachment lines ready to use for a variety of purposes, as outlined above. For that purpose, more streamlined, artsy third-hands were fine. No reason why aesthetics should not be married to engineering, from time to time.